[Lowfer] Software Defined Radios

[Douglas D. Williams]

Zack,

I happen to use an active e-probe antenna which covers from around 3kHz to
above 30Mhz. To use a computer sound card as a VLF receiver, you can
connect your antenna directly to the sound card's "line in" or "microphone"
input. With my setup, I do not need a preamp. The sound card's microphone
input is plenty sensitive. At my QTH, a low pass filter is a must, or I get
a lot of mixing and distortion from local MW stations. I happen to use a
50kHz low pass filter, as my sound card has a 96kHz sampling rate, which
will allow it to display a maximum frequency of 48kHz. I believe there are
sound cards available that have even higher sampling rates, and there are
also some (mostly older) sound cards that have 48kHz sampling rates, which
would only allow them to display a maximum of 24 kHz.

I would be careful about attaching large wire antennas to computer sound
cards, as voltages can develop that could possibly fry your computer. If
you are going to do it, I would certainly disconnect during any threat of a
storm.

One other thing I have discovered is that a lot of "crud" can be carried
along the coax shield to the outside antenna. I get much better results on
VLF when I leave my station completely ungrounded in the shack, and my
antenna, which is 50' from the house, is grounded at the mast. I also think
I will experiment with some common mode chokes, which will necessarily be
quite large to be effective at VLF, but thankfully they will be for receive
only.

D. KB4OER
On Tue, Jan 17, 2012 at 9:17 AM, Zack Widup <w9sz.zack at gmail.com> wrote:

> I have yet to try to use a sound card as a VLF receiver. What kind of
> antenna(s) do you use? Do you use a preamp between the antenna and
> sound card? How high in frequency will it go?
>
>