Last time we looked at the performance of a 40m dipole at 7.5 m or 3/16 wavelengths high. We discovered that its DX potential was limited but the high angle radiation made it useful for NVIS service. Now we will look at the change of performance as we increase the height of the antenna up to a maximum of 1 wavelength. At 40m obtaining a support structure of greater height than this is unlikely to be practical for most amateurs.
First we will look at the effect on impedance as the height incleases.
At 7.5m (3/16 wavelength) height band centre impedance is 62.8 ohms.
At 10m (1/4 wavelength) height band centre impedance is 81.2 ohms.
At 15m (3/8 wavelength) height band centre impedance is 92.1 ohms.
At 20m (1/2 wavelength) height band centre impedance is 71.8 ohms.
At 30m (3/4 wavelength) height band centre impedance is 70.0 ohms.
At 40m (1 wavelength) height band centre impedance is 75.2 ohms.
Clearly, for a dipole it would be worth considering using a 75 ohm coax cable feeder and a transformer or tuner at the shack end, but a match to 50 ohms would be within the range of most transmitter output circuits, even at the band edges.
Now we’ll look at the radiation patterns at these heights.
First the elevation patterns:
As we saw earlier, at 7.5m height the signal is largely radiated vertically, but as the antenna height increases the angle decreases towards the horizon. It is easy to see why for good DX performance a height of at least 1/2 wavelength is recommended. The gain figures are as follows:
Now for the azimuth plots:
The following table gives the figures for takeoff angle, gain, fron to side ratio and beamwidth for each height. For the lower heights the figures for 45 degrees are also given to allow a comparison of performance to be more easily made.
Again, a height of 1/2 wavelength is seen to be a reasonable compromise between performance and the size of mast or tower needed.
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:
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:
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:
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:
Here is the plot in 3D:
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.