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HRPT helical dish feed positioning

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  • Terence
    Dear all, I am experiencing signal strength problems after refurbishing my HRPT dish set-up. If aerials and their design are your field (they certainly aren t
    Message 1 of 11 , Jun 1, 2009
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      Dear all,
      I am experiencing signal strength problems after refurbishing my HRPT dish set-up. If aerials and their design are your field (they certainly aren't mine it seems) then perhaps you may be able to help.

      I was initially only going to put a short request on the forum and then provide any respondent with the full details; but if I have either hit on something useful to others, or done something that others need to be warned not to try, then perhaps I should grasp the nettle and all should know .......... so I've gone for broke ...... and here it all is.

      I have been using the information on Gerald Ihninger's site, linked from the GEO website (http://members.inode.at/576265/equipment.html) to rehash my dish set-up, as well as reading details on the 'Paul W1GHZ' page itself linked from Gerald's site. Paul's site is very comprehensive, but rather technical for my limited understanding.

      My HRPT dish had suffered greatly from inclement weather and become
      irretreivably distorted, so I took everything down for refurbishment as last Winter approached. I have stripped and serviced the G-5500 Az & El rotators (for the third time on this set - I still have the original KR-4500A setup as backup) and replaced the Az potentiometer, tracking having become erratic. Being now retired, with more time on my hands - and less funds, my seemingly promising idea was to use the perfect, but very old, and for some years now redundant, 1 mtr. SDUS WEFAX dish to replace the damaged Timestep 90cm HRPT dish.

      I used the formulae on Gerald's page to calculate the required focal
      length to position the feed and check the F/D ratio (but I am rather
      uncertain of the latter and the Gain and beamwidth calculations and their relevance to what I have done).

      The old Timestep dish figures are as follows:

      Dish nominal 90cms diameter ( I used a Radius (R) of 44cms for the focal length (f) calculation)
      Dish Depth (d) 15 cms
      Thus f= R-squared/4D = (44x44)/4x15 = 32.26 cms
      The dish's actual focal dimensions were 38.1cms to reflector plate, with the feed coil length being 15.25 cms from reflector plate towards the dish centre. (Thus the nearest point of the coil to the dish-centre was 22.85 cms). This seemed approximately consistent with Gerald's comment that the focal point should be somewhere between the reflector and the first turn of the coil, although it is a little more at 5.8 cms.
      Its F/D calculates to 32.26/88 = 0.367

      The old 1mtr WEFAX dish I treated as being 97cms in diameter and it has a Depth of 15.5cms.
      Its theoretical focal length is therefore (48.5x48.5)/4x15.5 = 37.94 cms.
      ( In fact, the old Rx rod/horn element in the WEFAX can feed was actually 53.3cms from the centre of the dish - the 15 cm difference between this and the calculated focal length confuses me).
      The F/D ratio thus comes out at 0.39, not too much different from the
      Timestep dish.
      I added the 5.8 cms difference apparent from the Timestep dish, and set the reflector plate at just over 43 cms from dish centre.

      I am not sure if I have done all this correctly, and am concerned about Paul W1GHZ site's comment on the focal length being more critical than he expected, with a variance of perhaps only 6mm giving a huge drop in DB, but would be grateful for any advice.

      The signal strength seems much lower than before, although I am without the electronic (and mental) equipment to properly quantify this. It is now only just possible for the Timestep HRPT Rx to receive sufficient signal to produce an image from the strongest NOAA satellites at very high elevations. Ironically, CHRPT from FY-1D seems almost as good as ever, affording a signal down to 10 degrees elevation when it was previously rarely any good below about 20 degrees. I suspect there is some significance to this, but am not well enough au fait with such matters to make a judgement.

      My gut feeling has been that the focal length needs to be increased to nearer the 54 cms of the original WEFAX feed-horn, but the maths as I understand them seem to say otherwise, and a 10 to 15 cm increase seems huge unless I have misunderstood the calculations. I now wish I'd made the feed support legs adjustable!

      I am completely out of my depth now and, suspecting I have missed something vital either in my calculations, or in the assumption that the WEFAX dish would be suitable, I would be most appreciative of any constructive help anyone might be able to give.

      Regards to all,
      Terence
    • Andy Eskelson
      First the timestep dish seems to be a different type than the old wefax dish. The basic dish is prob a parabola shape, but the wave then (if I understand your
      Message 2 of 11 , Jun 1, 2009
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        First the timestep dish seems to be a different type than the old wefax
        dish.

        The basic dish is prob a parabola shape, but the wave then (if I
        understand your description correctly), hits a sub reflector which
        directs the wave back to the dish. The focal length is going to depend on
        the shape of this sub reflector as well, so this can change the focal
        point. I agree with your figures that the focal point should be somewhere
        near 33.75 cm I used d^2/16xdepth so 90^2/ 16*15

        However all this is moot.. You already know that the old dish has a focal
        length of some 53cm that is prob due to the shape being slightly
        different.

        As you are fitting the reflective feed onto this dish, you need to
        consider the illumination of the reflector. The simple way to check this
        is to draw it out, it may need tweaking a bit, but it will get you in the
        right area.

        Construct a scale drawing of the dish and it's actual focal point.
        Draw a couple of lines from the edge of the dish to the focal point.

        This forms a triangle, find the point on this triangle where the distance
        between the two lines is equal to the diameter of the reflector plate.
        Then read off the distance from the dish (or the focal point whichever
        direction you are working from).

        That will tell you the minimum distance that you can position the
        reflector plate at. any closer to the dish and the wave from the dish
        will cover a greater area than the reflector plate, so you will lose
        signal.

        (Think of this is optical terms and you will soon see the problem.)

        Use that as your starting point and tweak from there.

