Broadband Hexbeam - In depth
On this page we will explore some of the technical details behind the Broadband Hexbeam. In particular we will look at which of the antenna dimensions are critical to particular performance parameters. Here's the "Executive Summary":
- 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
- Wire gauge is not critical, but requires Driver dimensions to be modified
1. How does it work?
The principles behind Hexbeam operation are no different than those of any 2-element "parasitic" beam. One element - approximately half a wavelength long - is "driven" by the transmitter; the other - also about a half-wavelength long - is placed close to the driven element. As a result of its proximity, currents are induced in this second element which result in power being re-radiated from it; because it is not driven directly this element is called "parasitic".
The relative magnitude and phase of the currents in the parasitic element result in the "re-radiated" power reinforcing the power from the Driver in some directions, whilst cancelling it in others - hence the antenna becomes "directional". In a Broadband Hexbeam the Front-to-Back ratio peaks at a frequency about 0.7% above the self-resonant frequency of the Reflector. The Azimuth plot on the right shows the directionality that is typical of the antenna.
The challenge to the antenna designer is to control the "mutual coupling" between the elements in such a way that the relative magnitude and phase of their respective currents optimises the antenna's performance. In the case of the Hexbeam this "mutual coupling" is controlled partly by the general spacing between the elements, and, critically, by the gap between the tips of the Driver and Reflector.
If the designer had total control of the relative magnitude and phase at all frequencies, excellent Hexbeam performance would be possible over a wide bandwidth - gains in excess of 6.8dBi (4.7dB more than a dipole) and F/B ratios over 40dB. But of course in the "real world" such control is not possible, so before getting too carried away we need to manage our expectations!
2. Yagi performance
The chart on the right shows the modelled performance of a full-size 2-element 20m monoband yagi in Free Space; it makes a useful reference for judging the performance of the Broadband Hexbeam.
This Yagi has a 386" Driver, a 418" Reflector, and a 133" boom length (0.16 wavelengths) - a typical design compromise between Gain, Front-to-Back ratio (F/B) and a reasonable match to 50 Ohms. The antenna has an 18.3ft turn radius and is assumed to be constructed from 3/8 inch diameter aluminium.
We see that the full size Yagi is a relatively broadband antenna. The Gain falls by only 0.7dB across the 20m band and never falls below 5.8dBi. The F/B varies by only 1dB across the band, peaking at 11dB. On the other hand, the SWR varies significantly from 2.1 at the lower band edge to 1.0 at the top of the band.
So, what can we reasonably expect from a Broadband Hexbeam just over half the Yagi's size?
3. Broadband Hexbeam performance
This chart shows the performance of a typical 20m monoband Broadband Hexbeam. Each Driver leg is 219.5" long and the total Reflector length is 415"". The spacing between the Driver and Reflector tips is 24" and it is constructed from #16 gauge bare copper wire; it has a turn radius of 10.7ft. The dimensions were chosen to provide a good compromise between Gain, F/B and SWR. This antenna will be used as a benchmark as we explore the effect that various dimensions have on Broadband Hexbeam performance.
The first thing we see is that the Hexbeam has less Forward Gain than the Yagi, and the Gain falls by 1.2dB across the band - a result that is typical of end-coupled wire beams. Neverthless, the Gain is never worse than 1.4dB (1/4 of an S point) below the Yagi and we might think this is a reasonable price to pay for the reduction in size.
The Hexbeam beats the Yagi "hands down" for F/B across the band, reaching a peak of 26dB and never falling below 14dB. What's more the SWR is "well-behaved" with a minimum of 1.4, rising to 1.7 at the band edges. A nice feature of the Broadband design is that it allows us to place minimum SWR at the same frequency as peak F/B.
For a relatively small antenna, the Broadband Hexbeam holds up well against the Yagi and it compares very well with commercial HF mini-beams. For example, according to figures published by Cushcraft, the 20m Gain of the MA5B is 3.6dBi and its 2:1 SWR bandwidth is 90 KHz.
