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Re: [loopantennas] Foil based loop antennas

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  • richard
    On Fri, 30 Apr 2010 09:33:54 EDT ... You can also use as length of heliax it nearly self supporting, LDF 4-50 is equivalent to 13 mm because of the ridges in
    Message 1 of 16 , Apr 30, 2010
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      On Fri, 30 Apr 2010 09:33:54 EDT
      toddroberts2001@... wrote:

      > In a message dated 4/30/2010 8:32:23 A.M. Eastern Daylight Time,
      > bitterlemon40@... writes:
      > I was thinking of making a loop antenna using copper foil. My
      > understanding is that 100mm copper foil would be equivalent to 50mm copper
      > pipe. However there is some talk about 'current crowding' at the edges of
      > the foil. There is a reference in following document that I would like to
      > follow up:
      > http://midnightdesignsolutions.com/MidnightLoop/Midnight%20Loop%20(Mar%2011a ).pdf
      > And the reference is to:
      > Resistance of Foil Conductors For Antennas, Rudy Severns N6LF, QEX
      > May/June 2002 Tech Notes
      > Maybe someone could scan that for me?
      > I have some interest in maximum Q loop antennas. There are indications
      > that a 2 turn foil antenna would be best in that regard. However it is all
      > very complex because of the proximity effect and distributed capacitance etc.
      >
      > Sean O'Connor
      >
      >
      > Sean,
      > I have had very good luck using a strip of copper flashing for a loop
      > element as far as efficiency goes. One thing I did find out is that
      > a copper foil loop will not hold up very well if placed outdoors.
      >
      > If placed outdoors in the open the wind tended to catch the strip
      > and it would twist and vibrate in the wind and eventually fail due
      > to metal fatigue.
      >
      > For an outdoor antenna I would only recommend using round
      > copper pipe as it is very rigid compared to the copper foil and
      > will stand up to the wind and weather much better.
      >
      > For an indoor or attic antenna a copper foil loop should work
      > fine as long as it is held up by a rigid framework with good
      > insulation. I would stick with a single-turn loop.
      >
      > Remember skin effect and use copper foil 10 mils thick or thicker
      > for lowest RF resistance.
      >
      > 73 - Todd WD4NGG
      >
      You can also use as length of heliax it nearly self supporting, LDF 4-50 is
      equivalent to 13 mm because of the ridges in it, but the nice thing is it an
      interference fit inside 15mm Cu tube, the same as 1/2" Cu tube, so yoiu can
      make the loop an strip back the outer cover and stuff it up the 1/2" tube so
      you can use ordinary pipe fittings,
      There is always a good supply of LDF4-50 , just ask any of the VHF/ UHF Dx
      guys, they may have a piece long enough for your needs./
      ie a length that water has got into, once that happens its pretty useless as
      coax ,but it doesn't matter for building loops as you are using the outer
      copper cover. It will also handle power.
      HTH

      --
      Best wishes

      Richard Bown G8JVNM

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    • capntripps2000
      I ve used 1/4 self sticking copper foil used for stained glass work for quick magnetic loop antennas setup in hotel rooms. These run aorund $5 for a 60ft
      Message 2 of 16 , May 1, 2010
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        I've used 1/4" self sticking copper foil used for stained glass work for quick magnetic loop antennas setup in hotel rooms. These run aorund $5 for a 60ft spool. I would run the foil along the hotel window spaced several inches inside the metal window frame. I used gamma matching at the bottom of the loop to match to the coax and a wide spaced air variable capacitor at the top where I split the foil. Using a battery operated soldering iron I tack soldered the wire connections to the foil. I also keep an assortment of capacitors in my bag in case I need to pad the tuning cap. Using up to 20 watts I worked 20-40M CW with good results. I've even worked as low as 80M if the window was large enough and also had surprisingly good results. When ready to check out of the hotel the foil comes off the window easily with no residue and I discard the foil in the trash.

        Stealthy and effective!

