[Elecraft] KAT500 Matching Range & Power Limits

[Jim Wiley]

Ed -

I think you are confusing the power handling capability of a tuner with 
it's ability to match a non-50 ohm load.  A tuner may be able to match a 
highly reactive load at low power, but be totally unable to handle the 
same load at higher power levels.  The match involves selecting the 
appropriate values of capacitance and inductance to transform a high SWR 
load to 50 ohms resistive so the transmitter final amplifier can deliver 
power efficiently.  However,  when substantial power is applied, the 
result of the needed transformations can easily result in voltages and 
currents that far exceed the ability of the components comprising the 
matching network to handle them.

Let's assume your tuner is matching a load that presents a 6 to 1 SWR.  
Let's further assume that you have a 100 watt transmitter.  At 50 ohms, 
100 watts represents about 70 volts RMS of RF signal. or about 101 volts 
peak.  No problem for a capacitor in the matching network to handle at 
50 ohms, but that 6:1 SWR means that the voltage could now be 420 volts 
RMS or almost 600 volts peak - still not too much of a problem.  But, 
suppose you increase the transmitter power from 100 watts to 1000 
watts.  The RF voltage at 50 ohms increases to 223 volts RMS, (315 volts 
peak), and when the4 6:1 SWR is taken into account, that voltage is now 
about 1338 RMS volts, or 1820 volts peak.  The same sort of thing 
happens with currents.  At 1000 watts, the RF current at 50 ohms is a 
little less than 5 amperes.  But at 6:1 SWR, there will be places where 
the current is almost 27 amperes - a significant amount to be sure.

Depending on where the tuner is physically located in respect to the 
standing wave on the transmission line, a voltage or current peak could 
occur directly at the output terminals of the matching network.  If that 
happens, the components of the matching network could easily be stressed 
well beyond their ability to cope.

So, while the coupler may be able to handle lower power levels without 
distress, the same is not true at high power.   In your example, 1000 
watts at a 14:1 SWR could result in peak voltages of about 4400 volts 
and/or  currents approaching 63 amperes RF.  Not many components can 
handle those voltages and currents, and those that can are physically 
very large, certainly much larger than those that can fit inside the 
cabinet of a KAT500.


- Jim, KL7CC



On 11/15/2012 12:56 AM, Ed G wrote:
> Hello,
>       I'm trying to understand what the KAT500 power limits mean in real
> life.  If I am tuning an antenna that initially shows a very high SWR (say
> 14:1), and I am using high power (say 1000 watts out), the KAT500 is going
> to open up the PTT line, and the tuning operation will take place with less
> than 100 watts.  Then I will show a 1:1 SWR, and I'm ready to go with my
> 1000 watts.  Operationally, what is the meaning of the 3:1 SWR limit for
> 1000 watts as shown in the KAT500 specs?  Will the KAT500 fault because it
> is matching a 14:1 SWR at high power?
> --Ed--
>
>
>