Martin Waller G0PJO builds an unusual antenna for the 40m band – an interesting project requiring both electrical and mechanical skills!

Over the years I have always enjoyed building antennas. The first antenna that I built was a 144MHz Ring-Base Antenna designed by F C Judd G2BCX and published in Practical Wireless back in September 1982. I still have it in my shed and it still works perfectly. One antenna that has always caught my eye has been the Isotron as sold by the Bilal Company in America. There doesn’t seem to be much published about them. In my library I have 16 books devoted to antennas and only two refer to the design, one being a review by Steve Nichols G0KYA and the other being a re-work of the same review. Searching around the internet turns up a few references – see below – and a couple of those references provided details for home construction. In particular, Ludovic Amathieu F5SWN provides details here:

www.f5swn.fr/?Radioamateur-en-ville/Isotron

The urge to build, and experiment, then took over. One limitation of Isotron antennas is that they are single-band antennas but, nevertheless, I wanted to try out the concept and see how it worked. The design here is for 40m – for other bands, you will need to figure out your own dimensions from the references I have given or other sources.

Required Materials

For the build I needed two lengths of aluminium sheet, threaded rod, stiff insulating material, 100mm diameter plastic tube, a few nuts and bolts, and some wire. All the materials, except for the stiff insulating material, were bought from the local DIY store. For the insulating material I used a couple of cheap nylon chopping boards sourced from the local supermarket.

Construction

I started the build with the two plates that form the capacitor. The design provides all the dimensions necessary to draw out an accurate pattern. I did this on a few sheets of A4 2mm square graph paper stuck together as necessary to obtain the required size. I then used cheap sticks of paper glue to stick each pattern to the aluminium sheet ensuring that one edge of the pattern aligned with the edge of the sheet to cut down the number of required cuts.

I chose to cut the aluminium using a standard jigsaw with the appropriate blade. To ensure that the edges were as straight as possible I clamped a steel rule to the sheets and used that as a guide. The sheet cut cleanly but do wear safety glasses while doing this because the jigsaw throws up lots of waste material. I then drilled small pilot holes where required.

The bending of the sheets proved to be the interesting and challenging bit. I wanted the bends to be accurate and sharp. After some thought I decided to clamp the sheet in my workmate with a scrap offcut of floor tile fractionally below the line of the bend on one side and a length of wood on the other side. The tile provided a solid object that was not going to give way when any force was applied. I thought about using wood but was worried that the edge may get damaged and hence spoil the bend. Then using a flat piece of wood to spread the force and a school protractor, I bent the sheet to approximately the right angle. I was never going to get the angle spot on using this technique but if the bends produced were clean and sharp, then further tweaks would be easy. Repeating this for all required bends produced a clean result that I was very pleased with. I then repeated the process with the second sheet, Fig. 1.

I then marked up the two side insulators. Again, I covered each part with 2mm square graph paper, cut with a jigsaw and drilled all pilot holes. Nylon chopping boards are very easy to work with hand tools and provide a very satisfactory result.

At this point I assembled the two plates and two insulators, using M3 nuts and bolts, to form the main loop. The loop came together quite readily with only minor tweak to the folds, Fig. 2. I left all the paper on as I wasn’t sure if I’d be needing further cuts or holes.

The construction and mounting of the coil required some more thought. The first threaded rod goes from the top of the loop down to the top of the coil and then the second threaded rod reflects this below the coil. The problem was how to join the rod to the coil former in a secure manner. I decided to cut two nylon disks slightly larger than the diameter of the former with the intention of using plastic cable ties to secure the join. The problem with this idea was that I would still need access to the centre of the former when winding and adjusting the coil. To allow access I cut out three segments from each disk and ended up with the shape as shown in the photo, Fig. 3.

Again, all construction of the final disk and cut outs was first drawn on 2mm square graph paper and stuck on the nylon cutting board before presenting the material to the saw.

I then cut the main coil forming tube to length. To ensure a parallel cut I placed a band of masking tape around the tube approximately where I wanted the cut. I then placed the tube vertically on the table and using a glass and a selection of mats to obtain the required height, held a pencil point against the masking tape and rotated the tube through 360° to provide my cutting line. The tube was then cut with a small tenon saw. I then test assembled the two ends and support rods, Fig. 4, with each end looking as shown in the photo, Fig. 5.

Next, I wound the main coil. I used a couple of nylon nuts and bolts to hold the coil in place and cable ties to secure it, Fig. 6.

In the final assembly the ends of the coil were secured to the threaded rods using solder tags. The antenna is fed via two further turns of wire that sit immediately below the main coils. These are then connected, via a short length of coaxial cable to the SO259 socket. The final assembly with simple support bars looked as shown in Fig. 7.

Tuning

To tune the antenna, I mounted it on my workbench in the middle of the garden. I understood from related literature that it needed a good earth, so I connected the end of the bottom rod to a substation, domestic type, earth rod. Then, with the aid of an antenna analyser, proceeded to measure and adjust, which basically meant removing one turn of wire at a time from the main loop and re-measuring. This was a little fiddly but wasn’t too difficult if done with patience. The original design suggested starting with 18 turns. I’m guessing that this is meant to be too many because it’s easier to remove turns than add them! It didn’t take long before I managed to get a nice dip just where I wanted it, Fig. 8.

Once I was happy with the number of turns I secured the coil windings with more cable ties. The feeding coil was held in place with masking tape just in case I needed further fine adjustment.

Did it Work?

I have to admit to being a little sceptical about the whole design but I connected up my transceiver and, to my joy, there were many stations to be heard with very little noise. Putting out a CQ. the Reverse Beacon Network quickly confirmed that my signal was getting out too, Fig. 9.

Since then I’ve using it to work around Europe with as little as 10W and I’ve been very pleased with it. The article I read before the build suggested that the received signal strength will be a few S-points down and I certainly found this to be true when compared to my Dutch design end-fed antenna. But overall, if you are pushed for space, then the Isotron is a workable solution.

Final Thoughts

Construction was straightforward but, on reflection, if I were to build a second, I’d use thicker aluminium sheet because the weight of the feeder coaxial cable was, at times, enough to slightly bend the bottom plate. I’m also a little concerned about wind damage because it does have quite a large cross-sectional area and I can image a decent blow possibly causing some damage. Again, this problem could be solved by using thicker aluminium sheet.

Further References

www.dc4fs.de/microvert.pdf

www.k1cra.com/ProductDetails.asp?ProductCode=ISOT10

https://tinyurl.com/ycw6d7ab

This article was featured in the December 2018 issue of Practical Wireless