Now let's explore which of the Broadband Hexbeam's dimensions are critical, and how they affect its performance.
The length of the Reflector on a Broadband Hexbeam has little effect on Gain and peak F/B, but it has a pronounced effect on the tuning of the beam (i.e. the frequencies where it delivers maximum Gain and best F/B ratio). It also has some effect on the SWR, as a result of the Driver / Reflector ratio changing, but it is small compared to the effect on tuning.
If we change the length of the Reflector on our reference antenna from 411" to 419" in 2" steps and plot the F/B ratio at each step, we get the results shown on the right. We see that for each 2" increase in Reflector length the frequency of peak F/B drops by about 68 KHz. And because 2" represents 0.5% of the total length, whilst 68KHz is 0.5% of the centre frequency, we conclude that Broadband Hexbeam tuning is linearly dependent on Reflector length at a rate of 34KH/ per inch.
By extrapolation we can calculate equivalent numbers for the other HF bands which are useful to remember if you have to tune your Broadband Hexbeam:
- 20m - 34KHz/inch
- 17m - 54KHz/inch
- 15m - 73KHz/inch
- 12m - 101KHz/inch
- 10m - 128KHz/inch
Getting the Reflector length right is essential if a HexBeam is to perform well. It is probably the most critical of all the antenna's dimensions.
The length of each Driver leg on our reference antenna was changed from 217.5" to 221.5" in 1" steps; the Gain, F/B and SWR were noted at each step. There was a negligible effect on Gain, F/B performance, or antenna tuning, but there was a significant effect on the SWR as shown on the right.
The shortest Driver length (217.5") has produced the lowest SWR; However, bearing in mind that the F/B performance is centred in the band, and that the Forward Gain peaks below the bottom band edge, the shortest Driver may not be the best choice operationally. The 219.5" Driver might be the better choice: it produces an SWR curve that is better centred, and its minimum SWR is not unacceptably worse at 1.43 vs 1.24.
At first sight it may seem strange that this parasitic beam has a Driver length that is significantly longer than its Reflector (2 x 219.5" vs 415"). This apparent anomaly is explained by the differing shapes of the elements. A detailed analysis of the modelling results shows that the 439" Driver is self-resonant at about 14.320 MHz - that's 280 KHz above the 415" Reflector's resonance of 14.040 MHz! What matters here is not the relative lengths, but the relative resonant frequencies.
We conclude that Driver length is less critical than Reflector length to good performance. Choosing to make the Driver resonate about 2% higher in frequency than the Reflector 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.
6. 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, but the major impact was on the peak value of F/B and, to a lesser extent, the SWR, as shown in the charts on the right.
We might be tempted to opt for a large End Spacing in pursuit of the best peak F/B performance. However:
- the F/B performance is little better at the band edges with the larger spacing;
- the SWR suffers;
- detailed analysis of the azimuth patterns shows that the high F/B numbers can be somewhat illusory. In some instances they result from deep, narrow, "notches" in a cardioid pattern which may not be particularly useful in day to day operation.
This cardioid pattern 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.
7. Wire gauge
Wire gauge affects all Broadband Hexbeam performance parameters to an extent, although its effect on tuning is minimal. Thicker wire improves forward Gain because it reduces copper losses; it modifies the SWR because it tunes the Driver higher in frequency; but it reduces peak F/B because it increases the Driver / Reflector end coupling.
The chart on the right shows the effect on F/B of stepping the wire gauge from #12 to #20, and we can see the reduction in peak F/B from 28dB to 25dB that the thicker wire has caused. We also see that there has been negligible change in antenna tuning. This surprising result follows because the U-shaped Reflector's resonance is relatively immune to wire diameter - this is discussed further in the Wire size and type section.
We conclude that there is no overriding reason to use one wire gauge rather than another. Choosing a thinner gauge such as #16 loses a little forward Gain, but it improves the peak F/B and results in a lighter antenna. If a wire gauge significantly different from #16 is used, the Driver dimensions should be scaled as explained in Wire size and type so as to preserve the SWR characteristics.