Mounting on Roof or at Ground Level?

Groundplane Antennas at Various Heights

Introduction:

Some years ago a friend told me the following story: "You know my triband groundplane antenna on top of the roof of my house, about 12m above ground level. On vacation I mounted it on top of my camper at about 5m height at different locations. It worked at outstanding performance!

I made a lot of new countries I never heard before from home site! Back home I compared the roof mounted groundplane antenna with an identical model on top of my camper. The same story! The lower mounted antenna worked better!"

Analysis:

After some hours of discussion (and some beers...) I decided to pick out an old PASCAL code which shows a graphical 3D-representation of MININECs output files. Simulations of full size groundplanes with four radials and vertical dipoles at usual heights from almost zero to 1.5 wavelength confirmed my hypothesis that a distorted radiation pattern could be the reason for that phenomenon. "Height" means the distance of the radial system to ground level.

Figure 1 shows the simple geometry of the simulation of a full size groundplane antenna at 0.5 wavelength height. The feed-point is at the joint connection between radiator and radial system. Radiator and radials are of 5.5m lengths (ca. 0.25 wavelengths on 14 MHz).

The conductivity of soil did not alter the results. I chose 5 mS/m for conductivity and 13 for dielectric constant. This corresponds to an average type of earth soil.

As usual the magnitudes of field strength are plotted in Decibels over isotropic radiator. For clearness I avoided any grid values. All patterns are shown from maximum gain to 20 dB down.

Figure 1

Example of input file for MININEC (K6STI, 1988, v. 1.0):

Groundplane Antenna, Height: 0.5 lambda
real ground
14.1 MHz
5 wires, meters
5, 0, 0, 10.0, 0, 0, 15.5, 0.01
5, 0, 0, 10.0, 5.5, 0, 10.0, 0.01
5, 0, 0, 10.0, 0, 5.5, 10.0, 0.01
5, 0, 0, 10.0, -5.5, 0, 10.0, 0.01
5, 0, 0, 10.0, 0, -5.5, 10.0, 0.01
1 source
1, 100, 0

Results:

Figure 2 shows a typical radiation pattern of a groundplane antenna as could be found in antenna handbooks. The radials are ground mounted. "Ground mounted" means that they are only 0.02 wavelength above earth level. The radiation pattern looks like a 360° "collar" with a deep null just overhead.

Figure 2: Height: 0.02 wavelength (0.5m at 14MHz).
Impedance: 42.7 + j4.9 ohms.
Max. Gain: -0.3 dBi at 24°.

At 0.25 wavelength the main lobe at 15° is somewhat more flat. This indicates a small amount of higher gain compared with the ground mounted antenna. As shown in Figure 3 a small secondary lobe grows at high angle.

Figure 3: Height: 0.25 wavelengths (ca. 5m at 14MHz).
Impedance: 24.2 - j6.4 ohms.
Max. Gain: 0.6 dBi at 15°.

But now have a look at Figures 4 to 6! At those usual heights most of the transmitting energy is radiated at high angles. There is only a small amount of radiated energy at low angles which could bring you up into DXCC highscore.

Figure 4: Height: 0.5 x wavelengths (ca. 10m at 14MHz).
Impedance: 25.7 - j0.7 ohms
Max. Gain: 1.9 dBi at 45°.

Figure 5: Height: 0.75 x wavelengths (ca. 15m at 14MHz).
Impedance: 26.0 - j3.2 ohms
Max. Gain: 2.9 dBi at 36°.

Figure 6: Height: 1.0 x wavelengths (ca. 20m at 14MHz).
Impedance: 25.5 - j1.9 ohms
Max. Gain: 3.0 dBi at 27°.

As could be seen in Figures 7 and 8 the situation becomes better at heights above 1.5 wavelengths. Most of the energy is radiated at low angles. But keep in mind that this occurs at least above 25m for 14 MHz

Figure 7: Height: 1.25 x wavelengths (ca. 25m at 14MHz).
Impedance: 25.5 - j2.2 ohms
Max. Gain: 2.8 dBi at 9°.

Figure 8: Height: 1.5 x wavelengths (ca. 30m at 14MHz).
Impedance: 26 - j2.6 ohms
Max. Gain: 3.3 dBi at 9°.

Conclusion:

Do not mount groundplane antennas at heights between 0.25 and 1.25 wavelength. At those levels above ground most of the energy will be radiated at angles of 27° to 45° into the ionosphere. This phenomenon seems to be independent to the number of radials or other counterpoises. Further simulations indicates that this is true for all other variants of vertical antenna systems too.

DF3LP, June 1998