Anyone ever use passive antennas?

[zs6hz]

Hello all,

I have a rather difficult situation to resolve in the Congo at the moment. My client has an office in town with a decent tower and has four MTR-2000 VHF repeaters on the tower. The client also has a mine site about 11 miles away, but unfortunately it is situated on the other side of a small hill and repeater access is unacceptably poor at the mine. Overall repeater coverage is excellent, except at the mine site itself due to the hill.

Someone has suggested that I put passive antennas on the top of the offending hill, and that this would improve signals into the mine site.

Now, as far as I understand it, a passive antenna is basically just two yagi antennas mounted back-to-back.

Has anyone used these? Do they work? Should I spend the time, effort, and money to try it out?

Any info would be gratefully appreciated.

Regards,
Marc

[kf4sqb]

Marc, what you are referring to is called a "passive repeater", and does indeed work. Try doing a search for passive repeater on the board, and see what you find. There should be a "calculator" to use when working with them posted somewhere on here by Nand.

[nmfire10]

A passive repeater usually needs isolation between one side and the other. Example being an antenna on the roof of a building and an antenna or radiax inside a building. Nothing gets in or out through the walls so it is isolated.

Two yagis on a hill are not going to have any isolation so that might present a problem.

[Jim202]

Problem with the passive repeaters using 2 antennas, is that the losses add up fairly fast. You also need
need to be close to the antenna being used for the passive repeater.

I tried one of these systems in my house while I worked for a cellular carrier. I put a gain antenna on my
tower to use as a doner antenna. Fed it with good coax and brought it into the house and used a gain
antenna up in the attic laying on the timbers there. As long as I was almost right under the second
antenna, the signal was good. As soon as I moved away more than say 6 feet, it all went away.

At microwave frequencies this works well. You can obtain real high gains with the dish antennas. Then
run a piece of waveguide between them and redirect the signal. At UHF or VHF, you don't have the ability
to get enough gain to make this work over very much of a distance. I have used and played with Nand's
simple program. It seems to give the results I would expect. He too I believe points out that as you
get further away from the 2nd antenna, you performance drops off fast.

Jim

[transistor747]

We've done many passive repeaters here along the Gulf Coast industrial belt. Usually to get UHF or 800MHz coverage into a control room for a process unit. They DO work. However, the comment above about losses adding up quickly is very true.
You are not going to be happy with a passive system such as you describe... particularly at VHF.
You are going to need something active.

[zs6hz]

Thanks for the replies, and for confirming my suspicions. I've used passive repeaters also for GSM 900/1800 and they're very useful in industrial areas where there is no signal into certain control roome etc.

One solution to my problem is to put up additional repeaters on site, and put a UHF link in between the new repeater and the existing repeater. In my case this will mean an additional 4 repeaters, as well as 4 links.

IIRC some of the Ham satellites used a wide band transponder that received signals from several sources simultaneously, and retransmitted the signal on a different band simultaneously. ie: signal A went in at 144.100 and came out on 440.100 while signal A went in at 144.250 and came out at 440.250 etc.

Is there a commercial version of this sort of device? Even a very low power output would be useful.

Regards,
Marc

[/\/\y 2 cents]

There is a better and more cost effective/better sounding way to do this. Please PM for a diagram.

Regards,

Steve

(803)474-4745

[RADIOMAN2002]

The following is a system that the NYPD came up with about 20 years ago the get the local precient frequency down in the subways. Yes you do need 2 antenna systems for each channel, unless they are very close in frequency. What the PD did was put 2 yaggi's up or a yagi and a medium gain omni on light poles outside a subway station. The first yagi was the down link to the subway platform, now they used a single pass filter can tuned to the repeater transmit frequency. Down in the subway they took a mobile receiver pre-amp and the ran it to just a magnet mount antenna stuck on a metal beam. The uplink started out with a magnet mount antenna, thru the can and preamp out to the street and another yagi or omni. Now this worked extremely well, even several hundred feet from the antenna's. Unfortunately it' didn't repeat our EMS channel, so we were in the dark so to speak.

On this subject, I came accross another instance where just a passive antenna, (10db omni repeater) mounted on a 200ft tower with 250ft of 7/8 hardline brought in at full quieting a repeater signal from my repeater 50 miles away, to my portable that was sitting on the ground near the end of the open coax.