Paul Keinanen wrote:
. . .
When the antenna size is below perhaps 1/10 wavelength, the radiation
resistance drops by the square of frequency, so the radiation
resistance can quite easily be well below 1 ohm at LF and below. The
loss resistance (include grounding and loading coil losses) can be
several ohms, thus the majority of the generator power is dissipated
in the losses and only a very small part is actually radiated by the
very small radiation resistance.
. . .

Neglecting ground effects, the radiation resistance of a four-sided loop
100 meters on a side at 100 kHz is 45 milliohms. The radiation
resistance of a four-sided loop 10 meters on a side at 10 kHz is about
400 picoohms (4 X 10^-10 ohms). If you used 2 mm diameter wire to
construct the loops, the first would have an efficiency of 0.7%, and the
second of 0.00000016 percent. These are very optimistic, since they
don't account for the considerable loss you'd incur by induction into
the ground and objects for quite some distance around. They also don't
account for losses in the required impedance matching network. And a
receiving antenna would have the same efficiency.

You can easily get these numbers with the free EZNEC demo program from
http://eznec.com. Or use a calculator and the simple equations you'll
find in any antenna text.

The OP wondered if anybody had ever thought of this before. The answer
is yes, the first time probably well over a hundred years ago. Anyone
doing the simple calculations sees immediately why it's not a great idea.

Roy Lewallen, W7EL

On Sun, 01 Oct 2006 13:12:54 -0700, Roy Lewallen wrote:

Paul Keinanen wrote:

snip

The OP wondered if anybody had ever thought of this before. The answer
is yes, the first time probably well over a hundred years ago. Anyone
doing the simple calculations sees immediately why it's not a great idea.

Yes, I'm beginnig to see that now. However, I am learning a great deal
in the process, and the equations I have found to explain the issue in
detail don't seem simple at all. But if it was simple it wouldn't be
interesting, now would it?

Right now I'm looking at;
http://en.wikipedia.org/wiki/Near-field
http://en.wikipedia.org/wiki/Radiation_pattern
http://en.wikipedia.org/wiki/Fresnel_zone

Lucky for me I'm trying to build up an interest in math. Cos here's plenty
of it...

Again, thank you all for your replies. It'll be a while until I really
have an intuitive understanding of the subject like you all do, but I am
patient.

--
Nos

To get the numbers I came up with, all you need is the equation for the
radiation resistance of a small loop and the resistance of copper wire.
It shouldn't be hard to find the equation for the small loop, and it's
very simple. At those frequencies, the resistance of the wire should be
nearly the same as the DC resistance, so all you need is a copper wire
table(*). Both should be readily available on the web.

Of course, if you want to know about other kinds of antennas and more
detailed effects like the field intensity in the presence of ground, the
equations can get difficult indeed, and some are too difficult to solve
directly. That's what the modeling programs are for.

(*) At higher frequencies or with much larger diameter wires, you'll
need to account for skin effect. This requires calculation of a square
root and can be done on a pocket calculator as can the others.

Roy Lewallen, W7EL

Ceriel Nosforit wrote:
On Sun, 01 Oct 2006 13:12:54 -0700, Roy Lewallen wrote:

Paul Keinanen wrote:

snip

The OP wondered if anybody had ever thought of this before. The answer
is yes, the first time probably well over a hundred years ago. Anyone
doing the simple calculations sees immediately why it's not a great idea.

Yes, I'm beginnig to see that now. However, I am learning a great deal
in the process, and the equations I have found to explain the issue in
detail don't seem simple at all. But if it was simple it wouldn't be
interesting, now would it?

Right now I'm looking at;
http://en.wikipedia.org/wiki/Near-field
http://en.wikipedia.org/wiki/Radiation_pattern
http://en.wikipedia.org/wiki/Fresnel_zone

Lucky for me I'm trying to build up an interest in math. Cos here's plenty
of it...

Again, thank you all for your replies. It'll be a while until I really
have an intuitive understanding of the subject like you all do, but I am
patient.

On Sun, 01 Oct 2006 20:36:23 -0700, Roy Lewallen
wrote:

To get the numbers I came up with, all you need is the equation for the
radiation resistance of a small loop and the resistance of copper wire.
It shouldn't be hard to find the equation for the small loop, and it's
very simple. At those frequencies, the resistance of the wire should be
nearly the same as the DC resistance, so all you need is a copper wire
table(*). Both should be readily available on the web.

Small magnetic loops with gains in -30 ..-60 dB range can be usable
for receiving due to the extreme noise levels on LF and VLF, but for
transmitting, they are far to lossy.

To get any significant communication distance, you would need a
vertical polarised signal. The popular antenna among 135 kHz
experimenters as well as in LF aeronautical beacons is a vertical
tower with as much top capacitance as you can put up.

Look at the antenna systems of old ships using the LF band, these have
multiple parallel wires erected between the masts in the bow and
stern. These wires form the top capacitance and a vertical wire going
directly from it isolator on the radio room to the top wires, which is
the actual vertical radiator. The top loading will increase the
current in the vertical conductor and hence vertical radiation.

Paul