        In your situation I would make up a simple signal source, point the dish
        at it and crank the refelector back an forth to tune for max smoke :-)
        That will save a lot of guesswork.

        Andy









        On Mon, 01 Jun 2009 10:10:42 -0000
        "Terence" <tohs@...> wrote:

        >
        > Dear all,
        > I am experiencing signal strength problems after refurbishing my HRPT dish set-up. If aerials and their design are your field (they certainly aren't mine it seems) then perhaps you may be able to help.
        >
        > I was initially only going to put a short request on the forum and then provide any respondent with the full details; but if I have either hit on something useful to others, or done something that others need to be warned not to try, then perhaps I should grasp the nettle and all should know .......... so I've gone for broke ...... and here it all is.
        >
        > I have been using the information on Gerald Ihninger's site, linked from the GEO website (http://members.inode.at/576265/equipment.html) to rehash my dish set-up, as well as reading details on the 'Paul W1GHZ' page itself linked from Gerald's site. Paul's site is very comprehensive, but rather technical for my limited understanding.
        >
        > My HRPT dish had suffered greatly from inclement weather and become
        > irretreivably distorted, so I took everything down for refurbishment as last Winter approached. I have stripped and serviced the G-5500 Az & El rotators (for the third time on this set - I still have the original KR-4500A setup as backup) and replaced the Az potentiometer, tracking having become erratic. Being now retired, with more time on my hands - and less funds, my seemingly promising idea was to use the perfect, but very old, and for some years now redundant, 1 mtr. SDUS WEFAX dish to replace the damaged Timestep 90cm HRPT dish.
        >
        > I used the formulae on Gerald's page to calculate the required focal
        > length to position the feed and check the F/D ratio (but I am rather
        > uncertain of the latter and the Gain and beamwidth calculations and their relevance to what I have done).
        >
        > The old Timestep dish figures are as follows:
        >
        > Dish nominal 90cms diameter ( I used a Radius (R) of 44cms for the focal length (f) calculation)
        > Dish Depth (d) 15 cms
        > Thus f= R-squared/4D = (44x44)/4x15 = 32.26 cms
        > The dish's actual focal dimensions were 38.1cms to reflector plate, with the feed coil length being 15.25 cms from reflector plate towards the dish centre. (Thus the nearest point of the coil to the dish-centre was 22.85 cms). This seemed approximately consistent with Gerald's comment that the focal point should be somewhere between the reflector and the first turn of the coil, although it is a little more at 5.8 cms.
        > Its F/D calculates to 32.26/88 = 0.367
        >
        > The old 1mtr WEFAX dish I treated as being 97cms in diameter and it has a Depth of 15.5cms.
        > Its theoretical focal length is therefore (48.5x48.5)/4x15.5 = 37.94 cms.
        > ( In fact, the old Rx rod/horn element in the WEFAX can feed was actually 53.3cms from the centre of the dish - the 15 cm difference between this and the calculated focal length confuses me).
        > The F/D ratio thus comes out at 0.39, not too much different from the
        > Timestep dish.
        > I added the 5.8 cms difference apparent from the Timestep dish, and set the reflector plate at just over 43 cms from dish centre.
        >
        > I am not sure if I have done all this correctly, and am concerned about Paul W1GHZ site's comment on the focal length being more critical than he expected, with a variance of perhaps only 6mm giving a huge drop in DB, but would be grateful for any advice.
        >
        > The signal strength seems much lower than before, although I am without the electronic (and mental) equipment to properly quantify this. It is now only just possible for the Timestep HRPT Rx to receive sufficient signal to produce an image from the strongest NOAA satellites at very high elevations. Ironically, CHRPT from FY-1D seems almost as good as ever, affording a signal down to 10 degrees elevation when it was previously rarely any good below about 20 degrees. I suspect there is some significance to this, but am not well enough au fait with such matters to make a judgement.
        >
        > My gut feeling has been that the focal length needs to be increased to nearer the 54 cms of the original WEFAX feed-horn, but the maths as I understand them seem to say otherwise, and a 10 to 15 cm increase seems huge unless I have misunderstood the calculations. I now wish I'd made the feed support legs adjustable!
        >
        > I am completely out of my depth now and, suspecting I have missed something vital either in my calculations, or in the assumption that the WEFAX dish would be suitable, I would be most appreciative of any constructive help anyone might be able to give.
        >
        > Regards to all,
        > Terence
        >
      • Patrik Tast
        Hi Terence, In this image Jerry Martes describes the differences betveen an offset and the usual dish
        Message 3 of 11 , Jun 1, 2009
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          Hi Terence,

          In this image Jerry Martes describes the differences betveen an offset and
          the "usual dish"
          http://www.poes-weather.com/~patrik/tmp/parabola%20shape.jpg

          Have you considered using a stack of yagi's for 1.7 GHz? We are about to do
          that (later this summer)

          Patrik

          ----- Original Message -----
          From: "Terence" <tohs@...>
          To: <GEO-Subscribers@yahoogroups.com>
          Sent: Monday, June 01, 2009 1:10 PM
          Subject: [GEO-Subscribers] HRPT helical dish feed positioning


