[AMRadio] AMRadio Digest, Vol 89, Issue 39

[Jim Wilhite]

There are a couple of wattmeters out there today that is almost in reach 
of the common amateur that exhibit very good accuracy.  Compared to the 
older versions Mirage, Heath and others, they are real good.  One is the 
LP-100A made by N8LP.  It will read the net power out and show the 
reverse power.  In that mode it is close to the power going out of its 
detector.

The thing can be purchased as a kit or wired and calibrated.  If you 
want to build it and verify its operation then send it back, he will 
calibrate it for you.

>From the telepostinc website:

Three autotanging scales with independently adjustable maximums.
SWR display resolution is .01
SWR error < .15  (5%) from about 100mW to 3000W, <.05 typical
Power accuracy is better than 5% at any frequency from ~1W to maximum, 
3% typical with NIST traceable factory calibration
Power display is actual power delivered to the load ( Fwd minus Ref 
power) or traditional Fwd power.


I don't know about the Coaxial Dynamics version, but I hear it is very 
accurate as well.  I read their description some time back but just 
don't remember.  The problem with either of these is the cost.  The kit 
version of the LP-100A is $335.  That is more than most guys want to pay 
for just an accessory, but if you want to know your peak power, it will 
do that with good accuracy.

The other way is as Don suggests for AM.  Get the good old thermocouple 
RF ammeter and use a scope.  A little math never hurt anyone.

Jim/W5JO

----- Original Message ----- >
> The only way to measure real actual transmitter power is to use a true
> RMS-reading voltmeter or ammeter working into an accurately known 
> resistive
> load.  One such instrument is the Bird APM-16 (very expensive). A 
> cheaper
> alternative is the old-fashioned thermocouple RF ammeter. I have tried
> several of  the popular Hammy Hambone "wattmeters" and never found two 
> that
> would give the same reading with the same transmitter running the same 
> input
> power.  The FCC admitted as much when they deleted the former  rule
> requiring amateurs to have on hand "accurate instruments" to measure
> transmitting power when running more than 900 watts, then going on to 
> state
> (as quoted approximately from memory) that "the 'modern day' amateur 
> has
> 'means other  than accurate measurement' for determining transmitter 
> power".