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