UHF Repeater Antenna Gain?

Our school district has a repeater system in place for custodial and maintenance operations. The repeater is a vertex 7000 with duplexer using a unity gain UHF antenna mounted at 80ft on top of the water tower on the high school property. We are having trouble receiving portable radios inside other district schools located (2) 1.5 miles away and (1) 3 miles away. Would an upgrade to a high gain antenna help receive the weak signals from portable radios?

Thanks,
Tom

It might, but your license may require modification to allow it.

Where are these other schools located in elevation relative to your repeater location? Are the much higher on a hill or in a valley? If they are, the gain antenna may do more hard than good.

A gain antenna may help, but it's not a sure thing. What may help is to put receivers in the schools where you have issues and link them to the repeater using a "voting" system. That would allow the strongest signal to be repeated, even if the main repeater system only gets noise. This answer assumes that the issue is the repeater can't hear the handhelds, but they can hear the repeater OK.

If the current signals are weak and noisy but there then going from a unity to 10Bd gain antenna would be good if your license allows the ERP. If you have complete dead spots a 10dB antenna may or may not provide weak readable signals in the current dead spots.

Keep in mind a 10dBd UHF antenna is about 20ft tall and not cheap. Plus your current antenna mount may need some serious upgrading to handle the weight and torque. An antenna upgrade with less than 10dBd gain will give less improvement and in my opinion not worth the effort.
prcguy

A couple of voting receivers engineered into the system and professionally installed with antenna systems and connected to an IP controller and connected to reliable Internet connections could easily cost $20k.
prcguy

n5ims said:

A gain antenna may help, but it's not a sure thing. What may help is to put receivers in the schools where you have issues and link them to the repeater using a "voting" system. That would allow the strongest signal to be repeated, even if the main repeater system only gets noise. This answer assumes that the issue is the repeater can't hear the handhelds, but they can hear the repeater OK.

Click to expand...

Adding 2 items to the above good suggestions...

1. Did it used to work from those locations and now doesn't?

2. Don't rule out a less than properly tuned duplexer may be the culprit.

LICENSE

Any license specs to review? School District Info? Lots of variables here, cabling, repeater wattage, height. etc

Thanks for the replies. This is a new installation, using an existing antenna and LMR 400 from a retired repeater system for the school buses. We have always used 5 watt handheld radios on simplex in the schools, but the intent is to allow inter-school communication through the repeater. The repeater is received well in 99% of the buildings, but reception of the handhelds at the repeater is good from 80% of the locations, the other locations open the repeater, but are unreadable or nearly unreadable. Our funds are limited, so it looks like the the voting receivers is out. Our licence is for 50 watts output and 80 watts ERP. Our current repeater transmits 35 watts after the duplexer. The two schools that are 1.5 miles to the south are 100 feet in elevation lower than the high school site. The school that is 3 miles west is at the same elevation. The duplexer has been professionally tuned.

Any recommendations are much appreciated.

Thanks,
Tom

What is the sensitivity of the receiver? You might be able to add a preamp which would not involve any of the licensing issues.

TSchwartz said:

Our school district has a repeater system in place for custodial and maintenance operations. The repeater is a vertex 7000 with duplexer using a unity gain UHF antenna mounted at 80ft on top of the water tower on the high school property. We are having trouble receiving portable radios inside other district schools located (2) 1.5 miles away and (1) 3 miles away. Would an upgrade to a high gain antenna help receive the weak signals from portable radios?

Thanks,
Tom

Click to expand...

Trying to second guess how a system is playing from a distance is like trying to predict which way a chicken will walk out in the barnyard. The real question here is can the portable radios hear the repeater? The repeater has more power out than the portable radios. So you have an unbalanced system to start with. If the portables have no problem hearing the repeater at all the schools, then another solution should be considered. Maybe adding some voter receivers to the repeater is part of the solution.

A higher gain antenna would help. But there are other issues and factors that come into play here. Like what is the path between the repeater and the different schools? Is it flat and few trees or is it hilly and a bunch of trees? What is the construction of the buildings? Is it cinder block , brick, or poured concrete with re-bar? Need to know what those school building walls are made of.

Now consider this. The higher the gain of the antenna, the lower the far field lobe from the antenna is. So knowing what the ground is doing out away from the water tower will determine just what you might need for an antenna. If you plot out the location of the schools from the water tank, it will give you an idea if an omni antenna is the right choice, or is a directional antenna the way to go.

Now that I have come up with a number of questions and some possible solutions, you need to do your own homework. It may take a combination of changes to make the repeater system play the way you need it.

One question I haven't asked is what is the size of the antennas on the portable? Are they the short stubby antennas that are only a couple of inches high, or are the antennas 6 to 8 inches long? If your using the short stubby antennas, get rid of them and put the long antennas on the portables.

You might get away with a 6dB gain antenna on the repeater if there is no more than 20w reaching the antenna and that would equal 80w ERP. More than likely the duplexer is a flat pack mobile type and if you add a preamp you need to add a cavity filter on the receive side with at least as much rejection at the transmitter frequency as the preamp has gain.

I would also loose the LMR400 and replace with an appropriate size Heliax. LMR has known problems with low level IMD generation in a full duplex system, especially if any moisture has made its way into the cable or connectors.

