Amateur Radio (G3TXQ)- Broadband Hexbeam Technical Details
On this page we will explore some of the technical details behind the new Broadband Hexbeam. In particular we will look at which of the antenna dimensions are critical to particular performance parameters. The conclusions will be very similar to those we arrived at for the Classic Hexbeam:
- Reflector dimensions determine the tuning of the antenna
- Driver dimensions largely determine the feedpoint impedance of the antenna
- End spacing mostly affects the peak F/B performance
.... however, we shall find a surprising result when we look at wire gauge.
A number of modelling experiments were carried out using a "reference" 20m beam whose
performance is shown in the chart to the right. Its dimensions were: Driver = 219"
Reflector = 207" and End Spacing = 24". We see that:
- F/B performance peaks at 24dB and falls to 12dB at the band edges
- The Forward Gain falls by about 2dB across the band - a figure typical of many 2-element beams - and appears to peak below the bottom band edge
- The SWR is nicely "centred" in the band, with a minimum under 1.4 and rising to about 1.9 at the band edges
1. Reflector
The length of the Reflector on the Reference model was changed from 205" to 209" in 1" steps; at each
stage the Forward Gain, SWR and F/B performance were noted across the band. The SWR changed as a result
of the Reflector / Driver ratio changing, but the critical dependency was the antenna tuning which
can be seen by reference to the F/B performance shown on the right.
We see that for each 1" increase in Reflector length the frequency of peak F/B drops by about 70 KHz - just as we found with the Classic Heaxbeam. A more detailed analysis of the modelling results shows that peak F/B is occurring at 14.184 MHz - about 100 KHz (0.7%) above the self-resonant frequency of the Reflector (14.088 MHz); in contrast the peak F/B on the Classic Hexbeam occured much closer to its Reflector resonance.
We conclude that: Broadband Hexbeam tuning is linearly dependent on Reflector length.
2. Driver
The length of the Driver on the Reference model was changed from 217" to 221" in 1" steps; at each
stage the Forward Gain, SWR and F/B performance were noted across the band. The length of the
Driver had little effect on Forward Gain or F/B tuning; however it had a major effect on the
feedpoint impedance as seen in the chart on the right.
The shortest Driver length (217") has produced the lowest SWR; However, bearing in mind that the F/B performance is centred in the band and the Forward Gain peaks below the bottom band edge, the shortest Driver may not be the best choice operationally. The 219" Driver might be the better choice: it produces an SWR curve that is better centred, and its minimum SWR is not unacceptably worse: 1.37 vs 1.19.
At first sight it may seem strange that this parasitic beam has a Driver that is significantly longer than its Reflector. This apparent anomaly is explained by the differing shapes of the elements. A detailed analysis of the modelling results shows that the 219" Driver is self-resonant at about 14.330 MHz - that's 242 KHz above the 207" Reflector's resonance! What matters here is not the relative lengths, but the relative resonant frequencies.
We conclude that: choosing to make the Driver resonance about 1.7% higher than the Reflector resonance results in a good match to 50 Ohms across the operating band. [It's no coincidence that this is the same Reflector/Driver ratio often used on the Classic Hexbeam; but in the case of that antenna, because Driver and Reflector are the same shape, the ratio translates directly to wire lengths]
3. End Spacing
The size of the End Spacing on the Reference model was changed from 16" to 32" in 4" steps; at each
stage the Forward Gain, SWR and F/B performance were noted across the band. The size of the End
Spacing had little effect on antenna tuning. It had some effect on Forward Gain and a slightly greater
effect on SWR, but the major impact was on the peak value of F/B as shown in the chart on the right.
We might be tempted to opt for a large End Spacing in pursuit of the highest peak F/B performance. However:
- The F/B performance is little better at the band edges with the larger spacing
- Detailed analysis of the azimuth patterns shows that the high F/B numbers are somewhat illusory. They result from deep, narrow, "notches" in a cardioid pattern which may not be particularly useful in day to day operation.
- The SWR suffers, as shown in the second chart to the right
The cardioid pattern mentioned above begins to develop with End Spacings between 24" and 28". So if we opt for a more "modest" spacing, like 24", we shall lose little practical F/B performance and will keep the SWR below 2:1 across the band.
4. Wire Gauge
The wire gauge on the Reference model was changed from #12 to #20 in 4 steps; at each
stage the Forward Gain, SWR and F/B performance were noted across the band.
Interestingly, as the F/B chart on the right demonstrates, the antenna tuning is largely independent of wire gauge. This may seem strange - usually employing thicker wire tunes an antenna lower in frequency. However, we saw in the earlier Critical Dimensions - Wire size & Type section that the unique Classic Hexbeam element shape causes the opposite effect: thicker wire tunes the antenna higher in frequency. We also saw that certain shapes, intermediate between the Classic Hexbeam shape and a linear dipole, exhibit almost no dependence on wire size. Fortuitously the Broadband Hexbeam Reflector shape is in this category.
The lower chart on the right shows how the SWR varies as wire gauge is changed. The thicker wire tunes the Driver slightly higher in frequency and therefore the minimum SWR moves up; but any "skew" at the band edges - as occured when we shortened the Driver dimensions - is more than offset by the significantly lower SWR overall.
