Amateur Radio (G3TXQ)- HexBeam performance
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". The Azimuth plot on the right shows the directionality that is typical of a HexBeam:
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 proximity of the ends 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. Managing expectations
This figure shows the typical performance of a 20m HexBeam. The Gain peaks at about 6.8dBi, the maximum F/B ratio is 16.2dB and the minimum SWR is 1.5. But notice that these "optima" do not occur at the same frequency; at 14.30 MHz this antenna exhibits a reasonable 1.5 SWR, but its gain is only 4.3dBi (2.2dB better than a dipole) and its F/B ratio is under 10dB. It's working much better at 14.14 MHz where the gain is 5.6dBi and the F/B is 16.2dB, but the SWR has risen to 2.2. This characteristic is not unique to the HexBeam - best F/B and maximum Gain rarely coincide.
The graphs demonstrate that low SWR is no guarantee of good performance - in fact with an unmatched HexBeam low SWR is almost certainly an indication that you are operating at frequencies of reduced beam performance! Low SWR should never be a primary design aim; it is far preferable to design for good performance, irrespective of SWR, and then sort out the SWR with external matching techniques or simply use a good Antenna Tuning Unit (ATU).
By the way, the curves also illustrate why we need to be wary about the performance data quoted by antenna manufacturers. If I were selling this antenna I could legitimately say that it has a Gain of 6.8dBi, a F/B ratio of 16.2dB and an SWR of 1.5:1 - with such good figures in a "half-size" beam you might be tempted to buy one, only to be disappointed when you find that there is no frequency at which it meets all these figures!
A fairer interpretation of the data is that the Gain is at least 4.6dBi (2.5dBd), and the F/B at least 10dB, over a 200 KHz range from 14.06 MHz to 14.26 MHz, whilst recognising that the SWR rises to 3.8 at the lower end of this range. At first sight you may be disappointed by these figures, but compare them with a typical commercial HF mini-beam and they look quite impressive. For example, according to figures published by Cushcraft, the 20m Gain of the MA5B is 1.5dBd and its 2:1 SWR bandwidth is 90 KHz.
Because the HexBeam maintains good performance over a relatively narrow bandwidth, it is essentially to tune the beam so that it performs well at your frequencies of interest. Other pages on this site will explain how to do this.