700 MHz public safety bandpass filter

[Tim-B]

It was, trust me, it was. Like I said, I can name the exact towers in South Louisiana that still overload scanners and the ones that used to overload them but now have upgraded equipment that is not a leaky. Back in the 800 MHz analog days I could hear the familiar sound of the overload as I got closer to the towers, peaking as I passed the tower, then fading as I got further. It sounded somewhat like rapidy rubbing sandpaper back and forth over smooth wood. The touble with digital is that you can't hear that sound like you can with analog so it's harder to diagnose why you are losing the signal but I have learned to notice little patterns in the interference in the digital signal that occur when getting closer to a cell site.

 

[Tim-B]

Let me add that before 700 MHz 4G LTE came along I could drive all over Louisiana and listen to the LWIN statewide 700 MHz system with only an Antennex/Laird Technologies 1/4 wave 3 and a half inch long antenna on a PSR-500 mounted in a holder that clips onto the air conditioner vents and it worked great given that Louisiana is saturated with tower sites for that system. When they started installing 700 MHz LTE in cellular systems then those same areas where I had problems with 800 MHz now had problems with 700 MHz and it all went south.

 

[jonwienke]

It was, trust me, it was. Like I said, I can name the exact towers in South Louisiana that still overload scanners and the ones that used to overload them but now have upgraded equipment that is not a leaky.

Interference is not the same thing as overload. That's not an overload problem, that's an issue of the transmitter radiating signals outside of its intended bandwidth. If a 700MHz transmitter is also radiating at 800MHz, then a bandpass filter on the receiver will not help you receive 800MHz signals, because the interfering signal (the spurious 800MHz emission) is still in the filter's passband. It's the same thing when a CBer with a dirty linear keys up--you can't filter out the adjacent channel interference on the receiving end, because the linear is broadcasting on multiple channels simultaneously. The only solution is to eliminate the spurious emissions at the source, which appears to be ongoing but not complete in your case.

A filter will not help you if the problem is slightly out-of-band emissions from some cell towers.

 

[jonwienke]

Another thought, If the problem was insufficient filtering on the receiving end, then the tower equipment upgrade would make no difference. 700MHz cell towers would always interfere with 800MHz reception, not some do and some don't.

 

[Tim-B]

For the mobile radio mounted in-dash I pretty much want to filter everything except 769-775. There aren't any 800 systems that I still listen to when mobile. The other services I monitor when mobile like FRS, GMRS, MURS, itinerant, Marine, Ham, and CB can all be monitored with a handheld with its own whip antenna.

 

[jonwienke]

OK, but the same principle applies. If some of the cell towers are emitting spurious radiation in the 769-775 frequency range, then a filter will not help you.

 

[Tim-B]

If actual Motorola and EF Johnson radios don't get interfered with then how are they different? What physical components are different in those radios (and the Unication pagers) that makes them work so much better than scanners?

 

[jonwienke]

You've compared them in your situation?

There is a big difference between having a strong signal on an adjacent frequency, and having a transmitter emitting spurious radiation on frequencies other than the one intended. Better receiver filtering will make a difference in the first case, but not the second.

 

[TDR-94]

Cellular sites were notorious for radiating outside their intended bandwidth.I'm sure numerous public safety complaints have helped to require tighter filtering on the cellular end,but the telecom industry has much more influence than PS or APCO.

 

[GMAN5UKP]

RF filters.

Try Stridesberg in Shereveport. They two make filters and preamps, tell them what you need and they will produce it.