        Andrew
        N3LCW




        --- In loopantennas@yahoogroups.com, "sean" <bitterlemon40@...> wrote:
        >
        > I was thinking of making a loop antenna using copper foil. My understanding is that 100mm copper foil would be equivalent to 50mm copper pipe. However there is some talk about 'current crowding' at the edges of the foil.
      • xtemporize
        CQ magazine for April 1967 has an article Skin Effect in High Frequency Design . I quote: The values from the graph (fig. 1) represent the depth at which
        Message 3 of 16 , May 1, 2010
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          CQ magazine for April 1967 has an article "Skin Effect in High
          Frequency Design". I quote: "The values from the graph (fig. 1)
          represent the depth at which currents will be reduced to 1/e (where
          e=2.302) of the surface currents, and it is customary to provide for at
          least 5 times the skin depth shown. For all practical purposes, 5 times is close to 100% conduction." The graph with the article is quite small,but it looks like the basic value for copper is 1 mil and for
          aluminum, 2 mil. So if I interpret this correctly and the article is
          true, 5 mils would suffice for copper, and 10 mil for aluminum. That
          would seem to make copper foil usable. Aluminum foil, I suppose you
          would have to use a much larger width to lower the overall AC resistance. I have thought about making a loop from plastics plumbing
          pipe, and then somehow covering it with copper foil, like maybe a couple
          lengths stretched along the whole length of the pipe, the loop being
          built in a square. I wonder how solderable this copper foil is, if at
          all. I found my two rolls of copper foil in a garbage dumpster after
          some kind of constructon job ( we have a very wasteful country ) but I
          think it is also sold for use in stained-glass hobby projects. Aluminum foil comes in widths up to maybe 18 inches and not expensive at all
          so maybe it could be used somehow. You would have to build some
          kind of box-like framework to attach it to, something looking kind of
          like a box kite. -Hue Miller / Newport, Oregon, USA
        • Jim Dunstan
          ... Hi Andrew A great idea. I m going to practice on the front picture window! By the way ... who caries the copper foil tape ? Thanks, Jim, VE3CI
          Message 4 of 16 , May 1, 2010
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            At 12:37 AM 5/2/2010 +0000, you wrote:
            >
            >
            >
            >
            >I've used 1/4" self sticking copper foil used for stained glass work for
            >quick magnetic loop antennas setup in hotel rooms. These run aorund $5 for
            >a 60ft spool. I would run the foil along the hotel window spaced several
            >inches inside the metal window frame. I used gamma matching at the bottom
            >of the loop to match to the coax and a wide spaced air variable capacitor
            >at the top where I split the foil. Using a battery operated soldering iron
            >I tack soldered the wire connections to the foil. I also keep an
            >assortment of capacitors in my bag in case I need to pad the tuning cap.
            >Using up to 20 watts I worked 20-40M CW with good results. I've even
            >worked as low as 80M if the window was large enough and also had
            >surprisingly good results. When ready to check out of the hotel the foil
            >comes off the window easily with no residue and I discard the foil in the
            >trash.
            >
            >Stealthy and effective!
            >
            >Andrew
            >N3LCW

            Hi Andrew

            A great idea. I'm going to practice on the front picture window! By the
            way ... who caries the copper foil tape ?

            Thanks,

            Jim, VE3CI
          • xtemporize
            The foil I found in the trash is 2 inch width. Pretty nice stuff. Interesting the idea of the disposable loop antenna using narrower stuff. Now re current
            Message 5 of 16 , May 1, 2010
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              The foil I found in the trash is 2 inch width. Pretty nice stuff.
              Interesting the idea of the disposable loop antenna using narrower
              stuff. Now re "current crowding", back around the late 1970s when
              AEA (company ) was offering a loop antenna built on a metallic band
              or stip, this may have been the reason MFJ company touted their loop
              antenna as being more efficient than a model not using a cylindrical
              conductor. Now that AEA is gone, I don't recall seeing MFJ advertisement
              text that still claims this advantage, since there's not really any
              competition. -Hue Miller
            • xtemporize
              It occurred to me that I could try a proof of concept experiment by taking a large cardboard box from a moving company, and running a strip of aluminum foil
              Message 6 of 16 , May 1, 2010
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                It occurred to me that I could try a "proof of concept" experiment by
                taking a large cardboard box from a moving company, and running a strip
                of aluminum foil around it. That would be a strip around 12 inches or more wide. I wonder if she high voltage developed would be a fire hazard
                with the cardboard? At least it would be safe at low powers. I recall
                once many years ago, trying to run a 80 meters, 100 watt transmitter
                into a short indoor antenna, made of aluminum foil by the way, and
                setting the window curtain on fire, from the high voltage flashover.
                -Hue Miller
              • Brent G DeWitt
                Your local home improvement store probably has copper tape as slug tape . The adhesive isn t conductive like EMI/EMC tape from 3M, but it s a lot more
                Message 7 of 16 , May 1, 2010
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                  Your local home improvement store probably has copper tape as "slug tape".
                  The adhesive isn't conductive like EMI/EMC tape from 3M, but it's a lot more
                  available.



