Tim,

Thanks for the response.

On Tue, 17 Feb 2009 11:02:42 -0600, Tim Wescott wrote:
On Mon, 16 Feb 2009 22:22:20 -0600, Frnak McKenney wrote:

Back in December I posted a question about ways to receive LF/VLF radio
signals.
--snip--
First, the need for impedance matching between an antenna and a
receiver.
--snip--
So any non-loop antenna I can construct will necessarily be a "short
wire" or "electrically small" antenna (two useful search terms). But how
does one go about calculating the impedance of a coat hanger or an
extension cord ("short piece of wire")?
--snip--

Why do you want a good impedance match?

Because I'm trying to snatch a signal that I have no experience with
"out of the aether", a signal that has to somehow excite an antenna,
feed into an upconverter, arrive at my receiver, and produce some
specific identifying evidence that I'm detecting the signal I hope
it will rather than (say) reporting that my neighbor is using his
electric razor. grin!

Each of these pieces (except perhaps the Mohican) are untested (by
me), and I don't have any tools that will help me easily distinguish
between (say) a bad upconverter, a poor antenna, or excessive noise.
Like many such situations, I'll know if I _succeed_, but if I don't
there won't be any clear indicators to help me figure out _which_
piece of the puzzle isn't fitting properly.

In short, anything that sounds like it might affect my results is of
interest to me.

Why don't you want to use a loop antenna?

It's a question of time and effort: it looks like it will take me
less of each to test the "wire" first. If it succeeds, I'm done; if
not, I can start experimenting with loops.

Which is, of course, a variant on one of my favorite puzzles: How
do you allocate your resources when you don't yet know what you're
doin... er, "under conditions of less-than-perfect information"?
grin!

At 5000m, atmospheric noise is very strong -- it would certainly
overwhelm any thermal noise that you'd receive if you did make a 1/2 wave
dipole (don't forget that your towers need to be at least 2500m tall to
get close to the ideal). Getting an appropriate impedance match is
mostly about maximizing your signal compared with your receiver's
internal noise; the strong atmospheric noise makes this less necessary.

Yeah. All that, plus the funding. But mostly the funding. grin!

This atmospheric noise also makes really efficient receiving antennas
rather unimportant. You want a good fraction of a wavelength for
_transmitting_, but it really doesn't make much difference for
_receiving_.

Which may well be true, but it seems puzzling. Why would it not be
important to deliver as much of the induced electron movement to a
receiver as possible?

The two common receiving antennas that I know of at that sort of
frequency are tuned loops and capacitive whips.

A loop can be fairly small -- my understanding (which I've never tested,
YMMV) is that one square meter is plenty. Loops are nice because you can
tune them, so they give you some additional selectivity on your receiver
front end. You can impedance match the loop to your receiver, but most
of the impedance your receiver sees will come from the wire in the loop,
not the radiation resistance of the loop.

Talking about "radiation resistance" in a receiver antenna also
feels a bit odd.

... Loops are also somewhat
directive, which helps to reduce the total static received, and if done
correctly (google "shielded loop") they can be arranged to reject sky
waves (I _think_ by polarization, but I'm not sure).

Yes. I'm reading up on loops so I have an alternative available in
caseXXXXXXX_when_ something goes wrong.

A capacitive whip is just a 1m long wire whip (like a coat hanger or
welding rod) feeding some high impedance amplifier like a JFET (or a
toob, if you want to be picturesque). Put the active element right at
the base of the wire for best signal. It's inherently wide band, and
hard to keep it from being so, so if you have some local interference
it'll kill your signal

This is the direction I'm starting in.

... (my first try at these didn't work in my shop
because of a nearby electric fence transformer, but it worked fine at the
end-user's more-urban location).

Gack! I don't suppose it used an old Model-T spark coil, or a
buzzer-and transformer equivalent? I remember in my youth (Dirt(tm)
_had_ been invented, but it was still considered cutting-edge
technology grin!) just how badly one could mess up AM BCB
reception with a string of those Christmas tree bulbs with built-in
bimetallic-switch flashers.


Frank
--
You'll never learn to do anything well until you're willing to
accept that you'll do it badly at first. --Anonymous
--
Frank McKenney, McKenney Associates
Richmond, Virginia / (804) 320-4887
Munged E-mail: frank uscore mckenney ayut mined spring dawt cahm (y'all)