Do "passive repeaters" work?

[kf4sqb]

First off, I may not have the correct terminoligey here. What I am talking about is the idea of simply connecting two antennas to each other, one outside a building, the other inside. If this actually works as intended, any suggestions as to what antenna to use? We are, of course, in a steel building. The repeater we need to improve reception from is app. 11 miles away, with an antenna height of app. 200 feet, and I think 100 watts into a DB224, on VHF. We have no problem using an HT on the repeater while inside this building, but our pagers, Minitor IV, of course, don't do so hot. Would something simple, like maybe an ASP 177 mounted through the roof for the outside antenna, and another ASP 177 mounted very close to it, upside down, inside the building, with maybe 2 feet of RG-58 between them?

[k2hz]

A passive system like you describe can work depending on how much area you need to cover and other conditions inside the building.

An application I did successfully was to provide coverage in a steel building truck bay area. The drivers wanted to use their MDTs to complete job reports after pulling into the bay and coverage was spotty so they sometimes had to stop outside before signing off their MDT.

We put a yagi aimed at the base station on a corner of the building roof with a short coax run to another yagi inside aimed across the truck bay area. It did make a significant improvement. This was successful because it was just to fill in the open truck bay area.

If you are looking at covering a large area and/or it is an office environment with wall and partitions, it may not be as successful. You need as much gain as possible in the outside antenna and the inside antenna located quite close to the users.

[kf4sqb]

Well, I'm not sure of the actual dimensions of the building in question, but it holds three fire trucks comfortably. Seriously, though, it does have three truck bays in the front of the building, with about six feet between trucks, and a few feet of extra space in the front and back of the trucks, and about half as much more length in the back of the building, where an office and meeting room are located. The rooms in the rear of the building are constructed of steel studs and sheetrock. Other than the sheetrock walls, the building is pretty well open. I know we've got one Yagi laying around. I may have to see if we can find another one, and see how that works.

[bernie]

My two bits worth:
Sure sounds like an interesting experiment.
You might do before and after tests with the squelch open to see how the coverage has changed.

[tvsjr]

Passive repeaters do work.

Your outside (donor) antenna needs to not "see" (electrically, that is) your inside antenna. Long yagis, corner reflectors, parabolics are good here.

Low-loss coax is your friend - use the biggest you can get away with (LMR will work, Heliax will too but is more expensive). Keep your run as short as possible.

Your interior antenna should be directional too - position it in a corner of the building and aim it back through. A panel antenna works great for this. You don't want anything omni that will "see" up, or you'll have problems.

I'd try for at least 40' of vertical separation.

[mr.syntrx]

I've done this for 800MHz CDMA mobile phone coverage in a packing shed. A spare 10 element yagi on the outside pointed at the mobile phone tower 400m away, and a rubber duck on the inside, with three feet of LDF5-50 between the antennas. It works really well.

[bellersley]

I used to have a passive repeater on my car for my cellphone. I lived in the sticks, and a portable would always have really spotty coverage. I took two glassmount cellphone antennas, put one inside and one outside. It worked pretty darn good actually.

[psapengineer]

Passive repeaters work really differently at 150MHz than at 800Mhz. So, let me think:

If your base antennas is delivering 100 Watts out and you lose 3db in the line and cavities and then have a 6db gain antenna you're at (30+20-3+6) +53dBm ERP.

At 155MHz the free space loss at 11 miles between two antennas is about 101 db.

If the Minitors can receive at 1uv with body losses then their sensitivity is about -107dbm.

So, if they are working marginally inside the building the loss coming through the building must be something on the order of (+53 - 101 -x = -107) about 59dB. This seems high; I'd expect it to be more on the order of 30 or 40 db but heck it may be a really tight buidling without windows and vents. Maybe the bay doors don't have windows.

For any antenna that radiates you lose about 60 db in the first 500 feet.
So, from a re-radiating antenna inside a large building you can expect to lose about 54 db or so on your way to the minitor.

If your outside antenna has let's say 6db of gain and your inside antenna has let's say 3db of gain then
(+53-101+6+3-54=) you should see about -93dBm of signal level inside the building. That may be 14 db improvement.

The gain (negative is loss) in the passive reflector is about -54+6+3 or about -45db

So, if the loss in the building is really 59db and the loss in the passive is about 45db it again may be a 14db improvement.

I'm surprized that the math shows this could work. Is the builidng really tight? If so it really may work. If not, something else is amiss.

Bob

[kf4sqb]

Yep, it's pretty tight. The only window is in a door facing away from the repeater, and it is a "security" window, with steel mesh embedded in it. I'm suprised as much RF gets in as does get in.

[RFdude]

Thanks psapengineer for putting numbers to the issue.

I think the sensitivity quoted for the Minitors is high. VHF HT's are -6 dB or so with a rubber duck antenna, and then they are -18 dB when worn on a belt. Don't know where the Minitor stands exactly, but -107dBm might be optimistic, especially for a public safety 95% coverage application. Going from 50% to 95% costs about 15 dB if I remember correctly which makes for -92 dBm required inside the station.

What about a base paging receiver with external antenna and amplified speaker (bull-horn) for the station interior. There are a few reputable companies that sell this sort of thing. We use this for a variety of indoor applications. Might loud too. Guaranteed to wake 'em up. For public safety, you would want DC backup power for the receiver and amplifier.

RFDude

[kf4sqb]

Waking up isn't a problem. I'm home in my own bed when the pager goes off in the middle of the night. I don't think I made this clear earlier, but this is a volly station. It's the fact that even if we are there, we may very well miss the page, unless one of our members who is still carrying a Minitor II happens to be there. I believe those old things would still alert in a triple-shielded room 80 miles from the paging transmitter. I know we've got one old three or four element yagi laying around. If we can find another for cheap, we may try it and see what happens.

[nmfire10]

It really might be more fool-proof to put a maxtrac or something in the building with an antenna outside. Program it with QC-II decode if desired. Put a bullhorn speaker in the bay and a regular speaker in the meeting room area. You can even have it drive a relay for a strobe light, buzzer, or something off the accessory connector.