          >
          > Dear all,
          > I am experiencing signal strength problems after refurbishing my
          > HRPT dish set-up. If aerials and their design are your field (they
          > certainly aren't mine it seems) then perhaps you may be able to help.
          >
          > I was initially only going to put a short request on the forum and then
          > provide any respondent with the full details; but if I have either hit on
          > something useful to others, or done something that others need to be
          > warned not to try, then perhaps I should grasp the nettle and all should
          > know .......... so I've gone for broke ...... and here it all is.
          >
          > I have been using the information on Gerald Ihninger's site, linked from
          > the GEO website (http://members.inode.at/576265/equipment.html) to rehash
          > my dish set-up, as well as reading details on the 'Paul W1GHZ' page itself
          > linked from Gerald's site. Paul's site is very comprehensive, but rather
          > technical for my limited understanding.
          >
          > My HRPT dish had suffered greatly from inclement weather and become
          > irretreivably distorted, so I took everything down for refurbishment as
          > last Winter approached. I have stripped and serviced the G-5500 Az & El
          > rotators (for the third time on this set - I still have the original
          > KR-4500A setup as backup) and replaced the Az potentiometer, tracking
          > having become erratic. Being now retired, with more time on my hands - and
          > less funds, my seemingly promising idea was to use the perfect, but very
          > old, and for some years now redundant, 1 mtr. SDUS WEFAX dish to replace
          > the damaged Timestep 90cm HRPT dish.
          >
          > I used the formulae on Gerald's page to calculate the required focal
          > length to position the feed and check the F/D ratio (but I am rather
          > uncertain of the latter and the Gain and beamwidth calculations and their
          > relevance to what I have done).
          >
          > The old Timestep dish figures are as follows:
          >
          > Dish nominal 90cms diameter ( I used a Radius (R) of 44cms for the focal
          > length (f) calculation)
          > Dish Depth (d) 15 cms
          > Thus f= R-squared/4D = (44x44)/4x15 = 32.26 cms
          > The dish's actual focal dimensions were 38.1cms to reflector plate, with
          > the feed coil length being 15.25 cms from reflector plate towards the dish
          > centre. (Thus the nearest point of the coil to the dish-centre was 22.85
          > cms). This seemed approximately consistent with Gerald's comment that the
          > focal point should be somewhere between the reflector and the first turn
          > of the coil, although it is a little more at 5.8 cms.
          > Its F/D calculates to 32.26/88 = 0.367
          >
          > The old 1mtr WEFAX dish I treated as being 97cms in diameter and it has a
          > Depth of 15.5cms.
          > Its theoretical focal length is therefore (48.5x48.5)/4x15.5 = 37.94 cms.
          > ( In fact, the old Rx rod/horn element in the WEFAX can feed was actually
          > 53.3cms from the centre of the dish - the 15 cm difference between this
          > and the calculated focal length confuses me).
          > The F/D ratio thus comes out at 0.39, not too much different from the
          > Timestep dish.
          > I added the 5.8 cms difference apparent from the Timestep dish, and set
          > the reflector plate at just over 43 cms from dish centre.
          >
          > I am not sure if I have done all this correctly, and am concerned about
          > Paul W1GHZ site's comment on the focal length being more critical than he
          > expected, with a variance of perhaps only 6mm giving a huge drop in DB,
          > but would be grateful for any advice.
          >
          > The signal strength seems much lower than before, although I am without
          > the electronic (and mental) equipment to properly quantify this. It is now
          > only just possible for the Timestep HRPT Rx to receive sufficient signal
          > to produce an image from the strongest NOAA satellites at very high
          > elevations. Ironically, CHRPT from FY-1D seems almost as good as ever,
          > affording a signal down to 10 degrees elevation when it was previously
          > rarely any good below about 20 degrees. I suspect there is some
          > significance to this, but am not well enough au fait with such matters to
          > make a judgement.
          >
          > My gut feeling has been that the focal length needs to be increased to
          > nearer the 54 cms of the original WEFAX feed-horn, but the maths as I
          > understand them seem to say otherwise, and a 10 to 15 cm increase seems
          > huge unless I have misunderstood the calculations. I now wish I'd made the
          > feed support legs adjustable!
          >
          > I am completely out of my depth now and, suspecting I have missed
          > something vital either in my calculations, or in the assumption that the
          > WEFAX dish would be suitable, I would be most appreciative of any
          > constructive help anyone might be able to give.
          >
          > Regards to all,
          > Terence
          >
          >
          >
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        • Dale Hardy
          Terence, I have found using a number of 1 square mirrors mounted around and just inside the circumference of the dish, then pointed towards the sun gives a
          Message 4 of 11 , Jun 1, 2009
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            Terence,
            I have found using a number of 1" square mirrors mounted around and just inside the circumference of the dish, then pointed towards the sun gives a very good indication of the focal length. Also having the feed position adjustable makes all the difference.
            cheers
            Dale


            ----- Original Message -----
            From: Terence
            To: GEO-Subscribers@yahoogroups.com
            Sent: Monday, June 01, 2009 8:10 PM
            Subject: [GEO-Subscribers] HRPT helical dish feed positioning






            Dear all,
            I am experiencing signal strength problems after refurbishing my HRPT dish set-up. If aerials and their design are your field (they certainly aren't mine it seems) then perhaps you may be able to help.

            I was initially only going to put a short request on the forum and then provide any respondent with the full details; but if I have either hit on something useful to others, or done something that others need to be warned not to try, then perhaps I should grasp the nettle and all should know .......... so I've gone for broke ...... and here it all is.

            I have been using the information on Gerald Ihninger's site, linked from the GEO website (http://members.inode.at/576265/equipment.html) to rehash my dish set-up, as well as reading details on the 'Paul W1GHZ' page itself linked from Gerald's site. Paul's site is very comprehensive, but rather technical for my limited understanding.

            My HRPT dish had suffered greatly from inclement weather and become
            irretreivably distorted, so I took everything down for refurbishment as last Winter approached. I have stripped and serviced the G-5500 Az & El rotators (for the third time on this set - I still have the original KR-4500A setup as backup) and replaced the Az potentiometer, tracking having become erratic. Being now retired, with more time on my hands - and less funds, my seemingly promising idea was to use the perfect, but very old, and for some years now redundant, 1 mtr. SDUS WEFAX dish to replace the damaged Timestep 90cm HRPT dish.

