Horizontal Versus Vertical Aerials For DX

Horizontal dipoles in free space provide 2.1 dB of gain over the isotropic model. However if you place the horizontal dipole 1/2 wavelength above ground, the gain increases to a nominal 8.1 dB over isotropic.

About the best that a typical vertical can offer for gain is 1.12 dB over isotropic.

As manmade noise is generally vertically polarized, verticals generally pick up more noise than the horizontally polarized horizontal dipole. In a location where all the utility cables are buried, a vertical on 20 metres will raise the noise floor around 8 to 12 dB on the receiver.

The big attraction for the vertical is that it has a low angle of radiation. This is attractive to DXer’s. In most cases keeping the launch angle of your signal close to the earth’s surface shortens the transit path between you and the distant station. Imagine that the peak radiated power for your dipole is 45 degrees. If the ionosphere f2 layer is 200 miles above earth, then your signal launching at 45 degrees will travel 280 miles before striking the ionosphere and reflecting back down to earth. If you vertical antenna is used, and if it has a low launch angle, then the signal travels more like 400 to 500 miles before hitting the f2 layer and reflecting back down. The typical dipole’s peak radiated power launch angle is around 25 degrees above earth if the antenna is 1/2 wavelength above ground. However, verticals provide a launch angle of around 10 degrees. The horizontal dipole, if operated below 1/2 wavelength above ground, say 1/4 wavelength, will have a peak radiated power at 45 to 50 degrees. While erecting a 20 metre antenna at 1/2 wavelength above ground (about 35 feet at 14.25 MHz) is within the reach of many amateurs, doing the same with an 80 metre dipole is not practical for most. Assuming your 80 metre dipole antenna is operated at 3.7 MHz, then 1/2 wavelength above ground is about 120 feet.

Since amateurs run low horizontnal dipoles at 1/4 wavelength or less above ground, the gain is reduced and the launch angle raised. That is just about the two worst things that can happen if chasing DX. If you erect your 80 metre dipole at 35 feet above ground, then the gain drops to about 3 to 4 dB best case and the peak radiated power will be almost directly overhead. If you examine the gain of a dipole 1/8 wavelength above ground, you will see that the radiated power is about 0 dB to -10 dB at the 20 degree angle and get progressively worse with decreasing angle. Suddenly that poor 1.12 dB gain of the vertical does not look so bad if operating the lower bands.

Vertical monopoles need a good ground plane to provide the 1.12 dB of gain. Without a good ground plane the gain of the vertical drops rapidly below 0 dB and the launch angle rises.

Vertical 1/2 wavelength dipoles have improved gain and typically yield about 2.5 – 3 dB of gain when the lower end is above the ground by at least an 1/8 wavelength.

Much of this is theory and assumes a clear site for your antenna farm with reasonably conducting ground. In an urban environment the proximity of buildings and trees has a major effect so experimentation is the key.