Is "ducting" a danger for first responders?

[ocguard]

There is a huge different between tropospheric ducting and co-channel interference from a nearby user.

What DelCo is experiencing is a problem where a DTV channel's broadcast signal is overwhelming the repeater inputs on several of their channels (remember, the bandwidth of a single DTV channel covers several LMR channels). It can be so powerful that the system, even with CTCSS protection, cannot reject the signal, and therefore, cannot receive or differentiate the 4 watt transmissions from portable radios attempting to connect with the repeater. And we aren't talking interference in the form of some other voice radio transmutation. We're talking a digitally encoded signal of several thousand watts.

This happened to York County, PA several summers ago when we were still operating on our 500mhz P25 system. A DTV transmitter, who's bandwidth covered our current control channel's input side, was overwhelming the receiver and preventing users from connecting to the system with portable radios. And since the guilty DTV channel's signal also covered several of the alternate CCs in the system, it took a bit of time to find one that could function.

This is far different and more dangerous than a nearby LMR user using the same channel and breaking through your CTCSS or whatever.

Just so everyone understands EXACTLY what's going on here.

 

[jim202]

Here is my question for everyone pushing new p25 technology with 3000 to 5000 or 6000 radios. Who is going to step in and help smaller fire departments needing to buy say 15 or 20 portables plus 2 or 3 mobiles and a base station that gets really really expensive really fast and that’s assuming the county keeps the vhf frequency for voice and alpha paging if we switch to 700 or 800mhz forget about it some fire companies are screwed. Plus police departments are a whole other set of issues

First off. STOP looking at only Motorola radios. There are several other companies that will work on a Motorola P25 system at much lower costs. Plus there is grant money available to help agencies if you write a grant request. This way your not having to foot the whole cost by your agency alone. There is no guaranty that you will get a grant, but it doesn't hurt to try.

Ask around for help on how to look at a grant for your replacement radios.

Jim

 

[zz0468]

There is a huge different between tropospheric ducting and co-channel interference from a nearby user.

What DelCo is experiencing is a problem where a DTV channel's broadcast signal is overwhelming the repeater inputs on several of their channels (remember, the bandwidth of a single DTV channel covers several LMR channels).

Just to be clear, a DTV signal is 6 MHz wide, and essentially a 6 MHz wide block of noise with tens of kilowatts of power.

Unlike old analog TV signals, the power density per MHz of a DTV signal is constant across the entire bandwidth. These DTV signals are quite capable of participating in passive intermod (PIM) in antennas, feedlines, and even filters. The most insidious thing about it is that, where most intermod requires an almost direct hit on your receiver frequency to be noticed, the mix product with DTV will be at least 6 MHz wide, sometimes more depending on the order of mixing taking place, and is unrecognizable as intermod. It manifests itself as desense, but the usual desense cures won't touch it.

Imagine what can happen with a 6 MHz wide block of noise colocated next to repeaters with 5 MHz T/R spacing, and the power density is high enough to excite all sorts of bi-metal contacts into conduction. You have intermod that looks like desense on everything, and the only thing that stops it is turning off the DTV transmitter.

A similar issue exists with 4G cellular signals, which can be several MHz wide, and stacked one on top of another making walls of noise 10 or 15 MHz wide.

 

[maus92]

Here is my question for everyone pushing new p25 technology with 3000 to 5000 or 6000 radios. Who is going to step in and help smaller fire departments needing to buy say 15 or 20 portables plus 2 or 3 mobiles and a base station that gets really really expensive really fast and that’s assuming the county keeps the vhf frequency for voice and alpha paging if we switch to 700 or 800mhz forget about it some fire companies are screwed. Plus police departments are a whole other set of issues

FEMA grants.

 

[WB3DYE]

This may be a stupid question but here goes: When we discuss ducting does it effect the lower freqs first or the higher first. if you have any documentation please post it here.

 

[krokus]

This may be a stupid question but here goes: When we discuss ducting does it effect the lower freqs first or the higher first. if you have any documentation please post it here.

Tropo is freq range specific, and depends on the dimensions of the duct(s).

 

[radioman2001]

Even going to 7/800 there are problems, just because it isn't a TV stations don't say it won't interfere. In California I can remember numerous issues with one 800 system interfering with another up and down the coast. So buying a new radio isn't going to fix the problem.
If the co-ordinators had been successful in getting TV off the 450-512 band you would still have a department in Boston stepping on Delaware.
You can't fix weather.

 

[W5lz]

Is ducting a -danger- to/for first responders? No, it isn't a danger. It certainly can be inconvenient though. To me, there is a big difference between danger and a PITA...

 

[RFI-EMI-GUY]

Years ago when I lived in Miami, regular TV broadcasts from TV Channel 6 through 13 suddenly became inundated with Cuban TV broadcasts.

