Broadband Hex Beam Antenna - 6 Band

Hello fello Hams and anyone else interested !

My name is Dave (call sign G7IYK) and I am based in Bristol south west UK.

This article documents the construction of a 6 band broadband hex beam HF antenna. I first must point out this antenna is not my design but is the work of the late Steve Hunt (call sign G3TXQ) who sadly passed away in December 2018. Steve takes the credit for the design of the broadband hex beam distinct from the original narrow band version.

In this article I am not intending to go into in-depth theory regarding the operation of the hex beam as this has been covered on several other websites easily Googled. This article focuses more on the construction of my particular antenna in the hope this will help others to build something similar.

My motivation for building a hex beam is simple - I wanted a directional, easy to build, robust, relatively discreate and lightweight HF antenna to cover bands 6,10,12,15,17 and 20m. To date I have been using a 130ft (40m) end fed antenna covering 10-80m. This antenna has surpassed my expectations in terms of performance and I have worked stations all over the world using this simple single wire multi-band design. The antenna consists of a 49:1 matching transformer (home made) driving approximately 130ft of wire incorporating a small inductor to better align the higher bands. My end fed is arranged as an inverted L and tends to radiate power in the plane of the wire having higher gain towards the front (away from the verticle section). Therefore, my end fed has lower rear gain and poor side gain. The antenna orientation works well for me into Europe but not so well over into the US.

Obviously the end fed is fixed so the solution to poor rear and side gain was the directional hex beam on a rotator. The hex beam has a front gain of potentially 10dBi. Other aricles have shown the hex beam performs very well compared to a Yagi and compares very well with commercial HF mini-beams.

The principle of the Hexbeam operation is not different to any other 2 element parasitic beam antenna. We have one approximately half wave driven element supplied with RF power from the transmitter. In addition there is a second approximately half wave element which is not driven but placed in close proximity to the driven element. As a result of the proximity, currents are induced in the second element which results in power being re-radiated from it. As the second element is not driven directly it is termed parasitic.

The relative magnitude and phase of the currents in the parasitic element results in the re-radiated power constructively and destructively combining with power from the driven element - hence the antenna becomes directional. In the case of the Hexbeam the mutual coupling between driven and parasitic elements is controlled by the spacing between elements and the gaps between the tips of the driver and parasitic reflector.

The center plate is constructed from a 300mm square piece of aluminium grade 6082T6 H30 5mm thick. Spreader arms are held in position with a pair of stainless steel U bolts and the center post in position with cast pipe base plate fitting for 35mm tube and a base diameter of 90mm. There is an identical pipe base fitting on the underside of the baseplate for a short section of aluminium tube to be inserted and subsequently coupled to the top of the rotator.

I am lucky enough to have a CNC machine with a bed about 850mm square so could machine the center plate. However, you could easily drill the plate with a hand drill it would just take a bit longer. The base plate was designed using Vectric Aspire software. I have provided a DropBox link to the Vectric design file, a scale PDF, a DXF design, a SVG design file and the GCODE for the CNC.

Link to files:

https://www.dropbox.com/sh/0801bttesie8n9r/AACwGRA...

Rather than inserting the spreader arms directly under the U bolts I got hold of a 1m length outside diameter 35mm aluminium tube and cut it into six sections one for each spreader arm. The aluminium tube sections are clamped under the U bolts and the fiberglass spreader arms slot into the aluminium tube sections.

The spreader arms are the key to the design and I thought long and hard about what to use. Initally I was considering three sections of glass reinforced tube slotted toether to form each arm. However, these tubes turned out to be quite expensive and I could only source them in 5m lengths resulting in quite a bit of expensive waste.

So in the end I decided on 5m long telescopic glass fibre windsock or flag poles, I bought all six poles for £116 from eBay.

The poles are 5m long but they only need to be 3.5m long so why did I just not buy the cheaper 4m poles. Well the last pole element is very thin so I bought the 5m poles and cut them down to 3.5m resulting in a much thicker more robust final section.

Note as described in the previous section the spreader arms are not inserted directly under the U bolts as the pressure would probably crush the glass fibre poles. Instead six 35mm OD aluminium tube sections are used under the U bolts and the spreader arms inserted - see the attached image.

Each spreader arm is pulled up into position using a support cord from the end of the arm to the top of the center post. I used 2mm Kevlar guy line. This line is a bit more expensive than cheap nylon but in my opinion is well worth the extra money as it is sure strong, does not stretch, is weather proof and very discrete. Each support cord is 3.25m long. There is also a single perimeter cord at the antenna front and another shorter cord about half way down the front two spreader arms. These two front cord just help keep the hex beam shape when the wire elements are added.

