[TowerTalk] AM Broadcast Harmonic Interference to 75 Meters
jd0 at broadsci.com
Sat Dec 5 13:34:22 EST 2020
> My problem is an extremely strong harmonic at
> 3870 Khz both day and night, plus there is crud every 10 Khz from about 3600
> Khz to 3960 Khz.
> The crud every 10 Khz
> indicates to me some serious mixing products being generated
You didn't say what the callsign of the station is, but it appears to be KAVY(AM). They are licensed for 2500 watts day, 178 watts night, both non-directional. That's about 11.5 dB difference - does the interference change by that ratio when they switch between day and night power?
Some general info to get you started:
1. AM broadcast stations must have all emissions that are 75 kHz or more removed from the carrier frequency (which would include all harmonics) attenuated 43 + 10 log (power) or 80 dB, whichever is the lesser attenuation. At 2,500 watts, that works out to -77 dBc, and at 178 watts, it's -65.5 dBc.
2. Unlike many other services (broadcast or otherwise), these attenuation values are based on measured field strength, not what leaves the transmitter. As such, in order to prove compliance, measurements are made in the field using a calibrated field intensity meter ("FIM"), or a spectrum analyzer or receiver with a calibrated antenna.
3. Related to day/night power level changes, many stations also have PSRA (pre-sunrise authority) and PSSA (post-sunset authority) which allows them to operate at some intermediate power levels between their day and night licensed levels. PSRA and PSSA operation can't exceed 500 watts. In the case of PSRA's and PSSA's aren't in the FCC's electronic database, so I can't tell you whether or not KAVY takes advantage of PSRA/PSSA to get more than 178 watts during those periods.
4. If you are going to monitor the level of the third harmonic and/or the "grunge" to see if they change in proportion to day/night (or potentially PSSA/PSRA levels), to find the sunrise/sunset times that stations in your area follow, go here:
5. The "grunge" every 10 kHz is unlikely to be a mix unless it is in being generated in equipment in/around your QTH.
6. If the "grunge" every 10 kHz is truly is being generated by the station, it would likely be a related to the third harmonic emission. That is, the fundamental "grunge" would be every 3.333 kHz. If that were the case, one would expect it to be audible since 3.333 kHz is in the AF passband. It would be helpful to know what kind of transmitter it is in order to come up with any theories or where to start troubleshooting. Related to the above, it would also be helpful to know if the amplitude of the "grunge" varied in proportion to day/night power levels.
7. I would start out first by confirming the third harmonic is meeting the -77 dBc requirement. To do that, your best bet is to find a ham who is also a contract engineer or consulting engineer to come out to your QTH with a FIM or spectrum analyzer and calibrated antenna to make some measurements. If you don't know anyone, you might send an email to someone at the local SBE (Society of Broadcast Engineers) chapter, which I would assume to be in Houston (www.sbehouston.org), hopefully they can refer you to someone.
Feel free to email direct.
--- Jeff WN3A
Jeff DePolo, Senior Engineer
Broadcast Sciences LLC
Valley Forge, Pennsylvania
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