20 m Contest Overload and Receiver IMD
You are operating a 20 m SSB contest from a quiet location with a tribander at 25 m and a transceiver with a 500 Hz roofing filter. The preamp is off and the attenuator is at 0 dB. You are trying to copy a weak station on 14.020 MHz at about S2 (-110 dBm). Two strong stations are calling CQ on 14.025 and 14.030 MHz, each about -20 dBm at your receiver input. You hear a broad rise in noise and two clear intermodulation products, one landing on 14.020 and another on 14.035. The receiver’s two-tone third-order intercept is +5 dBm. You have a 10 dB and a 20 dB front-end attenuator available, a narrow DSP filter, and a separate low-noise receiving antenna. The weak station is workable but marginal, and you want to keep the run frequency if possible.
1. Calculate the approximate input-referred third-order intermodulation level and explain why the weak station is being masked.
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For two equal strong tones, the input-referred third-order product level is approximately 3P - 2IIP3, where P is the level of each tone and IIP3 is the input intercept. Here P = -20 dBm and IIP3 = +5 dBm, so the product level is 3(-20) - 2(+5) = -60 - 10 = -70 dBm. The weak station is at -110 dBm, so the IMD product is about 40 dB stronger than the desired signal and falls directly in the 500 Hz passband. The 500 Hz roofing filter cannot remove the product because it is generated after the first mixer or in the front end, not at the strong stations’ original frequencies. This is receiver-generated IMD, not splatter from the strong stations. Without front-end attenuation or a different receive antenna, the weak station cannot be copied reliably no matter how narrow the DSP filter is set.
2. What does a 10 dB front-end attenuator do to the strong tones, the IMD product, and the weak station? Would a 20 dB pad be better?
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A 10 dB pad reduces each strong tone to -30 dBm and the weak station to -120 dBm. The new IMD level is 3(-30) - 2(+5) = -90 - 10 = -100 dBm. The IMD product drops by 30 dB while the desired signal drops by only 10 dB, so the IMD-to-signal ratio improves by 20 dB. However, the weak station is now at -120 dBm, close to the receiver noise floor, so copy may not improve much. A 20 dB pad would put the tones at -40 dBm and the IMD at 3(-40) - 10 = -130 dBm, while the desired signal falls to -130 dBm; the IMD and signal are then comparable, but sensitivity is severely reduced. The better choice is usually the smallest pad that makes the weak signal copyable-often 10 dB-combined with the narrow DSP filter and, if available, the separate receiving antenna that hears the strong stations less strongly.
3. If the separate receiving antenna is not available, what operating strategy would you use during the contest?
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Move the weak station out of the IMD-shadowed part of the band if possible, or ask the strong stations to spread out in frequency; increasing the spacing reduces the chance that their products land on your frequency. Use the 10 dB pad first and check whether the weak station is still copyable; if not, try the 20 dB pad only for short listening periods and rely on your transmitter’s power and the other station’s receiver to complete the contact. Reduce RF gain rather than audio gain, because audio gain amplifies both signal and noise. If the receiver has a variable attenuator or IPO, use it to keep the front end out of overload. Log the IMD conditions and consider operating a different band until the run frequency clears; a marginal contact is not worth missing a multiplier elsewhere.