The AEA Isoloop (I think it is not still being made) was about 3' in
diameter and had a wide aluminum strap for a loop conductor. The loop
was WELDED to the stators of a butterfly capacitor -- that is a
capacitor made with two stator sections and a common rotor. So there was
NO sliding contact or other resistive pieces.
It was fed by a small loop made of coax inside the larger loop with no
physical connection to the main loop. It tuned from 14 to 30 MHz and
would handle up to 100w.
In horizontal configuration I found it just about as good a transmitting
antenna as a full-size dipole, and (probably because of the perfect
balance) quieter on receive. Of course the bandwidth without retuning
was tiny (maybe 10 kHz on 20m) and the tuning was touchy.
But it shows how good a loop can be if the design is optimal.
73,
Victor, 4X6GP
Rehovot, Israel
Formerly K2VCO
http://www.qsl.net/k2vco/
On 27 Apr 2017 19:13, michaelstringfellow wrote:
I have considerable experience with both commercial and home-brew loops.
As others have said, it is tough to obtain maximum efficiency - the two
largest contributors to loss being resistance in the tuning capacitor and
ground losses. A couple of wires soldered to the tabs of a receiving
capacitor with a sliding contact to the rotor will have large losses and
several commercial antennas are made this way. Since small loops use
vertical polarization, unless you are over really good soil or sea water,
they need to be high enough to minimize earth losses - typically 0.1 to 0.2
wavelengths.
I modified a portable aluminum loop by enlarging it to 5-foot diameter and
improving the capacitor arrangement. It worked acceptably well and I was
able to make contacts on 20, 30 and 40 meters with a KX3 barefoot. However,
I find that almost any full-size antenna works better and the difference is
usually one to two S points. When operating portable, I now deploy a
telescopic 40-foot fiberglass pole with a wire antenna - vertical or loop.
I continue to use two small transmitting loops in my HOA restricted home
shack.
Mike
AF7ON
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