While a 6AL5 and transformer could be cheap, the venerable HP 410B and
410C used a smaller diode in the AC probe and they also made a 10:1
voltage divider and a coax tap adapter for this very purpose. So it can
be had ready made.
One other technique is to use a water proof dummy load submerged in an
insulate water bath. Then monitor the temperature rise of the water. Its
best if the water is circulated and you know the amount of water and the
thermal mass of the dummy load. It can be calibrated by heating the
dummy load with AC or DC that's more easily measured than RF power and
setting the applied AC or DC power to achieve the same rate of
temperature rise of the water. Then there's no math, the test power
equals the applied Rf power. Called calorimetry and there are commercial
products for that too though probably neither readily available or
suited for 100 watts.
Then there directional couplers with RF out that can go to an RF power
meter and head like the HP 432 and 436 families. Where the head can only
stand 10 milliwatts. And they can be used with a power attenuator. The
432 and previous work with thermistors biased to get their resistance to
50 ohms and the meter works by showing the reduced bias power required
to keep the thermistor resistance to 50 ohms. The later 436 family uses
thermocouples and load resistors sensing the temperature rise of the
load resistors and are much revered in the microwave industry, but a new
one with power head can cost the price an Eagle.
73, Jerry, K0CQ
On 4/2/2011 9:46 AM, Roy wrote:
Don't dismay...there's a way.
A 52-ohm dummy load with a rectifier and filter cap yields an accurate dc
voltage in proportion to the power being dissipated. Today's vom's and dvm's
are pretty accurate (average the readings of all your meters). The rectifier
can be a 6AL5 with its plates connected in parallel, and its heater
connected to its cathode, so the output voltage rating is high enough for a
kw. The plates are connected to the hot side of the dummy, and a 0.01 disc
in parallel with say a 1-uF connect to the cathode for a positive dc voltage
out with a hold time for peak readings. It's inherently self accurate and
requires no calibration.
73, Roy K6XK
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