Quote:
Originally Posted by G0HZU_JMR
With the scope or diode method the following will apply: Using a x10 scope probe is a bit dodgy because it will have to be compensated correctly and the response known to be flat to 30MHz and beyond. If it peaks up sharply at VHF then it could make the error conributions worse from harmonics up at VHF. Also, I wouldn't want to use a x10 scope probe at 100W at 29MHz because a typical low cost x10 scope probe will be starting to look like a dissipative load by 29MHz.
The scope will look like a dissipative load by 29MHz and parts inside could get stressed. It gets much worse by 50MHz.
If you use the diode detector method then use a decent Schottky diode like the HP2800 or 1N5711. Don't use one of the cheap/classic germanium glass diodes, the detector performance up in the linear region will usually be inferior.
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Could we break this down a little, and as per OP, stick to 100W at no more than 30MHz,
at least to begin with.
Re the Power Meter (PM) load; the 50R flange resistors, which
are intended for terminator applications, seem to be the consensus choice for a new "amateur build" PM, and for sure, the actual value of that component needs to be checked as accurately as possible (probably, in both the "as received condition" as well as after some period of ageing at elevated temperature (100'C max spec).
Coming directly off the 50R load (
no T pieces in the feed), we have an attenuator feeding either a diode probe or a scope. There seems to be consensus that the capacitance involved in using a scope complicates the issue; my understanding here is that the capacitance is effectively reducing the resistive value of the end of the attenuator, so giving a false, low reading which is also dependent on frequency?
If that is correct, is the optimum design for the attenuator
in this role, the one that gives just enough resistive attenuation so that the scope can handle the voltage safely, so the effects of C are minimised?
What (and why) would the optimum sensitivity setting of the scope be for such a measurement?
Looking at the diode detector option, I had anticipated that the low capacitance of the old point contact diodes would make them attractive for this role, and we are not short of power, so we can easily put enough voltage on them to be sure we are well in the linear region.
Re harmonics, impossible to assess in the absence of a spectrum analyser?
I'm sorry if I'm being slow to appreciate the subtleties here, but judging by the number of articles on the internet about RF Power Measurement, which have been criticised here, and the old previous thread on this forum, it does seem to be a subject where many have underestimated the problem of RF power measurement. But, it's an interesting subject and of significant practical use for anyone who is licenced!
As an aside, quite lot of amateurs are using ATU's made by MFJ. These are certainly built to a tight budget, but generally seem to be assessed as OK for 100W. Most models have a Power Out/SWR meter fitted. I searched the internet yesterday looking for comments on how accurate those meters might be; found absolutely nothing!
B