[600MRG] More finding weak points...

[Dave Riley]

More weak points... Indirect Measurements...  ( Dave Riley )

Have been spending time calibrating a home brew RF amp probe by sending 
a few watts @ 475 in to a good known 50 ohm dummy load with a voltage 
tap at the connector to see in the scope the RF sinewave shape and the 
p-p voltage value for figuring simple formula...
See http://www.w1tag.com/RFA.htm for a great primer and re-learner from 
the early days... Works on any Z line...

The vertical antenna transmit loop lead goes thru a fairly small core, a 
RED 1", wound with several turns of enamel wire terminated in a resistor.

A piece of coax is connected to the resistor which comes down to a 
little box with a shottkey diode detector, var. resistor, and cap. and 
then on to the scope vertical input...

The resistor across the toroid is variable so I make sure that I measure 
the RF AC voltage and use it to compute RMS volts then enter in to the 
simple formula,  e-rms(sq)/R, rms voltage squared divided by the 50 ohm 
dummy load resistance gives the RF current value in AMPS... I'm driving 
a 100ua. scale meter so when I know that there is 10 watts adjusted 
output of transmitter then I set the variable resistor so that the meter 
reads 20ua... In this case is my 1W eirp number...

So 20ua. = about 10 Watts   If we raise the RF to 40ua then we doubled 
the RF AMPS so we quadruple the power to 40 watts... And so it goes...
So 50 ua is about the 50W RF power here which is 5W eirp, depending on 
your antenna efficiency..

So I think I'm within a DB now.... Couldn't tell for sure on ham 
wattmeter or swr meters...

Now I know where 1 watt and 5 watts ( erp levels ) are on this simple 
meter with good assurance ( using same loop tx antenna)...
Compare results against other users on the WSPR server for rough 
verification...

In the XSH days we could use 20W erp so I knew about where 20W eirp was 
on the RF ammeter...  It was from the Heathkit SB600 here running very 
cool at almost a 100W out, loud and clean and cool...
http://www.radiocom.net/600M/SB600M3.jpg   It was neutered down to a 
single band, gets 5W eirp with one tube pulled and reduced drive but 
ready to get back to 20W eirp, maybe...

Respectfully submitted for feedback...   DaveR @ aa1a