[time-nuts] OT: Power level reference

Mike S mikes at flatsurface.com
Tue Dec 1 17:53:02 UTC 2009


At 11:41 PM 11/30/2009, Joseph Gray wrote...
>We all have our various highly accurate frequency and perhaps time
>references. Is there a relatively simple and inexpensive method of
>making an accurate RF power level reference?
>Joe Gray
>KA5ZEC

Since you're a ham, I'll assume you're primarily concerned with the 
simple case (50 ohm source impedance, sine wave, HF-VHF). Use an 
o-scope to measure the p-p voltage across a precision 50 ohm load (I 
use the Caddock thick film resistors available through Digikey, which 
are 1%, with minimal inductance).

The power is then W=((Vpp/2)^2)/100

Not time-nuts precision, but good "bang for the buck," and you can get 
better accuracy than a Bird wattmeter (which is only 5%), assuming you 
have a decent scope. Without an o-scope, you can make a peak detector 
and use a voltmeter, with some loss of low level accuracy.

You asked for "simple and inexpensive." For higher frequencies, 
different impedances, non-sine waves, the other more expensive 
solutions are needed.  





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