If the main problem is handhelds getting back to the repeater the above steps would make a slightly noticeable improvement for maybe 1 to $2k installed.
prcguy

Just out of curiosity, when the repeater was installed utilizing the existing coax, etc, was it professionally done and tested at the site itself or was it just brought there and plugged in? Reason being even if the duplexer was tuned good on "the bench", and nothing was tested at the site there could be receiver issues caused by a boat load of different things at the site.

Otherwise, if you can afford it i'd suggest to indeed purchase a good quality 10db omni antenna. Depending on what is up on the tank for mounting some fabricated mount may need to be built to hold an antenna of that size.

Get rid of that LMR 400. If the tank is 80 ft, i'd be willing to bet there is probably over 100 ft of coax in total length. Go with a Heliax type cable, 1/2 inch or better yet 7/8 Heliax would be ideal.

Yea - replacing the LMR400 might bring the receive up enough to hear the portables, too. +1 on the Heliax recommendations above.

I would try some other things before replacing the antenna, as you are looking at an easy grand plus license and coordination fees.

I would also still try the preamp. Many times receivers are quite deaf.

If their only allowed 80w ERP they would have to back the repeater down to provide about 8w at the antenna input for a 10dB stick. But they would have more ERP than now and the repeater receive would be improved by the antenna gain. The reduced transmit power also helps improves transmit to receive isolation and the transmitter will run cooler.
prcguy

12dbsinad said:

Just out of curiosity, when the repeater was installed utilizing the existing coax, etc, was it professionally done and tested at the site itself or was it just brought there and plugged in? Reason being even if the duplexer was tuned good on "the bench", and nothing was tested at the site there could be receiver issues caused by a boat load of different things at the site.

Otherwise, if you can afford it i'd suggest to indeed purchase a good quality 10db omni antenna. Depending on what is up on the tank for mounting some fabricated mount may need to be built to hold an antenna of that size.

Get rid of that LMR 400. If the tank is 80 ft, i'd be willing to bet there is probably over 100 ft of coax in total length. Go with a Heliax type cable, 1/2 inch or better yet 7/8 Heliax would be ideal.

Click to expand...

My suggestion

First thing I would do is new antenna and feedline.
Even if it's just a few years old (and I'm guessing it's more than that), new antenna, new feedline, new connectors on both ends, done right by someone who knows what they are doing, is the first thing I would try.

And here's another question.
Was this antenna for the old bus system close in frequency to the system it's being used on now?
Maybe it is, and I missed the OP telling us.
But wouldn't it be a funny if they are using an antenna tuned to 450-470 on a 423 something system.
Or better yet, the retired bus system was VHF.
It happens.

Excellent recommendation on the 6" portable antennas vs the 4" stubby antenna. When signal testing the the 6" portable antenna made a big difference. I have a 10 pack on order so we'll go ahead and change all of the stubby antennas out at the distant schools. The old repeater system was in the 460 MHz range and the new system is in the same range. The new repeater and duplexer was tuned on the bench, but not at the site. The existing coax and antenna were installed in the mid 90's. It sounds like I should budget for new coax and an antenna.

Thanks for all the replies,

Tom

Swap

TSchwartz said:

Excellent recommendation on the 6" portable antennas vs the 4" stubby antenna. When signal testing the the 6" portable antenna made a big difference. I have a 10 pack on order so we'll go ahead and change all of the stubby antennas out at the distant schools. The old repeater system was in the 460 MHz range and the new system is in the same range. The new repeater and duplexer was tuned on the bench, but not at the site. The existing coax and antenna were installed in the mid 90's. It sounds like I should budget for new coax and an antenna.

Thanks for all the replies,

Tom

Click to expand...

Yeah, the feedline is 20 years old.
You are for sure getting some loss.
New antenna and coax is going to make a difference, no doubt.

A couple of things should be done first before you replace the antenna. As has been said replace the LMR400 with 7/8' hardline. Yes it's expensive, but the loss factor if half of the LMR. That gains you right there 3db minimum, next you may want to go and also replace your portable stubby ant for 6 in whips (assuming you are using UHF), as that's another 6db of gain overall (generally speaking stubby ant is -9db over isotropic), and there is also a 5/8 wave version antenna which is more gain too. It's longer so that might not go well with the troops, but you could use them on the outer locations. Adding a preamp is a good idea, just make sure it's not too high a gain as it will bring in a lot of what is called noise, which can make the reception even worse.
Now one thing to remember with a high gain base antenna, you may actually degrade the local reception due to the power points of the antenna are high up. Think of an antenna signal as a doughnut around the antenna, gain is squeeshing that doughnut flatter.
The FCC max power out requirement can be satisfied by adjusting and lowering the transmitters power output. If you are licensed for 80 ERP with a unity gain antenna, with a 6db antenna you have to lower your power to roughly 20 watts. Now that's into the coax after the duplexer at the antenna.
Follow what I have said above and if that still doesn't work then you may have to go with more expensive solutions. Personaly if the above doesn't satisfy your requirement I would put a small repeater in each location instead of a voter.The cost of the phone for the voter over it's life could cost more than the repeater.

Maybe no one thought about User Training.

Using the handheld radio properly will go a long way to better radio operation.

+ on the standard length,6", antennas on the handhelds.

Part number for antenna

radioman2001 said:

... and there is also a 5/8 wave version antenna which is more gain too. ...

Click to expand...

I'd be interested in using an antenna like that on my ham XPR.
Do you have a Moto or other manufacturer's part number?