[Elecraft] KPA1500 wattmeter adjustment

[Ed Cole]

I would guess most RF power meters are lucky to be within 5% accuracy of 
full scale.

As a comm tech in the 2-way commercial industry, the Bird43 is pretty 
much a standard field instrument for measuring power and reflected power 
(to check SWR on cables, antennas, filters, etc.).

If you want higher precision measuring ac voltage across an accurate 
50-ohm termination is the better choice.  The ac voltmeter is your scope 
and its accuracy depends on the frequency range its built for.

For fairly low power you can get quite accurate RF loads.  I keep a 
couple for doing comparison checks with my Bird (have two of them).

Since I do ham radio up to 10-GHz the Bird and most ham SWR meters are 
useless.  I have a surplus HP432A microwave power meter for that.  Its 
not calibrated (recently).

I have several directional couplers for measuring reflected power.  A 
couple are rated to nearly 1000w for the 900-MHz mobile phone market.  I 
checked directional and coupling factors using the HP432A at 1296 and 
those are installed on output of my 600w 1296 amp for continuous 
monitoring forward and reflected power.  I use a small pcb with shottky 
diode detectors to provide indication on a ma meter.  I record the dc 
voltage derived with RF power measured by the HP432A.  Milliammeter is 
then set with a precision bench PS at the observed voltage from the 
detector.

They will get checked often with one of the Birds for a quick test.  The 
Birds will be calibrated with the HP432A using attenuators to bring 
level down to 1mw.

I only have a 150MHz scope at present so can't really use it for RF 
calibrations at mw frequencies but the HP432A can be compared at HF or VHF.

So probably most hams are not going to send their RF power meters out 
for calibration.  But there probably are 1% who might.  Solution would 
be for them to offer to test meters for other hams at a nominal fee (or 
even for free) if two-way shipping is paid for.  That is done by a few 
microwave hams for others.

73, Ed - KL7UW