Joe,

Thanks for replying. I wan't entirely certain which of the four I
should respond to, but I'm assuming this was the vinfal version.

On Wed, 18 Feb 2009 06:11:29 -0800 (PST), J.A. Legris wrote:
On Feb 16, 11:22*pm, Frnak McKenney
wrote:
Back in December I posted a question about ways to receive LF/VLF
radio signals.
--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--
My other question has to do with how to interpret signal strength.
--snip--
a
signal of at least 100uV/m. *Does this mean that I should expect to
see 100uV from any one-meter hunk of wire strung out horizontally in
the optimum direction? Or is there something more subtle going on I
need to be aware of?

A field strength measured in 100 uV/meter is just that, but the
problem getting the energy out of the air and into a receiver.

Yes. It's not like I can just hang a bucket out the window and
bring it back full of electrons wiggling at just the right speed.
grin!

A short linear antenna has a very low radiation resistance ( 1 ohm)
which is a poor match to a practical transmission line, whose
characteristic impedance is typically 1000's of times larger. The
radiation resistance of an antenna is the component of its complex
impedance that is associated with the power captured. Balanis (Antenna
Theory Analysis & Design (1997), p.137) gives a formula for the
radiation resistance of a short dipole:

R = 80 * pi^2 * (W/L)^2 ohms

where W is the length of the antenna and L is the wavelength. The
value for a monopole is roughly half as much again.

Um... 1.5 * 80 * (%pi^2) * (1/5000)^2 is... 471 micro-Ohms? That 's
pretty low; why would anyone match that to a JFET input?

Why do you request a non-loop antenna?

I started there, ran into some questions, and wanted to clear up the
confusion in my own head before moving any further. It's not as
though I'm prejudiced against them; heck, some of my best friends
have radios with loop antennae. grin!

... A small circular loop antenna
also has a low radiation resistance but it can be increased by adding
turns. Balanis (p.209) gives a formula for the radiation resistance of
a small loop:

R = 20 * pi^2 * (C/L)^4 * N^2 ohms

where C is the circumference of the loop, L is the wavelength and N is
the number of turns.

Better still is to use a ferrite loop antenna. You may be able to get
one out of an old AM radio and adapt it to your receiver. The
resulting formula is identical to the above, multiplied by the
relative permeability of the core, u (SQUARED !), so you can use a
very small-diameter loop and/or fewer turns, getting improved
selectivity and sensitivity (i.e. high Q) in a tuned circuit:

R = 20 * pi^2 * (C/L)^4 * N^2 * u^2 ohms

Oddly enough, I now have ten old transistor radios that I picked up
at FrostFest a few weekends back for $1 each. I was looking for
ferrite and wide-ratio tuning capacitors, as they seem to be in
scarce supply these days. I don't know where today's kids are
getting their crystal radio parts from these days; it certainly
isn't Radio Shack.


Frank
--
"A man should never be ashamed to own that he has been in the
wrong, which is but saying, in other words, that he is wiser
today than yesterday." -- Jonathan Swift
--
Frank McKenney, McKenney Associates
Richmond, Virginia / (804) 320-4887
Munged E-mail: frank uscore mckenney ayut mined spring dawt cahm (y'all)