Re: LF: questions about a tx square loop fpr 470KHz

[Dimitrios Tsifakis <dtsifakis@gmail.com>]

Buongiorno Mauro,

a few months ago, I build a single turn loop, 0.7 m by 0.7 m, using very thick electrical copper wire. I resonated it on 160 m using a couple of beefy silver mica capacitors in parallel. Because I didn't have a suitable variable capacitor, I did the tuning by adjusting the loop. The impedance of this is, as others have mentioned, a tiny fraction of an ohm, and it is very much dominated by the losses of the copper cable. Radiation resistance is orders of magnitude smaller! The bandwidth was also very small but it was enough to have an SSB QSO with a friend, not too far away.

I did the matching using a ferrite toroid (FT114-43 from memory) that had a number of turns primary (connected to 50 ohm cable) to a single turn secondary (loop wire through the toroid. I adjusted the turns until I got as close to 50 + j0 impedance as possible. It worked remarkably well given its size.  

Won't replace a full size antenna but is certainly worth the effort! It also had the benefit of being able to null out sources of interference, such as plasma TVs. The nulls are sharp but the peaks are not, which I found convenient. As others suggested, I would not bother with two turns, one would be a lot easier to predict.

Ciao & 73, Dimitris VK1SV/VK2COW

2015-05-01 7:16 GMT+10:00 IK1WVQ Mauro <ik1wvq@stmb.it>:

73 to all,

I would like to know your opinion about this:

I can made a square TX loop for 470KHz of 7 * 3 meters (see picture) in the terrace of the building where I live (6 floors).

the questions are:

- it is enough a 6mmq copper electric wire ??  (20W is the output power of my tx).
- single turn or two turns spaced 10cm max ?? (two turns =  less pF required to take the resonance).
- Is this a good idea to start to trasmit on 470KHz WSPR ??

thanks in advance for your attention.

73 de Mauro IK1WVQ

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