                  73,



                  Brent - AB1LF



                  From: loopantennas@yahoogroups.com [mailto:loopantennas@yahoogroups.com] On
                  Behalf Of Jim Dunstan
                  Sent: Saturday, May 01, 2010 9:05 PM
                  To: loopantennas@yahoogroups.com
                  Subject: Re: [loopantennas] Re: Foil based loop antennas





                  At 12:37 AM 5/2/2010 +0000, you wrote:
                  >
                  >
                  >
                  >
                  >I've used 1/4" self sticking copper foil used for stained glass work for
                  >quick magnetic loop antennas setup in hotel rooms. These run aorund $5 for
                  >a 60ft spool. I would run the foil along the hotel window spaced several
                  >inches inside the metal window frame. I used gamma matching at the bottom
                  >of the loop to match to the coax and a wide spaced air variable capacitor
                  >at the top where I split the foil. Using a battery operated soldering iron
                  >I tack soldered the wire connections to the foil. I also keep an
                  >assortment of capacitors in my bag in case I need to pad the tuning cap.
                  >Using up to 20 watts I worked 20-40M CW with good results. I've even
                  >worked as low as 80M if the window was large enough and also had
                  >surprisingly good results. When ready to check out of the hotel the foil
                  >comes off the window easily with no residue and I discard the foil in the
                  >trash.
                  >
                  >Stealthy and effective!
                  >
                  >Andrew
                  >N3LCW

                  Hi Andrew

                  A great idea. I'm going to practice on the front picture window! By the
                  way ... who caries the copper foil tape ?

                  Thanks,

                  Jim, VE3CI





                  [Non-text portions of this message have been removed]
                • capntripps2000
                  I bought several foil rolls on Ebay but they can also be found in arts and crafts stores that carry stained glass supplies. I found some locally after I
                  Message 8 of 16 , May 2, 2010
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                    I bought several foil rolls on Ebay but they can also be found in arts and crafts stores that carry stained glass supplies. I found some locally after I bought mine on Ebay.

                    re; cardboard and fire: unless the loop arcs over somewhere or there are metal particles in the board I wouldn't worry about a few watts of RF. Personally, I'd avoid the cardboard anyway. It reminds me of the Harry Lythall SM0VPO packing crate loop antenna:

                    http://www.sm0vpo.com/antennas/crate.htm


                    Andrew
                    N3LCW




                    --- In loopantennas@yahoogroups.com, Jim Dunstan <jimdunstan@...> wrote:

                    >
                    > A great idea. I'm going to practice on the front picture window! By the
                    > way ... who caries the copper foil tape ?
                    >
                    > Thanks,
                    >
                    > Jim, VE3CI
                    >
                  • Rick Karlquist
                    I would just like to clear up 2 common misconceptions: 1. Tubing vs foil (or any other non-close shape): RF current spreads out on the conductor and
                    Message 9 of 16 , May 2, 2010
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                      I would just like to clear up 2 common misconceptions:

                      1. Tubing vs foil (or any other non-close shape):
                      RF current spreads out on the conductor and concentrates on
                      the edges. Tubing has no edge, therefore the RF
                      current will spread out more uniformly. If the tubing
                      is circular (as opposed to square) the current is forced by
                      symmetry to be uniform and there is no current crowding.
                      This is why round tubing will have the less loss than
                      foil of comparable dimensions.

                      2. Reducing conductor loss in a small loop ultimately results
                      in an impractically high Q with very narrow bandwidth. If
                      you are stuck with a small loop, it just isn't going to be
                      very efficient and there isn't a lot that you can do about it.

                      Summarizing: foil will be lossier than comparable tubing.
                      It may not be much better than wire. So your foil loop will
                      not be especially efficient. OTOH, if foil is convenient to
                      use, go ahead, because loop efficiency is much more about loop
                      size than conductor size.

                      Rick N6RK
                    • david feldman
                      I found on amazon.com JVCC brand 36 yard by 1/4, 1/2 and 1 1 oz (thickness measure) tape with conductive adhesive (USD $12, 22, 39 respectively.)
                      Message 10 of 16 , May 2, 2010
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                        I found on amazon.com JVCC brand 36 yard by 1/4, 1/2 and 1" 1 oz (thickness measure) tape with conductive adhesive (USD $12, 22, 39 respectively.)

                        http://www.amazon.com/JVCC-CFL-5CA-Copper-Conductive-Adhesive/dp/B000WQBRJA/ref=sr_1_3?ie=UTF8&s=hi&qid=1272817726&sr=1-3

                        Maybe it is used in laptop display repair (for shielding the inverter and fluorescent lamp circuitry)?

