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Re: New at Applications map.

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  • Rui Geraldes
    Thank you. Now that s all more clear. Regards Rui ... each end ... use ... trying to ... (quoted for ... what will ... shunt to ... end of ... weak. Try ...
    Message 1 of 7 , Mar 1, 2009
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      Thank you.
      Now that's all more clear.
      Regards
      Rui

      --- In VLF_Group@yahoogroups.com, "Alan Melia" <Alan.Melia@...> wrote:
      >
      > Coax cable loss is only true for matched cables, that is 50 ohms at
      each end
      > that was why I suggested that you need some matching if you want to
      use
      > coax. Coax only works properly is matched at both ends. You are
      trying to
      > use its screening properties but you do still need to match it.
      >
      > As an example assume your antenna has an capacitance of 20pF
      (quoted for
      > short VLF antenna and probably of the right order) just work out
      what will
      > happen if you feed that to 10m of coax which WILL look like 570pF,
      shunt to
      > ground. You will get 20/570 of the induced emf across the receiver
      end of
      > your coax, or 1/30th of the available signal. That is why it sound
      weak. Try
      > disconnecting the braid at one end and leaving it floating when
      listening to
      > the strongest signal you hear....it should increase in level quite
      > significnatly
      >
      > (Capacitance does not vary with frequency is is a function of the
      cable
      > geometry) the capacitance I was refering to was for 50ohm cable
      which is
      > greater than 75 ohm. Cable loss is unimportant at these
      frequencies, the
      > loss is the lumped capacitive "potential divider" of the high
      antenna
      > impedance and the low (high capacitance) of the cable. Most VLFers
      mount the
      > receiver or at least the first stages out at the antenna so that
      something
      > like an emitter follower can be used to drive the coax. You need
      quite a lot
      > of current to charge and discharge the cable capacitance.
      >
      > As a quick test if you run twin "bell" cable instead of the coax
      you will
      > probably hear more signal and you will probably not pick up as much
      > "domestic noise" as with the coax with the braid disconnected.
      >
      > Actually the capacitancce of the cable has nothing to do with its
      high
      > frequency loss this is dielectic losses in the polythene inner
      insulator.
      > Microwave cables use PTFE, but at those frequncies the skin effect
      where the
      > signal only travels in a very shallow portion of the metal core and
      sheath
      > is a major cause of the incresed losses with increasing frequency
      >
      > Ok you though experiment, but it an incorrect analogy. Your antenna
      is NOT
      > like a very low impeance mains feed. To simulate the antenna put a
      20pF
      > capacitor between the mains and the inner core then measure the
      VOLTAGE
      > across the coax at the either end....you should measure 1v rms !! I
      used to
      > charge coax up to 15kV and discharge it with a spark gap for my
      work on
      > submerged cable systems in the 1960s !!
      >
      > I hope that explains it
      >
      > Alan G3NYK
      >
      > ----- Original Message -----
      > From: "Rui Geraldes" <rui_geraldes@...>
      > To: <VLF_Group@yahoogroups.com>
      > Sent: Saturday, February 28, 2009 8:07 PM
      > Subject: [VLF_Group] Re: New at Applications map.
      >
      >
      > I am sorry Alan, but with all my respect, I disagree with you.
      >
      > According RG6 catalogue I have, the RG6 have the following
      > characteristics:
      >
      > Impedance: 75 + / -3 Ohm
      > Velocity of Propagation: 85%
      > Capacitance: 52 pF / m
      > Return Loss: 20dB min
      > Attenuation: [@68F.(20 C)] dB/100meters
      >
      > 5MHz: 1.90db
      > 55MHz: 5.25db
      > 83MHz: 6.40db
      > 187MHz: 9.35db
      > 211MHz: 10.00db
      > 250MHz: 10.82db
      > 350MHz: 12.63db
      > 400MHz: 13.61db
      > 500MHz: 15.09db
      > 600MHz: 16.73db
      > 750MHz: 18.54db
      > 1000MHz: 21.49db
      > 1450MHZ: 26.16db
      > 1800MHZ: 27.73db
      > 2100MHZ: 31.15db
      > 2250MHZ: 34.03db
      >
      > If in 100m at 5.000.000Hz (5Mhz) the cable "lose" (attenuate) only
      > 1.90db, I suppose the attenuation at 2.5Mhz will be less, and at
      > lower frequency will be less again...
      >
      >
      > As far as a know "the capacitance" (the efect of the capacitance) of
      > the coaxial cable change with the frequency.
      >
      > That's one of the reasons why the coaxial cable lose more signal at
      > high frequencies.
      >
      > So if we are using the cable at a very low frequency the effect of
      > the coaxial capacitance must be very low.
      >
      > I am using the coaxial cable to reduce the interference of one mw
      > transmitter located at 3km from my position and to "reduce" the
      > reception of the local hum interference.(50Hz)
      >
      > One easy test that can be done (do at your won risk) are connect the
      > central cable of 100m cable roll to the mains (220VAC in my case)
      and
      > leave the end of the cable open not allowing any contact between the
      > shield and the central conductor, and measure the AC current in
      lower
      > Amps, and you will find that the current are zero.
      >
      > In other words, the 50Hz (60Hz) of the mains are not affected by the
      > capacitance of the 100m coaxial cable, and in the same way, the
      > 15000Hz will no be affected to.
      >
      > Regard to the impedance, I know the impedance are one big problem I
      > have, but the use of an normal audio transformer maybe will help to
      > improve the reception.
      >
      > Can you please explain-me what are wrong with my interpretation?
      >
      > Regards
      > Rui
      >
      >
      >
      > --- In VLF_Group@yahoogroups.com, "Alan Melia" <Alan.Melia@> wrote:
      > >
      > > Hi Rui I have not looked at the files area but from your
      > description I guess
      > > you are shorting out (by-passing) most of the signal with the
      coax.
      > RG-58
      > > coax is about 100pF per metre so 99% of your signal may be going
      > through
      > > that and not reaching your receiver. If you need to bring the
      > signal in
      > > over screened line you need an impedance converter at the bottom
      of
      > the
      > > antenna ...... something like an emitter follower as a minimum. A
      > quick
      > > check is to remove the coax ground at the "receiver" the signal
      > level should
      > > rise significantly.....but you may bet a lot of household
      > interference.
      > >
      > > Alan G3NYK
      > >
      > >
      > >
      > > ----- Original Message -----
      > > From: "Rui Geraldes" <rui_geraldes@>
      > > To: <VLF_Group@yahoogroups.com>
      > > Sent: Saturday, February 28, 2009 7:58 AM
      > > Subject: [VLF_Group] New at Applications map.
      > >
      > >
      > >
      > > Hi all members,
      > >
      > > For now I am using a very rudimentary equipment for the VLF
      > > reception, but in a non far way future I will improve the
      receiver.
      > >
      > > For now, I am using the sound card from my desktop computer, the
      > > Spectrum Lab program and as aerial, I am using in vertical
      position
      > > (on the roof) one 50mm pvc pipe with 2m long with 150 turns of
      6mm²
      > > isolated electrical cable. The signal come to the computer by one
      > RG6
      > > coaxial cable.
      > >
      > > I suspect I can receive some "stations" but apparently I have some
      > > drift between the frequency marks and the "transponders" I
      receive.
      > >
      > > The level of the signal are very low so I need ad one preamp.
      > >
      > > Please see some examples at Rui folder.
      > > Regards
      > > Rui
      > >
      > >
      > >
      > >
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