[Antennas] Varactor-Tuned Loop Antennas

Thu Aug 28 09:18:51 EDT 2008

Guys one thing.
Any flux coupled transformer has to cope with hysteresis losses, big or
small, never zero!
Any material influencing a coil's inductance introduces losses inherent in
itself.
So, from that POV, Chris is absolutely correct: there is no material in
nature that is totally lossless!
61 ferrite is intended for use in switch mode power supplies, one of my past
occupations. I know it quite well. You'll definitely have very small losses
at low frequency, likely quite acceptable at 160 meters. It's a nickel zinc
material, whereas the ferrites we usually employ are nickel manganese.
Improvements here mean you did something to a mismatched condition rather
than anything else.
Just for the info from my experience: even when using transmission line
transformers- coiled coax lines- in which the core's only role is to
increase the fringe inductance of the winding at the low frequency end, the
core heats up! That means that it absorbs energy even in that secondary
role. This state of affairs led to the Bulk Resistance value given by
ferrite manufacturers, which indicates how the core heats up by dissipating
energy absorbed from the windings into heat.
To all this add normal Ohmic losses and skin effect losses and you'll see
how close Chris is to the truth.
Alex	4Z5KS 
-----Original Message-----
From: antennas-bounces at mailman.qth.net
[mailto:antennas-bounces at mailman.qth.net] On Behalf Of Rick Karlquist
Sent: Thursday, August 28, 2008 2:11 AM
To: Chris Trask
Cc: richard at karlquist.com; antennas at mailman.qth.net
Subject: Re: [Antennas] Varactor-Tuned Loop Antennas
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
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