The GP antenna is a very popular antenna. You can use it for every freqency you want as long it’s cut to the correct length. It’s a excellent HF antenna or 27 mhz CB antenna (PR-27). The typical groundplane antenna is 1/4, 1/2 or 5/8 wavelength long. The different length have different characteristics.
The 1/4 will be shorter than the 1/2 or 5/8 wavelength antenna and will have a higher take-off-angle (shorter skip distance). On the other hand the antenna may work better in a valley. (The signals will shoot out of the valley. A 5/8 will maybe shoot the signals straight in to the mountains). Read more about take-off-angle here: http://megahertz-radio.com/2008/11/28/skip-distance-and-radiation-angle-toa/
The design of the antenna:
Before you build this antenna, choose your desired length after your needs. If you have the space and area for a big antenna you should make the 1/2 or 5/8 antenna. It will be a better long distance antenna. If yo want to make local contacts in your area you go for the shorter 1/4.
Here is a little calulator: http://www.csgnetwork.com/antennagpcalc.html
The calulator is for the 1/4 antenna. The 1/2 antenna is double the length and so on.
Radials and where to install the antenna:
Many places I see a CB antenna without any radials. Well It will look good, but that is not a very efficient antenna. The radial system is HALF the antenna. So a decent radial system will make this to a good antenna. So a home made GP antenna will often be better than a one you buy.
If you mount the antenna on the ground (GROUNDplane antenna…) you will need a lot of radials. The radials should be from 1/10 wavelength up to 1/2 – 1 wavelength long. The length is not important when them are on the ground. You should lay down ATLEAST 8 radials. If you have the land, the “optimal” is 120 radials (!).
The best way to make an efficient GP antenna is to have elevated radials. (atleast 2m above the ground). A good way is a roof-mount. This will also place the antenna over noisy obstacles and give a better “line-of-sight” coverage. The most important note for elevated installation is the radials NEED to be correct lengt. (ex. 1/4 long). Opposite of the groundmounted antenna where you can use random length. You dont need more than 4 radials with this setup and will give you an excellent radial system.
Read more on radials: http://www.steppir.com/files/radial%20systems%20for%20vertical%20antennas.pdf
The GP antenna is fun and easy to build. with some meters with wire and a fishing-pole you can make this in minutes and you dont need a balun or a matching network. Just hook it up to your 50 ohm coax and straight to your radio. Before you transmit you need to check your SWR or you can blow your finals. If you have a low/high SWR the antenna is to short or to long. If you like to build antennas, I recommend that you buy a MFJ 259B antenna analyzer. (link)
More about SWR (link)
