Hi All
Elecraft has a new minikit, a directional coupler model CP1. This kit can be
built as a 20 db or 30 db directional coupler. The coupler is usually used
with a spectrum analyzer but I find it works perfectly with my OHR WM-2 QRP
wattmeter.
Just connect the CP1 in series with your dummy load and connect the WM-2 to
the forward output of the CP1. Set the reverse switch to terminated and the
forward switch to the output connector. I you have a 20 db CP1 then select
0-1 watt on the WM-2 and the meter will read 0-100 watts with very good
accuracy. If you have a 30 db CP1 then use the 0-100 mW range on the WM-2
for a 0-100 watt scale.
This setup is great for testing and calibrating a KPA100 or KPA100/KAT100. I
use a OHR 100 watt dummy load or a TenTec #239 300 watt dummy load. Either
of these dummy loads work great with this setup and they are both available
in kit form. For up to 20 watts you can use the Elecraft DL1. The 20 db
version of the CP1 is exactly 20 db in reduction so the accuracy is very
good.
Elecraft specifies the 20 db version at 25 watts but I have not seen any
problems with it at 100 watts in this 50 ohm environment. If you build the
CP1 as a 30 db coupler the accuracy is not quite as good as the divide ratio
is really 29.8 db but it is rated at 250 watts. You could tweak the 100 mW
range of the WM-2 to account for the divide ratio on the CP1.
You can do a quick check of the calibration with a scope and a X10 probe.
Check at 10 watts, 50 watts and 100 watts. 10 watts is 63 volts pk to pk, 50
watts is 141 volts pk to pk and 100 watts is 200 volts pk to pk across the
50 ohm load measured with the scope. Remember scopes measure peak to peak
not RMS. You should use a X10 probe as most scopes will not measure this
high of voltage without the probe and the X10 probe also isolates the input
capacitance of the scope so you do not load the dummy load with capacitance
making it reactive.
Don Brown
KD5NDB
BTW As Elecraft is no longer supplying a hand mike for the K2. I have found
the hand mike from TenTec works fine if you change the 4 pin connector out
to the 8 pin that fits the K2
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