Re: K3s driving a KPA1500 - how much drive power is nominal for 1500 watts? #kpa1500

Bob McGraw - K4TAX

Re dummy load.  First measure it with an ohm meter. It should measure 50 ohms  +/- 5% or better.  Then measure it with your antenna bridge from the lowest to highest frequency you intend to use.   An MFJ-259 or other known accurate bridge is suitable.  Make sure you use a known good, short length, low loss coax as a jumper.  The bridge should show 50 ohms and an SWR value of less than 1.2:1.  Higher values indicated some value of reactance.  A good dummy load will be resistive at all frequencies and show no reactance.  The R value is resistance and the reactance is the “j” part of the impedance.  Ideally it should be zero as measured on a VNA.  

I’ve found that both the air cooled loads and the “gallon of oil” types will change values as they heat.    Heat one with some RF and measure it again while hot.  If you find it to be different when hot than when cold, just be forewarned what you have to consider when making power measurements. 

It doesn’t take an expensive or precision load. It just takes knowledge and knowing what you are dealing with. 

Bob, K4TAX

On Feb 5, 2020, at 9:26 PM, Mike VE3YF <mike@...> wrote:

I have only received my KPA1500 recently, But I decided to put it through the paces with the K3S being the exciter.

I found that into a Cantenna Dummy Load that has not been tested for 50 Ohm impedance that below is what I get from the KPA1500 Display and a LP-100A meter, but never really used the LED's as the indication of power.

20m - 10m approx 30w output on K3S yield approx 1425 -1475w Output
40m - approx 27w output on K3S yield approx 1450w Output
80m - approx 25w output on K3S yield approx 1450w Output

30m and 160m antennae are only rated for 1kw, so didn't do the test on those bands.

Mileage will vary from station to station, and as Bob mentions the Dummy Loads actual impedance is a factor, but so far I am happy with the KPA1500 performance, and see no need to go out and get a lab spec Dummy Load for something I rarely use.

73 De Mike

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