Help with Antenna Selection

TheOCAstro · Post by **TheOCAstro** » Thu Feb 02, 2006 1:50 pm

Hi All,

Our in-house radio guru for our company has been out sick and may never return so I'm taking on the task of upgrading our communications sysstem.

We are running a uhf 460 repeater system (on a mountain), base station in-house, several mobile units and numerous hand-held units our communications system needs noticible attention.

Based on what I've read on here for sure we need a new antenna with new cable.

The current cable is probably whatever was on sale at Home Depot that weekend, so I've already decided on the heliax 7/8 with the heliax connectors and weatherproofing. (if this is wrong, do tell)

The current antenna is a Antenex 5db, omni on a 20 foot tripod which sits on top of a 14ft roof. I'm not impressed with the performance of this antenna at all, as even on simplex its getting poor range. Immediatly around the building there are no obstructions or large metal buildings that could cause problems and there is a clear line of sight to the mountain top.

I've gone onto the Motorola web site and researched anennas and found more choices then I know what to do with. So I'm asking your expertise on what brand & Model and if you know a better place to buy from then the big M.

nmfire10 · Post by **nmfire10** » Thu Feb 02, 2006 1:57 pm

What is the elevation difference between the antenna site and the users in the field. It sounds somewhat counter-intuitive but it is quite possible you have too much gain on that if the thing is up high enough. It can actually shoot it right over everyone's head. It's easier to explain in a picture, but it can happen. A simple 1/4 wave antenna with no gain may actually work better.

If the coax and connectors suck, that right there is probably the root of the whole problem. Nothing will work right and the losses can be HUGE. Upon proper installation of real coax and real connectors, you might find it's like a whole new system.

If this is a repeater, you'll want to have your duplexer checked as it may be quite out of tune. That again will screw everything up really bad.

And last but not least, the station itself could use a good PM. Have someone hook the baby up to a service monitor and see what's cooking. If it takes 3 volts to break squelch, something is probably wrong. It can also tell you if the transmitter is working, on frequency, and how much power.

You are lucky in that there is no tower involved here. The complexity and expense of any service goes up exponentially. This sould be pretty simple to get it working like it should.

TheOCAstro · Post by **TheOCAstro** » Thu Feb 02, 2006 2:22 pm

The elevation for the repeater site is roughly 5600 ft, the elevation of the field units are roughly 137 feet. Station Antenna is roughly 171 feet.

I like your idea of changing out the coax first and then getting the base maintenanced and see how it goes from there. I was directing all my anger and frustration at the poor antenna and who knows it might be alright after all.

nmfire10 · Post by **nmfire10** » Fri Feb 03, 2006 7:32 am

Well, 5,000ft is definately high enough for a lot of gain to be a problem. However, it isn't set in stone.

Repair the known problems before you spend more money on a maybe problem. If you know the coax sucks and the thing needs a tune-up, definately do that first. If it works, then you don't need to replace the antenna.