Understanding, designing, constructing and using radio antennas

# 25 Jun 07 Modeling a 40m Dipole at Low Height

Varying the height of an antenna above ground affects many of its characteristics, among which are its feed impedance, its gain, the angle above the horizon at which maximum gain occurs, and the length of the elements for resonance. We will model a 40m dipole and investigate its performance at 7.5m, 10m, 15m. 20m, 30m and 40m above ground. These heights embrace the practical limitations placed on most amateur radio antenna installations. They represent a range of from 3/16 to 1 wavelength height.The elements for our dipole are as follows:

Wires for a 40m dipole at 7.5m height

For resonance at 7.15MHz we need a 2mm diameter wire to be 20.11m long. We will feed it at the centre. Now we find the SWR of the antenna:

SWR for a 40m dipole at 7.5m height

The impedance at 7.15MHz is 62.8 ohms, rising to 57.7 – j34.5 ohms at 7.0MHz and 68.3 + 35.1 ohms at 7.3MHz. This results in an SWR below 1.94:1 across the band for a 50 ohm feed with 1.26:1 at band centre. The match to 75 ohms is much better, being below 1.78:1 across the band and 1.19:1 at the centre.

The radiation pattern is interesting. In elevation we have the following:

Elevation plot for a 40m dipole at 7.5m height

The outer circle indicates a gain of 7.24 dBi. At 45 degree elevation the gain is down to 5.56dBi, and it reaches 3.0 dBi at 29 degrees. Clearly, this antenna is not going to be very efficient at sending a signal to the horizon. However, it would make a good cloud warmer or Near Vertical Incidence System (NVIS) antenna.

The azimuthal pattern at 29 degrees elevation is:

Azimuth plot for 40m dipole 7.5m high at 29 degrees

Here is the plot in 3D:

3D radiation pattern of a 40m dipole at 7.5m height

The pattern is not as omnidirectional as is commonly assumed, with 3.02 dBi gain perpendicular to the dipole and -4.34 dBi off the ends. That’s a difference of 7.36 dB front/side ratio! The beamwidth is 90.0 degrees.

At 45 degrees elevation the pattern is similar, but the gains range from 5.56 dBi to 1.48 dBi, a front/side ratio of 4.08 dBi.

So, an antenna at this low height of 3/16 wavelength is quite directional and will need careful alignment. It also aims most of its signal high in the air, rather than towards the horizon where it would do a DXer most good. It will probably work well enough for short range contacts in directions perpendicular to the wire.

Next we will investigate the performance of this antenna at greater heights.