Topband: "Magnetic" receiving loop.
w8ji at contesting.com
Tue Dec 20 11:02:01 EST 2005
So far I have the loop itself done: two turns of RG-6 with
the shields opened up at a point opposite
from the feedline attachment. It has deep (about 7 S-units)
and very narrow (<10° in azimuth) nulls
where it should.>>
That's good. The sheilds have to be opened exactly opposite
the feedpoint otherwise they ruin the balance. By the way,
the shield is actually the antenna picking up the
signal....not the wire inside the shield.
Next steps are a resonating variable capacitor and a preamp.
Maybe I'll need a 3rd turn... but the
books recommend keeping the overall length <0.1 wavelength.
I'd like to use this on other bands
when chasing down RFI sources, so I put a switch in to
choose the number of turns.>>
Increasing the number of turns does not change the effective
aperature of the loop or the sensitivity. It only changes
the impedance, or ratio of terminal voltage to current. The
received power remains essentially the same, since it is a
function of the physical area of the loop and not the length
of copper in that area. The same is true for transmitting
loops. Multiple turns actually increase losses for a given
overall volume of conductor area, and that's why compact
high efficiency transmitting loops are always single turn
loops (unless they just can't be tuned without the
additional inductance of a second turn). If multiple turns
increased effective aperature, we'd see multiple turn
transmitting loops dominating the high efficiency loop
The overall length of the conductor in a multiturn loop
doesn't mean much except for the terminal impedance. It is
the loop circumference that cannot be large in terms of
wavelength, not the length of the multiple turn conductor.
The only thing that can happen if you use too long of a wire
is you may not be able to tune and match the antenna.
Antennas are largely an ampere over linear spatial distance
problem. One foot or 1000 feet of wire in a one foot box is
just a one foot box antenna, only the loss resistance and
impedance can be changed.
More information about the Topband