Passive Repeater Engineering

[KB5ILY]

I have been tasked by County Administration to engineer a system at our County Fairgrounds that could allow PD, Sheriff, State Police, EMS & Fire be able to hear their respective radio traffic while inside the four metal-skinned exhibit buildings during the week-long County Fair.

The frequencies involved for the various agencies include a dozen VHF repeater pairs plus the 851mhz to 859mhz range as part of Arkansas Wireless Information Network (AWIN).

With so many frequencies & tower sites involved, using anything other than a passive repeater system would be very cost prohibitive.

I am thinking that a wide-band omnidirectional antenna be mounted above each of the four buildings (there is too much distance between the buildings to allow more than one building to use a single antenna), a smaller wide-band antenna mounted inside the exhibit area and connected to the outside antenna with low-loss coax.

Has anyone done a similar system ? Any suggestions ?

 

[N4DES]

It has been pretty much proven that passive systems are hit and miss when the users are not stationary. With 6GHz point to point microwave systems they work quite well due to the very narrow beamwidth and it being stationary. With trying to use passive systems in LMR you just can't get the required gain and then due to just the pathloss alone the effort typically yields zero results.

Unfortunately you will have to throw $$ at it and being this is for public safety usage there should not be a resistance to doing it right the first time.

 

[sfd119]

I know many state systems have Site On Wheels or mobile trunking sites. Does Arkansas have something similar you can bring up for the event?

 

[krokus]

I know many state systems have Site On Wheels or mobile trunking sites. Does Arkansas have something similar you can bring up for the event?

For access to the state system, I second this suggestion.

For a passive repeater to have a chance of working, there needs to be a strong signal on the external building area.

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[Project25_MASTR]

So to engineer for a passive repeater, you have to take into account the path loss from the site antenna to the external antenna, subtract the line loss and then the loss from the internal antenna to the officer's portable. It's hit or miss.

Easier thing to do would be to either see if an agency has a MOV or comms trailer that could be temporarily setup or to put mobiles with mag mount antennas on the roof (maybe a wireless speaker mic for mobility).


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[TampaTyron]

A more common solution here is to use Pyramid Mobille extenders on a mutual aid simplex frequency. So..... Mobile trunking radio configured as a base is set to talkgroup of chouse and the pyramid is set to whatever mutual aid simplex they have in their radio's..... Cheap and easy.TT

 

[KB5ILY]

Thanks for the insights . Site On Wheels would be great for the 800 system, but I don't know what their VHF coverage is. Arkansas State Police has one (maybe two) and Arkansas Dept. of Emergency Management has two.

The problem with using a state Site On Wheels during this type event is that there are 75 counties in the state and all of them have their county fairs during the first 3 weeks of September after Labor Day, then the District fairs Sept. 27-Oct. 3, leading up to the state fair Oct. 9-18.

None of the agencies in the county have an MOV/comms trailer (would be nice if they did), there is only about 22k people in the county, with around 10k in the county seat and the rest in smaller communities and unincorporated areas.

The only working Passive system in this area that I know of is 25 years old, 75 miles away from me on an 800 trunk system and the engineer that built it has been retired atleast 20+ years (& has probably passed on by now as I haven't heard anything out of him in 20 years), so I can't call and pick his brain.

Again, thanks for your comments, they are helpful. I will try to update what I come up with in the coming weeks.

 

[mmckenna]

A properly set up bi-directional amplifier will do this, but when you are getting into multiple channels on multiple bands, you are looking at a lot of money. Centralized site feeding indoor systems via fiber optic cable is the way this is being done more and more, but that's for big buildings.

I think the right solution for you is to take a look at some of those mentioned above. A local simplex channel to a base that can reach the system is likely the cheap or free way to do this.
Anything else is going to be an expensive engineered solution that will take some time to fine tune. Nothing you are going to be able to pull off in the next few weeks on a shoestring budget.

 

[jim202]

Most of the county fairs that I have had anything to do with over the years set up their own coms right on site. The use of the "National Interoperability" channels in a simplex mode worked fine. It took the load off the normal dispatch channels and allowed the extra traffic from the fair to operate by themselves.

Normally a mobile command vehicle (MCV) is brought onto the fairgrounds and set up in a central location. It was staffed by as many operators / dispatchers as needed. Generally there would be a common law enforcement channel and a EMS / fire channel. If anything was needed to be relayed back to the normal 911 center, the MCV had additional radios for that purpose.

Portable radios provided good coverage to those on foot and the mobile radios in the MCV covered the entire fairgrounds with no problems. If you don't have a MCV available, then maybe it is time to lash something together. Find an old bus, gut it, put in a generator, air conditioning, benches along each side, mount antennas on the roof and beg, borrow or what ever some mobile radios and your in business. A couple of energetic bodies should be able to whip it together in a couple of weekends. If you can get AC shore power, you won't need the generator for the fair. The couple of air conditioners might be the hardest item to obtain.

The one major downside to using a MCV on VHF frequencies is that when you transmit on one channel, it will desense or kill the reception of your other VHF receivers when any of the VHF transmitters is keyed up. Use low power on the mobiles to reduce the problem. Your only talking with 2 to 5 watt portables. No need to use 50 or 100 watt output power mobiles. Also try to separate the VHF antennas as far from each other as possible. The antenna mounting real estate is limited so you don't have much to work with.

 

[byndhlptom]

Passive repeater

Do you need both rx and tx repeated? I.e. can the handhelds hear OK, but not get back to dispatch? Or can they not hear, but can talk OK. If only one way help is needed, a low level amp combined with the inside/outside antennas may work

Have you (the county) considered a tempory base set up in an office space at the fairgrounds. You could run an outside antenna(s) (leave it there for next year). With a lot of dispatching transitioning to IP based links (linking to 800 system), it may be a doable option. A PC and a couple of mobile radios may get you by.....

Some of the cost could be offset by making it a "permanent" portable setup for other disasters/situations, as in a step to putting together a MCV of some kind down the road.

$.02