[Elecraft] KAT3 use questions

[Fred Jensen]

Stan Gibbs wrote:

> When I hit the ATU button with the KAT3 enabled, it does its thing
> for a second or two and then flashes a SWR value on the display as
> expected.  I want to operate on 80m with a VERY limited choice of
> antennas.

It shows you the "best" match it could obtain.  With really mismatched
conditions, the "best match" may depend on where the tuner
was when the tuning algorithm started.  If you tap ATU again, you *may*
get a better [or worse] match.  Under better match conditions, this 
disappears and the first tap will likely get you as good as the 
L-network and algorithm can do.
> 
> My questions  are: 1) If the displayed SWR is, say, 4.1:1, does that
> tell me that the ATU successfully matched the load and that I can
> operate the radio without fear of damage?

The K3 will power back if it doesn't like the match.  Mine won't do full
power into a 4:1 match anywhere and it complains when I try to TX. 
It'll do full power into a 2.5:1 on higher bands, but not so good on 80 
or 160 where I get HI SWR on occasions.

> 2) What does the display indicate when it cannot achieve a reasonable
> match and I should not operate the radio?

It tells you the SWR and then goes back to the freq display.  When you 
TX, you may get HI SWR, it may dial the power back, or both.  Either is 
an indication that you're not going to be able to transmit.

The L-Network in the ATU can match a wide range of impedances but there 
are limits.  If the best it can do is 4:1 SWR, you really need to do 
something to the antenna.  Changing the length of the coax will change 
the impedance the tuner is working into and you might get a better 
match, but ...

keep in mind that, if the feedpoint impedance at the antenna is wildly 
mismatched to the transmission line, the SWR on the line will be very 
high regardless of the length of the line, so will losses, and the ATU 
may not be able to completely match it to the radio.  "Antenna Tuners" 
have nothing to do with the antenna.  They just match the impedance seen 
at the coax connector to the impedance the PA wants to see [usually 50 
ohms].

73,

Fred K6DGW
- Northern California Contest Club
- CU in the 2010 Cal QSO Party 2-3 Oct 2010
- www.cqp.org