Ok, an update to this thread ...
We decided to ditch the vertex because of the wideband issue.
We will now be using a Motorola Astro/P25 radio that runs in the 480 mhz range, and a kenwood TK-790.
So, given the comments above, we will be getting two antennae, and put one on each side of the truck.
Are both of them 1/4 wave, 18" tall ? Anything more specific than that ?
Thanks.
Radio antennas are a tuned device. They resonate at specific frequencies. Like a tuning fork, they have a natural frequency they operate best at. Same is true with your antennas on top of the fire truck, they need to be a specific length to work properly.
There are some specific formulas that can be used for figuring the length of a quarter wave antenna based on it's intended operating frequency. Those work great on paper, but the exact length depends on the base design, the ground plane, other near by items (lights, speakers, anything conductive), so figuring this out with a calculator or me telling you how long they should be isn't the proper way of doing it.
-I will say that a 1/4 wave antenna for 480MHz is going to be around 5 inches. A 1/4 wave antenna for VHF is going to be around 18 inches.-
This might get you close enough, but close enough isn't a good way to do radios for public safety use. Guessing this isn't a responsible way to go about cutting antennas
If (and only if) you know the -exact- manufacturer and model of the antennas, you can check on the manufacturer website, they'll often have cutting charts that will usually get you close.
If you don't have that information, or you just want to do this right, you need to get the whole truck, with the radios installed, to a radio shop that can either:
1. use an antenna analyzer to properly tune the antennas.
2. at least use an SWR meter to check how well they are radiating the RF.
-or-
You can get a SWR meter that is designed to work on VHF and UHF (one designed for CB isn't going to work right) and you can tune them yourself. Unless you have done this before, it can be a headache. Not impossible, but not a lot of fun if you don't have some exprience.
The good news is that 1/4 wave antennas are quite broad banded, and getting them close (using the cutting charts for the -exact- model antenna) works fairly well.
Not tuning the antennas properly can affect performance and damage the RF amplifier stage of the transmitters. If the antennas are not tuned right, they won't properly radiate the RF energy from the radio. RF energy that cannot be radiated will get reflected back into the radio. The radio will handle a bit of reflected power, but if it's too much, the radio won't. The radio will end up turning that reflected energy into heat, and heat kills electronics. Damaging the RF amp stage of your radio will cause it to fail (-always at the worst possible time-) and the radio won't put out enough power to function properly. Basically you'll end up with a radio that might receive just fine, but won't transmit worth a darn. Repairing this will cost in the hundreds of dollars.
If this was amateur radio or another hobby radio service, I'd say go for it and try it on your own. Since this a fire service and peoples lives may very well depend on your radios working properly, I'd strongly suggest going to a radio shop and getting this done correctly.
