[600MRG] Indirect RF Power Measurements wrap up...

[Dave Riley]

Indirect RF power measurement wrap up - Dave Riley AA1A

So who has an accurate RF ammeter??        ribbit       Thought so, me too..

The Bird 250 watt slug for 2-30 mhz. reads low @ 475 but by how much?? 
Who knows??

A low cost general purpose 'O' Scope serves us well on 600m cuz vertical 
peak to peak calibration is fairly certain so is all we need to look at 
an RF sine wave which appears across the input of a decent dummy load...

So then send the new HexFet amplifier RF output pours in to the 50 ohm 
dummy load with the scope leads hooked up across the input...
Then start WSPR at 100% duty cycle (  continuous ) in to the 
transverter/PA amp and then creep up on the output control so as to see 
some Drain current ( 3 amps here ), and see an RF sine wave on the 
scope... Bring the output level up to cause 141 volts _peak to peak_,  
and at those values you should have 50 watts @ _ONE_ amp for a 
reference... Scale down to 5 watts if just using the transverter...

See John Andrew's, W1TAG web site for some easy to understand directions 
which includes winding a small toroid inductor so as to place over the 
RF hot side of the coax in order to detect RF current in AMPS.. I added 
a small capacitor across the output so as to build up the output level 
of the RF sine wave to about 20 volts p-p, thence to the Shotkey 
detector, voltage divider, and by pass caps for a DC signal to feed a 
sensitive microammeter, in my case a 0-100 ua. full scale...

The TX RF is running at 50 watts so I can now adjust the meter/detector 
variable resistor to read half scale or 50 microamps = _ONE AMP RF..
_By placing the ONE amp value at the middle of the scale we maintain 
fair linearity so that as we go from .5A to 2A of RF current we can 
fairly well rely on that value...

This transmitter is set to drive a 50 ohm load in to the primary of a 
toroid coupling transformer then direct to the TX LOOP which is 30 ohms 
R secondary and +/- 0 X ( j=0 ) at resonance...

Now the RF ammeter is ready to read 1 amp @ 50 ohms and there is the 50 
watt reference which is just a little more power than I need to assume 
our 5 watt eirp signal and I'm confident to within ONE DB now...

When the Bird meter read 35 watts is when I know that it is really 50 
watts ( in a 50 ohm load )... Then the coupling transformer is wound for 
a 50 ohm to 30 ohm turns ratio, the vacuum variable tunes the loop nice 
and sharp, phase is maintained between high voltage produced @ the 
capacitor as  seen on NE2 bulbs n series and the input current to the 
transformer...

Beware of stray reactances, clean em out and set the R values as close 
as you can to 50 ohm reference... Now I go from dummy load to the input 
of the loop tuned circuit and they look the same.. 1 AMP, 50 ohms, 
brightest bulbs, dependable and repeatable...

Extra help was stumbling on an old Autek Vector Analyzer VA-1 which not 
only goes down to about 450 khz. but reports R, X, Z, SWR, etc...
Cheap and simple but it gets you on the target fast, sometimes bull's eye...

Watta Country!!!  Having much fun the second time around in ham radio 
only this time we know a thing or two cuz we've seen a thing or two, 
like in the insurance adds...

Happy New Year...  DaveR aa1a