Such an antenna was always my most want to have item for portable operations and SOTA. I always drooled around the hyEndfed antennas but would never pay over 100 EUR for such an antenna. In the end, it’s just a transformer and some wire. I respect the quality and all, but amateur radio is about building your own items too.
Building such an antenna is quite easy. If you follow some important rules. I made a couple of mistakes and I want to share them with you, so you don’t have to go through the same trouble I did.
- Use FT mix 43 toroids. FT140-43 works well. First I used T130-2 but it did not work!
- Tune your antenna with the coil and the appendix (part after the coil) attached!
- Take longer wire lengths than described in the original article! My 20m wire portion is 10.28m, and the appendix is 2m long. Maybe because I am using SOTAbeams ultralight antenna wire or my coil material reacting with the coil
Let’s get started…
I’ll try to keep it short and simple. I followed these instructions:
The winding of the toroid is quite easy, just follow the pictures in the original article. I used 100pF capacitors and it works well. Make sure you take more wire for winding. I take a piece of wire, wind it two times around the core and then mark the endpoint. Unwind it and measure the length used and divide it through 2. Then you’ll have the length for one winding. Multiply that with the windings you need and add another 5cm for the cross-section and let’s say 1% to the overall length for the soldering connections. It should be enough. Remember, every time the wire goes through the center of the core, it counts as one turn! I winded plumbers tape around the core to prevent sparking. Although I only use 50W maximum for now, its more a precaution. Hams say, this FT140-43 should go up to 100W SSB without a problem.
For this antenna, we need a coil of 34uH. Since I own a 3d printer, I did not want to use parts of a PVC pipe, rather a nice and narrow design. For designing the coil, I used those tools. I used both to get a value that is somehow similar on both websites:
With the length and diameter set, I designed a form where I can wind the wire up. For that, I ordered 0,6mm enameled copper wire. When winding, make sure you wind the coil tight and keep it under tension all the times, else the wire will unwind and you’ll have a mess!
There is no rocket science behind that. Coils were always scientific for me, but if you follow those instructions, it works!
The transformer box
I created the transformer box with fusion. Using ultra thin banana plugs I ordered from Banggood, I could design the box to fit those plugs. Also, with the small size of the box, accommodating the coil, it requires almost no place in your backpack. I like that strain relief too, my previous transformers were ugly but this one… Well, I am really proud of it 😉
The banana plugs cost 0,60$ a pair and can be ordered here:
I can really recommend those plugs, they are great for linked dipoles too.
Such a small antenna combination is great for portable QRP operations and SOTA.
Choke that feedline!
Make sure you choke your feedline before it goes into your antenna analyzer and your radio later on. Although I get slightly better SWR when removing the choke on certain bands, you should choke the feedline before it enters your radio. There are different opinions on where to place the choke and if to use at all. Some say you don’t need to choke endfed halfwaves, only random wires, then some say you have to. Then they say, you need to choke right at the transformer to keep your feedline from radiating. Then others say you need to choke at your radio’s site so the feedline can act as a counterpoise… Well, that’s hamradio, there is no definite solution. You have to try it out. I use my choke on the radio’s site and it works OK
For choking I use 10 turns of RG174 through a FT130-43
Tuning the antenna
This was the most frustrating part for me, as I cut the wire too short initially. And I was tuning the 20m part without the coil attached. When tuned and attaching the coil everything went berserk. So, I had to attach a piece of wire to tune it again. I never like to extend wires but that’s how it goes. I don’t want to throw away 10.1m of wire just because I need to add 20cm!
Anyway, tune for desired resonance on 20m and check your 10m band. It should have good SWR too. For tuning the 20m you’ll have to either trim from the side where the connection to the transformer is or on the side to the coil. That is a time-consuming effort but it’s worth it!
Once the 20m band is tuned, now go for the 40m band. But be warned, the bandwidth for the 40m band is very narrow, so make your decision where you want your most resonant portion. Although the antenna will also work on 1.6 SWR and you will not experience a difference from 1.6 to 1.3, still it should be taken into consideration. I folded back the upper portion of the wire through a shrink tube to get resonant. I can adjust the wire if needed later on by just sliding it up and down.
It’s not a 1.1 SWR but for me it is acceptable. I don’t have to carry a tuner around anymore for SOTA activations when speed and simplicity is the key. Also, I measured the SWR in my yard, where powerlines and a lot of QRM is present, so this may have worsened the SWR. I have to re-measure it in the next use when in the field.
3d model download
You can download the 3d printing files from this link:
Field testing the antenna
Yesterday I went for a quick SOTA activation to try out the wire. Although the conditions on the 20m band were horrible, I could make a QSO with England. When I plugged in the antenna I was astonished about the loudness of the signals! They all came in so strong and clear, quite the opposite when using my 26m random wire hooked up on a tuner. Very different. The setup is quick and easy, the 40m band was full of Russians, I could not find a free spot to call CQ for SOTA.
All in all, I am very satisfied with the result!
If you have any questions, let me know!