[Antennas] Varactor-Tuned Loop Antennas

[Rick Karlquist]

Thanks for posting this.  You obviously put a lot of work into
this.  The workmanship is nice.

The premise of this article is that the use of a loosely coupled
transformer will result in elevated noise temperature and loss
of sensitivity.  While this could possibly happen with some
designs, it is not inevitable.  With proper design, the less-
than-unity coupling simply results in some leakage inductance
in series with the loop inductance and can be tuned out by
varying the varactor voltage slightly.  With proper choice
of magnetic material, inductance is basically non dissipative
and doesn't result in power loss.  In any event, there is
plenty of signal out of the loop (at least on the low bands) to
overcome the receiver noise so that the sensitivity is easily
limited by external noise.

Instead of the voltage balun at the output, I prefer a true
transformer because it completely decouples currents on the
outside of the coax shield from the antenna and matching
network.  The voltage balun relies on physical symmetry to
work, because the center tap is connected to the antenna
circuitry.

You can make a bias tee for the varactors (which draw essentially
zero current) with a 10k resistor and a blocking capacitor.
The transformer shown is unnecessary.  The control unit
with the 2 stage regulator is unnecessarily complicated.  The
second regulator can be replaced with a pot and the first
regulator is often unnecessary, depending on the power supply.
AFAIK, the MVAM109 diodes cited in the article are unobtainable in small
quantities (except possibly as surplus), and may be completely out of
production by now.

Here is a Powerpoint of the loop antenna talk I presented
at the International DX Convension in Visalia in April:

http://www.n6rk.com/Loop_Antennas_as_submitted.ppt

This design is much simpler, using only a single transformer.
The circuit uses parallel resonance, which completely circumvents
the leakage inductance issue.  It covers 1.6 to 5 MHz and
uses readily available varactors.  It happens to be a so-called
shielded loop, made with coax.  I have built and tested
this design.  I will be presenting this design at the Pacificon
antenna seminar in October.

Rick N6RK


Chris Trask wrote:
>     I finally completed the work that I have been doing on a simple
> varactor-tuned loop antenna with interchangeable antenna elements.  The
> documentation is available at:
>
>     http://tinyurl.com/6lht4y
>
>     A little more work needs to be done here as the inductance of the
> transformers is limiting the high end of the tuning range.  However, the
> results obtained at this point show that the sensitivity is far better
> than
> I had anticipated, and together with the very good S/N performance this
> design rivals the performance my earlier active loop antenna that was
> published in QEX back in 2003.
>
>     Corrections and constructive comments are always appreciated.
>
> Chris
>
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