[Elecraft] Measuring Losses in an ATU

[K9MA]

On 11/3/2016 22:24, Ron D'Eau Claire wrote:
> 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".

The core losses could also be significant.

> 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.

The L network is the lowest Q matching network (without a transformer), 
so it has the lowest circulating currents.  T and Pi networks have 
higher Q, but don't require a continuously variable inductor.

At extreme impedances, T and Pi network tuners can indeed be so lossy 
that most of the power is dissipated in the tuner itself.  One of mine 
once achieved a match to a shorted feedline, with predictable results.

73,

Scott  K9MA

> 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
>
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-- 
Scott  K9MA

k9ma at sdellington.us