Ken Hough's Website
|This section includes reports on contruction and use of quarter wave ground plane antennas for 2m and 70cm.|
Quarter wave ground plane (QWGP) antennas typically comprise a vertical quarter wavelength element above a number (usually four) downward pointing counterpoise (ground plane) elements.
The sensitivity (gain) of this kind of antenna is similar to that of a comparable dipole arrangement, but can provide some practical advantages.
Dipoles have characteristic load impedances of 60 to 70 Ohms, whereas the load impedance of QWGP antennas is somewhat lower and by adjusting the angle of the counterpoise elements, can be set to exactly match 50 Ohm coax feeder.
The radiation pattern of a QWGP antenna is claimed to be rather higher (ie a greater part of radiation is towards the horizon) than is the case for a vertically mounted dipole, so that in practical terms, the actual sensitivity of a QWGP should be rather better than that of a vertically mounted dipole.
I found this practical implementation for VHF/UHF ground plane antennas in the ARRL Handbook.
The design is based on an SO239 socket with the vertical element being soldered into the centre part of the SO239 and the counterpoise elements being bolted or soldered onto the flange.
I've made 2 metre and 70 cm versions which both work well, and run with SWR values at close to 1:1. I used 2mm diameter brass wire for the elements.
With the feed point being below the ground plane elements, there is no need to include a choke balun on the coax feeder. These antennas can be suported simply by slipping the SO239/PL259/and coax feeder inside a vertical plastic tube.
|Dimensions for VHF and particularly for UHF are difficult to predict accurately. For this reason, I began by making all elements slightly longer than specified, and then gradually trimmed them back to get resonance (ie lowest SWR) at the desired frequencies.|