End Fed Half Wave (EFHW) Antenna

 

 Ever since I saw a video of someone with this antenna operating portable on 20 Watts I have been compelled to build one. He was making contacts all over the place with it just slung over a couple of trees. This is a 1/2 wave resonant antenna for 40m, but should also be good at the harmonic frequencies too, making it somewhat multiband. There are lots of resources on line about these antennas and I recommend some research is done before attempting the build. The 49:1 unun in particular needs to be studied because it isn't straight forward. The 9:1 and 16:1 ununs and transformer types I have employed before will not suffice for this project. They will be too lossy and inefficient at this ratio with a resonant antenna.  Capacitors need to be employed to improve SWR and performance on the higher bands. (counter-acting the inductance at higher frequencies)

(Diagram of 49:1 unun)

I am only using low power so just one toroid core should suffice. It seems that the higher the power, the more cores you will need, and the transformers can get hot. I have seen people opt for the FT240-31 core, but after researching the differences between type 31 and type 43, I decided to go with the recommended type 43 core. The capacitor required is a 100pF high voltage type which I found to be expensive compared to the 220pF of similar spec. I acquired some 220pF 15kV ones. If I connect two in series the capacitance will be halved and the working voltage will be doubled resulting in a 110pF at 30kV rating (belt and braces). Also, if your wallet allows, the theory is that using two toroids makes the transformer more efficient, especially at lower frequencies. Your choice. Theory also dictates that a good ground is required, however, with the right length of coax, the coax screen can act as a counterpoise because the current flow will be minimal at resonance. Some experimentation is required here. It might be that a counterpoise wire will be required on the ground connection, anything from 1m to 10m long. A good ground plane system can also be used if available. My finished project can be seen below.

I am assuming it will be good up to 50 Watts, but if the SWR drifts at higher power, this could be a sign of over-heating.

The box will have to be at least 100mm square. Mine is about 115mm square. There is a terminal post on the left for the antenna radiator. A link was made to a terminal post on the right to accomodate a ground / counterpoise connection.The two capacitors can be seen connected at the SO-239 socket, with some hot glue as a make-shift insulator on the joint.  I have left two large holes at the bottom for cooling / ventilation. I need to cover these with some kind of gauze to keep nasties out. I initially intend to house the box indoors. Just needs the 20m radiator wire erecting and we should be good to go. I want to construct a pulley and weight arrangement at the far end of the wire to maintain tension and prevent breakage in high winds.

Results and Testing

I can just fit a 20m wire from near the top of the roof to the diagonal corner of the garden. There is about 3m of radiating wire in the loft making connection to the box and counterpoise. A post was erected in the corner with the pulley attached at the top.

A piece of nylon rope was threaded through the pulley and connected to the wire.

The weight, a plastic bottle with just enough water to hold the wire tight, was tied to the rope and tension was achieved.

The theory is that any extra tension on the rope from high winds, temperature expansion / contraction or perching birds will allow the weight to rise and fall accordingly, relieving the strain on the wire. I think the antenna would perform much better if higher from the ground. At the moment it stands at about 12 feet at its lowest, rising to about 25 feet. 

Results

After experimenting with various counterpoises, 2m, 3m and longer lengths, I settled on a 1m length of wire. I really didn't know where to check the SWR for adjusting, so I decided to check at 14MHz, which is a full wavelength. The reading was about 1.5 after snipping about 6 inches off the radiating wire. Further cutting of about 2 inches caused the SWR to rise a small amount, so I decided not to trim the wire any more. I also experimented with varying the capacitance value from 220pF to 110pF, 73pF and no capcaitor at all. Some interesting and weird results were obtained. Using 220pF flattened the SWR peaks, but raised the overall SWR over the bands. 73pF raised all the readings. So I settled with the design spec of 110pF. At one point I was also getting gradually rising and falling SWR values at the higher frequencies which I couldn't explain. I guess it was something to do with changing the capacitor values, or a dodgy solder connection. The analyser results pretty much reflect the results from the radio SWR readings. Obviously, looking at the graphs, the SWR will be lowest where the impedance is 50 ohms.

(Analyser results - 1.8MHz to 30MHz sweep)

Final SWR Readings

1.8MHz = 2.9

3.5MHz = >10 (useless)

  7MHz  = 1.9

10MHz  = 1.6

14MHz  = 2.1

18MHz  = >10 (useless)

21MHz  = 2.2

24MHz  = 3

28MHz  = 1.7

Conclusions

The final results I obtained are far from perfect with respect to SWR, but the antenna is very effective when used with the internal ATU. It was immediately apparent that the received signals sounded much better and clearer compared to my other antennas. It would also seem that the antenna works better at long distances rather than closer to home, judging by the signal reports. Contacts with USA for example, are much easier to get now, but I can still make contacts across the UK. I will have to do without the 3.5 and 18MHz bands for now until I come up with a solution. The problems must lie with the transformer build. I might try a different approach in the future. Generally very impressed with this antenna, once again proving that the simplest methods are usually the best. 

 

 

Update - September 2023

Just for the hell of it I decided to take apart the 49:1 unun and add another ferrite core to see if there would be any difference. Apart from a possible increase in power handling ability, its hard to see any real improvement with performance. No improvement with the higher SWR readings on the lower frequencies. However, I can now tune the 18Mhz band. I found a small improvement by disconnecting the counterpoise. I also noticed that after moving things around in the loft, there was a change in readings. This reinforces the fact that you shouldn't have any objects close to your antenna. My situation isn't ideal because part of my antenna is in the loft! On the positive side, all the frequency bands above 3.5MHz fall below the 3:1 threshold. This is still my best performing antenna. So my advice would be that if you are only running up to about 50 Watts, then I think you would be OK with just one ferrite. Above this, you probably need two. Below is the full band sweep from the antenna analyser.