            I used the formulae on Gerald's page to calculate the required focal
            length to position the feed and check the F/D ratio (but I am rather
            uncertain of the latter and the Gain and beamwidth calculations and their relevance to what I have done).

            The old Timestep dish figures are as follows:

            Dish nominal 90cms diameter ( I used a Radius (R) of 44cms for the focal length (f) calculation)
            Dish Depth (d) 15 cms
            Thus f= R-squared/4D = (44x44)/4x15 = 32.26 cms
            The dish's actual focal dimensions were 38.1cms to reflector plate, with the feed coil length being 15.25 cms from reflector plate towards the dish centre. (Thus the nearest point of the coil to the dish-centre was 22.85 cms). This seemed approximately consistent with Gerald's comment that the focal point should be somewhere between the reflector and the first turn of the coil, although it is a little more at 5.8 cms.
            Its F/D calculates to 32.26/88 = 0.367

            The old 1mtr WEFAX dish I treated as being 97cms in diameter and it has a Depth of 15.5cms.
            Its theoretical focal length is therefore (48.5x48.5)/4x15.5 = 37.94 cms.
            ( In fact, the old Rx rod/horn element in the WEFAX can feed was actually 53.3cms from the centre of the dish - the 15 cm difference between this and the calculated focal length confuses me).
            The F/D ratio thus comes out at 0.39, not too much different from the
            Timestep dish.
            I added the 5.8 cms difference apparent from the Timestep dish, and set the reflector plate at just over 43 cms from dish centre.

            I am not sure if I have done all this correctly, and am concerned about Paul W1GHZ site's comment on the focal length being more critical than he expected, with a variance of perhaps only 6mm giving a huge drop in DB, but would be grateful for any advice.

            The signal strength seems much lower than before, although I am without the electronic (and mental) equipment to properly quantify this. It is now only just possible for the Timestep HRPT Rx to receive sufficient signal to produce an image from the strongest NOAA satellites at very high elevations. Ironically, CHRPT from FY-1D seems almost as good as ever, affording a signal down to 10 degrees elevation when it was previously rarely any good below about 20 degrees. I suspect there is some significance to this, but am not well enough au fait with such matters to make a judgement.

            My gut feeling has been that the focal length needs to be increased to nearer the 54 cms of the original WEFAX feed-horn, but the maths as I understand them seem to say otherwise, and a 10 to 15 cm increase seems huge unless I have misunderstood the calculations. I now wish I'd made the feed support legs adjustable!

            I am completely out of my depth now and, suspecting I have missed something vital either in my calculations, or in the assumption that the WEFAX dish would be suitable, I would be most appreciative of any constructive help anyone might be able to give.

            Regards to all,
            Terence






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          • Terence
            ... Dale, that s a really clever trick I d not heard of. It s a little impractical at present, with the dish being mounted quite high on the roof (for which I
            Message 5 of 11 , Jun 2, 2009
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              --- In GEO-Subscribers@yahoogroups.com, "Dale Hardy" <photon1@...> wrote:
              >
              > Terence,
              > I have found using a number of 1" square mirrors mounted around and just inside the circumference of the dish, then pointed towards the sun gives a very good indication of the focal length. Also having the feed position adjustable makes all the difference.
              > cheers
              > Dale

              Dale,
              that's a really clever trick I'd not heard of. It's a little impractical at present, with the dish being mounted quite high on the roof (for which I had to fight hard for planning consent) but, if I do have to bring it down again I will certainly use it as a double-check.

              Many thanks,
              Terence.
            • Terence
              ... Andy, Yes, just a wider segment from a larger parabola (I hope) ... I have just been told by someone that the old can feed had its effective focal point
              Message 6 of 11 , Jun 2, 2009
              • 0 Attachment
                --- In GEO-Subscribers@yahoogroups.com, Andy Eskelson <andyyahoo@...> wrote:
                >
                >
                > First the timestep dish seems to be a different type than the old wefax
                > dish.

                Andy,

                Yes, just a wider segment from a larger parabola (I hope)
                >
                > The basic dish is prob a parabola shape, but the wave then (if I
                > understand your description correctly), hits a sub reflector which
                > directs the wave back to the dish. The focal length is going to depend on
                > the shape of this sub reflector as well, so this can change the focal
                > point. I agree with your figures that the focal point should be somewhere
                > near 33.75 cm I used d^2/16xdepth so 90^2/ 16*15
                >
                > However all this is moot.. You already know that the old dish has a focal
                > length of some 53cm that is prob due to the shape being slightly
                > different.

                I have just been told by someone that the old can feed had its effective focal point near the aperture, and the dimension from the rod Rx element to the aperture was 13 cms, which, if subtracted from the 53 seems to confirm the 40cm odd basic focal length calculation.
                >
                > As you are fitting the reflective feed onto this dish, you need to
                > consider the illumination of the reflector. The simple way to check this
                > is to draw it out, it may need tweaking a bit, but it will get you in the
                > right area.
                >
                > Construct a scale drawing of the dish and it's actual focal point.
                > Draw a couple of lines from the edge of the dish to the focal point.
                >
                > This forms a triangle, find the point on this triangle where the distance
                > between the two lines is equal to the diameter of the reflector plate.
                > Then read off the distance from the dish (or the focal point whichever
                > direction you are working from).
                >
                > That will tell you the minimum distance that you can position the
                > reflector plate at. any closer to the dish and the wave from the dish
                > will cover a greater area than the reflector plate, so you will lose
                > signal.

