VHF Analog Simulcast system??

[RRR]

I recently became aware of a new public safety "Analog simulcast" 3 site VHF system, with 7 or 8 freqs, that has been installed. I haven't had a chance to go and try to monitor it, but, how does that work? I understand with digital modes, how simulcast can be tuned for radios to pick it up properly, but how does this work on analog?

Are there VHF analog simulcast public safety systems in use around the country?

Trunking? Vote scan? With no digital, there wouldn't be talkgroups, so I am just curious how that would work.

 

[PriorMike]

In the next county over from me, fire is on an analog VHF simulcast system. I think there are 6 sites; not entirely sure. 2 channels; the first is the actual simulcast one. Paging for all departments and initial response/operations takes place on it.

The second channel is a simplex TAC channel for fireground use. I think that dispatch might be able to communicate on TAC from what I've heard on the scanner, but there are no repeaters on it.

Seems to work good for them. Not sure what the infastructure is.

 

[GTR8000]

Simulcasting analog is very common and has been in use for many years. You can simulcast analog conventional, or analog trunking. Simulcast conventional is most commonly used for Fire/EMS paging, but can also be used to provide wide-area coverage without the use of a trunked controller. Simulcast trunking can be found on countless Motorola SmartZone systems going back 15-20 years.

 

[RRR]

How does it keep from distorting the signal at the receiver, with multiple transmitters on the same freq?

 

[nd5y]

How does it keep from distorting the signal at the receiver, with multiple transmitters on the same freq?

On analog FM the distortion from simulcast isn't usually that noticeable. It doesn't destroy reception like it does on digital scanners that weren't designed properly.

 

[MTS2000des]

How does it keep from distorting the signal at the receiver, with multiple transmitters on the same freq?

Launch timing, and site placement.

We ran an analog simulcast trunking system prior to migrating to Astro 25 from 1990 to 2015.

 

[KevinC]

How does it keep from distorting the signal at the receiver, with multiple transmitters on the same freq?

Timing and phase. Timing from a GPS and phase from proper alignment of the transmitters.

 

[RRR]

Thanks for the informative replies!

Quite interesting such a system is being deployed in 2019, but, whatever works!

 

[PriorMike]

It could be a situation that the county/township/whomever 'own' the system, and the agencies are responsible for their own radios. An analog system would ensure that just about any radio would work.

 

[jaspence]

VHF FM also has what is referred to as "capture effect". Simply put, the strongest signal wins. See Capture effect - Wikipedia

 

[zerg901]

Voting and Simulcast networks

Tait seems to sell alot of synchronized simulcast systems in the USA

 

[GTR8000]

Quite interesting such a system is being deployed in 2019, but, whatever works!

There are more of these simulcast analog conventional "systems" being built out than you might realize. They are becoming a popular alternative to a full blown P25 trunked system, especially in areas where they simply do not have the money for incredibly expensive trunking infrastructure and subscribers. You can build out a multi-site, multi-channel simulcast network using a few analog frequencies for a fraction of the cost of trunking, achieving the same wide-area repeated coverage using existing (cheap) analog radios. It may not be as spectrally efficient as trunking, or come with all the bells and whistles, but it just plain works.

Motorola has been building out these types of systems for various counties in the northeast using VHF frequencies in the 161 MHz range, under their WQHE714 Public Coast Stations, Auctioned (PC) license.

 

[kayn1n32008]

Launch timing, and site placement.

We ran an analog simulcast trunking system prior to migrating to Astro 25 from 1990 to 2015.

The city about 45 minutes from me is about to retire a 4 site simulcast, mixed mode EDACS system. Transitioning to a province wide phase one system. It has served the city very well over the years.

 

[PriorMike]

There are more of these simulcast analog conventional "systems" being built out than you might realize. They are becoming a popular alternative to a full blown P25 trunked system, especially in areas where they simply do not have the money for incredibly expensive trunking infrastructure and subscribers. You can build out a multi-site, multi-channel simulcast network using a few analog frequencies for a fraction of the cost of trunking, achieving the same wide-area repeated coverage using existing (cheap) analog radios. It may not be as spectrally efficient as trunking, or come with all the bells and whistles, but it just plain works.

Motorola has been building out these types of systems for various counties in the northeast using VHF frequencies in the 161 MHz range, under their WQHE714 Public Coast Stations, Auctioned (PC) license.

Huh, I thought Motorola was the 'digital Borg' when it came to public safety.

Nice to know they'll entertain other options. I don't think I've seen it on their website...

 

[riccom]

you want a good example of voting and simulcast vhf analog, Phoenix Fire dispatch is a good place to start!

 

[speedway_navigator]

Keep in mind that digital modulation is still using an analog carrier so frequency and timing requirements are similar.
Pre GPS days were more difficult to keep frequency and timing control.