[Elecraft] Measuring Losses in an ATU

[Ron D'Eau Claire]

Since, at HF, there is very little dielectric losses most of the loss is
resistive. That means the greatest loss is at whatever condition of
frequency and impedance transformation produces the greatest circulating
currents. You are quite right; RF resistance is different from dc
resistance. It's generally much higher due to the "skin effect".  

One of the widest range ATU circuits is the T-match, but it is capable of
producing a low SWR to the transmitter at settings the result I huge
circulating currents in the coil. More than a few Hams have discovered the
coil in their MFJ T-match tuner collapsed in a heap as the poly insulating
bars on the coil melted at a fraction of the rated power. It's easy to have
one consume more than 50% of the RF power applied. 

73, Ron AC7AC

-----Original Message-----
From: Elecraft [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of K9MA
Sent: Thursday, November 3, 2016 4:28 PM
To: elecraft at mailman.qth.net
Subject: Re: [Elecraft] Measuring Losses in an ATU

I haven't tried it, but one way to measure the loss of the ATU would be to
connect a second, identical ATU "backwards" to a 50 Ohm load. This second
ATU would be tuned to present the highest or lowest resistive load possible.
One way this could be done would be by connecting a resistor across the
transceiver output, matching it, and reading out the ATU component values.
The second ATU would be set to the same values, which should result in a
match from the first ATU without retuning.  The measured loss would be twice
that of a single ATU.

That said, it's very likely the maximum ATU loss would be at the highest
frequency of interest and highest or lowest impedance the ATU can match.
Highly reactive loads might make it worse.  I suppose an automated test
could explore all the possible combinations.

This could all be simulated, of course, if good models of the components are
available.  I suspect most of the loss is in the inductors, but one needs to
know more than their DC resistance.

73,

Scott  K9MA

--
Scott  K9MA

k9ma at sdellington.us

______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:Elecraft at mailman.qth.net

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html Message
delivered to ron at elecraft.com