Broadband Hexbeam - Reflector experiments

This page describes some of the experimental work carried out on my 10m Hexbeam testbed to investigate methods of extending the antenna's bandwidth by decreasing the "Q" of the Reflector. This work led directly to the new Broadband Hexbeam.

1. "Fatter" Reflector

Traditionally, dipole elements have been made broad-band by building them out of "fatter" elements, or by connecting in parallel a number of elements with slightly varying length. Some quick tests showed that the "fatter" element appoach was easier to implement and adjust.

Two reflector wires at knee Two reflector wires joined at tip

Rather than using larger diameter wire with its associated weight penalty, I decided to add a second wire to the Reflector structure, spaced a small distance from the first, in much the same way as the elements of a Cage Dipole. The simplest way to do this on the classic Hexbeam support structure is to join the two wires at the Centre Post terminal and at the element tips, but to keep them apart at the "knee". The two photos on the right show the arrangement used on my testbed antenna.

Broadband inverted W Before assembling a complete beam, I investigated the effect of a second wire on the impedance of a "W" dipole. The results are shown in the chart on the right and were encouraging. The key characteristic that determines the bandwidth is how quickly the Reactance changes with frequency. You can see that the addition of the second wire has had no effect on the Resistance of the dipole, but it has reduced the rate of change of Reactance quite significantly.

We note that:

Broadband Hex performance Encouraged by these results I constructed a complete 10m beam with a 2-wire Reflector. The wires were joined together at the centre post terminal and at the tip of the Reflector; they were spaced by 2" at the "knee". The chart on the right shows the results compared to a beam with a single-wire Reflector.

We note that:

In an effort to find more-practical, but still effective, ways of implementing the technique, I built a range of different broadband Reflectors. With a Reflector up at 20ft I measured its resonant frequency, and its +12/-12 Ohm reactance bandwidth - a useful guide to the F/B bandwidth it will deliver. All Reflectors were 2754mm (108.4") long; in the case of the 2-wire designs the shorter of the two wires was 2754mm long

Various 2-wire geometries were tried; in all cases the wires were joined at the centre-post terminal and at the Reflector tip, and separated at the mid-point "knee" by horizontal distance "a" and vertical distance "b".

The results are presented in the table in order of increasing bandwidth:

Reflector Resonant frequency +12/-12 Ohm Bandwidth (KHz)
Single wire reference 28,440 KHz 460 KHz
RG174 coax -107 KHz 500 KHz
2-wire a=2.5" b=0.5" -680 KHz 571 KHz
300 Ohm slotted ribbon -265 KHz 576 KHz
450 Ohm "window" -440 KHz 600 KHz
2-wire a=1" b=1" -110 KHz 600 KHz
2-wire a=2" b=1" +193 KHz 622 KHz
2-wire a=1.5" b=3"" +626 KHz 672 KHz
2-wire a=2" b=1.5" +460 KHz 711 KHz
2-wire a=0" b=4" +710 KHz 720 KHz

We note that:

2. Modified Reflector shape

Hybrid Hexbeam shape The major contributor to the HexBeam's high Q is its low Radiation Resistance (about 22 Ohms). This is caused by the current cancellation that occurs where the inner halves of the dipole elements come together at an acute angle at the Centre Post. If this part of the Hexbeam geometry can be avoided, the radiation Resistance will increase and the Q will drop.

The drawing on the right shows one way of achieving this. EZNEC work shows that there is little point in making the same change to the Driven element: it has little effect on the F/B bandwidth; it damages the array SWR significantly; and it would probably require some extra mechanical support to be provided at the feed point. In the following discussions this geometry is referred to as the "Hybrid" shape, to distinguish it from the "Classic" shape. Good results were achieved on my 10m test array by placing the "cross connect" at a point on the support spreader 20" out from the Centre Post

The altered geometry means that, for a given Reflector length, the "knees" will be further out from the centre post and thus require slightly longer support spreaders; however this is offset to an extent because the Hybrid shape pushes the array tuning down in frequency and so the Reflector can be shorter for a given frequency. Further shortening can be achieved by using insulated wire. By placing most of the "End Spacing" on the Driver side of the support structure it should be possible to arrive at a "symmetric" design which lies neatly in the horizontal plane.

Hybrid performance chart The chart on the right compares the measured performance of a Hybrid shape Hexbeam compared with the Classic shape. In both cases the Reflector was constructed of #16 bare copper wire; but in the case of the Hybrid the Reflector had to be shortened by 1" in order to get the tuning in-band. The "cross-connect" on the Hybrid design was positioned at a point on the support spreaders 20" out from the Centre Post, and the Driver/Reflector tip spacing was 10.5"

We note that:

Hybrid with 450 Ohm window reflector Hybrid with RG58 reflector Of course there is no reason why these two bandwidth extension approaches - "fatter" elements and Hybrid shape - should not be combined. Consequently I constructed a total of 11 arrays using various "fatter" Reflector wire types in Hybrid and Classic shapes. The photos on the right show a couple of examples: the first has a Reflector of RG58 coax in the hybrid shape; the second has a Reflector of 450 Ohm window in the hybrid shape and a Driver of 300 Ohm window in the classic shape.

F/B bandwidth of various Reflector types Each array was tested at a height of 20' for its F/B performance. The bar chart summarises the results in order of increasing bandwidth.There are no surprises here. The fatter the wire the better the bandwidth, and the Hybrid shape outperforms the Classic shape for each wire type. Some of the arrangements are more practical than others: the 2-wire Classic design delivers good performance, but its tuning is quite unpredictable; the Hybrid shape using 450 Ohm window does best of all, but construction is more difficult with this type of wire.

An attractive option is the Hybrid shape using RG58 coax: it delivers a 36% increase in bandwidth; it is relatively easy to handle; and the tuning is quite predictable. Scaled to other frequencies, it would cover all of the 15m band and all but 25KHz of the 20m Band.

The effectiveness of the Hybrid shape led me to model an "extreme" version in which the Reflector is kept as far from the Centre Post as possible. A quick calculation showed that this would increase the turning radius by only 15%, and EZNEC modelling predicted that it would outperform any of the options considered so far, even when implemented with relatively thin #16 gauge wire. Practical tests on a 10m test antenna confirmed the modelling results and led to the design and evaluation of a complete 5 band prototype; you can read about it on the Broadband Hexbeam page.

Please note that none of the earlier techniques have been evaluated on a multiband design, nor have their dimensions been optimised. I would recommend that you only try them if you have facilities to evaluate an antenna's performance accurately and are prepared to experiment! If not, stick to the well-proven Broadband or Classic designs.