On a nice sunny summer day in 2008 I was doing annual, FCC-required National Radio Systems Committee measurements on WJON(AM) in St. Cloud, Minn.

The station had passed many times, and there was no reason to believe this time would be any different.

Surprise, the NRSC test failed! We encountered an RF mixing product at 1300 kHz.

Things became perfectly clear after a moment. Just months before, KYES(AM) on 1180 kHz had been built and turned on.

(click thumbnail) Before-and-after measurements using an Agilent N9340B spectrum analyzer.

The two transmitter facilities are 15 miles apart. You would think that wouldn’t present a problem, but it did. KYES runs 50 kW with a two-tower directional pattern pointed at WJON. To make matters worse, WJON is only a 1,000 watt station on 1240 kHz but has a five-eighths-wavelength tower, with a gain of about 3 dB over a standard quarter-wave tower. It makes a wonderful receive antenna for 1180 kHz.

Yes, this was a “perfect storm” to create a problem. See the “before” photo of the initial measurement using an Agilent N9340B spectrum analyzer. WJON is in the center and KYES is to the left by just 60 kHz. A mix product in the WJON transmitter, between WJON and KYES at 1300 kHz, is transmitted from the WJON tower and peaks above the NRSC mask by 5 dB.

Ouch!

FCC Rule 73.44

(b) Emissions 10.2 kHz to 20 kHz removed from the carrier must be attenuated at least 25 dB below the unmodulated carrier level, emissions 20 kHz to 30 kHz removed from the carrier must be attenuated at least 35 dB below the unmodulated carrier level, emissions 30 kHz to 60 kHz removed from the carrier must be attenuated at least [5 + 1 dB/kHz] below the unmodulated carrier level, and emissions between 60 kHz and 75 kHz of the carrier frequency must be attenuated at least 65 dB below the unmodulated carrier level. Emissions removed by more than 75 kHz must be attenuated at least 43 + 10 Log (Power in watts) or80 dB below the unmodulated carrier level, whichever is the lesser attenuation, except for transmitters having power less than 158 watts, where the attenuation must be at least 65 dB below carrier level.

It is clear WJON did not comply with the rules. Some would say KYES was the cause and should have been required to fix the problem at WJON. Others would disagree.

No matter how you look at it, WJON ultimately is responsible for maintaining compliance with FCC rules at its transmitter plant, or risk losing its license.

For those who are unfamiliar with this kind of problem, the transmitter is almost always where mixing of signals takes place. This is a well-known phenomenon. Each transmitter has its own “turn-around loss” in its power amplifier where the mix occurs. The good part is that KYES did not induce enough RF current to make the WJON antenna current meter read above zero.

(click thumbnail) The station hired Kintronic Labs to design a filter to nudge WJON into compliance with FCC rules.

What to do

The station hired Kintronic Labs to design a filter to nudge WJON into compliance with FCC rules. Kintronic came back with a design to put a 1240 kHz pass/1180 kHz reject filter on the 50 ohm side of the WJON antenna coupling network. There was no attempt to filter 1300 kHz. The idea was to attenuate 1180 kHz by at least 10 dB to reduce the mixing that results in a 1300 kHz spur.

The filter is a fixed vacuum capacitor with tapped coil in series to pull/trap 1180 kHz to ground. A variable vacuum capacitor in parallel with the series trap is used to parallel resonate the arrangement to 1240 kHz. This makes the network a nearly infinite resistance at 1240 kHz with very little degradation even at 1230 and 1250 kHz.

WJON Engineer Mark Young built an aluminum box under the existing antenna coupling network inside the shack at the base of the WJON tower. He cut a hole between the two boxes to allow a single copper tube through. This connected to the incoming 50 ohm transmission line via a J-jack and J-plug. The arrangement made it possible to tune and test the network without taking the transmitter off the air except for installation of the J-jack.

In the end

The filter worked as expected. You can see the result in the “after” display of the spectrum analyzer.

This goes to show that you never know what might happen from year to year at a transmitter site. I uncover a problem or two every year when doing NRSC measurements for AM clients.

Yes, annual measurements are not required on FM stations, but these kinds of measurements should be made on a regular basis just to confirm compliance with FCC rules. Mixing can and does occur in FM transmitters as well.

Mark Persons WØMH is certified by the Society of Broadcast Engineers as a Professional Broadcast Engineer and has more than 30 years’ experience. He has written numerous articles for industry publications over the years. His website is www.mwpersons.com.