Dear J.B., LF Group,
I think the problem here is the size of
the coupling loop, which is only slightly smaller diameter than the main loop, is
too large. If the coupling loop had unity coupling factor to the main loop, the
complete loop would be a 15:1 transformer. If the RX Zin is 50R, the transformed
load across the main loop terminals would be 50 x 15^2 = 11.2k. With a loop
inductance around 2.1mH, the loaded Q is then only about 6 or so. In fact, the loading
will be reduced by the less-than-unity coupling factor between the main loop
and the coupling loop; this is difficult to calculate due to all the variables
involved, but a rough guess would be an overall impedance ratio of about 450:1,
making the shunt resistance about 22k, and the loaded Q of the loop around 12, still
very low. Going further with the rough calculations, 1uV/m field strength would
then only give you about 0.2uV at the RX input at best. This comes out similar
to an un-tuned 40m delta-loop, which ought to have about 80m^2 area. So that is
reasonably consistent with what you are observing.
With an unloaded Q of 380, the shunt
resistance across the loop terminals should be about 680k. Matching this to the
RX input (to achieve maximum signal level) requires an impedance transformation
ratio of about 14000:1. The loaded Q of the loop would then be 190, and the
voltage at the RX for 1uV/m signal ought to be around 0.6uV, so about 9 or 10dB
improvement, plus much sharper selectivity. If the unloaded Q of the loop is lower,
the output will also be slightly lower, and the optimum impedance
transformation ratio lower too. The way to get these higher impedance
transformation ratios is to make the coupling loop smaller, I would guess
somewhere around 1 - 1.5m in diameter would be about right. As the others have
said, I don’t think screening and balancing the coupling loop will make a
significant practical difference, so a plain bit of wire would be as good as
anything, and easy to play around with to get the optimum size.
The other suggestions of using a capacitive
tap, or a series transformer, should also work well – I would estimate a
capacitor of around 50 – 70nF would be right, or a series matching
transformer with a 1:2 or 1:3 step up ratio feeding the 50 ohm cable.
Cheers, Jim Moritz
73 de M0BMU
-----Original Message-----
From:
[email protected] [mailto:[email protected]] On Behalf Of J. B. Weazle McCreath
Sent: 21 May 2005 00:52
To: [email protected]
Subject: LF: Loop antenna problem
I've finally finished the
construction of my LF loop and I appear to have a
problem with it regarding the
connection to the receiver. The loop itself
is in the form of an octagon, with
an overall diameter of 10.5 feet, and it
consists of 15 turns of number 12
THHN stranded wire. The spacing of
the turns is 0.25 inch and the
tuning is done with a 3 gang, 400 pF. per
section, variable capacitor with all
sections connected in parallel….
…The problem
is with the pickup loop, which is of the classical shielded
construction as described in many
antenna books. Mine is made from
RG-213 coax and it's positioned
roughly six inches inside of the main
loop windings, but in the same plane…
73, J.B. Weazle McCreath, VE3EAR -
VE3WZL
