[Lf] Loop antennas

[Andre' Kesteloot]

Re - loop antennas - Marcus wrote:

>I'll repeat those readings on a 4m diameter aluminum loop, 17mm >tube,
assoon as I finish my remotely controlled
capacitor decade >circuit (1nf steps from 0 to 350nf). Seems that the 1
turn solution >will perform well -and the
best part - no pre-amp, but the decade C >:o(

In theory and to a reasonable approximation, the SNR of a loop depends
only upon it's effective area and Q, and
not on the number of turns. This assumes a preamp or receiver input
which does not add significantly to the noise
generated by the loop itself, which is usually possible. So a 1 turn
loop is quite reasonable, with obvious mechanical
advantages. But there are also difficulties. Tuning capacitance has to
be very large at LF. Impedances are very low,
so resistances must also be kept very low in order to avoid severely
reducing Q. At U of H, we found wide range,
switched tuning capacitors difficult to implement with a single turn
loop, mostly due to trouble finding low-resistance
switches. In the LF range, and loop inductances of the order of 10s of
uH, 0.1 ohm will be significant. Bearing in
mind multiple switches will probably be needed, this can be a real
problem. We found relays were really the only
option for remote switching, and even then you have to be careful - reed
relays had losses around 0.1 - 0.5 ohms,
much higher than their DC resistance. The best bet are relays intended
for UHF antenna changeover duty.

However, problems can be greatly eased by going to larger loop sizes -
with the increased signal level, and the high
noise levels on LF, optimum Q is not then required, and tuning
arrangements are less critical. I am currently using 2
square, single turn loops with 3m sides, made from 1.5mm^2 wire, mounted
on the same pole, roughly at right
angles. One of the two loops is selected by a relay, giving 360 degree
coverage. This is connected to a tuner in the
shack by about 25m of RG58 coax. the tuner has (for 136kHz), a 700uH
inductor in series with the loop, tuned by
about 2nF to ground. A high impedance preamp input is connected to the
hot end of the tuning C. Although this is
far from optimum from the noise viewpoint, external noise appears to be
the limit on sensitivity. This antenna has
worked well for receiving the various transatlantic signals. It can be
tuned over a wide range by switching in
different tuning capacitors and inductors, and has also worked well for
73kHz.

Cheers, Jim Moritz
73 de M0BMU