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16 Jul 07 Modelling a 40m Dipole at Various Heights

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.

SWR of 40m dipole 7.5m high

SWR of 40m dipole 7.5m high

At 10m (1/4 wavelength) height band centre impedance is 81.2 ohms.

SWR of 40m dipole 10m high

SWR of 40m dipole 10m high

At 15m (3/8 wavelength) height band centre impedance is 92.1 ohms.

SWR of 40m dipole 15m high

SWR of 40m dipole 15m high

At 20m (1/2 wavelength) height band centre impedance is 71.8 ohms.

SWR of 40m dipole 20m high

SWR of 40m dipole 20m high

At 30m (3/4 wavelength) height band centre impedance is 70.0 ohms.

SWR of 40m dipole 30m high

SWR of 40m dipole 30m high

At 40m  (1 wavelength) height band centre impedance is 75.2 ohms.

SWR of 40m dipole 40m high

SWR of 40m dipole 40m high

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:

Elevation plots for a 40m dipole

Elevation plots for a 40m dipole

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:

Azimuth plots for a 40m dipole

Azimuth plots for a 40m dipole

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.

Takeoff angle, gain.front to side ratio and beamwidth for a 40m dipole at various heights

Takeoff angle, gain.front to side ratio and beamwidth for a 40m dipole at various heights

Again, a height of 1/2 wavelength is seen to be a reasonable compromise between performance and the size of mast or tower needed.

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