Author: Vin Robinson G4JTR

In the Spring of 2022, I experimented with a 40M long wire, ie a ½ wave on 80M, fed with a small commercial 49:1 unun transformer. This was positioned on the fence top, 1.8m above the flower bed 1.5m back from the footpath in front of my property. The first 1/3 sloped up at 45deg. to the top of the VHF pole on the gable end of my house and then southwards at 10M high to the tree at the end of the back garden. I describe this as an inverted hockey stick. This was the same path as my previous centre fed 30M long multi-band HF doublet. Fig. 1 shows the new antenna. I slowly tweaked its length so that eventually it showed a low swr on the harmonically related bands from 3.5 to 29MHz. Where the match was more than 2:1, my Yaesu FT-710 internal tuner match matched it easily. This applied mainly to the 10, 18 & 24MHz bands. The TX output went via the external LDG AT-200 tuner, normally in the unpowered straight thro’ setting. On 5.3 & 50MHz the tuner was powered up and easily matched these 2 bands.

The antenna is “X” number ½ waves, based on 3.57MHz, which are near resonant, as follows:-

3.57 x 1 = 3.57MHz
3.57 x 2 = 7.14MHz
3.57 x 3 = 10.71MHz
3.57 x 4 = 14.28MHz
3.57 x 5 = 17.85MHz
3.57 x 6 = 21.42MHz
3.57 x 7 = 24.99MHz
3.57 x 8 = 28.56MHz

The above is theoretical. The actual rigging of the wire changes the resonance points, so for example 1.2:1 on 24MHz, and 1.8:1 on 7.199MHz. & 1.0:1 on 7.0MHz, 2.5:1 on 28.9MHz.

The antenna is fed with 15M of RG213 coax which was laid across the garage roof and then pinned along the fence top to the unun. This is convenient as the feed point was easily accessible. I no longer have the old open wire or window line feeders hanging over the garden and waving about in the wind. After a while I added a 20m length counterpoise along the fence top southwards which appeared to make no difference at all.

The on air performance was about the same as my previously used open wire fed doublet and on the lower bands, the locally derived noise level from VDSL broadband on the overhead phone lines was about the same. Ie. dreadful! The noise floor on 3.5MHz c was about -75dBm in a 2.4KHz bandwidth.

So far, so good. I was on the air with a 10 band antenna performing as well as expected and after several months of use I was happy that it was working well. The solar cycle was improving, I started using the higher bands with some successful DX contacts. All SSB, of course, not the FT8, “easy DX mode!!!”

All went well until Club Member, Mike Dunstan, G8GYW, published an article in October 2023 PW entitled, “Building an unun for an EFHW Antenna” In that article, using a Nano VNA, he investigated the 3 different types of ferrite used, and the thermal performance of each. The article focussed on efficiency of the ferrite components when wound as a 49:1 auto-transformer. Like many operators, I had never given this a thought. I have previously used 1:1, 4:1. & 9:1 wound ferrite devices. At no point did I consider insertion loss, mismatch loss and overall efficiency. The truth is, of course, that any item placed between the TX antenna port and the antenna feed point will have an insertion loss and a mismatch loss, reducing efficiency of the whole system.

We are all familiar with using low loss coax to keep efficiency high, and ATUs although generally efficient and essential for non-resonant antennas, can occasionally lose quite a lot of power. The same applies to any wound ferrite component. These are commercially sold or home-made and used ubiquitously in most EFHW antenna systems, yet I have rarely seen in the specs for these devices, an efficiency plot versus frequency band width.

Mike’s VNA showed my home brew 9:1 balun, using the supposedly right ferrite material and form, was almost useless! My commercial 49:1 transformer turned out to be 74% efficient on 7.1MHz & only 52% efficient on 28.5 MHz! Not so good at all. So far I have only found one 49:1 manufacturer who actually shows efficiency graphs 3 to 30MHz, for their products, MyAntennas. https://myantennas.com/wp/

So, I wondered, how many other similarly deluded EFHW users are out there using inefficient antennas. It turns out that the huge upsurge in popularity of this antenna in the last 5 years has generated huge numbers of YouTube videos on EFHWs & 49:1 transformers. It appears the many of these are USA guys who show, “This is how I put up my antenna & how well it works” with videos of the VNA SWR plots showing perfect dips at the amateur band and of DX being worked. Many brilliantly show how to get it wrong! There are, however, some really excellent videos on the correct theory and best practice. Some of these are listed below. After many weeks of research, I have concluded the following.

1. The last few years has seen the solar cycle improve beyond expectation, fooling lots of people into thinking their wonderful new EFHW is working brilliantly!
2. The form factor of a ferrite device as well as its material are critical for high efficiency at the designed power level.
3. The type of 49:1 winding, is less important but 14 close wound turns tapped at 2 turns as an auto-transformer is easiest to construct and is efficient. The primary invariably has 100pF cap across to improve efficiency at the HF end but varying this value slightly can improve the result further. Mine has 150pF.
4. The easiest way to test efficiency is the make or buy 2 identical devices, and join them back to back and measure the power loss through both and divide by 2. I confirmed Mike’s VNA measurements using my Bird Thruline into a dummy load.
5. If using a VNA, the measurement need to be set up very carefully because of stray connection effects and results from measuring the “naked” ferrite on a bench may not be the same as the enclosed product. Enclosures are a “black art” Efficiencies in excess of 100% can be seen when things go wrong.
6. The easiest way to go for one of these antennas is to buy a proven commercial product. Most are either 100W or 400W rating. 85 to over 90% efficiency across 1.8 to 30MHz is normal with these products. MyAntennas is the only company I’ve found which shows an efficiency plot.
   A Uk Antennas unun 400W model, borrowed from a Club Member, encased and with backing plate and u bolts attached, was tested and gave a very good result of 92 & 85% efficiency at 7.1 & 28.5MHz respectively.
   I wound my own 49:1 tapped autotransformer, using a 2643251002 ferrite and boxed it as a 100W device. This gave 88 % & 84.2% efficiency at 7.1 & 28.5MHz respectively which I decided was good enough.
7. I also learnt, do not tweak the antenna length to get the best SWR spread until you have completely finished positioning all components and finished all other adjustments. My finished antenna is shown in Fig. 1 below when erected in mid. 2022.

