[Lowfer] Varactor-Tuned Loop Antennas

[Chris Trask]

>
> Did you measure Q?  Did you look for intermodulation with big signals?
> Neat design.
>

    The tuning Q for the 1m diameter loop is 30 at 5.0MHz and 60 at 12MHz,
an almost constant 200kHz 3dB bandwidth.

    Couldn't measure the IMD performance properly.  With a 1W amplifier
added to both generators, there wasn't enough of an IMD product above the
spectrum analyzer noise floor to get a proper measurement, and with the
input attenuator cranked all the way down it's more likely that the IMD
products are from the spectrum analyzer itself.  The nature of this beast is
such that it's very difficult to rig up an IMD bridge so as to cancel the
signals themselves and just be left with the IMD products of the varactors.

    I don't want to make a direct connection to the loop as that would upset
the impedance and possibly the IMD performance.  When I get a transmitter
version of this built, I'll try setting them up about ten feet apart and see
if I can make that measurement.

    I was quite surprised at the IMD and BW performance of this, neither of
which was expected to be this good.  It's very likely that the balanced
varactor tuning topology is cancelling a good portion of any IMD products.
But, I do need to look into where the extra inductance is coming from that
seems to be limiting the maximum tunable frequency, and if my experience so
far is any indication it's probably the transformers buiult on the balun
cores as that and the excessive losses were the reasons for using the toroid
for the input transformer.

Chris

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