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Tech Tips: Test a Microwave Shot at the Receive Site
When building a new 950MHz microwave shot, it's good to know the fade margin you've achieved on the new link. Actually, this is a good thing to know regardless of whether the shot is new or old. Knowing how much signal is getting into the receiver -- accounting for all the various losses in the system -- can really help in troubleshooting a bad link. You can do this in a couple of different ways, but the easiest is to purchase some 50Ω type-N attenuators (in 10dB, 6dB and 3dB sizes) and simply put them in front of the receiver input, thus attenuating signals coming in. Keep adding attenuators until such time as you either a) squelch the receiver, or preferably b) hit the noise floor in the receiver output that you consider unacceptable for on-air use. (The receiver should squelch there anyway, right?)
I would consider a very strong link to have a 30dB fade margin; 20dB would be a good link; and at 10dB I would probably add an amplifier ahead of the receiver. (Some good sources are Advanced Receiver Research; Angle Linear; and Miteq.) Another thing I sometimes do (using the spectrum analyzer) is to look at other signals coming off the receive antenna on different frequencies. Take some notes on how strong they are, too. If one day you suddenly find that your STL signal is weak, take a look at the others you previously measured. If they've all gone down in level by 10dB (as an example) the problem is the near end. If only your signal has gone down by 10dB, the problem is at the far end. If there are any co-channel or adjacent-channel signals, it's a good idea to take notes about them as well. You would certainly want to know if a co-channel signal suddenly went up in level, or if a new one just showed up one day.
You can extend this idea to the transmit antenna as well. After the link is all up and running, at some point turn off the transmitter, and connect the spectrum analyzer to the transmit antenna, temporarily using it as the receive antenna. Make some notes about what you see coming in from the direction at which it's pointed. The strength of one or more of the received signals might help you troubleshoot path problems at a later date.
Irwin is RF engineer/project manager for Clear Channel Los Angeles. Contact him at firstname.lastname@example.org.
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