Amateur Radio (G3TXQ)- HexBeam Reflector

This figure illustrates the effect of changing the length of the Reflector on our benchmark 20m HexBeam, whilst keeping all other dimensions the same. The light blue and dark blue curves show the Gain and F/B ratio respectively of the beam with a Reflector length of 222.3". The light green and dark green curves show the Gain and F/B performance when the Reflector length is decreased to 219.25"

Note that the Gain curves peak at about the same value of 6.8dBi and that the F/B maxima are very similar at just over 16dB. So changing the Reflector length has had little effect on the basic performance parameters of the beam.

However, shortening the Reflector has had a major impact on the frequency at which maximum Gain and maximum F/B are delivered; in fact they have both been increased by about 220 KHz. Clearly, the length of the Reflector is determining the frequency at which the beam performs best; in fact the F/B ratio peaks very close to the frequency where the Reflector is self-resonant. Intuitively we might expect this to be the case because the Reflector is "parasitic" (i.e. not directly driven) and therefore the induced currents which cause the antenna's directivity are maximum at its self-resonant frequency.

We note that the 3.05" change in Reflector length represents about 1.5% of the total, and that 220 KHz is also approximately a 1.5% change in frequency; in other words the frequency of best performance is inversely proportional to the Reflector length. To illustrate this in more detail, we can plot small changes in Reflector length against the frequency of maximum F/B as shown in the figure to the right.

We see that there is a linear relationship between these two parameters, and that a total change in length of 4" has produced a frequency shift of 270KHz at 20m, or about 68KHz per inch. By extrapolation we can calculate equivalent numbers for the other HF bands:

• 20m - 68KHz/inch
• 17m - 107KHz/inch
• 15m - 145KHz/inch
• 12m - 202KHz/inch
• 10m - 256KHz/inch

These are useful numbers to remember when you are tuning your HexBeam.

To conclude:

Getting the Reflector length right is essential if the HexBeam is to perform well at your preferred frequency of operation. It is probably the most critical of all the HexBeam dimensions