HF Public Safety Trunking

[scannerboy01]

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Not sure if this is the correct forum to post in, but I thought this would be the most appropriate one. I was thinking if it would be possible to have an HF trunked system? I know that hams can have digital on HF, but I'm curious to know if public safety can use trunking on HF?

 

[kd5vjn1307]

Thats a really good question. Unfortunatly I don't have an exact answer. I do know that when I did communications in the military, we mostly used frequency hopping type systems, which is close to trunking in the sense that it used a set of frequencies and switch between them, but that was as you communicated. Trunking systems in most cases use a repeater/ controller system using a polling system, (multiple sites/ repeaters pick up the signal and the system picks the strongest system to relay back to dispatch) and/or multicast ( where the dispatch transmission goes out on all sites/repeaters to cover the most area) in HF communications its used for medium to long distance communication, so the need for repeaters to cover area usually isn't needed. Simplex operation is sufficent. As for the part of trunking where the system pool the available frequency and disburses them amongst the demand for channels among users, that actually doesn't sound like a bad idea. I just haven't heard of any instances where that has taken place. Could it happen? I wouldn't see why not. Why its hasn't could be another question all together.

 

[Token]

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Not sure if this is the correct forum to post in, but I thought this would be the most appropriate one. I was thinking if it would be possible to have an HF trunked system? I know that hams can have digital on HF, but I'm curious to know if public safety can use trunking on HF?

That is a multi part question.

Is it possible to have an HF trunking system? Technically yes, it is possible.

Can public safety use trunking on HF. Again, it would be technically possible, but public service is not, as a general rule, on HF.

There are no trunking systems on HF that I am aware of. Sure, you might look at some of the digital modes like ALE as having a “trunking” potential, but none are on HF that I know of that are like what you are thinking about (i.e. no traditional trunking systems as found in VHF and UHF FM and digital communications networks).

Trunking really does not have an application on HF for several reasons.

FM is very infrequently used on HF, particularly seldom below 25 MHz. AM and SSB modes are the norm for voice. Squelch does not work particularly well for SSB modes, so it would be hard to have a radio squelched between calls in the talkgroup.

The entire HF range of frequencies contains less bandwidth than the 148 to 178 MHz VHF-Hi band, and this narrow range is shared by many, many, users world wide (instead of just the 50 to 150 miles as would be the VHF-Hi band), so there are more limited regions to have multiple frequencies for selection for the talkgroup.

HF is affected by propagation much more than VHF or UHF. So it would not be uncommon for your link freq to be wiped out by distant stations, meaning your talkgroup could easily become broken.

T!

 

[scannerboy01]

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I guess it is technically possible, but not practical. I just had the thought run through my head at lunch and thought I'd ask. Thanks for the answers.

 

[joen7xxx]

While there are some state HF public safety frequencies, there are not that many available. ALE is sort of like trunking but the frequencies are selected on the basis of band and propagation path between stations.

As a matter of fact, the FCC tightened the requirement for non-governmental land mobile operation on HF to a critical few licensees. A lot of exploration drilling companies and others who worked in the wilderness had HF licenses prior to the advent of more affordable satellite comms. Now you must demonstrate that no other means of communication will work. The Red Cross has a few HF frequencies for disaster communication.

 

[jim202]

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Not sure if this is the correct forum to post in, but I thought this would be the most appropriate one. I was thinking if it would be possible to have an HF trunked system? I know that hams can have digital on HF, but I'm curious to know if public safety can use trunking on HF?

Lets look at the question on a practical point of view. In trunking, you use multiple frequencies at the same site. In order to limit the number of antennas on the tower, the transmitters are all combined in a cavity system to prevent interaction and interference between the different frequencies being used.

The transmitter combiner at the HF frequencies would require a room by itself probably the size of a average bedroom. Then you still need room for the radio equipment.

Next you have the antennas on the tower. At say a 7 MHz frequency, you need at least 65 feet for a simple half wave dipole. Then you need either a second dipole antenna for the receiver system or more cavities to be able to use a single antenna.

Frequency separation will be a problem between the different channels. Don't forget that at 800 MHz they use 45 MHz. At the UHF band they use a normal split of 5 MHz. between TX and RX channels. On the VHF band you could make it less like 1 MHz. But now the cavities start needing a higher Q to provide the selectivity. In other words, they get much larger. Like 6 to 12 inches in diameter and about 38 inches long. As you go lower in frequency, the cavities become larger not only in length, but also the diameter. They will become huge at say 7 MHz.

Maybe by now you start to see the hardware issues of why you don't see any trunking below the VHF band. We haven't even talked about the lack of any frequencies being available. Plus you have all the propagation issues to deal with on the HF segment. Not hardly what you want for 2 people trying to talk a couple of miles apart.