                The significance of the proportional size of the reflector had not occurred to me. I will do as you suggest.
                >
                > (Think of this is optical terms and you will soon see the problem.)
                >
                > Use that as your starting point and tweak from there.
                >
                > In your situation I would make up a simple signal source, point the dish
                > at it and crank the refelector back an forth to tune for max smoke :-)
                > That will save a lot of guesswork.

                Yes, it took me no time at all to realise that bottle-screw type adjustment on the support legs would have been worth the effort, but I was trying to keep the weight down.
                >
                > Andy

                Thanks Andy,
                Terence

                >
              • Terence
                Hello Patrick, both dishes were prime focus, so I assumed they were just varying sized segments of the same parabola. I have seen one or two off-set dishes
                Message 7 of 11 , Jun 2, 2009
                • 0 Attachment
                  Hello Patrick,
                  both dishes were prime focus, so I assumed they were just varying sized segments of the same parabola. I have seen one or two off-set dishes used, but sorting out the angles - and changing feed type - seemed more daunting than sticking with prime focus and the original helical feed.

                  Yes, I considered a yagi some years ago, since it was likely to be easier to obtain planning consent, but I recall reading somewhere that it would never be possible to obtain as good reception with a yagi as a dish, unless the yagi was very large, so went no further.

                  I would be extremely interested to hear how your experiment goes, as might others I am sure.

                  Kind regards,
                  Terence.



                  --- In GEO-Subscribers@yahoogroups.com, "Patrik Tast" <patrik@...> wrote:
                  >
                  > Hi Terence,
                  >
                  > In this image Jerry Martes describes the differences betveen an offset and
                  > the "usual dish"
                  > http://www.poes-weather.com/~patrik/tmp/parabola%20shape.jpg
                  >
                  > Have you considered using a stack of yagi's for 1.7 GHz? We are about to do
                  > that (later this summer)
                  >
                  > Patrik
                  >
                  > ----- Original Message -----
                  > From: "Terence" <tohs@...>
                  > To: <GEO-Subscribers@yahoogroups.com>
                  > Sent: Monday, June 01, 2009 1:10 PM
                  > Subject: [GEO-Subscribers] HRPT helical dish feed positioning
                  >
                  >
                  > >
                  > > Dear all,
                  > > I am experiencing signal strength problems after refurbishing my
                  > > HRPT dish set-up. If aerials and their design are your field (they
                  > > certainly aren't mine it seems) then perhaps you may be able to help.
                  > >
                  > > I was initially only going to put a short request on the forum and then
                  > > provide any respondent with the full details; but if I have either hit on
                  > > something useful to others, or done something that others need to be
                  > > warned not to try, then perhaps I should grasp the nettle and all should
                  > > know .......... so I've gone for broke ...... and here it all is.
                  > >
                  > > I have been using the information on Gerald Ihninger's site, linked from
                  > > the GEO website (http://members.inode.at/576265/equipment.html) to rehash
                  > > my dish set-up, as well as reading details on the 'Paul W1GHZ' page itself
                  > > linked from Gerald's site. Paul's site is very comprehensive, but rather
                  > > technical for my limited understanding.
                  > >
                  > > My HRPT dish had suffered greatly from inclement weather and become
                  > > irretreivably distorted, so I took everything down for refurbishment as
                  > > last Winter approached. I have stripped and serviced the G-5500 Az & El
                  > > rotators (for the third time on this set - I still have the original
                  > > KR-4500A setup as backup) and replaced the Az potentiometer, tracking
                  > > having become erratic. Being now retired, with more time on my hands - and
                  > > less funds, my seemingly promising idea was to use the perfect, but very
                  > > old, and for some years now redundant, 1 mtr. SDUS WEFAX dish to replace
                  > > the damaged Timestep 90cm HRPT dish.
                  > >
                  > > I used the formulae on Gerald's page to calculate the required focal
                  > > length to position the feed and check the F/D ratio (but I am rather
                  > > uncertain of the latter and the Gain and beamwidth calculations and their
                  > > relevance to what I have done).
                  > >
                  > > The old Timestep dish figures are as follows:
                  > >
                  > > Dish nominal 90cms diameter ( I used a Radius (R) of 44cms for the focal
                  > > length (f) calculation)
                  > > Dish Depth (d) 15 cms
                  > > Thus f= R-squared/4D = (44x44)/4x15 = 32.26 cms
                  > > The dish's actual focal dimensions were 38.1cms to reflector plate, with
                  > > the feed coil length being 15.25 cms from reflector plate towards the dish
                  > > centre. (Thus the nearest point of the coil to the dish-centre was 22.85
                  > > cms). This seemed approximately consistent with Gerald's comment that the
                  > > focal point should be somewhere between the reflector and the first turn
                  > > of the coil, although it is a little more at 5.8 cms.
                  > > Its F/D calculates to 32.26/88 = 0.367
                  > >
                  > > The old 1mtr WEFAX dish I treated as being 97cms in diameter and it has a
                  > > Depth of 15.5cms.
                  > > Its theoretical focal length is therefore (48.5x48.5)/4x15.5 = 37.94 cms.
                  > > ( In fact, the old Rx rod/horn element in the WEFAX can feed was actually
                  > > 53.3cms from the centre of the dish - the 15 cm difference between this
                  > > and the calculated focal length confuses me).
                  > > The F/D ratio thus comes out at 0.39, not too much different from the
                  > > Timestep dish.
                  > > I added the 5.8 cms difference apparent from the Timestep dish, and set
                  > > the reflector plate at just over 43 cms from dish centre.
                  > >
                  > > I am not sure if I have done all this correctly, and am concerned about
                  > > Paul W1GHZ site's comment on the focal length being more critical than he
                  > > expected, with a variance of perhaps only 6mm giving a huge drop in DB,
                  > > but would be grateful for any advice.
                  > >
                  > > The signal strength seems much lower than before, although I am without
                  > > the electronic (and mental) equipment to properly quantify this. It is now
                  > > only just possible for the Timestep HRPT Rx to receive sufficient signal
                  > > to produce an image from the strongest NOAA satellites at very high
                  > > elevations. Ironically, CHRPT from FY-1D seems almost as good as ever,
                  > > affording a signal down to 10 degrees elevation when it was previously
                  > > rarely any good below about 20 degrees. I suspect there is some
                  > > significance to this, but am not well enough au fait with such matters to
                  > > make a judgement.
                  > >
                  > > My gut feeling has been that the focal length needs to be increased to
                  > > nearer the 54 cms of the original WEFAX feed-horn, but the maths as I
                  > > understand them seem to say otherwise, and a 10 to 15 cm increase seems
                  > > huge unless I have misunderstood the calculations. I now wish I'd made the
                  > > feed support legs adjustable!
                  > >
                  > > I am completely out of my depth now and, suspecting I have missed
                  > > something vital either in my calculations, or in the assumption that the
                  > > WEFAX dish would be suitable, I would be most appreciative of any
                  > > constructive help anyone might be able to give.
                  > >
                  > > Regards to all,
                  > > Terence
                  > >
                  > >
                  > >
                  > > ------------------------------------
                  > >
                  > > Paypal - Paypal - Paypal - Paypal - Paypal
                  > >
                  > > GEO now accept Paypal for subscription renewal and shop purchases
                  > >
                  > > See the web site for details
                  > >
                  > > *** RENEWAL of SUBSCRIPTIONS REMINDER ***
                  > >
                  > > Check your status by looking at the label on your latest issue or contact
                  > > members@... by email
                  > >
                  > > ** AUTOMATIC CREDIT CARD RENEWAL **
                  > >
                  > > GEO will be operating automatic credit card renewal to save you time and
                  > > reduce our administration. If you wish to convert to automated renewal
                  > > contact members@... by email. Also a tick box is provided on
                  > > the application form to indicate your preference. The form can be found
                  > > here :- http://www.geo-web.org.uk/join.pdf Your credit card details will
                  > > be held securely.
                  > >
                  > > Contact details ### GEO is run by volunteers ###
                  > >
                  > > David Anderson members@... Membership enquiries
                  > > Ray Godden webmaster@... GEO website
                  > > Les Hamilton editor@... GEO Quarterly Editor
                  > > John Tellick info@... General enquiries
                  > > Peter Wakelin meteo@... Meteorological matters
                  > > Francis Bell francis@... Public Relations and Education
                  > > Clive Finnis shop@... *** GEO SHOP ***
                  > > Yahoo! Groups Links
                  > >
                  > >
                  > >
                  >
                • Patrik Tast
                  Hi Terence, Oh no, the yagis are small (4 x 700 mm long) and extremly light (~1 kg). I built my stack using copper wires and fiberglass tubes from old fishing
                  Message 8 of 11 , Jun 2, 2009
                  • 0 Attachment
                    Hi Terence,

