[Antennas] Varactor-Tuned Loop Antennas

Wed Aug 27 19:11:12 EDT 2008

Chris Trask wrote:
>> You couldn't be more incorrect. Committing coupling to the magnetic
> material rather than to close coupling between the wires themselves does
> in
> fact result in power losses in the core material, both in terms of bulk
> losses due to magnetic skin effect and eddy current losses. Those losses
> occur regardless of whether you are using the core for low power receiving
> or higher poser transmitting applications. And under no circumstances can
> you "tune out" the resistive losses of the core by simply adjusting the
> varactor to another voltage/capacitance. The varactor is a variable
> capacitance, not a negative resistance.
>> If what you said were to be true, then I could use a 1/4" diameter
> toroid to couple 1KW of power to a short circuit and not experience any
> heating of the core.
>> And it doesn't happen in some designs, as though some applications
> were
> to somehow negate any and all eddy current and bulk losses. It happens in
> all designs.
>>> Chris

What you say is true in general for lossy cores such as 43. But
this is a special case.
The core I used was Fair-Rite 61 material resulting in a coil with
a measured unloaded Q of around 400 in the 160 and 80 meter bands.
It has negligible loss for receive purposes, as the loaded Q is less
than 5. In actual operation, I get plenty of signal to overcome
the receiver noise. I don't need an external preamp.
Rick N6RK