Re: KX3 ATU

Posted by vk4tux on Sep 19, 2012; 4:48am
URL: http://elecraft.85.s1.nabble.com/KX3-ATU-tp7495809p7562956.html

Don, When you say "The ATU will tune the antenna to an acceptable SWR", you are contradicting yourself, because as you rightly said the tuner does not change the antenna feedpoint swr.

So we tune the transceiver to the feedline length feeding the antenna. that feedline length is important
regarding the 'measured' swr. A 1/2 wl will show the antenna feedpoint swr or impedance (with analyser).
At the connection to the transceiver before any builtin or ext tuner. The indicated swr on the transceiver or ext tuner display shows the feed side of the tuner swr.

In theory the swr along the feedline maintains value, but it's a very different story when you measure it.
The 1/2 wl feedline connected to a high swr antenna feedpoint shows high swr at the tx feedline end, but cut the feedline at the 1/4wl point and measure there for a much different 'indicated' result.

As the swr detectors in the transceiver or atu use this method also, it becomes obvious that feedline/coax length becomes a factor in showing the transceiver a good match when the antenna feedpoint impedance is other than 50 ohms..

The best thing you can do in a multiband antenna setups where antenna feedpoint impedance gets way off desired on certain bands, is to use the lowest loss feedline possible., because this is where most of your losses are, not the tuner. Remember only resistance can lose power as heat, not reactance.

Every signal pass back up and down the feedline until sync radiation release occurs, loses power with every pass depending on the feedline specifications.

 

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Lucia,

The Elecraft tuner (I don't care if it is a KAT2, KAT1, KXT1 o KXAT3)  
cannot change the the SWR of the antenna itself.  The ATU will tune the
antenna to an acceptable SWR so the PA  transistor in the transceiver
are happy.  If you measure the SWR between the Tuner and the antenna,
you will not see any change.  The internal tuner does change the
impedance that is seen by the PA transistors, and that is the goal of
antenna tuners.

73,
Don W3FPR

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