I’d like to start an on-going discussion about something that I’ve seen nothing on in the various forums and mail lists. It’s got to be a problem for others besides us. I’m referring to intermod caused by broadband signals such as cellular and DTV that manifests as desense.
A few examples… A site with a full performance UHF DTV transmitter with 8VSB modulation, which to an analog receiver is a 6 MHz wide wall of noise. Virtually every UHF repeater on the site with a 5 MHz TR spacing gets desense whenever it’s own transmitter is keyed, but in a test with the DTV transmitter turned off, no such desense occurs. Duplex links with spacing wider than 6 MHz don’t get the desense at all.
Another example is 700/800 MHz P25 sites that are experiencing desense with or without their own transmitters being keyed up. Universally, where this has been seen, is sites with nearby cell sites operating 600 MHz (T-Mobil) and 700 MHz (Verizon?) LTE with carriers as wide as 20 MHz, similar to the ‘wall of noise’ DTV signals mentioned above.
The 600 and 700 MHz cellular works out as a 3rd order mix to hit the 806-815 MHz input band on the P25 sites. Additional filtering on the receivers reduces the desense only by the amount of the insertion loss of the filter, so it’s an external environment thing that can’t be filtered out.
I'm seeing other examples, but these should give you an idea what I'm talking about. Back in the good old days, an intermod mix required an almost direct hit on your receiver before you’d notice it, then you could listen and identify what it was. Now, it’s intermod manifesting as desense that’s absolutely intractable and there seems to be little that can be done.
Has anyone else observed this sort of thing? I’d be interested in hearing how widespread it is, and what, if anything, people have done to reduce it.
Mel - WA6JBD
A few examples… A site with a full performance UHF DTV transmitter with 8VSB modulation, which to an analog receiver is a 6 MHz wide wall of noise. Virtually every UHF repeater on the site with a 5 MHz TR spacing gets desense whenever it’s own transmitter is keyed, but in a test with the DTV transmitter turned off, no such desense occurs. Duplex links with spacing wider than 6 MHz don’t get the desense at all.
Another example is 700/800 MHz P25 sites that are experiencing desense with or without their own transmitters being keyed up. Universally, where this has been seen, is sites with nearby cell sites operating 600 MHz (T-Mobil) and 700 MHz (Verizon?) LTE with carriers as wide as 20 MHz, similar to the ‘wall of noise’ DTV signals mentioned above.
The 600 and 700 MHz cellular works out as a 3rd order mix to hit the 806-815 MHz input band on the P25 sites. Additional filtering on the receivers reduces the desense only by the amount of the insertion loss of the filter, so it’s an external environment thing that can’t be filtered out.
I'm seeing other examples, but these should give you an idea what I'm talking about. Back in the good old days, an intermod mix required an almost direct hit on your receiver before you’d notice it, then you could listen and identify what it was. Now, it’s intermod manifesting as desense that’s absolutely intractable and there seems to be little that can be done.
Has anyone else observed this sort of thing? I’d be interested in hearing how widespread it is, and what, if anything, people have done to reduce it.
Mel - WA6JBD
