It was a significant challenge as I am an HF CW operator and the only aerials I had built before were made of wire for use on the HF Bands. I had the engineering skills and the tools (hammer, screwdriver, drill, hacksaw, file, mole grips and vice) to build the helix but honestly I never had a clue about the aerial itself and certainly did not have the equipment to set it up and test it.

My first task was to ask the UHF operators at my club if any of them had ever built a Helix aerial before and to my dismay none had as they all preferred the Yagi aerial for 13cms. Disappointed by this discovery, my next task was to turn to the internet and “Google” “Helix Antenna”. This search turned up a myriad of designs but nothing really that suited my needs as most were for 8 turn wire Helix’s, though I did learn a lot about this type of aerial.

All the Helix designs I had looked at had a coil circumference of 1 wavelength but there were many different techniques for tuning the aerial to the 50 ohms impedance required for feeding it. I did come across a paper by L B Cebick the antenna Guru which recommended that the circumference of the coil be one and a quarter wavelengths instead of 1 wavelength as this gave the antenna more gain.

Now I had a good idea of how I was going to build the aerial all that was left was to work out the measurements and what materials I was going to use to build it. I had decided to use 5mm brake pipe for the coil as this was easy to bend, would be self-supporting and make the aerial broadband. For the ground plane, by using 3mm aluminium plate it would be possible to incorporate the mounting point for the aerial with the ground plane as one piece thus making it easier to fabricate. For a cover for the aerial, I found a 160mm inside diameter white resin tube used for kitchen ducting and dutifully checked it out in the microwave and it seemed suitable for the purpose.

I was designing the aerial for a centre frequency of 2250MHz which gave me a wavelength of 130mm which meant that the circumference of the coil was 162.5mm thus giving a coil diameter of 51.7mm, with this information I could set about marking out, cutting and drilling the ground plane/fixing plate. The centre of the “N” type socket had to be positioned on the radius of the coil and the minimum diameter of the ground plane had to be 100mm but I made this 160mm to be a tight fit in the cover I had chosen.

With the ground plane finished, it was time to sort out the coil which was going to be the delicate bit as the turns had to be quarter of a wavelength apart and I had to find a suitable former to wind the coil.

It was time to measure and cut the length of brake pipe which was 162.5mm X 3.5 + 10mm for fixing to the “N” type socket. After I had cut the brake pipe, I marked out the position of a full turn on the brake pipe, as this was important after I had wound the coil as all marks had to line up in a straight line vertically down the coil to ensure it was the correct diameter. I found a 40mm OD plastic tube and wrapped it with masking tape and marked it up as shown in the diagram below so it gave me the position where the coil had to be as I was winding it. I had left a 10mm section at the end of the tube before the start of the winding which was for clamping the brake pipe to the plastic tube with Mole Grips. After the coil was wound it was a simple adjustment to line up the marks on the coil and the 10mm end piece was bent so that the coil could be soldered to the “N” type socket.

I was still undecided about the technique I was going use to tune the aerial and was given access to a professionally built Helix and decided to copy the method they had used on their Helix so made the tuning plate and pop riveted it to the ground plane of the aerial. The completed aerial including the top cover I had made was then passed to Bob G7AVU, he had earlier agreed to tune the aerial for me as he had all the relevant equipment needed for the job and UHF was his forte.

This was the point where the boys were separated from the men or the HF men were sorted out from the UHF men as Bob could not tune the aerial. I had used a small piece of fiberglass rod as a support on the antenna not knowing that this was a big NO NO at UHF frequencies and also I had fitted the Helix too close to the ground plane. Bob removed the offending fiberglass and raised the Helix and then tuned the aerial. It proved to be very broadband as I had designed it for, covering 2200MHz to 2300MHz (perfect for the requirement) with SWR of 1:1.2 and SWR of 1:1 on the centre frequency of 2250MHz. When Bob fitted the cover on the Helix, the SWR rose to 1:2.8 which showed that the cover was reacting at UHF frequencies so I have had to replace this with something that is transparent at these high frequencies.

I would like to sincerely thank Bob G7AVU for all the advice, help and support that he has given me during the building of this Helix aerial as I have learned a lot in the process and made up my mind - "I am sticking to HF!".