[Antennas] Varactor-Tuned Loop Antennas

[Chris Trask]

>
>There you are guys!!!!
>
>Nobody, but nobody, including Jerry Sewick managed to make a reasonable
>matching transformer for 50Ώ to 10kΏ operation.
>
>The inherent leakage self-inductance is adding reactive effects . part of
>these can conceivably be tuned out, but resistive losses... forget it!
>

     That problem is what motivated me to perform the matching by way of a series of transformers using bifilar or trifilar twisted wires rather than separate windings as the coupling coefficient is much better.  That first transformer was a serious design hurdle as the low impedance at that point demanded a better than average combination of wire size, turns count, and core selection.  Ferrite materials, both 43 and 61, proved to be inadequate for the task.

     I generally refrain from using toroids for wideband transformers as the magnetic path will have a parallel-equivalent resonance that limits the high-frequency end.  This is especially true when you are using larger toroids for power applications, particularly toroids made of ferrite rather than powdered iron material.

     I had spoken with Jerry many years ago after he retired from AT&T, but it was on another subject entirely.  He and I worked in different arenas in wideband transformer design.  He was focused on high power matching and coupling where core heating de to inadequate coupling is a problem, and I was focused on low power applications where the nonlinearities and losses of the core resulted in higher NF and IMD products in receiver front ends.  

     Until recently, I haven't had a need to use his method of parallel wires on toroids, but I have a transmitter version of this loop matching topology that requires it due to the heavier wire gauge.  I've been working on that in parallel with this varactor-tuned receiver version as most of the problems encountered are common to both.  Before too long, I should have a 100W version completed.





Chris

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