Amateur Radio (G3TXQ)- HexBeam End Spacing
The chart on the right illustrates the effect of changing the spacing between the tip of the Driver and the tip of the Reflector from 10" to 17" on our benchmark 20m HexBeam.
We note several features:
- There has been a substantial increase in the peak F/B performance from 16dB to 30dB
- The bandwidth over which the F/B ratio exceeds 10dB remains at 200KHz for both spacings
- The maximum Gain has not changed
- The SWR performance has deteriorated somewhat
- The frequencies of best F/B and maximum Gain have both increased by 40KHz
We can explore further the relationship between end spacing, peak F/B ratio, SWR and Frequency by plotting these parameters agains various end spacings. The chart below shows the results. The horizontal axis of this chart represents the various end spacings expressed as a ratio of our "standard" spacings of 6.7" and 4.3". Note that the SWR values have been scaled by a factor of 10, and the Frequency values have been "normalised" to 14MHz to make the trends easier to see.
We notice that:
- The F/B performance increases steadily with increased end spacing, and then drops abruptly. Best F/B occurs at end spacings about 80% higher than our "standard" figures
- Increasing the end spacing causes the SWR to rise
- Increasing the end spacing raises the frequency at which the beam performs best - by 80KHz over this range of spacings. This frequency shift is linearly dependent on the spacing.
- Not shown in the chart, but evident from the supporting data, is that the frequency range over which the F/B ratio exceeds 10dB remains at just over 200KHz for all the end spacings considered.
We conclude that End Spacing is the critical factor in determining the peak F/B ratio of a HexBeam, and that improved F/B performance can be achieved over a narrow range of frequencies by increasing the end spacing from the values often quoted; but note that this option will result in a worse SWR, will detune the beam from the original design frequency, and will not improve the F/B bandwidth. Whether the improvement in peak F/B is worth the deterioration in SWR will depend on individual operating preferences.
In passing, note that it is the total distance between the Driver and Reflector tips which matters - it is not important how this gap is apportioned between Driver side and Reflector side. Designers usually arrange to have more of the gap on the Driver side so that (Driver + Driver Insulator) = (Reflector + Reflector Insulator); this ensures that the wires line up horizontally on the support structure. Note also that, when referring to "end spacing", some designers mean the length of the insulators while others are referring to the perpendicular distance between the end of the wire and the centre of the spreader; if you are following a particular set of published dimensions, make sure you know what the designer intended!
Finally, be aware that the way the tips of the Driver and Reflector are terminated can change their capacitive coupling and hence alter the F/B performance and the antenna tuning. Practical experiments and EZNEC simulations show that the common approach of using a connector block to join a wire element to the end insulator is the equivalent of extending the wire by an inch. When following a published design, you should treat the termination method as an integral part of the design and follow closely the designer's recommendation.