Looking for a little help.
I installed this antenna, 11-04261 VHF-5I NAV/COM ANTENNA
http://www.aircraftspruce.com/menus/av/ ... osite.html
It's mounted on a piece of 1/8" plywood, which is in turn zipped to the airframe above the bagage compartment. Radio is basic hand held i'com.
Just seems it hasn't improved range over stubby i'com antenna
Question: Does the antenna cable have to be specific length?
If so, how do I figure that out?

The picture at aircraft spruce looks like a dipole antenna. What type of cable are you using? A dipole antenna requires a balun between the cable and the antenna if you're using coaxial cable. Also, dipole antennas are highly directional, picking up the signal the best when it's perpendicular to the axis of the antenna, and poorest when the axis of the antenna is pointed at the source. Finally, if the antenna is mounted inside of the airframe and the airframe is made of metal, then the signal will be attenuated. Even if you have a rag airplane with metal tubes, then the metal tubes will have some properties of a Faraday Cage and will diminish the signal.

Edit: Oops, I forgot to answer your question. The length of the cable shouldn't be of concern for the lengths likely to be encountered in an airplane.

The antenna cable does not have to be any specific length, shorter is always better but anything under 25 feet is fine. Strapping it to a piece of 1/8 inch plywood to mount it in a fabric plane should work fine. The antenna will work best if it is vertical, if you have to mount it horizontal put it crossways in the fusalage. you didn't say what you used for coax cable, any 50 ohm cable will work but 75 ohm cable TV cable will NOT work, RG-58 is very common, unless you have experience installing coax connectors it's best to try and buy a cable with connectors already installed, they are very easy to put on wrong or have a strand of shield short the connector out when you put it on. If you're in Boise or close this weekend give me a call, I'm in the phone book, I can check it out for you.

The only really good place for that antenna in my opinion is behind a plywood leading edge or as noted, in a composite airplane.

VHF radio signals are normally vertically polarized. It's the nature of the frequency more than the nature of the transmitting antenna. Short wave signals, below 50mhz begin to be horizontally polarized.

Most aircraft VHF antennas are a straight rod cut to length for more or less the center of the aircraft bands. A little longer for a strictly COMM antenna. If you mount a rod type (called a vertical) antenna, you hook up the center conductor to the antenna, mount the antenna to a non conductor of some type, and then attach the shield to the metal fuselage or in the case of a composite airplane, a ground plane (some sort of a conductor much longer than the antenna) imbedded in the structure. This is a unbalanced to unbalanced connection. No balun needed. Like Kevbert said, no 75 ohm cables allowed. Transmitting equipment manufactured according to current international agreements uses 50 ohm connection.

Use RG-8, RG-58, or RG-74 (274? I forget) anyway, a 50 ohm cable.

In almost all cases ( watch out, here come an opinion) a vertical antenna will outperform a dipole at 108-136 mhz.

As has been mentioned, the length of the antenna is critical, but the coax length is not. Just keep it as short as possible.

What (I think) you're missing is a ground plane. Your antenna needs a "ground reference" to properly transmit RF. Instead of the plywood you installed, you should have a metal base, just skinny aluminum is fine, the base of your antenna when fastened to it will make the aluminum your "ground plane".

Also what I learned (via my 'lectronics guru) is that the tube frame will not appreciably attenuate your RF if your antenna is inside, but the aluminum UV protection you sprayed on your fabric WILL. Our tests showed an 8db attenuation inside the SCub fuselage. (I wanted to mount my ELT antenna inside).