ECE119 Transmission Media - Antenna Design - Quarter Wave Vertical

Reference: http://www.qsl.net/aa0ni/antennafaq.html

The quarter wave vertical

This antenna is almost as popular as the dipole. The center lead of the coax (short for coaxial cable) goes to a conductor (pipe/wire) that is one quarter-wave long (half as long as the whole half-wave dipole). The equation for this conductor is 234 divided by the frequency in MHz. That's only half of the antenna. The other half consists of two or more wire radials that hook up to the shield of the coax. These are also the same length as the first conductor. If this antenna is placed on the ground with the first member pointing straight up (perpendicular with the earth), then the other members are laid upon the ground. If you want to permanently install them, then you should bury those elements a few inches below the surface of the ground so as not to hamper your ability to mow the yard.

Another name for this antenna is the 'ground plane antenna'. It doesn't have to sit down on the ground, although it does okay there. Since the radials represent ground, the whole antenna could be raised up into the air (either mounted to a tower or mast, or hung from a tree if made from wire). The antenna may perform better if the radials are hung at a 45 degree angle below horizontal. Two radials is enough to make it work, but more can be added if desired.

I've made a quarter-wave ground plane antenna for the 2-meter band. Using 12 gauge solid copper wire, a small piece of plywood or plexiglass, and some lightweight RG-58 coax (with the type of connector you need for your radio - PL-259 or BNC attached to the end). The elements were about 19". The center conductor goes to the vertical element. Instead of two separate wires for the ground radials, I took one wire and made it a little bit longer (about 1/4 inch). I bent that wire so that it was 90 degrees/square in the middle. After removing the insulation at the bend, I soldered the shield of the coax to that wire. To give the whole antenna a little bit of support, I placed the coax and the wires on top of a small 3x3 piece of plywood or plexiglass. With a drill, I drilled out small holes on each side of the wires and coax so that I could tie some strings through the board and around each of the wires and coax. The I took a hot glue gun and ran a bead of hot glue up and down each wire and the coax between them and the board/plexiglass.

To raise this antenna in the air, you can either put a small loop in the top of the radiating element and tie a string to it that goes through your favorite tree, or you can attach it to a mast or piece of wood that puts it up higher than the roof of your house.

The ground plane antenna is easy to make, and it does a good job. When it transmits, it sends most of it's signal down near the distant horizon in all directions. Some folks like to say that it radiates poorly in all directions. That's also true. It does not have the directivity that the dipole and full wave loop have.