VHF Repeater Antenna Question

[Dbl_naught_spy]

Looks like I may have to install a repeater in a school we just had built in our district, Cleveland County, NC. Have talked a bit about it in the licensing forum regarding some licensing questions and some radio questions.

New question, related to the antenna. I have sitting here a brand new Kenwood TKR-750 repeater. Same one we use at other schools that we have VHF repeaters at. I will most likely use this one for this new school. We are licensed at 159.840 MHz for our last repeater (last 50W frequency). I need advice on an antenna (2 actually as I will do a 2 antenna set up without a duplexer). On the other schools, some have 2 YAGI style antenna's, one has a YAGI style with a small fiberglass omni-directional it looks like, mounted to it, one has what looks like 2 omni-directional antennas. It's a hodge podge. I'm new to the radio stuff, at least as far as going down the rabbit hole of installing an antenna and repeater. I'm more of a component level repair guy. I've looked at the Antenna Farm site and see lots of antennas that will handle that frequency (our other frequency will also be in the 151MHz to 158MHz range). Any advice on what would work best for this kind of set up?

 

[prcguy]

You may never get it to work with a dual antenna system, especially without some sharp band pass cavities on both the transmitter and receiver. If you could get some serious vertical separation on the antennas you might have a chance but it might not be practical due to the height needed.

Last time I tried this at VHF was using a GE Master II repeater with about 1.5MHz split, single 1/4 wave cavity on both transmitter and receiver, about 20ft vertical separation and it worked like crap. The receiver in a Master II is probably way better than the Kenwood receiver.

Looks like I may have to install a repeater in a school we just had built in our district, Cleveland County, NC. Have talked a bit about it in the licensing forum regarding some licensing questions and some radio questions.

New question, related to the antenna. I have sitting here a brand new Kenwood TKR-750 repeater. Same one we use at other schools that we have VHF repeaters at. I will most likely use this one for this new school. We are licensed at 159.840 MHz for our last repeater (last 50W frequency). I need advice on an antenna (2 actually as I will do a 2 antenna set up without a duplexer). On the other schools, some have 2 YAGI style antenna's, one has a YAGI style with a small fiberglass omni-directional it looks like, mounted to it, one has what looks like 2 omni-directional antennas. It's a hodge podge. I'm new to the radio stuff, at least as far as going down the rabbit hole of installing an antenna and repeater. I'm more of a component level repair guy. I've looked at the Antenna Farm site and see lots of antennas that will handle that frequency (our other frequency will also be in the 151MHz to 158MHz range). Any advice on what would work best for this kind of set up?

 

[kayn1n32008]

You are going to lose your mind, and your hair, trying to do split antennas. Save yourself the hassle and buy a duplexer with proper isolation.


Sent from my iPhone using Tapatalk

 

[mmckenna]

Yeah, duplexer.

As for the antenna, if you are just covering a campus, you don't need/want high gain antennas. Telewave ANT150D.
Nice low gain with an even pattern. It'll cover down into the buildings well.

 

[Citywide173]

What kind of area are you looking to cover? Just in-building and immediate grounds? How many square feet? Do the other schools suffer any "dead spots?"

Many years ago (1990-91), I assisted in the design of an in-building system (34 stories) where distance beyond the footprint didn't matter to the customer but had to have 100% in building coverage. We put the repeater (R100) on the 17th floor with Radiax split up to the roof and down to the basement. On the roof, we put a small antenna (rated for double the power) and in the basement, we installed a terminator. The in-building coverage was excellent and the out of building coverage was well beyond what we had designed (30+ mile mobile coverage.) That system is still in use today. Seems like an irrelevant story, but you might want to consider a leaky cable run from the radio to the roof to enhance coverage directly below the antenna while not focusing on just the antenna. You might even find that your power requirements are less.

And I also support going with a duplexer to save headaches.

 

[Dbl_naught_spy]

Wow. We've got 4 already, all 2 antenna set ups. I didn't figure it'd be that bad. I'll definitely go duplexer then.

There are dead spots right now, that is part of the problem. Lots of static in the comms too (analog VHF at the moment). We need coverage for the entire building as well as a horse arena and some close by riding trails. All on the same property. Not exactly sure of the square footage. I did go back by today with the EVX-261's I ordered to try on UHF and they worked much better than what they have now. I had ordered them to try and avoid the repeater and I get really good coverage except for one dead spot in the tack room/office of the horse arena. Other than that I think we would be OK. I'm afraid though with this being a special needs school (some of the kids are violent) they are going to want that one 6'x6' space covered too which means a repeater setup just for that. Unless there is another option that I'm not familiar with.

 

[Citywide173]

A control station with an outside antenna dedicated to that spot might fit the gap for that 6x6 area

 

[RadioGuy7268]

For your small 6 x 6 area - consider mounting a single radio there with an external antenna. You could do a base/control station (a mobile radio mounted to a power supply) and then run the antenna to an area that has no problems with coverage.

VHF repeaters are not for rookies. Split antenna systems using VHF yagi antennas? Sounds like a bad dream. Do a google search on "exposed dipole" antenna - is that maybe what you are describing as a Yagi?

 

[lmrtek]

VHF repeaters are a good choice if you want school bus communications over an entire county but VHF radios work poorly
inside school buildings so it is a poor choice for that.
.... ....
UHF radios (470-512Mhz) do a much better job inside schools.
.......
Yes it is possible to use two vhf antennas instead of a duplexer but you need ALOT of vertical separation of the antennas IE 40 feet or more.
And even then, you would be lucky to be able to use the repeater at even it's lowest tx power setting without desense of the receiver.
.....
I suspect that the other repeaters in your schools are also performing
poorly as a direct result of this lack of vertical separation.
........

 

[WA0CBW]

You might find adding duplexers and changing the antennas might fix your coverage issues.
BB

 

[brushfire21]

The frequency split was never mentioned, but highly recommend a duplexer versus split antenna and the ANT150 Telewave would be a great option like someone suggested above. UHF would be preferred for in building coverage, but VHF may work as well. Don’t forget good quality Heliax or double braided copper cable, don’t skimp!

 

[Dbl_naught_spy]

I admittedly don't know my antennas but I don't think any set up we have has the same type pair. I thought I had a picture of one of them on my phone but I apparently deleted it. Will try to get a picture for opinions. It's looking like though the powers that be are willing to accept that we won't have great comms from the tack room office desk. There is a phone there and the lady running the program is rarely in there. Plus she is only at the school 2 days a week as is. I don't think it's going to be as big a deal as I anticipated. The UHF digital units I tried work great otherwise.

But I would still like opinions on the other set ups. I will have to do something with them eventually. The frequency offset on most of them I think is 5Mhz approximately. I will have to look at one of the saved files to be sure. It appears the guy before me used the 50W frequencies that we have and then one of the lower 5W frequencies to pair with it.

 

[mmckenna]

On most UHF systems, 5MHz split is standard. One of the nice advantages for UHF compared to VHF.

Good duplexers and good antenna make a big difference. Try to avoid "mobile duplexers" if you can. While the full sized ones will cost quite a bit more, there's a good reason for it. With a 50 watt repeater and a good antenna set up, you should get pretty good coverage.