Re: New at Applications map.
- Thank you.
Now that's all more clear.
--- In VLF_Group@yahoogroups.com, "Alan Melia" <Alan.Melia@...> wrote:
> Coax cable loss is only true for matched cables, that is 50 ohms at
> that was why I suggested that you need some matching if you want to
> coax. Coax only works properly is matched at both ends. You are
> use its screening properties but you do still need to match it.
> As an example assume your antenna has an capacitance of 20pF
> short VLF antenna and probably of the right order) just work out
> happen if you feed that to 10m of coax which WILL look like 570pF,
> ground. You will get 20/570 of the induced emf across the receiver
> your coax, or 1/30th of the available signal. That is why it sound
> disconnecting the braid at one end and leaving it floating when
> the strongest signal you hear....it should increase in level quite
> (Capacitance does not vary with frequency is is a function of the
> geometry) the capacitance I was refering to was for 50ohm cable
> greater than 75 ohm. Cable loss is unimportant at these
> loss is the lumped capacitive "potential divider" of the high
> impedance and the low (high capacitance) of the cable. Most VLFers
> receiver or at least the first stages out at the antenna so that
> 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
> 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
> frequency loss this is dielectic losses in the polythene inner
> Microwave cables use PTFE, but at those frequncies the skin effect
> signal only travels in a very shallow portion of the metal core and
> is a major cause of the incresed losses with increasing frequency
> Ok you though experiment, but it an incorrect analogy. Your antenna
> like a very low impeance mains feed. To simulate the antenna put a
> capacitor between the mains and the inner core then measure the
> across the coax at the either end....you should measure 1v rms !! I
> charge coax up to 15kV and discharge it with a spark gap for my
> 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
> 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)
> leave the end of the cable open not allowing any contact between the
> shield and the central conductor, and measure the AC current in
> 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?
> --- 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 is about 100pF per metre so 99% of your signal may be going
> > 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
> > antenna ...... something like an emitter follower as a minimum. A
> > check is to remove the coax ground at the "receiver" the signal
> level should
> > rise significantly.....but you may bet a lot of household
> > 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
> > For now, I am using the sound card from my desktop computer, the
> > Spectrum Lab program and as aerial, I am using in vertical
> > (on the roof) one 50mm pvc pipe with 2m long with 150 turns of
> > isolated electrical cable. The signal come to the computer by one
> > coaxial cable.
> > I suspect I can receive some "stations" but apparently I have some
> > drift between the frequency marks and the "transponders" I
> > 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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