Antenna Recommendation for UHF-T

Question

My organization (a school) has an FCC license for three frequencies used on our campus for handheld radios (safety) and CB radios (busses). We wanted to record and reflect (retransmit) these signals out over the web so that even those without a radio can listen in the event of an emergency.

I went out and got a scanner at Radio Shack and plugged in our three frequencies. This works like a champ! We hooked it up to a computer and with Icecast and Darksnow, we now have a working "listening post" on the web. The problem started when we tried to move this fully working setup from our office (on the top of a hill) into our data center (a metal box inside a concrete box inside a metal box in a valley). We lost all reception. The scanner has a coax-input however, so I got 50 feet of coaxial cable and ran it to a spot outside the building. Our reception improved significantly, but still isn't great.

The cable isn't plugged into anything right now, because I'm not sure what type of antenna to buy. Our frequency assignment is tight, three frequencies in the 490-500 Mhz range. I think we need something that is fairly omnidirectional. Ideally it would be something compact that won't draw attention or look like an eyesore. I'm having a hard time interpreting all of the information about antenna types.

Will a UHF TV antenna work? Should I be looking for something more specific?

Answer 1

Why buy something when you can make something? Since you already have coax out there, how about making an antenna from the coax?

The "metal tubing" is the shield of the coax, and the "solid metal or tubing" is the center conductor. So you just strip a long length of the coax, then carefully fold the shield back over the coax. Keep in mind the shield will get shorter as you expand its diameter to fit over itself, so strip it a bit longer then trim it to length.

The whole arrangement should be half the wavelength long (so each section is a quarter wavelength). As an approximation:

$$ \text{length in feet} = 460 / f $$

Where \$f\$ is the frequency in MHz. For your application, \$ 460/495 \approx 0.93 ft \approx 11.2 in \$.

You can cut it a bit long then fine-tune the length if you have the appropriate equipment or through trial-and-error, but that may be unnecessary.

Also, take care to weatherproof the end of the coax. You don't want water getting inside the coax. There are any number of products out there, but some epoxy thinned with alcohol brushed on the end also works great.

Of course this isn't the most perfect antenna design. But if you are getting some signal without any antenna at all, this will be at least an order of magnitude better, and probably, good enough.