Amateur Radio (G3TXQ)- Broadband Hexbeam
We saw from the Basics - Shape page that a consequence
of the Hexbeam's geometry is a relatively narrow performance bandwidth; typically the F/B exceeds
10dB over a band equivalent to only 1.4% of the centre frequency, and the SWR is above 2:1 for
a significant proportion of this band. This narrow bandwidth is largely
determined by the Q of the Reflector which I measured at about 30 for a 10m element constructed
from #16 wire.Compare this with a linear dipole which has a Q of about 10. If we can find a way
of reducing this Q we should end up with a broader-band antenna.
I spent many hours modelling Reflectors and evaluating ideas on a 10m testbed - you can see the detail on the Reflector Experiments page. I tried using thicker wire of various types, including 2 varieties of coaxial cable and "caged" wires. I also tested alternative Reflector shapes. Of all the ideas evaluated, by far the most effective and easiest to implement was to change the shape of the Reflector as shown in the diagram on the right. Even when using relatively thin #16 wire this shape has a radiation resistance of 44 Ohms and a Q of about 17. It requires an increase in turning radius of about 15%. Modelling a Hexbeam with this geometry produced very encouraging results: F/B > 10dB and SWR < 2:1 across all of the 20m, 17m, 15m and 12m bands, and approximately 1 MHz of 10m. The modelling suggested there was little to be gained by making the same change to the shape of the Driver element; in fact, retaining the classic shape for the Driver delivers a better match to 50 Ohms and avoids a further increase in the turning radius. The chart below compares the Free Space performance of a 10m version of this new hybrid antenna with a classic Hexbeam.
Construction and testing of a 10m monoband version of the new antenna confirmed the modelling results, and so a full 5-band test beam was constructed. The 20m, 17m and 15m results were immediately satisfactory, but it took some time to optimise the 12m and 10m performance; the proximity of these bands often causes problematic interactions which are not always predicted by the modelling, and the final wire dimensions for these bands were a result of "cut and try" on the testbed. The final dimensions using #16 bare copper wire were:
| 20m | 17m | 15m | 12m | 10m | |
| Driver (half-length) | 218" | 169.5" | 144.5" | 121.7" | 106.8" |
| Reflector (total) | 412" | 321" | 274.4" | 232" | 204.4" |
| End spacing | 24" | 18.5" | 16" | 13.5" | 12" |
| Vertical spacing from 10m elements | 38" | 15" | 9" | 5" | 0 |
The band feedpoints are interconnected with 50 Ohm coax, and the array is Top fed
This 5-band design requires a horizontal distance of about 130" from the Centre Post to the tips of the spreaders. If you are unable to accomodate this increased size, don't be tempted to stick to the classic shape for 20m and adopt the new shape for 17m thru 10m: modelling shows that the 20m performance bandwidth suffers dramatically, probably due to the mid section of the 17m Reflector providing an RF coupling path between the "knees" of the 20m Reflector.
The following charts show the measured performance of this design with the baseplate at a height just under 20ft. At this height, ground reflections cause the 10m and 12m peak F/B figures to be suppressed compared to the Free Space values, and the 20m and 17m figures to be enhanced. The wire dimensions may need some further slight adjustment to optimising the tuning once the antenna has been evaluated at more representative heights.
Audio Recordings
Here are some audio clips which demonstrate the F/B performance of the new design. They comprise 3-5 seconds with a beam pointed towards a station, interlaced with 3-5 secs with a beam pointed in the opposite direction. Interestingly, analysis of the audio waveforms suggests that the F/B ratios are in excess of 30dB - a higher figure than I measure with a line-of-sight source on my test range.
Hexbeam F/B 20m Iceland
Hexbeam F/B 20m USA
Hexbeam F/B 17m USA
Hexbeam F/B 17m USA
Hexbeam F/B 20m USA
Hexbeam F/B 20m USA
Hexbeam F/B 20m USA
And finally here is a clip demonstrating the difference between my Broadband Hexbeam at 20ft and my 132ft inverted V doublet with its centre at 20ft. The first half of each 10 second period is the Doublet and the remaining period is the Hexbeam. A half second gap has been inserted to emphasise the changeover.