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LF: RE: RX loop designs

To: [email protected]
Subject: LF: RE: RX loop designs
From: "Michael J Underhill" <[email protected]>
Date: Sat, 1 Feb 2003 16:33:13 -0000
Importance: Normal
In-reply-to: <[email protected]>
Reply-to: [email protected]
Sender: <[email protected]>
Yes please, Jim.

By the way, in experiments with my version of the CFL, I have made
double tuned HF transmitting loops with similar bandwidth improvements
to yours but having at least four times the Q values that I guess you
have. I have not yet achieved the claimed CFL bandwidths with the same
efficiencies and sensitivities that I get from my other loops. Ganged
tuning is necessary and it is difficult to keep this accurately enough
in step to cover even a single HF band. At the moment considered 'not
worth the hassle' for the HF bands.

73 de Mike G3LHZ

[email protected]

-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of James Moritz
Sent: 01 February 2003 11:33
To: [email protected]
Subject: LF: RX loop designs

Dear LF Group,

With the recent chatter about LF loops, I finally got round to writing
up 3 of my receiving loop designs, which I have been using for the last year or so. They seem to be significantly different to the designs other people have come up with, and work well, so could be of interest.
The usual problem with a tuned loop is that the bandwidth is very
narrow, so remote tuning is needed. Resistively loading the loop increases BW, but reduces the loop output and SNR, so a bigger loop is needed, and also out-of-band selectivity is reduced. My idea was to add an additional tuned circuit coupled to the loop to produce a bandpass response, which can be
designed to give a flat-topped or slightly double-peaked response
covering the whole 136kHz band without re-tuning. This has turned out to work well, and the article describes one 1m^2 loop with a bandwidth of about 4kHz, and a 2m x 2m loop using a single turn of tubing with a bandwidth of about 18kHz. With the simple pre-amp also described, the noise floor of both these antennas is well below the band noise even on a quiet day.
The article also includes a wideband loop based on a low-pass rather
than bandpass design, which covers 10kHz -200kHz, also a single turn 2m x 2m loop. This is slightly less sensitive than the bandpass designs, but more than adequate for general use over the LF/VLF range. The relationship between field strength and loop output for this antenna is predictable, so it can also be used for field strength measurement.
The article I have written is 5 pages, with several diagrams, and is a
.pdf file of about 340kB. It is too big for the reflector, but if you are interested, let me know, and I will e-mail it to you as an attachment.
Cheers, Jim Moritz
73 de M0BMU

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