End Fed Half Wave Antenna
I have built my first end fed half wave antenna, and it seemst to be working quite well at least on 20m. I’m trying to explain what I was doing; if there are mistakes in my understanding and explanations, please let me know. Here is what I did.
If I understand correctly, an EFHW is the same as a regular dipole, except that it is fed at the end, as the name suggests, instead of in the middle. At the ends of the antenna, the current is nearly zero, and the voltage is high. Therefore, the impedance at the end is a lot higher than in the middle of the antenna wire. So to use a 50 Ohm feedline, we will need a transformer. The common choice seems to be either a ratio of 49:1, or 64:1. I was going for 49:1. This means one only needs a transformer with a ratio of windings of 7:1. While in theory that is exceedingly simple, there are tricks to be found in the depths of the internet on how to effectively wind a transformer for this purpose. I found one here: https://elginradio.files.wordpress.com/2017/10/49-1-unun.jpg?w=500. I used a FT140-43 toroid to wind 2 primary and 14 secondary turns. This used up just about 1m of enameled copper wire. This I then put into a small enclosure that I had lying around, of about 2.5x10x6cm.
I used a BNC female connector for the feedline connection. The antenna wire is connected via a screw at the top. The screw is connected to the output of the transformer with a small wire lug. There is also a connection to ground via a srew.
The wire is a AWG26 piece of wire that I also had lying around. It was too short, so I soldered another piece to it. No idea how long that will last, but it seems pretty good. The wire is about 10m long to get me an antenna for the 20m band (as one would expect).
It turned out to do pretty well. The VSWR using around 12m of RG8X as feedline looked like this:
One of the ends of the wire has a lug soldered to it to go to the matching unit, the other has a simple wire connector. Bot have metal thimbles such as these from The Wireman. I used zip ties and some hot glue to secure the wire in place at the thimbles. I made another wire to connect to the first one with the matching connector, so that I can make a 40m EFHW from the antenna in cases where I have enough space.
This turns out to be a very nice antenna, for really little money. In total, I would estimate about $10-$13 for the matching unit, if you have to buy everything. If you happen to have a few parts, then it’s cheaper in the sense of money you have to spend right now. For me it was about $4 I had to spend in addition to the stuff I had lingering around in some boxes anyway. The total weight of the antenna is around 120g. I am looking forward to take this outdoors, once the conditions here get better.