The support cords are attached to the end of the spreader poles using a simple S hook which I picked up in the home improvement store B&Q. The support cords are attached to the top of the center post using threaded cup hooks (see later section) also picked up from B&Q.

The center post I probably gave the most thought to .....

Each of the wire radiating elements connected to the center post via a pair of M5 nut/bolts and M5 ring wire connectors soldered to the radiating elements. All of the radiating elements are fed from the top of the post using RG58 coax. The coax first feeds the 20M element and then short sections of coax then feed all the subsequent elements down the post terminating in the 6M element. The coax enters the center post from the bottom with a grommet runs up the length of the post before exiting via another grommet and connecting to the 20M feed bolts.

The post itself is made from 32mm OD, 3mm thick and 100cm long clear Acrylic tube. My intitial thought was to run the sections of feed cable inside the tube and decided on a clear tube to make this easier. In the end I found trying to bolt the sections of connecting coax from the inside of the tube near impossible so decided in the end to mount the connecting sections of coax on the outside of the tube towards the rear of the antenna. The tube being clear still made it much easier to feed the bolts through from the inside. I used one of the spreader arm offcuts and a stiff piece of copper wire to hold the bolts so they could be fed through the mounting holes from the inside and a star washer on the inside allows the external bolt to be tightened without the bolt just spinning in the hole, the star washer grips the Acrylic from the inside. To attach the M5 ring connectors a second external M5 nylock bolt was used.

The 32mm tube was a little too small to fit directly into the post mount so I used a short section of the 35mm aluminium tube to slot into the post mount and then slot the 32mm clear post into the aluminium sleeve - see attached image.

In order to better weatherproof the external connections I covered all the connection with a liquid insulation tape which just paints on and dries in about an hour (not shown in the images).

Here are the distances of each band with respect to the bottom/base of the clear tube. These measurements are likely to change slightly if you use different spreader arms (to me) as the spreader arm radius of bend will be different :

6M 8cm

10m 16cm

12m 24cm

15m 35cm

17m 52cm

20m 94cm

I used a plastic pipe cap and some sealent to seal off the top of the center post tube preventing rain water from running down inside the tube.

The wire I used for both the driven elements and the reflector elements was 19 strand poly flex weave antenna wire which I sourced 100m of from eBay for about £49. This wire is fantastic for antenna construction as it is so easy to work with. It's main attribute is that it doesn't tangle or twist and it is very robust and very suitable for outdoor use. It is also very easy to cut and solders very well. I soldered the wire to the M5 ring connectors but you could crimp if you like. You can use other wire but I would recommend the poly flex weave. The wire lengths given below are assuming this wire is used and the lengths will vary slightly for other types of wire especially if the wire is not coated in an insulator.

The wire lengths given here are the lengths of the radiators and reflectors including the length of the M5 ring connector which can be considered to be part of the wire element.

The tip spacers are made from the same 2mm kevlar guy line used to support the spreader arms. Each driven element and reflector is terminated with an M5 ring connector. The ring connectors are connected to the tip spacers with small cable ties.

All dimensions in the table are metric and in cm

Note the (x2) for the driven elements as there are a pair of driven elements per band but only one reflector.

The wire sets are attached to the spreader arms using pipe clamps and small cable ties. The six pipe clamps are threaded onto the spreader arms before they are arched into position. Each pipe clamp should be the same distance from the center post along the spreader arm for each of the bands.

My technique was to attach the driven elements to the reflector and then thread the combined wire and tip spacer assembly through all six pipe clamp / cable tie assemblies and bolt the driven elements to the center post. Then move all the pipe clamps out evenly from the center until the wire is taught finally nipping up the pipe clamps but not so much as to crush the spreader arms. The entire wire / tip spacer conbination should sit horizonal when viewed from the side and so should sit in a flat plane.

The performance of the antenna has been really very good - although I should point out I have no other experience with directional antennas.

Compared to my 130ft end fed in the direction with the highest gain the Hex beam and end fed perform about the same although the noise floor of the Hex beam is lower. However, compared to the rear of the end fed or to the sides the Hex beam performs much better. Obviously, this is not suprising as the Hex beam has quite a bit of forward gain and can be rotated whereas the end fed is a bit deaf to the sides and rear.

So I generally use the end fed initially to spot a contact and figure out where they are from. Then spin the Hex beam around to the correct bearing and then switch over from end fed to Hex beam. I use the end fed exclusively for 40m and 80m as the Hex beam only suports 6m - 20m.