[600MRG] More on accurate RF power measurements...

[Dave Riley]

This info is in short form. We can pick the details over as we go.

A while back I became aware that something was wrong with the RF power 
measurements around here, especially @ 630m.
As it turns out the Bird meter was not telling the truth and neither was 
the Daiwa crossed needles. Both are OK at HF but not for far below that.

After doing some homework, digging back in to the W1TAG website, I 
relearned toroidal RF current sensors and how to understand them.

The first try produced a good quality pickup, a _red/white_ toroid core 
with many turns to slip over the coax center lead on it's way from the 
TX to the 50 ohm dummy load.
The toroid core output leads were then sent to a shottkey detector, 
bypass capacitor, a variable voltage divider, thence on to a 0-100 DC 
microameter.

When all was done and the final ribbons adjusted and tied, the middle of 
the meter was set to 50 watts by using the indirect method of RF measuring,
that is by looking at the RF sine wave on the TX output coax with a good 
calibrated O-Scope with a X10 probe, reading the peak to peak sine wave 
value and converting it to RMS.
+47dBm = 50 volts AC RMS = 50 watts, in a 50 ohm line, very handy in our 
case.  ( 141 ac volts peak to peak )
This was all done by looking in to a known good 50 ohm load.

This resource is a sheet of paper which compares dBms - volts RMS - 
watts @ a 50 ohm reference.  Thanks to Mini Circuits.
https://www.minicircuits.com/app/AN40-012.pdf

Now I am very confident as to where 50 watts of 630m RF is located on 
the micrometer, adjusted to the center of the meter scale.
Next is to scale down the power to say 10 watts using the indirect 
method and then logging the microammeter reading as such.

Since this TX loop antenna is close to a net -10db gain, I can look at 
the 50W on the meter which is the same as our 5 watt eirp limit, luck of 
the draw.

This is all settled home brew science around here and seems very 
reliable. Probably within 5-10%.

Today's journey is to accurately read lower power levels. Say down to 
100 milliwatts or even less. Using the SDR-IQ receiver with Spectra Vue 
software looking in to a known 50 ohm load and a step attenuator that 
has a short piece of hook up wire  running from it's input.

DO NOT HOOK UP YOUR RF directly in to the poor little receiver. Final 
notice.

Right now I'm looking at top power ( 50 watts RF ) coming from the RFPA 
and adjusting where it shows up on the Spectra Vue scale while adjusting 
the 50 watts to look like say -20dbm on the SV scale... -30dbm should 
then look like 5 watts, and -40dbm should look like 500 milliwatts, 
we'll see...

Crossing fingers and touching wood, this is tinker DaveR @ aa1a, Gudday.




-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.qth.net/pipermail/600mrg/attachments/20180201/2f36b073/attachment.html>