An Antenna Study - Part 2
Last month we presented the effect of radials and poor ground on radiation patterns of vertical antennas (See Antenna Study – Part 1). This month we will compare 1/4, 5/8, and 1/2 wave verticals.
The radiation patterns of these antennas are presented in Figures 5a, 5b and 5c. For these plots, the base of the antenna is at ground level and all had 20 1/4 wavelength radials over average ground. Figure 5a is a comparison of a 40-meter 1/4 vertical plotted with a 1/2 wave vertical.
Figure 5b is the patterns of a 1/4 wave vertical against a 5/8 wave vertical.
Figure 5c is the patterns of a 5/8 wave vertical plotted against a 1/2 wave vertical.
If your interest is DX, consider using a 5/8 wavelength vertical. Note the gain is higher than other verticals and provides significantly more gain at the lower DX angles than the others. You would be fortunate if you could mount your vertical over swamp water.
A 5/8 wavelength vertical radiates most of the transmitter power at lower angles. This is illustrated in Figure 6.
To make the transmitter happy, install a base-loading coil in a 5/8 wavelength antenna to cause the antenna system to be resonant. If you have a problem installing radials, consider using a 1/2 wavelength vertical. This could be fed with a parallel tuned circuit or the familiar J-pole configuration with the 1/4 wavelength matching section laid along the ground. The matching section may be replaced with a parallel tuned circuit and the use of a ground rod, since this is a high impedance and the poor ground will have little effect on efficiency.
Next month we will examine the sloper antenna
Originally posted on the AntennaX Online Magazine by Ted Hart, W5QJR
Last Updated : 17th March 2024