Fig 1. The EFHW antenna configuration

Adding the 1.9MHz band to the Antenna setup

The 40M long antenna wire is ¼ wave on 1.9MHz, which can be used on this band if a suitable counterpoise is installed. As previously stated, I had run a 20M counterpoise down the fence, connected to the earth side of the SO-239 socket on the transformer primary input. It made little difference to the 3.5 to 29MHz SWR curves. It was essential however, when used on 1.9MHz. To do this I needed a bypass arrangement to bypass the 49:1 transformer. Fig. 2. Shows this. For remote switching I would need to switch both the input and output of the transformer box, requiring a suitable DPDT relay system and power. As I rarely use this band, I opted for an easy but unusual configuration. I installed a separate coax to the end of the antenna wire. The counterpoise was permanently connected to the screen of the feeder. The main coax to the 49:1 was left in place not connected to the counterpoise. All I needed was to switch the end of the antenna wire to the 49:1 terminal or the 2nd coax. Initially I did this by moving a banana plug between 2 x 4mm sockets. I tried a simple relay but the added capacitance messed up the SWR spread when using the 49:1 EFHW. I ended up with a miniature toggle switch, a DPDT type to spread the current. (see Pic) The internal capacitance was negligible and the SWR spread was not affected. Apart from 5MHz, no high SWR is present at the feed point so I am fairly certain that very high RF voltages are not present at the switch contacts. So far, I have not experienced any suggestions of flashover. All I have to do to use 160M is the go out of the front door to the fence, part the polythene cladding of the box and push the toggle switch, and hope I took my door keys with me!

The optimum solution for the 160m addition is to use a low capacitance DPDT relay designed for RF. Then build the 49:1, the relay and all external connections including relay power into 1 enclosure, keeping all internal leads short. Then set it up and test it and only then tweak the SWR spread, then tweak the counterpoise. It’s acts as a sort of fine tuning for the earthy side of the radio system, see below.

Fig. 3 shows a pic of the fence top arrangements. Please excuse the compost bag waterproofing! The reverse side is black and when folded down over the connections, is waterproof and is an inconspicuous cover allowing air to circulate.

I had fun and games with the 1.9MHz SWR match. Initially it was way off but the LDG tuner just handled it. I lengthened the counterpoise to about 30M but the match got worse. I started to shorten it and at 24.5m the SWR fell to 1.0 around 1915KHz. With a steep rise each side, ie a high Q match but very useable. Looking at the earthy side of the system, the counterpoise is connected to the 2nd coax screen, which is connected to the LDG ATU earth and shack earth wire and is also connected to the 49:1 un-terminated coax screen etc so I guess tweaking the counterpoise length acts as a fine tune to a mass of earth wiring which together provides the ¼ wave antenna with a resonant system. The results on 160 have been really good, working all around the UK in the evenings.

EMF calculations using the RSGB web App

I modeled this as an inverted L. I do not use this antenna for 50MHz, so using 28.85MHz as the worst case frequency and using only SSB at 100W for 3 minute overs, the horizontal compliance distance is 1.1M. The feed point is about 1.5 away from the public footpath outside the front drive so I deem the antenna compliant.

Conclusions

1. I have an 11 band antenna which is near resonant on 9 amateur bands and works well on 5MHz, 50MHz yet to be assessed, if necessary.
2. Except for 5 & 50MHz, where the swr requires a slight tweak, the internal ATU in my FT-710 handles the match well. The LDG ATU handles 5 & 50MHz.
3. The antenna length is easy to tune without a VNA. (remember those things called a Twin meter VSWR bridge??)
4. Using an efficient 49:1 device is essential and buying from the right source, especially for 400W, or making and enclosing one using 2643251002 ferrite for 100W service is important.
5. The addition of 1.9MHz is a straight forward for fence top end fed systems. The dual coax version in my case is a simple work around but the optimal solution using shack powered coax relays would be preferable.
6. I am the only person I know who feeds one antenna with 2 separate feeders! and I also I have 2 antennas which use the same piece of wire at the same time! This is known as a G4JTR Bodge. But it works.

Acknowledgements

Many thanks to Mike Dunstan, G8GYW, who has helped me understand much of the above and has provided me with many efficiency measurements using the NanoVNA on several 49:1 ununs. This has led to a repeatable process for doing this, giving results confirmed by back to back power meter measurements.
Thanks to Brett, 2E0HFW for loan of the efficient UKAntennas 400W unun and another locally made unun.

References

There are hundreds of YT videos, these below are some of the better ones and there are many more.

• https://www.youtube.com/watch?v=TZjY2Jim7fY
• https://www.youtube.com/watch?v=Xe0wvbOQeok plus many more by Colin MM0OPX
• https://www.youtube.com/watch?v=xfqlun3bdI0
• https://www.youtube.com/watch?v=iMbN_uDeyU0 – start about 15mins into to the vid.