700/800/900 MHz......Why?

CopperWhopper67 · Oct 17, 2018

Most large scale radio systems, especially trunked ones, seem use the UHF High Band frequencies to broadcast signals. That doesn't make sense to me. Wave propagation of 700/800/900 is very linear and tends to avoid wrapping around terrain like VHF Low (and to a lesser extent VHF High) can, which severely decreases it's range. What's the motivation behind using these high of frequencies?

AI7PM · Oct 17, 2018

"Project 25 Cities". (P25) Driven by DHS and grant money. 7-800 has it's advantages in metro areas as it covers in and out of buildings better than VHF.

Unfortunately, grant money and salesmanship sold it in places where retaining VHF or UHF would have been far superior. As usual, the if it's good for NY and DC, it must be good for the rest of the country mentality figured in.

Wyoming is an example of where they went P25, but stayed on VHF to keep the superior coverage in their terrain, yet use multi band radios in their cities to allow for the better in building coverage on 7-800.

mancow · Oct 17, 2018

CopperWhopper67 said:
Most large scale radio systems, especially trunked ones, seem use the UHF High Band frequencies to broadcast signals. That doesn't make sense to me. Wave propagation of 700/800/900 is very linear and tends to avoid wrapping around terrain like VHF Low (and to a lesser extent VHF High) can, which severely decreases it's range. What's the motivation behind using these high of frequencies?

Spectrum availability.

CopperWhopper67 · Oct 17, 2018

AI7PM said:
"Project 25 Cities". (P25) Driven by DHS and grant money. 7-800 has it's advantages in metro areas as it covers in and out of buildings better than VHF.

Unfortunately, grant money and salesmanship sold it in places where retaining VHF or UHF would have been far superior. As usual, the if it's good for NY and DC, it must be good for the rest of the country mentality figured in.

Wyoming is an example of where they went P25, but stayed on VHF to keep the superior coverage in their terrain, yet use multi band radios in their cities to allow for the better in building coverage on 7-800.

Ohhhh, ok. I see now. Thank you. That clears it up a lot.

And just as a side question, maybe you would know. Are there any plans for professional radio to venture into Gigahertz territory?

mmckenna · Oct 17, 2018

CopperWhopper67 said:
And just as a side question, maybe you would know. Are there any plans for professional radio to venture into Gigahertz territory?

For mobile/portable two way radio?
Yes, but not like you are familiar with. Think cellular style broadband services. It's not channelized like standard LMR radio systems, but broadband like LTE.

For most two way portable/mobile radio, 900MHz is where most of the traditional radio services pretty much stop.

As for 700, 800, 900, building penetration is a big bonus. Getting signals into reinforced concrete buildings with low-E energy efficient glass can be difficult. Higher frequencies penetrate better.

Spectrum availability is an issue in larger metropolitan areas. One issue with VHF is non-standardized repeater in/out splits, which can make getting a bunch of VHF frequency pairs that can work together well difficult. UHF, with the standardized 5MHz split is a bit easier. When you get up into 700, 800 and 900, standardized wide spaced repeater in/out splits are the rule, and that works well with multichannel trunked systems where you need to combine a bunch of repeaters into one antenna systems.

But, yeah, a lot of areas could do better with VHF or UHF.

As for low band, noise floor becomes an issue, as well as skip. Then there's the issue with efficient antennas on portable radios.

ko6jw_2 · Oct 17, 2018

The Alaska state wide trunked system is VHF with the exception of the Municipality of Anchorage which is 700 Mhz. Trunking is not allowed on VHF low band. Some systems such as the Los Angeles City Fire Department are 800 Mhz, but received a waiver from being required to be trunked (very smart). The choice of bands does relate to the availability of spectrum coupled with a complete ignorance of radio propagation in some cases. The result is the need for many more sites than would be required in a VHF system.

kayn1n32008 · Oct 17, 2018

mancow said:
Spectrum availability.

This.

In my province VHF is a Charlie Foxtrot. It’s very heavily utilized, add in the Wild West of unlicensed users(oil field, where they can’t even stay away from LAD 2&3) and UHF is congested with SCADA, it is extremely difficult to find available pairs for something like a wide area public safety trunk system. We ended up on 700MHz partly due to a lack of adequate VHF spectrum and politics(700Mhz is relatively new in Canada) wanting to use this chunk of spectrum dedicated to public safety.

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westcoaster · Oct 17, 2018

kayn1n32008 said:
This.

In my province VHF is a Charlie Foxtrot. It’s very heavily utilized, add in the Wild West of unlicensed users(oil field, where they can’t even stay away from LAD 2&3) and UHF is congested with SCADA, it is extremely difficult to find available pairs for something like a wide area public safety trunk system. We ended up on 700MHz partly due to a lack of adequate VHF spectrum and politics(700Mhz is relatively new in Canada) wanting to use this chunk of spectrum dedicated to public safety.

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The LAD channels are just like CB right? Good to go all across Canada eh!
It is true about the wild west. I see that here on the coast as well.

You forgot to mention the non-existent enforcement....
I have had an antenna waggling around on my truck for 21 years and have yet to be asked for a radio license....

kayn1n32008 · Oct 17, 2018

westcoaster said:
The LAD channels are just like CB right? Good to go all across Canada eh!

It is true about the wild west. I see that here on the coast as well.

You forgot to mention the non-existent enforcement....

I have had an antenna waggling around on my truck for 21 years and have yet to be asked for a radio license....

I did forget the lack of enforcement. Apparently, ISED won’t be paying any attention to mobile only licenses, unless there is a complaint.

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zz0468 · Oct 17, 2018

CopperWhopper67 said:
What's the motivation behind using these high of frequencies?

The move to 800 goes back to the 70's. More spectrum was needed for various radio services (it was called radio back then), and there was quite a bit of unused UHF television spectrum that was made available.

Same thing happened more recently, only it was 700 mhz that was made available. More TV spectrum to chisel away at.

Little thought was given to propagation characteristics when 800 was first made available. Engineers figured it out as they went.

700/800/900 MHz......Why?

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