Hexbeam end spacers
During my practical experiments on Hexbeams it became evident that the way the Driver and the Reflector elements are connected to the end-spacing cord can produce capacitive "end loading" which affects the antenna tuning and F/B performance. To understand better the magnitude of this effect, I measured the resonant frequency of a 10m Hexbeam driver when terminated with a range of different end connection arrangements.
1) Expecting that use of a metal connecting block, and possibly the thickness and type of end-spacing cord, would affect the end loading, I began by simply taping the wire element to the thinnest cord that I had - 0.3mm nylon. The arrangement is shown on the right. It is not strong enough to be a practical connection method, but it represents the closest situation I could manage to a freely suspended wire, and it formed a useful baseline against which to compare other methods. The resonant frequency was measured as 28,890KHz.
2) Next, I joined the wire to the thin nylon cord using a brass connecting block and a large brass screw. as shown on the right. The block was placed flush with the end of the wire. The resonant frequency was measured as 28,630KHz - a drop of 260KHz caused by the end loading.
3) Then I moved the block back 3" from the end of the wire, using the rigidity of the wire to maintain its correct position. The resonant frequency recovered by 160KHz to 28,790KHz - just 100KHz below the baseline frequency.
4) The block was placed flush with the end of the wire again, and a small hexagonal set-screw used in place of the large brass screw. The resonant frequency was measured as 28,760KHz - 130KHz below the baseline, but 130KHz higher than the same arrangement using the large screw.
5) I next reverted to the baseline taped arrangement, but used 1.5mm dacron as the spacing cord in place of the nylon. The resonant frequency was 28,896KHz - almost exactly the same as with the thin nylon.
6) To confirm that the dacron was having no effect I repeated arrangement 2, but using dacron cord. The resonant frequency was 28,620KHz - again substantially the same as with the thin nylon.
7) Finally, I dowsed the dacron cord with water and repeated the measurement. The resonant frequency dropped by 50KHz to 28,570KHz
There have been occasional reports on the Yahoo Hexbeam Users' Group of antenna VSWRs rising appreciably during/after heavy rain and recovering after a couple of hours; that seemed to point to wet end spacing cords. Thinking that the resonant frequency of a Driver might not be a sensitive enough indicator I constructed a complete 10m beam and measured its VSWR with dry cords and then after soaking the cords with "dirty" rainwater. This chart shows the results.
There appears not to be any major effect other than a 100kHz shift in tuning. This test was necessarily short and is not fully conclusive: there may be contaminents in rainwater in other parts of the world which we don't experience in my part of UK, or it may be that other beams employ different cord material. However, I have yet to experience the major changes noted by others, either on my main operational beam or my test antennas.
We conclude that the end connection arrangement can have a significant impact on the tuning of the antenna. The smaller the bulk of metal involved, and the further it can be placed from the end of the wire, the smaller the impact. Rain on the spacing cord is also likely to have some effect. However, the thickness of the spacing cord, and the material used, appears not to be a factor. Finally, when following a published design you should treat the termination method as an integral part of the design and follow closely the designer's recommendation.