[Antennas] Varactor-Tuned Loop Antennas

Thu Aug 28 09:25:54 EDT 2008

There you are guys!!!!
Nobody, but nobody, including Jerry Sewick managed to make a reasonable
matching transformer for 50Ώ to 10kΏ operation.
The inherent leakage self-inductance is adding reactive effects . part of
these can conceivably be tuned out, but resistive losses... forget it!
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 3:05 AM
To: Chris Trask
Cc: richard at karlquist.com; antennas at mailman.qth.net
Subject: Re: [Antennas] Varactor-Tuned Loop Antennas

Chris Trask wrote:
>>
>> What you say is true in general for lossy cores such as 43. But

>> this is a special case.

>>
>
> There is no such thing as a "special case" when it come to losses in

> magnetic materials. I don't know where you got that from, but it is an

> outrageous fairy tale. Since the Fair-Rite catalogue clearly lists and

> charts to loss tangent for their materials, including 61, perhaps you

> would

> care to enlighten us as to where this miracululous lossless material

> (which

> defies all manner of physics) came from?

>
> Chris

The Fair-Rite catalog says that the loss tangent is 3 X 10^-5 at
1 MHz and the permeability is 125. This predicts a Q of 267, assuming
copper losses are negligible, which they are in this case. (This
has been verified by winding an identical transformer with smaller
gauge wire and seeing negligible change in Q). Actual
measurements on the HP4342A Q meter show that the material is
somewhat better than this, even at several MHz, achieving a Q
in the 300's to as much as 400. Do you have a problem with my Q
measurements? What do you think the correct value of Q is?

Since the antenna load impedance is between 5,000 and 10,000 ohms,
any leakage inductance or even loss resistance in series will have
negligible effect. The transformer should work OK even if the
coupling coefficient is only 25%. Again, this is a special case.
I'm not saying coupling coefficient never matters.

Rick N6RK

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