[AMPS] power meter errors

[Dick Green]

I know of at least two power meters that compensate for readout errors due
to mismatch, and have other useful features as well. Unfortunately, both are
rather hard to obtain.

The Nye RFM-003 RF Power Monitor reads either net forward or reflected
power, depending on a rear panel switch setting. I'm not sure, but I believe
the coupler measures total power output and reflected power, converts them
to DC, then circuitry inside the meter subtracts the reflected power reading
from the total power reading in order to display the correct net forward
power. For example, when I transmit at 100W on 160M to my 80M vee, the rig's
power output meter reads 40 watts (the solid state rig pulls back on the
output due to high SWR). In contrast, the Nye reads about 30 watts forward
power and about 10 watts reflected power. The RFM-003 has a computing SWR
meter (no "calibrate" knob), accurate SWR readings at low power (few meters
can do that), high/low autorange, Peak, Peak-Hold and Average readings, and
automatic lock-out to protect the amp when excessive SWR or reflected power
is detected. There's a 3KW model and a 5KW model with interchangable
couplers. QST raved about these meters a few years ago. Shortly after that,
you couldn't buy one anywhere. Evidently, they're not being made anymore.
The going used price is $350-$400. I was lucky to get one for $275 a few
years back and would buy another (or a spare coupler) in a second.

The other meter is the Tandem Match, by John Grembenkemper, KI6WX (ex
KA3BLO), which appeared as a project in the January 1987 issue of QST about
10 years ago. This device uses a Bruene directional coupler and matched
Schottky barrier diodes to measure power output and reflected power. Solid
state circuitry computes SWR and subtracts reflected power from total power
output to display forward power. It is very accurate for both forward power
and SWR readings at low power output, even for QRP levels and less. It isn't
autoranging, but you can design your own range switch with as many positions
as you like, from less than one watt full scale to 1500 watts full scale.

I was one of the few people to attempt the Tandem Match project soon after
the article was published. I had quite a few problems due to errors in the
schematic, and corresponded with John several times to get corrections and
answers to questions. The January 1988 issue of QST, Technical
Correspondence, contains a letter from John with corrections for the errors
(a photocopy of the revised schematic could be obtained from HQ.) There was
no PC board available for the complex circuit, so it was very difficult to
build. I used a combination of point-to-point and wire wrap construction,
which I think led to some instability in the unit. Also, John was not a QRO
enthusiast and the coupler was not designed for much more than a couple of
hundred watts. I burned mine out with a kilowatt from my SB-221. I
corresponded with John about it, then replaced the coupler's ferrite core
torriods and mini coax with powdered iron core torroids and RG/8. Now it
works fine at 1500 watts, although I suspect that accuracy may not be quite
as linear as it was before. One of these days, I'll check it against some
lab equipment. It's good enough for typical medium-to-high power use,
though. When in doubt, I consult the RFM-003.

In the July 1993 issue of QST, Technical Correspondence, John reported that
a PC board had been developed and was available from FAR Circuits. I got the
board and rebuilt the circuit. Once again, I found numerous errors -- some
omitted bypass caps, some labelling errors on the parts screen, and quite a
few foil side errors (mostly incomplete cutting between pads and the ground
plane, various shorts, omitted parts, etc.) Needless to say, I built it very
carefully. I had to stop when we remodelled our house three years ago, and
never got back to it. I still have to mount the connectors and detector
diodes on the board, and figure out some sort of enclosure. I bought LCD
meters for the readouts, although I'm thinking that the old analog meters
might make amp tuning easier. Maybe I'll use the LCD meters for digital
readout and LED bars for tuning. Anyhow, I'll get back to that back-burnered
project one of these days!

Hope this information is of use.

73, Dick, WC1M





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