On Wed, 28 Jan 2009 13:03:45 -0000, "christofire"
wrote:

My next question is about using varicap diodes to tune the loop. Most
designs use a conventional capacitor, but a varicap would enable the loop
to be located (say) in the loft, and the tuning varied from the shack
downstairs. Is this technology worth perusing (ie Googling)? Or has it
been found to be a waste of time?

Varicaps are fine and you will find devices containing pairs like the BB212,
specifically intended for tuning domestic receivers. They're used in all
the modern radios that are digitally controlled. The trick is to use them
in balanced pairs so strong signals don't push a single diode into
conduction.

Hi All,

The strong signal issue goes well beyond the possibility of
conduction. When you think about a voltage controlled capacitor, you
have to account for the effects of the voltage of the signal you are
tuning (and those you are not tuning). The BB212 application is found
characterized for signal levels less than 100mV (typically far less).

Why? A half volt signal level at the terminals of the device (taking
the BB212 for example) biased at 1V will swing the capacitance 100pF
above and below where it would be thought to be tuned. Of course,
correspondingly smaller signals have correspondingly smaller swings -
it all depends on how much you can tolerate.

This swing is also a problem for PIN diodes used as voltage variable
attenuators.

73's
Richard Clark, KB7QHC