Immediately a finger was pointed at Castro and the rumor was that he had "cranked up the power" which was silly.

Interestingly, TV channel 6 in Princeton FL went off the air completely, not simply captured by co channel interference.

I suspect the State Department was interested in the phenomenon and the intelligence that could be gathered, and so requested the station cease operations for a few hours. I went to monitor VHF band and police radio transmissions from Cuba were very strong.

Tropo Ducting at 800 MHz was a regular thing when Privacy Plus and Smartnet systems were new because Motorola in their wisdom had only one CTCSS type "connect tone" of 105.88 Hz in the early systems. We called it "tailgating" because when a talkgroup on a local system became active, other users on distant signals would be repeated at the end of local transmissions causing confusion and excessive traffic.

 

[GTR8000]

Even going to 7/800 there are problems, just because it isn't a TV stations don't say it won't interfere. In California I can remember numerous issues with one 800 system interfering with another up and down the coast. So buying a new radio isn't going to fix the problem.
If the co-ordinators had been successful in getting TV off the 450-512 band you would still have a department in Boston stepping on Delaware.
You can't fix weather.

Well no, not really. T-Band channels were assigned to 11 metro regions in such a way as to minimize the negative effects of both line of sight propagation as well as ducting events. Boston area was assigned TV channels 14 and 16, while the D.C. area was assigned TV channels 17 and 18. So the likelihood of a T-Band ducting event causing interference between those two areas is zero.

The far bigger issue is TV stations that still operate on T-Band, because those signals take up the entire 6 MHz slice of spectrum that is assigned to each TV channel. That is why a ducting event from a TV station to a public safety agency can wipe out the entirety of their operations, on both the outputs and inputs.

Take Morris County, NJ as the example. They operate a 12 channel P25 system exclusively within the TV channel 15 footprint (476-482 MHz). They experience semi-frequent ducting events from a TV station (in Virginia, I believe), which renders their entire system unusable for hours at a time. Even though there are 12 frequencies available ranging from 477.475 to 478.675, the TV station carrier is filling that entire block.

That is not to suggest that 700 or 800 are perfect bands, as nothing is perfect. However the likelihood of harmful interference due to ducting events is vastly reduced, thanks to not having signals that take up wide blocks of spectrum, as is the case with TV stations on T-Band. You may get the odd interference from a distant station that happens to be on the same 12.5 kHz frequency as the local users, but that sort of event normally only affects specific frequencies at a time. P25 trunked systems are less likely to be affected by these sorts of events, as they employ technology such as NAC validation to reject invalid signals, as well as have the ability to temporarily disable repeaters that are receiving too much sustained interference (illegal carriers). The negative effects of ducting events are greatly mitigated with digital systems.

 

[maus92]

Worcester County in Maryland is having trouble with ducting wiping out most of their freqs on their new Harris P25 system. The offending system is in Virginia Beach that shares 7 of their 8 channels. The county is searching for new 800 freqs that experience less interference, but since there are very few 800 channels available, their options are limited. Worcester County has a long coastline along the Atlantic and inland bays - sort of a worst case scenario during the summer when temperature inversions and humidity is high.

 

[RFI-EMI-GUY]

Worcester County in Maryland is having trouble with ducting wiping out most of their freqs on their new Harris P25 system. The offending system is in Virginia Beach that shares 7 of their 8 channels. The county is searching for new 800 freqs that experience less interference, but since there are very few 800 channels available, their options are limited. Worcester County has a long coastline along the Atlantic and inland bays - sort of a worst case scenario during the summer when temperature inversions and humidity is high.

It would be interesting to determine if Worchester's signals are being diffracted above the horizon just as Virginia Beach's signals apparently are when this ducting occurs. I read an article some time ago about an FM broadcast station that dealt with this by installing a transmit antenna lower on the tower with some down tilt. They would manually select this antenna when the band was enhanced to put more signal into their local area.

 

[maus92]

It would be interesting to determine if Worchester's signals are being diffracted above the horizon just as Virginia Beach's signals apparently are when this ducting occurs. I read an article some time ago about an FM broadcast station that dealt with this by installing a transmit antenna lower on the tower with some down tilt. They would manually select this antenna when the band was enhanced to put more signal into their local area.

They enlisted Federal Engineering to survey the system as designed and implemented by Harris, and they recommended some antenna changes, and other tweaks (like BDAs) to improve in-building coverage in a few areas. But apparently it did not fix their issues entirely. The Region 20 RPC has identified 4-5 frequencies that that are not used by the Orion system in Virginia Beach that might be available. The interesting thing is that Virginia Beach doesn't seem to be experiencing interference from the Worcester system - perhaps the builders of the Orion system were "less careful" (or more careful?) with their design. IDK. Another mitigation that Worcester is using is operating public safety users in FDMA mode - not sure if they will switch them back to TDMA now that the summer is over.