Re: Procedure to measure rf watts

Richard Knoppow
 

At communication frequencies I use a General Radio 1800A mainly because I have the proper adapter for the probe. The 410B is fine but its adapters for any of the more common coaxial connectors are pretty scarce. Otherwise its fine. The 1800A takes a shell for a GR 874 connector which screws into the end of the probe, the banana plug acting as the center conductor. I then use an N type T with UHF adaptors on two ends and the probe in the center. I have a couple of dummy loads which have been measured on a GR RF bridge so I know their actual impedance at the measurement frequency. In general a DC resistance measurement comes close and they are pretty non-reactive. I can then calculate the power with reasonable accuracy.
Both the 1800A and 410B have single diode rectifiers for RF and both have the usual characteristic of being square law at small voltages transitioning to peak reading at high voltages. The scales are calibrated in the RMS value of a sine wave and both have different scales for low and high voltages as required by the characteristic of the diode. I have calibrated my MFJ tuner/SWR meter/power meter using this arrangement. It seems to be reasonably accurate. A scope could be used but as you say requires some calculation because it reads peak-to-peak.
Nothing is as simple as it seems or as you would prefer.
OTOH, a scope can tell you an awful lot about what is going on in the transmitter.

On 5/22/2019 5:39 PM, Bob Albert via Groups.Io wrote:
I use my venerable HP410B.  The ac probe has wide bandwidth and the most sensitive range of 1 V should suffice for all but the weakest power from a transmitter.  Full scale corresponds to 20 mW.  Half scale, or 0.5 Volt, 5 mW.
Sure you can use an oscilloscope.  You will have some calculation to do.  Further, you would be able to ascertain whether you are working with something close to a sine wave.
Ideally, use both instruments, one for checking waveform and the other for a quantitative reading.  The load, of course, needs to be purely resistive.  If not, your 'power' reading will be wrong.
In any case, an oscilloscope isn't the best tool for precise measurements.  The trace width and screen nonlinearities need to be considered.  A spectrum analyzer might be a good idea, since it would show each frequency component and allow one to decide if any are large enough to influence the result.  Most spectrum analyzers have 50 or 75 Ohm input already in place; the pitfall there is to make sure you don't burn out the termination.  Ten Watts is too much, and most aren't safe above 1 Watt or less.  A dummy load and attenuator might be a good idea.
When I measure the power out of my ham transmitter I can get an accurate reading with the HP voltmeter.  I use either a dummy load or an antenna with close to 1:1 SWR.  Without the linear amplifier I get around 65-70 V reading (100 W or so) and with the amplifier about 220-250 Volts (over 1 kW), depending on tuning and load quality.  The HP pointer movement is so well damped that I can get peak readings when operating pulsed, such as a series of CW dots.
I also have an oscilloscope to see the wave; I couple it to the system with a loop pickup.  I connect a coaxial cable to the 'scope and short the other end around one of my voltmeter leads.  Thus, inductive pickup.
When I use audio modulation I can measure peak envelope power on the voltmeter.  The 'scope will enable me to adjust the modulation such that I don't get clipping.  I can also verify the performance of the speech processor.
Bob
On Wednesday, May 22, 2019, 4:01:03 PM PDT, Randy.AB9GO <@AB9GO> wrote:
Hello everyone,
I have a need to check the calibration on some low power watt meters (10 watts or less, 21 megahertz or less) and was wondering if anybody had a favorite procedure using their scope to measure peak-to-peak rf voltage without letting out the magic smoke.  My thought is to use a t connector, hook one side to the transmitter, one side to a dummy load and then the center connector straight to the 10X oscilloscope probe. Any other precautions I should take?
Thank you,
Randy.
--
Richard Knoppow
dickburk@...
WB6KBL

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