[Antennas] Varactor-Tuned Loop Antennas

[Chris Trask]

>
> 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.
>

    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

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