[Elecraft] Power Calibration ?

[Mike Harris]

Hi folks,


> Francis Belliveau wrote regarding the KPA100:
>
> "My question for the group is:  Wouldn't it be just as accurate to read
> the RF Voltage across the load?"
> ==========
> Keep in mind that the RF voltage across a 50-ohm resistive load will be
> 70.7 Vrms with 100 watts applied.  If the RF probe can handle that much
> voltage, it should work fine.
>
> 73, de Earl, K6SE

Quite so, my Fluke probe is only good for 32 volts.

The idea of using a coupler is common practice.  I used to run an 11 metre
dish earth station (Intelsat Std. "B") and many station measurements are
made using couplers, 4 and 6GHz.  I still have access to the test equipment
and will soon be able to measure the true coupling loss and bandwidth.  The
only thing I won't be able to check is if I'm getting saturation in the
small transformer at the 1W level.  If that becomes apparent then a bigger
core is the answer.

For what it's worth how it's done is to wind ten turns on an FT37-43 core,
slip the core over the wire between the centre pin of the input PL259 socket
and the resistor board which makes a single turn giving a turns ratio of
1:10.  One end of the ten turn winding goes to the "earthy" terminal of the
load and the other end goes to the centre pin of an output socket.  I have
used a BNC to ensure I don't mix up in and out.  The only thing to remember
is that there has to be a small 50 ohm load permanently on the output socket
or the main load is no longer 50 ohms.  Actually it should be nearer 50.5
ohms after this modification.

Will post the results when I have them.

Regards,

Mike VP8NO (IOTA SA-002)
GQRP 10148
K2 #1400 + lots of bits