A year ago I was helping out temporarily at Bext with technical service and a number of interesting challenges came up. One was from a customer whose low power FM transmitter shared a site with a mobile carrier. The carrier’s technician was complaining about the transmitter’s 8th harmonic getting into his  radios. The FM transmitter in question was clean far beyond FCC requirements of -80dB below assigned carrier level (as I remember, about -96dB) but on a spectrum analyzer, you could see a tiny bump down near the noise floor in the 800 MHz band. I used the analyzer and its tracking generator to trim a hunk of coax and knock down the 8th harmonic of the offending FM transmitter about 20 dB. That’s a pretty short piece of coax, by the way. Problem solved and nobody had to build a Faraday cage.

A June article in the BDR mentions this conflict between the very sensitive LTE mobile site receive inputs and their FM transmitter neighbors. These radios are attempting to discriminate data at -120 dBm and lower from distant tiny transmitters inside metal cars and buildings.

I first ran into an application for a stub with a TV translator site in the Oregon high desert in the early 80’s, where I was receiving a faint UHF signal beyond line-of-sight and picking up some birdy from my channel 5 output. With an N-connector “T” adapter and a quarter-wave length of RG-213, I fashioned an open-ended stub that resonated at the mixer product, attenuating it enough to remove the problem from the visual band of my UHF input.

Another recent article in Radio World magazine, this by Mark Persons, suggests putting a bandpass filter in the form of a quarter-wave shorted (not open) stub after a solid-state amplifier feeding a tube amplifier. This is brilliant. It serves to attenuate voltage spikes that might enter since only the resonant frequency passes without great loss–DC and pulses slow and fast are shorted out. By the way, this won’t work with TV or other broadband applications due to the high-Q of the stub.

Check out articles on construction of these stubs online.