                    Oh no, the yagis are small (4 x 700 mm long) and extremly light (~1 kg). I
                    built my stack using copper wires and fiberglass tubes from old fishing
                    rods.
                    I have not yet had the time to test it but EZNEC show that we should get ~22
                    dB gain.

                    Here is an image when it still was under construction when I attached it to
                    my homebrew Az/El rotator
                    http://www.poes-weather.com/~patrik/1.7GHz/CIMG2916.AVI (3 mb)
                    http://www.poes-weather.com/~patrik/1.7GHz/ (665 kb)

                    Here are the length and distances & positions of the elements
                    http://www.poes-weather.com/~patrik/1.7GHz/hrpt-yagi-wires.txt

                    Regards,
                    Patrik



                    ----- Original Message -----
                    From: "Terence" <tohs@...>
                    To: <GEO-Subscribers@yahoogroups.com>
                    Sent: Tuesday, June 02, 2009 8:18 PM
                    Subject: [GEO-Subscribers] Re: HRPT helical dish feed positioning


                    > Hello Patrick,
                    > both dishes were prime focus, so I assumed they were just
                    > varying sized segments of the same parabola. I have seen one or two
                    > off-set dishes used, but sorting out the angles - and changing feed type -
                    > seemed more daunting than sticking with prime focus and the original
                    > helical feed.
                    >
                    > Yes, I considered a yagi some years ago, since it was likely to be easier
                    > to obtain planning consent, but I recall reading somewhere that it would
                    > never be possible to obtain as good reception with a yagi as a dish,
                    > unless the yagi was very large, so went no further.
                    >
                    > I would be extremely interested to hear how your experiment goes, as might
                    > others I am sure.
                    >
                    > Kind regards,
                    > Terence.
                    >
                    >
                    >
                    > --- In GEO-Subscribers@yahoogroups.com, "Patrik Tast" <patrik@...> wrote:
                    >>
                    >> Hi Terence,
                    >>
                    >> In this image Jerry Martes describes the differences betveen an offset
                    >> and
                    >> the "usual dish"
                    >> http://www.poes-weather.com/~patrik/tmp/parabola%20shape.jpg
                    >>
                    >> Have you considered using a stack of yagi's for 1.7 GHz? We are about to
                    >> do
                    >> that (later this summer)
                    >>
                    >> Patrik
                    >>
                    >> ----- Original Message -----
                    >> From: "Terence" <tohs@...>
                    >> To: <GEO-Subscribers@yahoogroups.com>
                    >> Sent: Monday, June 01, 2009 1:10 PM
                    >> Subject: [GEO-Subscribers] HRPT helical dish feed positioning
                    >>
                    >>
                    >> >
                    >> > Dear all,
                    >> > I am experiencing signal strength problems after refurbishing
                    >> > my
                    >> > HRPT dish set-up. If aerials and their design are your field (they
                    >> > certainly aren't mine it seems) then perhaps you may be able to help.
                    >> >
                    >> > I was initially only going to put a short request on the forum and then
                    >> > provide any respondent with the full details; but if I have either hit
                    >> > on
                    >> > something useful to others, or done something that others need to be
                    >> > warned not to try, then perhaps I should grasp the nettle and all
                    >> > should
                    >> > know .......... so I've gone for broke ...... and here it all is.
                    >> >
                    >> > I have been using the information on Gerald Ihninger's site, linked
                    >> > from
                    >> > the GEO website (http://members.inode.at/576265/equipment.html) to
                    >> > rehash
                    >> > my dish set-up, as well as reading details on the 'Paul W1GHZ' page
                    >> > itself
                    >> > linked from Gerald's site. Paul's site is very comprehensive, but
                    >> > rather
                    >> > technical for my limited understanding.
                    >> >
                    >> > My HRPT dish had suffered greatly from inclement weather and become
                    >> > irretreivably distorted, so I took everything down for refurbishment as
                    >> > last Winter approached. I have stripped and serviced the G-5500 Az & El
                    >> > rotators (for the third time on this set - I still have the original
                    >> > KR-4500A setup as backup) and replaced the Az potentiometer, tracking
                    >> > having become erratic. Being now retired, with more time on my hands -
                    >> > and
                    >> > less funds, my seemingly promising idea was to use the perfect, but
                    >> > very
                    >> > old, and for some years now redundant, 1 mtr. SDUS WEFAX dish to
                    >> > replace
                    >> > the damaged Timestep 90cm HRPT dish.
                    >> >
                    >> > I used the formulae on Gerald's page to calculate the required focal
                    >> > length to position the feed and check the F/D ratio (but I am rather
                    >> > uncertain of the latter and the Gain and beamwidth calculations and
                    >> > their
                    >> > relevance to what I have done).
                    >> >
                    >> > The old Timestep dish figures are as follows:
                    >> >
                    >> > Dish nominal 90cms diameter ( I used a Radius (R) of 44cms for the
                    >> > focal
                    >> > length (f) calculation)
                    >> > Dish Depth (d) 15 cms
                    >> > Thus f= R-squared/4D = (44x44)/4x15 = 32.26 cms
                    >> > The dish's actual focal dimensions were 38.1cms to reflector plate,
                    >> > with
                    >> > the feed coil length being 15.25 cms from reflector plate towards the
                    >> > dish
                    >> > centre. (Thus the nearest point of the coil to the dish-centre was
                    >> > 22.85
                    >> > cms). This seemed approximately consistent with Gerald's comment that
                    >> > the
                    >> > focal point should be somewhere between the reflector and the first
                    >> > turn
                    >> > of the coil, although it is a little more at 5.8 cms.
                    >> > Its F/D calculates to 32.26/88 = 0.367
                    >> >
                    >> > The old 1mtr WEFAX dish I treated as being 97cms in diameter and it has
                    >> > a
                    >> > Depth of 15.5cms.
                    >> > Its theoretical focal length is therefore (48.5x48.5)/4x15.5 = 37.94
