- Tailoring Patterns for HF Verticals

Predicting the theoretical radiation characteristic of an HF whip antenna is very useful for determining how actual performance will be at a given frequency. However, looking at individual patterns frequency by frequency, the overall cycle of pattern development is not always obvious. Knowing this cycle can be very useful in choice of frequency and antenna.

Without going to the trouble of plotting radiation patterns for HF whip antennas, it is possible to have an idea what the radiation characteristics will be like. This is because the radiation pattern of a monopole operating in 1/2 wave mode is always the same.

Similarly, patterns at 5/8 wave, 1 wave, 1 1/2 wave, etc., show the same characteristics.

Multi-wave Radiation Characteristics

Knowledge of this pattern, will assist in determining the radiation characteristics of a given HF whip.

An HF whip antenna with a frequency range of 2-30 MHz (via an automatic antenna tuning unit) operates in different modes at different frequencies due to the physical length of the antenna.

Because wavelength is determined by physical antenna length, the choice of antenna length (or height, in the case of a vertical) can therefore be significant. At 5/8 wave and below, a symmetrical pattern is achieved across the range. After this the pattern starts to break down into lobes, losing the desirable lower angle radiation.

Obviously for 2 MHz, where a quarter wavelength is 36m (119ft), it is desirable to have as much physical length as possible, as this is the most effective way of achieving gain. However, a 9m antenna is quarter wave at 8 MHz, half wave at 16 MHz, 5/8 wave at 20 MHz, 3/4 wave at 24 MHz and 32 MHz at 1 wavelength.

At 22 MHz and above communications reliability tends to be adversely affected by the sun spot cycle and this will often determine the upper level of usefulness for the HF band. For these reasons the most effective antenna height is 8-10m (Moonraker series 23, 80 and 100 and type 29W, giving useful radiation patterns up to around 20 MHz.

To determine the radiation characteristics of an antenna of a given length: multiply 35.6 by the required mode factor, then divide by the antenna length. The mode factor for quarter wave is 2, half wave is 4, 5/8 wave is 5, 3/4 wave is 6 and 1 wave is 8.

Therefore frequency of operation for a 5.5m antenna operating in 5/8 wave mode will be 32.4 MHz, whereas that for a 10m antenna in 5/8 wave will be 17.8 MHz

Electrical loading is available to maximise performance on the lower frequencies for antennas that are physically short at these frequencies. This loading can have the effect of increasing antenna efficiency at the lower frequencies AND improving the radiation characteristics at higher frequencies.

These properties are dependent on the inductance value and the physical position of the inductor relating to the overall length of the antenna. As a rule of thumb, the inductor should be located between 50 to 80% from the base (feedpoint of the antenna).

This is why we recommend that physically short antennas, especially those required for emergency operation and those that will mainly be used at 10 MHz and below, are loaded to the highest operating frequency below 10 MHz. Such loading provides improved performance on the 2187.5 kHz emergency frequency and on the higher frequencies also.

In loaded antennas the inductive loading itself acts as a choke, effectively cutting off the upper part of the antenna at frequencies at twice the fundamental. Relating this to our 9m antenna, this time with loading for 6 MHz at 75% from the base feedpoint, operation would follow the normal radiation characteristics from 2 to 10 MHz. From 12 to 30 MHz, however, the effective antenna length would be reduced to 6m.

As the radiation characteristics for a 6m antenna are quarter wave at 12 MHz and half wave at 24 MHz, the result is improved performance for operation at the high end of the band. Therefore the assumption that unloaded is always best does not always give the best solution. There are some cases where judicious loading can give you the best of both worlds. However, before selecting a loaded antenna, consideration must be given to the operation frequencies required within the context of total communications needs. We can always advise.