[TenTec] Eagle -- power measurement

[Dr. Gerald N. Johnson]

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
>
>