Re: LF: RE: Re: VLF_8.79 kHz

[Andy Talbot <andy.g4jnt@googlemail.com>]

I dunno....

Some 99p headphone  'dongles' from Ebay deserve that sort of treatment.

 

Andy
www.g4jnt.com

On 25 February 2010 11:16, Chris <c.ashby435@btinternet.com> wrote:

You should NEVER 'stick an antenna wire into your sound card'.

Chris, G4AYT, Whitstable, Kent.

----- Original Message -----

From: M0FMT

To: rsgb_lf_group@blacksheep.org

Sent: Thursday, February 25, 2010 10:38 AM

Subject: Re: LF: RE: Re: VLF_8.79 kHz

Ken.

Why not just stick an antenna wire into your Sound card mic socket and install either Argo Spectran or Spec lab FFT application. You then have DC to 48kc/s VLF coverage with the ability to detect very very weak signals. Using SDR software you can then record the result in real time spectrum allowing it to be processed by others.

This type of set up is used by many people to copy SAQ on 17.2kc/s.

 

It is the method we are using here for some simple tests on 7.8125kc/s and or 9.765625kc/s depending upon xtal used.

 

You are about 11km from this location if you are keen on this exercise I can let you know when my setup is working. Now that the weather is getting better I will be able to get out working on the antenna.

 

Two arrangements will be tried a vertical loop up abt 12/13m and an earth electrode method. I have no real idea of how to load these antennas because my best calculations say if I use a kW the coil about 1 Henry will dissipate most of the power which means it be getting very very hot and probably fail and need over 2 km of wire. Totally impractical! Some ideas have been kicked around on the is reflector and off list there may be something I can try.

73 es GL petefmt

--- On Thu, 25/2/10, Ken <ken.h.wright@btinternet.com> wrote:

From: Ken <ken.h.wright@btinternet.com>
Subject: LF: RE: Re: VLF_8.79 kHz
To: rsgb_lf_group@blacksheep.org
Date: Thursday, 25 February, 2010, 10:05

Hi Jim.
I think I could knock up a quick 9kHz RX here in Luton over the weekend if
that would help. I have been following this thread with interest, could you
put up some diagrams etc of your parallel capacitor and other antenna
configurations and formulae.

73.
Ken
M0KHW    IO91TV

-----Original Message-----
From: owner-rsgb_lf_group@blacksheep.org
[mailto:owner-rsgb_lf_group@blacksheep.org] On Behalf Of James Moritz
Sent: 24 February 2010 21:14
To: rsgb_lf_group@blacksheep.org
Subject: LF: Re: VLF_8.79 kHz

Dear LF Group,

A couple of years ago I did some measurements of Rloss of the 10m high, 40m
long inv-L antenna at my home QTH, and also a near-identical antenna set up
in the middle of a field away from trees and buildings. The attached graph
shows Rloss plotted against frequency over the range 10kHz to 600kHz for
both antennas. At all frequencies, the loss resistance of the open-field
antenna is much lower than the home QTH antenna, which is surrounded by
numerous small trees. The ground in both cases was 4 x 1m ground rods, close

to the feed point of the antenna. The actual ground around both antennas was

very similar - a waterlogged clay soil.

At 10kHz, the open  field antenna has Rloss of 50R, against 380R for the
home QTH antenna. Both antennas show a decreasing Rloss with frequency -
this suggests dielectric losses are dominant (the antenna voltage increases
at lower frequencies for a given current) in both cases. The text books say,

for electrically small antennas, that dielectric losses will dominate at low

frequencies, while at high frequencies the skin effect will eventually cause

resistance to start increasing - in the case of the open field antenna, a
turn-over point might have been reached at a few hundred kHz.

The 50R figure suggests that antenna efficiency might actually be higher at
9kHz than people are expecting - at least in an open field site. A suitable
loading coil would be a problem. The antenna capacitance was around 350pF -
in these experiments I used a ferrite-cored coil of around 0.7H with a Q of
about 150, but this had a loss resistance of about 300R. If you tolerated
loosing half the TX power in the loading coil, a similar inductance with a Q

of around 1000 would be needed. Increasing the top-loading capacitance of
the antenna would definitely be useful...

Cheers, Jim Moritz
73 de M0BMU