Hey, What’s This at 1300 kHz?
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.
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
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
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
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.