General Antenna Question.
Vertical mounted antennas, such as on a vehicle.
I know there's a up side to a bigger antenna,
Is there a down side (other than the awkward physical size) of using a full wave length antenna versus using a quarter wave length antenna.
Thanks in advance for your help and patience.
A general question deserves a good general answer.....
In a nut shell, the antenna is only 1/2 of the system; the other half is the ground plane. In the case of your vehicle, it acts as the ground plane. Placement of your antenna on your vehicle will also affect your transmit and receive. If you mount the antenna on the rear of the vehicle, you will have a directional (or beam) system. Your transmit and receive will work better in the direction of the most metal, or in your vehicle's case, towards the front if mounted on the rear.
For the best all around performance, mount the antenna in the middle of the vehicle and make sure it has a good contact with the metal of your vehicle. Fiberglass bodies, vinyl tops, etc., will greatly reduce your antenna's performance not to mention possibly damage your transmitter. If you have a fiberglass body, you will need to drill through it to the frame or roll cage and attach it to there.
As for wavelengths, they say the best is a 5/8 wave, or a .64 wave if you can find or make one. 5/8 = .625. A full wave antenna tends to radiate most of it's signal up in the sky. At 5/8 you have the "flatest" take off angle of your signal, which in theory helps it cover the distance the best.
As for power, for every doubling of power you will gain 3dbs. Each number on your radio's S-Meter (if it has one) is 6dbs. So if you are giving someone 3 S-units on his radio's meter at 25W of transmit power, you will need to increase your transmit power to 100W to give him 4 S-units on his meter, given you don't change anything else.
With all that said, my best advice is for you to buy an antenna that is designed for the frequency band(s) you wish to talk on (450-470) for GMRS, and mount it as close to the center of your vehicle as you can, making sure it has good contact with the sheet metal of your vehicle's body. A good ground and matched SWR on your antenna system will do more for you than anything else.