Amateur Radio (G3TXQ)- HexBeam Driver
The chart on the right illustrates the effect that changing the length of the Driver has on the performance of our benchmark 20m HexBeam, whilst keeping all other dimensions the same. The red, green and dark blue curves show the Gain, F/B ratio and SWR of the beam, respectively, with the original Driver length (218.6"). The yellow, light blue and purple curves show the corresponding parameters when the Driver length is decreased by 4" to 214.6"
Note that the F/B curves are almost identical, and the gain curves are so close that it is almost impossible to see the red curve below the yellow one. Clearly, changing the Driver length has had little effect on the basic performance parameters of the beam, nor has it changed the frequency where this performance is delivered.
However, shortening the Driver has had a small but significant effect on the SWR, improving it marginally at higher frequencies and worsening it at lower frequencies. Unravelling what has caused the change in SWR is important because it provides an understanding of how the Driver length affects the feedpoint impedance of the beam, which in turn allows us to tailor the Driver to meet particular matching requirements.
This chart shows the resistive (R) and reactive (jX) components of the same HexBeam for the two Driver lengths. The results are interesting. The resisitive components are almost identical, increasing from a small value of about 5 Ohms at low frequencies and increasing rapidly through the frequencies of best beam performance to a final value of about 40 Ohms. However, the reactive components are quite different; they have the same basic shape, but the curve for the shorter driver is offset downwards by a consistent 27 Ohms - the equivalent of introducing a series capacitor at the feedpoint of 420pF.
The next chart plots the beam's performance and its feedpoint impedance (measured at the frequency of best F/B) for several values of Reflector / Driver ratio (R/D). It shows that:
- Antenna performance is essentially independant of R/D over a wide range of values
- The reactive component of the feedpoint impedance falls linearly with increasing R/D
- The reactive component of the feedpoint impedance is zero where R/D is just over 1.02
In passing, notice that the beam still performs well at R/D values less than 1; in other words when the Driver is longer than the Reflector! This may seem rather strange if you are used to dealing with conventional Yagis where the correct Reflector / Driver ratio is key to establishing the proper phase relationship between their currents to produce directivity. This is not the case with the HexBeam (and other beams such as the VK2ABQ) where the end coupling is the critical factor.
Typically, values of R/D between 1.015 and 1.02 are used in HexBeam designs because they result in reasonable SWR figures. But the foregoing data shows that more extreme ratios can be used, without detriment to antenna performance, where necessary to meet certain matching requirements. A very useful ratio is 1.04 which results in a feedpoint impedance of about 25 - j25 Ohms - a value which can be translated to an SWR near unity using a "Beta Match" .