                    >> > cms.
                    >> > ( In fact, the old Rx rod/horn element in the WEFAX can feed was
                    >> > actually
                    >> > 53.3cms from the centre of the dish - the 15 cm difference between this
                    >> > and the calculated focal length confuses me).
                    >> > The F/D ratio thus comes out at 0.39, not too much different from the
                    >> > Timestep dish.
                    >> > I added the 5.8 cms difference apparent from the Timestep dish, and set
                    >> > the reflector plate at just over 43 cms from dish centre.
                    >> >
                    >> > I am not sure if I have done all this correctly, and am concerned about
                    >> > Paul W1GHZ site's comment on the focal length being more critical than
                    >> > he
                    >> > expected, with a variance of perhaps only 6mm giving a huge drop in DB,
                    >> > but would be grateful for any advice.
                    >> >
                    >> > The signal strength seems much lower than before, although I am without
                    >> > the electronic (and mental) equipment to properly quantify this. It is
                    >> > now
                    >> > only just possible for the Timestep HRPT Rx to receive sufficient
                    >> > signal
                    >> > to produce an image from the strongest NOAA satellites at very high
                    >> > elevations. Ironically, CHRPT from FY-1D seems almost as good as ever,
                    >> > affording a signal down to 10 degrees elevation when it was previously
                    >> > rarely any good below about 20 degrees. I suspect there is some
                    >> > significance to this, but am not well enough au fait with such matters
                    >> > to
                    >> > make a judgement.
                    >> >
                    >> > My gut feeling has been that the focal length needs to be increased to
                    >> > nearer the 54 cms of the original WEFAX feed-horn, but the maths as I
                    >> > understand them seem to say otherwise, and a 10 to 15 cm increase seems
                    >> > huge unless I have misunderstood the calculations. I now wish I'd made
                    >> > the
                    >> > feed support legs adjustable!
                    >> >
                    >> > I am completely out of my depth now and, suspecting I have missed
                    >> > something vital either in my calculations, or in the assumption that
                    >> > the
                    >> > WEFAX dish would be suitable, I would be most appreciative of any
                    >> > constructive help anyone might be able to give.
                    >> >
                    >> > Regards to all,
                    >> > Terence
                    >> >
                    >> >
                    >> >
                    >> > ------------------------------------
                    >> >
                    >> > Paypal - Paypal - Paypal - Paypal - Paypal
                    >> >
                    >> > GEO now accept Paypal for subscription renewal and shop purchases
                    >> >
                    >> > See the web site for details
                    >> >
                    >> > *** RENEWAL of SUBSCRIPTIONS REMINDER ***
                    >> >
                    >> > Check your status by looking at the label on your latest issue or
                    >> > contact
                    >> > members@... by email
                    >> >
                    >> > ** AUTOMATIC CREDIT CARD RENEWAL **
                    >> >
                    >> > GEO will be operating automatic credit card renewal to save you time
                    >> > and
                    >> > reduce our administration. If you wish to convert to automated renewal
                    >> > contact members@... by email. Also a tick box is provided on
                    >> > the application form to indicate your preference. The form can be
                    >> > found
                    >> > here :- http://www.geo-web.org.uk/join.pdf Your credit card details
                    >> > will
                    >> > be held securely.
                    >> >
                    >> > Contact details ### GEO is run by volunteers ###
                    >> >
                    >> > David Anderson members@... Membership enquiries
                    >> > Ray Godden webmaster@... GEO website
                    >> > Les Hamilton editor@... GEO Quarterly Editor
                    >> > John Tellick info@... General enquiries
                    >> > Peter Wakelin meteo@... Meteorological matters
                    >> > Francis Bell francis@... Public Relations and Education
                    >> > Clive Finnis shop@... *** GEO SHOP ***
                    >> > Yahoo! Groups Links
                    >> >
                    >> >
                    >> >
                    >>
                    >
                    >
                    >
                    >
                    > ------------------------------------
                    >
                    > Paypal - Paypal - Paypal - Paypal - Paypal
                    >
                    > GEO now accept Paypal for subscription renewal and shop purchases
                    >
                    > See the web site for details
                    >
                    > *** RENEWAL of SUBSCRIPTIONS REMINDER ***
                    >
                    > Check your status by looking at the label on your latest issue or contact
                    > members@... by email
                    >
                    > ** AUTOMATIC CREDIT CARD RENEWAL **
                    >
                    > GEO will be operating automatic credit card renewal to save you time and
                    > reduce our administration. If you wish to convert to automated renewal
                    > contact members@... by email. Also a tick box is provided on
                    > the application form to indicate your preference. The form can be found
                    > here :- http://www.geo-web.org.uk/join.pdf Your credit card details will
                    > be held securely.
                    >
                    > Contact details ### GEO is run by volunteers ###
                    >
                    > David Anderson members@... Membership enquiries
                    > Ray Godden webmaster@... GEO website
                    > Les Hamilton editor@... GEO Quarterly Editor
                    > John Tellick info@... General enquiries
                    > Peter Wakelin meteo@... Meteorological matters
                    > Francis Bell francis@... Public Relations and Education
                    > Clive Finnis shop@... *** GEO SHOP ***
                    > Yahoo! Groups Links
                    >
                    >
                    >
                  • Dale Hardy
                    Another important consideration, if you are using a helical feed, is a tuning stub mounted under the coil. Further improvements can be had by tuning
                    Message 9 of 11 , Jun 2, 2009
                    • 0 Attachment
                      Another "important" consideration, if you are using a helical feed, is a ' tuning stub ' mounted under the coil. Further improvements can be had by 'tuning' the co-ax feed, I can help with a method if you need further info.
                      Dale


