Hi Guys
I’ve picked up somewhere, such as radio club, that when using wire radials as the ground plane for a vertical antenna,
that the radials should be 1/4 of the wavelength (or at least 1/4 wavelength).
From this I assume the ground plane being improvised is a circular shape 1/2 wavelength in diameter?
Cheers
If you are asking about using wire (usually four or more) to make a ground plane, then yes, the wires should be 1/4 wavelength long (or longer). It also depends on what frequency antenna you are making and where you are mounting it.
For a typical homemade VHF/UHF 1/4 wave groundplane antenna that is mounted multiple wavelengths above ground, you use four stiff wires (brazing rod or brass rod is common) for the groundplane and angle them down about 45 degrees. The reason to angle them down is to raise the feedpoint impedance, to better match the 50 Ohm impedance of the radio. The exact length of the four wires is not critical. The length of the radiator is and will need to be trimmed for the desired frequency.
For a vehicle mounted 1/4 wave or 5/8 wave VHF/UHF antenna, the metal body of the vehicle is the groundplane. This is why fiberglass bodies present a problem. You either need to add a sheet of metal for the groundplane, or you use a "no groundplane antenna", which is simply an end fed 1/2 wave antenna (like a dipole, only fed at one end instead of the center).
If you are talking about a ground-mounted HF antenna, then you want as many ground wires as possible and you want to bury them, if possible. With a ground mounted vertical, or one mounted close to ground level, you get very high ground loss. In other words, a lot of your RF signal is wasted by heating the earth.
If you can mount your HF antenna well above ground level, only four wires for the groundplane work as well as many wires at ground level (like the VHF/UHF antenna above). Of course, with the longer wavelengths, mounting a vertical HF antenna high in the air can be a problem. On the upper HF bands, it is more practical.
All of the above assumes we are talking about an antenna for single band use. Multiband antennas get more complicated.
For more details and even formulas, look into any edition of the Amateur Radio Handbook, or one of their Antenna Handbooks. With some Googling, you can also find this information on the net.