Hi.
What is the intended "Purpose" of the measurement?
If it's to check some aspect of the modulation, then you can use any
means (a simple wire antenna) to get enough signal to see, without
frying your SA.?? Hint, always start on the highest attenuation (lowest
sensitivity) setting with unknown signals, especially when connecting to
equipment you don't have all the info for...
If however, they are asking you to check for harmonics, and if they are
within specification, take care.? Simple solutions will not do, as they
will not have a flat response from the TX frequency out to the third and
beyond harmonic.?? You can pre-calibrate simple pic-off's, but that
takes care too, and you have to insert them into the TX to antenna
feeder somewhere.
Ideally, there would be a broadband directional coupler permanently in
line for such checks, but they cost, so I guess get left out of the
build spec'.?? Or, one kept on site and calibrated, so that it can be
inserted when needed.? (Obvious TX down time needed of course.)
Using any (even a formally calibrated) antenna to make such measurements
needs the calibration/correction data, as you will need to correct for
the antenna factor, to get the true "in-the-air" signal levels.
If the TX has a sample port, ask to see the calibration data, so you
know what sort of level comes out of that.?? Also, if it has data out
into the land of harmonics.?? Make sure too, that the sample port is
actual RF (not rectified RF just used as a power level check.)
If it's a harmonic check that is wanted, and even if the sample port
shows it's clean, any "off air" received harmonics could also be caused
by "rusty bolt" effects, on other structures near the affected RX, or
even on the TX's own antenna/and support.
Though, if it's own antenna (the antenna itself that is) is that bad,
some harmonic energy will come back down the feeder, and in turn be
reflected back up, so should show up in any Forward Power Sample signal.
Transmitter (especially broadcast TX) checks, need to be done carefully,
and with a documented method, so you know you are getting a "true"
measurement, comparable to those when it was first commissioned.?? Even
for quick n dirty checks, you need to know quite a bit of detailed info
before you begin.
Sadly, even with the kit I have available, and my job is high power RF
(up to 30kW at present) & I'd hesitate to do such checks on any
broadcast TX, without a lot of background info, and data first.
Plus, a written agreement to pay for any damage caused to your SA, by
their equipment!
Lastly, an 8566 is a large heavy beast (as I'm sure you know!)? I would
not want to cart one into a small Broadcast TX hut.? A 8591 or similar
is much better, and some versions have the ability to be programmed up
with correction factors for antennas and couplers before the event via
the memory card.? But that's another kettle of fish...
Best Regards.
Dave B G0WBX.
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