                        Not known if it can be cleanly removed from window glass...

                        >>N3LCW
                        >
                        > Hi Andrew
                        >
                        > A great idea. I'm going to practice on the front picture window! By the
                        > way ... who caries the copper foil tape ?
                        >
                        > Thanks,
                        >
                        > Jim, VE3CI
                        >

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                      • david feldman
                        Another one - 2 wide tape (thickness unspecified) also 36 yard roll, however, it does include in description text it s aggressive adhesive system... so
                        Message 11 of 16 , May 2, 2010
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                          Another one - 2" wide tape (thickness unspecified) also 36 yard roll, however, it does include in description text "it's aggressive adhesive system..." so maybe not so easily removed? Anyway, USD 49 per 36 yard roll makes interesting possibility for wrapping lengths of PVC (not sure if it would survive outdoor flexing and getting around 45/90 degree angles...)

                          http://www.1stmaskingtape.com/detail.php?pid=11119&cid=575

                          > I found on amazon.com JVCC brand 36 yard by 1/4, 1/2 and 1" 1 oz (thickness measure) tape with conductive adhesive (USD $12, 22, 39 respectively.)
                          >
                          > http://www.amazon.com/JVCC-CFL-5CA-Copper-Conductive-Adhesive/dp/B000WQBRJA/ref=sr_1_3?ie=UTF8&s=hi&qid=1272817726&sr=1-3
                          >
                          > Maybe it is used in laptop display repair (for shielding the inverter and fluorescent lamp circuitry)?
                          >
                          > Not known if it can be cleanly removed from window glass...
                          >
                          >>>N3LCW
                          >>
                          >> Hi Andrew
                          >>
                          >> A great idea. I'm going to practice on the front picture window! By the
                          >> way ... who caries the copper foil tape ?
                          >>
                          >> Thanks,
                          >>
                          >> Jim, VE3CI
                          >>
                          >
                          > _________________________________________________________________
                          > The New Busy is not the old busy. Search, chat and e-mail from your inbox.
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                        • Christopher E. Brown
                          Why go through the hassle? Wrapping copper tape on a PVC form leaves you with a fairly stiff loop, and alot of unknowns. Air con copper tube (available in
                          Message 12 of 16 , May 2, 2010
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                            Why go through the hassle? Wrapping copper tape on a PVC form leaves you
                            with a fairly stiff loop, and alot of unknowns.



                            "Air con" copper tube (available in multiple sizes at any home supply or
                            plumbing house).

                            Thin wall copper pipe, easy enough to make an octagonal loop with a couple
                            sticks and some 45 elbows.



                            If you want flexable/portable there are a number of receive only designs
                            that used coax. You could also use a fine strand DC power cable, #4 can
                            be had that rolls up almost as easy as rope.

                            Losses would be higher, but I suspect losses would be lower in #4 power
                            cable than with copper foil wrapped on a PVC form.





                            Don't get me wrong, the copper foil trick is neat for a temp antenna, used
                            in myself on 6M once I just would not use it for a high current low band
                            antenna where I had another choice.





                            On Sun, 2 May 2010, david feldman wrote:

                            >
                            > Another one - 2" wide tape (thickness unspecified) also 36 yard roll,
                            > however, it does include in description text "it's aggressive adhesive
                            > system..." so maybe not so easily removed? Anyway, USD 49 per 36 yard
                            > roll makes interesting possibility for wrapping lengths of PVC (not sure
                            > if it would survive outdoor flexing and getting around 45/90 degree
                            > angles...)
                            >
                            > http://www.1stmaskingtape.com/detail.php?pid=11119&cid=575
                            >
                            >> I found on amazon.com JVCC brand 36 yard by 1/4, 1/2 and 1" 1 oz (thickness measure) tape with conductive adhesive (USD $12, 22, 39 respectively.)
                            >>
                            >> http://www.amazon.com/JVCC-CFL-5CA-Copper-Conductive-Adhesive/dp/B000WQBRJA/ref=sr_1_3?ie=UTF8&s=hi&qid=1272817726&sr=1-3
                            >>
                            >> Maybe it is used in laptop display repair (for shielding the inverter and fluorescent lamp circuitry)?
                            >>
                            >> Not known if it can be cleanly removed from window glass...
                            >>
                            >>>> N3LCW
                            >>>
                            >>> Hi Andrew
                            >>>
                            >>> A great idea. I'm going to practice on the front picture window! By the
                            >>> way ... who caries the copper foil tape ?
                            >>>
                            >>> Thanks,
                            >>>
                            >>> Jim, VE3CI
                            >>>
                            >>
                            >> _________________________________________________________________
                            >> The New Busy is not the old busy. Search, chat and e-mail from your inbox.
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                          • bm40g
                            Hi All, If you tried to price the cost of copper lately, its clear that for an economic solution to the truely efficient magnetic loop antenna, a non-copper
                            Message 13 of 16 , May 16, 2010
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                              Hi All,

