Q Bob, K4RFK, asks, “I live in Cape Coral, Florida. The local radio station, WINK-WNOG, transmitting on 1200 kHz AM, can be heard on my transceiver on several HF bands. I called the station and informed them of the situation and they said it was my problem. Can you help?”
A The type of interference you describe can come from several possible sources. The first could be the broadcast transmitter. Their transmitter could be transmitting some signals other than the one they are licensed to use. These signals are usually exact multiples (2.4 MHz, 3.6 MHz, etc.) of the transmitting frequency, but modern transmitters can also transmit other signals due to the internal mixing processes. These unwanted signals are collectively called spurious emissions, or simply spurs.
Much like the regulations that govern the Amateur Radio Service, there are stringent FCC regs governing the levels of broadcast spurs. Even if the spurs are below the FCC limits, however, the regulations require that radiated signals not cause harmful interference. The FCC, however, has stated that simple reception of a spurious emission does not constitute harmful interference. The signal is only considered “harmful” if it causes repeated disruptions of communication.
It is also possible that the problem is in your station receiver. A very strong signal can overload a receiver, resulting in spurious responses. These can range from image responses, to receiver responses, to various internal spurious signals in your receiver’s local oscillator or phase-lock-loop circuitry. There are a few things you can try to diagnose whether this may be occurring. The easiest is to try an entirely different receiver. Its internal design will be different than the receiver on which you are hearing signals, so it will have a different set of spurious responses. It is best to use a receiver built by a different manufacturer for this test. If the interfering signals disappear, then you clearly have a problem.
If you are unable to use a different receiver, there are some tests you can try that may help diagnose the problem. Switch in your receiver’s attenuator. If you select a 10-dB attenuator and the unwanted signal drops by significantly more than 10 dB, it is a clear indication that the signal is an unwanted receiver spurious response. Also, note how the receiver tunes in the unwanted signals when in the CW or SSB mode. If the tuning seems “fast,” meaning that the beat note of the carrier seems to be changing in pitch at a faster rate than normal as you tune across it, this also is a clear indication that a receiver spurious response is the cause of the interfering signals.
Not all receiver responses will necessarily be found with the above tests. In that case, you may want to try a filter on the receiver to eliminate the broadcast spurs. You can use the search engine at http://www.arrl.org/tis/tisfind.html to search the database for companies that sell broadcast band filters.
