[600MRG] More on accurate RF power measurements...

[Edward R Cole]

Simplest method is measuring RF voltage or RF current with a known 
resistive load.  I use a Bird 25w coaxial load on output of my 100w 
NDB transmitter (modified to be three stage linear amp driven with 
less than a mw.

Using a Bird 43 power meter with the 100H element (rated 2-30 MHz) 
see a reading of about 85w forward power.  The Bird Corp provides a 
calibration curve for off frequency measurements in the meter 
operating manual.  From that curve they show 3% under reading at 
400-KHz.  Also the meter calibration is only within 5% at full 
reading (100w).  So its possible to have a reading - 8% below 100w or 92w.

My NDB has a RF ammeter which makes measuring output 
"better".  Running 1.4 amps into 50-ohms produces 98w (P=I^2*R).  Of 
course this depends on accuracy of the load.  I measured the Bird at 
49-ohms with dc ohmmeter (Fluke) and my MFJ-269B, so P= 96w with 
I=1/4 and R=49.  Bird would read 92% of 96 = 88w.  Pretty close to 
the 85w observed with the Bird.  I have compared the RF ammeter 
reading with RF voltage read by a 100-MHz rated oscope.  Its close 
enough without using magnifier glass to read the meter.

Good enough accuracy for amateur use.

For SWR measurements, I rely on my modified MFJ-269B.  It will run 
from 400-850 KHz and provide Z = R + jX readings plus calculate 
SWR.  From memory that is about 1.5:1 into the 100-foot RG-213 
connected to my base loading coil at the antenna.  I get a more 
accurate reading of antenna match at the coil and match at the NDB is 
slightly different.  SWR losses at 470-KHz are in significant, so 
mostly keeping a reasonable load for the output transistors.

73, Ed - KL7UW
High winds preventing outside work this past week; 630m antenna 
laying on ground for now.


At 08:51 AM 2/1/2018, Warren Ziegler wrote:
>Wavenode makes digital wattmeters:
><https://wavenodedevelop.com/>https://wavenodedevelop.com/
>
>I emailed them and found that they make a coupler (sensor) that for 
>ultrasound that covers 60kHz to 2 MHz for 0-2000 Watts for $75.
>You can connect the sensor to a Wavenode meter, alternatively they 
>supply a calibration table that tells you the d.c. output voltage 
>from the sensor vs. RF power through the sensor. (The d.c. output is 
>approx 0 to 12v over the RF power range.)
>
>N.B. The 60kHz to 2MHz sensor is not listed on their webpage, I 
>found out by emailing Wavenode.
>
>73 Warren
>
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73, Ed - KL7UW
   http://www.kl7uw.com
Dubus-NA Business mail:
   dubususa at gmail.com 
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