On Mon, 23 Aug 2004 04:24:52 GMT, Richard Clark
wrote:

Workmanship and quality materials tests those reputations vastly more
for smaller antennas than standard sized ones. Those 1 meter loops
used for HF are not rated for the lower bands for very good reasons,
and they claim (and I believe them) high standards for their product.
However, if you could resonate them in the 160M band, you'd be lucky
to see 1% efficiency.

You would be lucky if you could get 10 % efficiency at 40 m for these
1 m loops. Since the radiation resistance is inversely proportional of
the fourth power of frequency and the skin effect losses proportional
to the square root frequency, one could expect to get nearly 1 %
efficiency at 80 m and well below 0.1 % efficiency at 160 m.

On the European 135 kHz LF band, the practical vertical antennas are
usually less than 0.01 .. 0.02 WL, the estimated efficiency is less
than 0.1 %, so more than 1 kW has to be driven into the antenna to
even get 1 W of ERP. This 1 W ERP limit is used by many countries and
still narrow band contacts of several thousand kilometers are made.

Unfortunately, trying to compensate the low efficiency in a small
magnetic loop with a high transmitter power is not very practical,
since the voltages would be huge.

Thus, if some exotic small antenna with inevitably low efficiency is
to be used, I would first check that it can constantly handle the full
legal limit power, so that it would be possible to compensate for the
lower efficiency.

The low antenna efficiency is not much of a problem in receiving on
LF, MF and lower HF frequencies, since the band noise is still well
above the receiver front end noise. However, on upper HF and above, a
low efficiency will degrade the reception, especially if the receiver
noise figure is high (which it often is in HF receivers that try to
maximise the intermodulation performance).

Paul OH3LWR