                              If you tried to price the cost of copper lately, its clear that for an economic solution to the truely efficient magnetic loop antenna, a non-copper support of the loop makes sense. Really, to get good efficiency for middle HF frequency (7 to 15 MHz), a three inch diameter copper tubular conductor in the 3 to 4 foot diameter range is a good target.

                              A tubular conductor that size from copper, or even aluminum is very heavy, and will require some significant mechanical design.

                              A PVC octagon made from 3 inch or 4 inch lightwieght (not the solid) PVC tubing is a reasonable cost.

                              If one then, rather than wrap the copper flashing, but laid it side by side in-line with the PVC support tube , a low resitance loop could be made. The issue with wrapping , is that the currents flow in the direction of the tubing, and every time a current has to jump to the next layer of wrapping, it incurs a resistance. So, a wrapped configuration would require soldering every wrap-to-wrap joint, and then you still get the solder surface resistance. By having the conductors in line, with say a tack solder here and there, much less resistance is encurred.

                              Now, current bunching is an issue, but I have not seen any modeling to suggest how much of issue at HF. The practicalities of the flat loop may just not occur until much higher frequencies. Or maybe not. Without any true high frequency analysis, its just not clear how wide the strip has to be before the current bunching increases the resistance by what percentage over the predicted surface area resistance. Indeed, what modelling I have done on this issue tends to indicate the currents stay on the inside surface of a conductor with significant thickness, especially near the drive end of the loop. This implies that the optimum conductor may actually be a half-tube. Where current bunching pushes currents to the edges, but the induction action in the antenna pulls the currents torwards the smallest radius section of the tube. The result being an even coduction across the surface of the half-round conductor.
                            • Richard (Rick) Karlquist
                              ... The problem with this approach is that high efficiency necessarily results in very narrow bandwidth and very high Q (ie high voltages across the gap). At
                              Message 14 of 16 , May 16, 2010
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                                bm40g wrote:
                                > If you tried to price the cost of copper lately, its clear that for an
                                > economic solution to the truely efficient magnetic loop antenna, a
                                > non-copper support of the loop makes sense. Really, to get good
                                > efficiency for middle HF frequency (7 to 15 MHz), a three inch diameter
                                > copper tubular conductor in the 3 to 4 foot diameter range is a good
                                > target.

                                The problem with this approach is that high efficiency necessarily
                                results in very narrow bandwidth and very high Q (ie high voltages
                                across the gap). At some point, it becomes impractical as an antenna.

                                > If one then, rather than wrap the copper flashing, but laid it side by
                                > side in-line with the PVC support tube , a low resitance loop could be

                                Right

                                > true high frequency analysis, its just not clear how wide the strip has
                                > to be before the current bunching increases the resistance by what

                                A tubular conductor uniquely has no edges for current to bunch up
                                on, and due to circular symmetry, the current is forced to be uniform.
                                Any non closed cross section will have current bunching.

                                > percentage over the predicted surface area resistance. Indeed, what
                                > modelling I have done on this issue tends to indicate the currents stay
                                > on the inside surface of a conductor with significant thickness,

                                What tool are you using to model? I did a bunch of measurements on
                                loops and found that the Q goes up as the conductor thickness increases,
                                but only to a point of diminishing returns. After that, you can't
                                get a higher Q no matter how thick the conductor. (This was for a
                                solid wire conductor). This maximum Q was only several hundred,
                                and could not be explained by radiation resistance. I hypothesized
                                that the currents were being bunched up by the magnetic field and
                                were not fully utilizing the conductor.


                                > especially near the drive end of the loop. This implies that the optimum
                                > conductor may actually be a half-tube. Where current bunching pushes
                                > currents to the edges, but the induction action in the antenna pulls the
                                > currents torwards the smallest radius section of the tube. The result
                                > being an even coduction across the surface of the half-round conductor.

                                That's interesting; two opposing effects. Hard to say which one "wins"

                                Rick N6RK
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