Topband: QST Jun 06 RX Loop
ford at cmgate.com
Sun May 14 12:49:56 EDT 2006
> As most on this e-mail reflector, I read the QST article about the rx
> loop with great interest. I was amazed at what great lengths the author
> went to in making sure his loop was well-"shielded".
> I feel that a non-"shielded" loop will work just as well, plus it would
> be easier to construct and to feed.
> I also feel that the most useful information that one can glean from that
> article is that a receiving loop can be used to minimized a noise source
> by nulling it out. His preamp design is also interesting.
> 73, de Earl, K6SE
Earl, and others...
I too was taken aback by the author's fantastic claims of 38dB nulls. Wow! Can this be true? I've built shielded loops before, at least the kind with the shield split at the top, and cannot confirm anything even beginning to approach this performance.
I modeled a small loop of the dimensions in the article using Nec2. Frankly, the performance near a sommerfeld norton ground may be greatly distorted in Nec2, but the nulls were nowhere near 38dB--like maybe 6-10dB instead--more on the order of what I experienced after building a split shielded loop.
The other amazing claim is the bandwidth. -3dB points at 1.764 and 1.896. The circuit does appear to be resonating the loop, which would normally suggest a bandwidth of about 5-10 kHz instead of the 130 KHz suggested by the author.
I may just have to build one of these. Since my "dead iron pile" behind the shed is already filled with fantastic antennas that simply don't work, I'm hoping somebody else takes the arrows in the back, builds one, and reports back with either confirmation of claims, or complete disgust. Possibly more important than confirmation of claims would be to measure the output of the loop. The author never mentions output. My guess is that it would be on the order of -25 to -30 dBi, possibly even worse.
ford at cmgate.com
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