How To Make A Simple Ground Plane Antenna
A ground plane antenna is a very simple antenna. It is a quarter wave vertical radiator and four radials that simulate a ground for the radiator. It directly matches to 50 ohm coax and should show a VSWR of 1.5:1 or better at its center frequency. The match may be varied slighty by varying the angle that the radials droop (see figures in PDF file) This angle is typically 30 to 45 degrees. The antenna has 0 dB gain and is non-directional, and it is useful for both scanner, CB, amateur FM and ATV use, and as a general purpose antenna. It is useful from HF through UHF, although it tends to be large below 30 MHz. While a ground plane cut for 439 mHz will have elements around 6.25 inches long, a ground plane cut for 28 MHz will have elements 8.5 feet long. The radiator (vertical portion) is equal in length to (234/F mhz) feet, and the radials (drooping elements) are made about 5% longer. These formulas are fairly accurate, but element diameter, proximity to other objects, and the presence of mounting hardware (brackets, pole, etc) will have an effect. The antenna SHOULD BE CHECKED FOR VSWR BEFORE USING WITH A TRANSMITTER, and any necessary adjustments made before use. The radiator element may have to be lengthened or shortened slightly, and if less than a few percent change is made, the radials can usually be left as is. The radiated signal is vertically polarized, if the radiator is vertical, as is the usual practice. Large ground planes for lower frequencies must be made of tubing or pipe and suitably constructed mechanically.
DO NOT ERECT OR USE THIS ANTENNA WHERE CONTACT WITH POWER OR TELEPHONE LINES IS AT ALL POSSIBLE. YOU COULD BE ELECTROCUTED.
The antenna should be mounted at a distance of at least its height above ground plus twice either the length of the longest element or ten feet, whichever is greater, from any other wires or metallic conductors.
This means that for example, if the ground plane has 9 ft elements and is to be mounted 30 feet in the air it should be at a distance of 48 feet from any wires. If it has 2 ft elements, then the minimum distance from any other wires should be 30 plus 10 feet, or 40 feet. This is to insure your safety and to obtain proper performance. If this is impossible then do not consider using this kind of antenna.
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