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.