Amateur Radio (G3TXQ)- HexBeam Wire type
1. Wire Gauge
The chart on the right illustrates the effect of changing the wire gauge from #16 to #10 on our benchmark 20m HexBeam.
We note that:
- Peak Gain, peak F/B and SWR all remain at similar values
- There has been a frequency shift upwards of approximately +80KHz
- The increased wire diameter has produced a small increase in F/B bandwidth
We will explore this relationship further by plotting the frequency at which best F/B is delivered, against a range of wire gauges. The results are shown in this second chart.
We see that there is an inverse linear relationship between wire gauge and frequency which leads to the following handy "correction factors" for adjusting dimensions that have been quoted for #16 gauge wire:
- # 20 multiply by 0.996
- # 18 multiply by 0.998
- # 16 multiply by 1
- # 14 multiply by 1.002
- # 12 multiply by 1.004
- # 10 multiply by 1.006
It is interesting that increasing the gauge number (decreasing the diameter) causes a decrease in frequency - the opposite of most other antennas. I have explored this characteristic by modeling "bent dipoles" with various included angles and noted the effect of changing the wire gauge. With a linear dipole, increasing the gauge number increases the resonant frequency. If the dipole is then gradually bent, the change in gauge has less and less effect until, with an included angle of 90 degrees, it has no effect at all. When the dipole is bent further to included angles less than 90 degrees, the effect re-appears but in the opposite sense! The HexBeam shape lies in this area.
I have seen no explanation of this effect in the technical literature. My guess (no more than that) is that at an included angle of about 90 degrees the decreased inductance of a thicker wire is offset by increased capacitive coupling between the two halves of the dipole. At smaller included angles, the capacitive effect predominates, and at larger angles the inductive effect predominates.
For those of you who do not like doing maths, the following table details the correction factors (in inches) that should be applied to the Driver and Reflector half-lengths for different wire sizes on the various HF bands; the correction factors for Driver and Reflector end-spacings are so small as to be negligible. Complete wire dimensions for a 5 band array constructed in #16 gauge can be found on Leo's website, to which these correction factors can be added/subtracted.
| Band | #20 | #18 | #16 | #14 | #12 | #10 |
| 20m | - 0.9" | - 0.4" | 0 | + 0.4" | + 0.9" | + 1.3" |
| 17m | - 0.7" | - 0.3" | 0 | + 0.3" | + 0.7" | + 1.0" |
| 15m | - 0.6" | - 0.3" | 0 | + 0.3" | + 0.6" | + 0.9" |
| 12m | - 0.5" | - 0.2" | 0 | + 0.2" | + 0.5" | + 0.7" |
| 10m | - 0.4" | - 0.2" | 0 | + 0.2" | + 0.4" | + 0.7" |
2. Insulation
Using insulated wire to construct the HexBeam will change the resonant frequency of the antenna. By how much is very difficult to predict - it depends on the diameter of the conductor, the thickness of the insulation and the dielectric properties of the insulation. Generally speaking it will lower the frequency of operation and dimensions will need to be shorter than the corresponding bare wire dimensions - typically by 1% - 2%.
3. MaterialI have modeled HexBeams constructed with Copper wire and with Stainless Steel wire and compared the respective performances. The reduced conductivity of Stainless Steel caused a drop in antenna Gain, and interestingly a shift in resonant frequency. I have not explored the frequency shift mechanism further but believe it may be a result of the change in skin depth caused by the reduction in conductivity. At HF frequencies, the skin depth is sufficiently low that you can treat copper-clad stainless wire as if it were solid copper.