                      The signal strength seems much lower than before, although I am without the electronic (and mental) equipment to properly quantify this. It is now only just possible for the Timestep HRPT Rx to receive sufficient signal to produce an image from the strongest NOAA satellites at very high elevations. Ironically, CHRPT from FY-1D seems almost as good as ever, affording a signal down to 10 degrees elevation when it was previously rarely any good below about 20 degrees. I suspect there is some significance to this, but am not well enough au fait with such matters to make a judgement.

                      .





                      [Non-text portions of this message have been removed]
                    • Terence O'Hanlon Smith
                      Dale, the feed is the as issued Timestep one on its original reflector plate. I ve simply remounted it on another dish on longer support legs. If it s of
                      Message 10 of 11 , Jun 2, 2009
                      • 0 Attachment
                        Dale,
                        the feed is the 'as issued' Timestep one on its original
                        reflector plate. I've simply remounted it on another dish on longer
                        support legs. If it's of interest, I've put an image of it at the top
                        of the old GSS animation page on my site (not weather related).

                        http://www.rifleman.org.uk/mslp.htm

                        I am receptive to any method of improving things, since they can't be a
                        whole lot worse at the moment, but I am loathe to interfere with what
                        was a perfectly satisfactory feed arrangement - until I moved it!

                        Regards,
                        Terence



                        Dale Hardy wrote:
                        >
                        >
                        > Another "important" consideration, if you are using a helical feed, is
                        > a ' tuning stub ' mounted under the coil. Further improvements can be
                        > had by 'tuning' the co-ax feed, I can help with a method if you need
                        > further info.
                        > Dale
                        >
                      • Terence O'Hanlon Smith
                        To Dale and Patrik ( & Jerry), I now have the mirrors, but the eight-eighths smoke is getting in the way! Patience is a virtue so they say ..................
                        Message 11 of 11 , Jun 3, 2009
                        • 0 Attachment
                          To Dale and Patrik ( & Jerry),
                          I now have the mirrors,
                          but the eight-eighths smoke is getting in the way!
                          Patience is a virtue so they say ..................

                          Terence.

                          Dale Hardy wrote:
                          >
                          >
                          > Terence,
                          > I have found using a number of 1" square mirrors mounted around and
                          > just inside the circumference of the dish, then pointed towards the
                          > sun gives a very good indication of the focal length. Also having the
                          > feed position adjustable makes all the difference.
                          > cheers
                          > Dale
                          >
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