Yaesu FT-450 Power Measurements.

Last week, after some lurking on E-bay, I managed to get a basic 20MHz CRT oscilloscope for just £25.

Now, I got a first in astronomy, not electronics, so I'm perfectly content to be an avid learner, not an expert.

I happened upon these two useful pages (a) and (b), which led me on the route to make my own RF sampler, and some measurements on my trusty Yaesu FT-450.

An aluminium box, two SO239 connectors, a 470 KOhm (top) and 470 Ohm resistor, plus a 10nF capacitor completing the connection to a BNC panel outlet. The overall RF sampled is ~ input Volts/1000.  With a 1.5mm copper link between the centre pins of the SO239s, and a piece of metal shim connected to the box sides lying very close, and separated by thin plastic, the analyser-measured SWR of the box into a dummy load is 1.18:1.

So, last evening, I set about some careful measurements with a calibrated scope.  As my scope is only a 20MHz bandwidth model, I stopped at 14MHz. 

Here are the results, which are quite revealing.  For uses like WSPR, the reported errors are significant.  Accurate power measurement is not particularly easy or cheap, but is of considerable importance to many of us.  There must be quite a market out there for someone who can make an affordable, accurate power meter!

1.8MHZ

Set 5W; Actual = 4.83W
Set 10W; Actual = 8.41W
Set 20W; Actual = 16.00W
Set 50W; Actual = 42.89W
Set 100W; Actual = 72.23W

Mean error: 16.26% below set value

3.5MHz

Set 5W; Actual = 4.83W
Set 10W; Actual =9.30W
Set 20W; Actual = 18.48W
Set 50W; Actual = 43.55W
Set 100W; Actual = 76.54W

Mean error: 10.87% below set value

5.2MHz

Set 5W; Actual = 4.83W
Set 10W; Actual = 8.41W
Set 20W; Actual = 16.00W
Set 50W; Actual = 42.23W
Set 100W; Actual = 80.98W

Mean error: 14.77% below set value

7.1MHz

Set 5W; Actual = 4.83W
Set 10W; Actual = 9.61W
Set 20W; Actual = 17.63W
Set 50W; Actual = 48.99W
Set 100W; Actual = 80.98W

 Mean error: 8.03% below set value

10.1MHz

Set 5W; Actual = 7.02W
Set 10W; Actual = 13.69W
Set 20W; Actual = 24.50W
Set 50W; Actual = 56.23W
Set 100W; Actual = 99.97W

Mean error: 22.37% above set value

14.1MHz

Set 5W; Actual = 7.83W
Set 10W; Actual = 14.44W
Set 20W; Actual = 26.00W
Set 50W; Actual = 60.04W
Set 100W; Actual = 115.53W

Mean error: 33.32% over set value