Re: Procedure to measure rf watts
Richard I pretty much agree with your comments. But I'd like to add that, at HF, precision connectors and fancy adapters are overkill. I have a T connector, UHF type, in the line from the rig. I plug into that an adapter from UHF to BNC and another adapter from there to binding posts. I connect the HP 410B to the binding posts.toggle quoted messageShow quoted text
Now of course I am aware that this is not good practice. But I suggest that, for frequecies below 30 MHz, it's close enough. There are no discontinuities more than an inch or so, much less than what rule of thumb (one tenth wavelength) says. My readings are close to what I expect. And if there is an error, well it can't be great and I don't think FCC will be breathing down my neck for running too much power.
Should I desire to work at 2 meters, then I might be more rigorous. (I am reminded of an old exam question for ham license that suggested using an HF SWR meter at 2 meters if that's all you have, and it's close enough.)
On Wednesday, May 22, 2019, 6:46:39 PM PDT, Richard Knoppow <email@example.com> wrote:
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.