Thanks for doing these experiments and making videos Colin. Thinking about this, the current is high in the primary and low in the secondary. Perhaps wind the two turn primary with the heavy wire and the rest with thinner wire soldered at the tap point so it is still autotransformer style. No need to use and fight with heavy wire for the low current part of the winding. Also winding the primary with a little spacing reduces current crowding which increases losses in the wire. Space the wire windings one or perhaps two wire diameters to reduce wire loss. 73 de w6akb.
Just a bit of trivia, Kapton tape which is great for high temperatures and high voltages can be wrapped around the cores and then you can use bare copper wire as long as you keep the turns apart. Plenty of fixed electrical wiring uses solid copper conductors so very easy to source. Something I have tried is to space the stacked cores slightly apart, to increase the surface area for cooling. The cores are very expensive lately so I prefer not to superglue them either. 73
Interesting… ferrite isn’t that electrically conductive anyway - it has to be like that or you would get eddy currents in the core… I wonder if tape is even required?
Excellent. You are right there are little to no searchable videos on how to do this on youtube. You verbalized and explained what you were doing very well. I am a blind ham so I have two questions that weren't obvious from the narration: How many cores are you using? Two? And where exactly does the cap go? Between the center of the coax and ground or between center and the antenna or between ground and antenna? Thanks for taking the time to show this process and talk through it so clearly. The EFHW antenna is such a simple and effective option but a 49:1 transformer is ridiculously expensive to buy online and as you say, nothing really on youtube explaining how to build your own transformer. This will be invaluable for my upcoming antenna projects this spring and summer. Hope to catch you on the air one of these days.
Hi Colin, thanks for comment and feedback. I will keep this in mind for future videos. This is a 3 core stack of FT240-52 and the capacitor goes across the centre pin and sheild on the primary side.
Nice Colin. The May/June 2024 issue of QEX has an excellent article on Optimizing EFHW UNUN's by VE2AZX. And, he references your previous video concerning the use of 43 core material. Thanks!
@@MM0OPXFieldRadio The author of the QEX article tested using flat copper strip for the primary of the autotransformer (since it's at high current) to maximally couple to the core. Small diameter wire for the secondary windings worked best (to lesson the capacitance).
Enjoy your posts, Colin. But, as an old time engineer, I have to say that tack soldering like you have done here is bad practice. This is because under stress the solder can fracture and the joint will separate. It is basic practice to make a solid mechanical join between the two wires before soldering them. In this case, creating a small half loop in the wire during winding creates a raised area that you can later scrape clean and pass the tap wire through that loop and bend it back on itself, snug up the connection with a squeeze of your needle nose pliers, and then solder the connection. In this way, you minimize mechanical stresses on the joint where the solder can be fatigued and fail. Think of it as soft solder makes a poor glue.🙂
Yes, I would completely agree with that. I'm well aware that solder isnt designed for mechanical strength. Will I change my existing transformer? No, as there isnt enough forces created to split the joint in the short or medium term and its never up long enough. Any future Autotransformers will definitely use this method 👍
Thank you for the demonstration. Good thing about the tapped vs bifilar is one can reuse left over clippings. BTW: did you ever try nail polish remover to remove the enamel?
Do you know where to find this box ? I mounted mine in a larger tube with vent holes. I’m also curious what soldering iron that is ? I couldn’t tell if it was cordless or not. Tnx Colin. You did a great job showing exactly what to do.
I get these boxes from Direct Trade Supplies here in the UK. The Iron runs on lighter gas (butane I think) its 125w Portasol Superpro. Its made under different brands including Snap-On but more expensive for nothing.
Hey Colin, I love your videos! I wonder if you have any insight as to why everyone uses an autotransformer design for EFHW antenna's. I recently did an experiment and wound a 49:1 with seperate primary/secondary windings, used a counterpoise on the secondary and found it to work very well, without the common mode issues from RF on the coax shield. The antenna is definitely quieter and doesn't bring anywhere near as much RF back into the shack. Just curious why everyone uses an autotransformer, as I'm extremely pleased with the antenna/counterpoise decoupled from the coax shield. Obviously, this makes a counterpoise mandatory. But if you're using a counterpoise anyway, or have common mode issues, this seems like a better way to go. But I'm curious why I can't find any examples of other people trying this.
@@MM0OPXFieldRadio As someone who, when I was a new ham, blew the front-end out of my radio with an ungrounded dipole, I can definitely appreciate this! I do see the value in keeping things simple. In my setup I grounded the counterpoise at the transformer through a choke (I wanted to keep the RF current on the counterpoise to avoid ground loss), but I realize that's a bunch of extra complexity.
Its because the winding acts as both primary and secondary winding and connected electrically. In a standard transformer primary and secondary are separated. Technically a normal Twisted primary EFHW is also an Autotransformer.
A transformer where the peimary and secondary are electrically interconnected, as opposed to the more common type where they are electrically separate windings. Either type will do the job in this application.
I've been wanting an end fed portable antenna for quite sometime now but I can't seem to get a wire working. I have a 49/1 transformer and had up to 160 feet of wire as the element. I spent the day putting this antenna up and trimming it. I went from 160 feet to just over 100 and no way no how could I get that antenna to work not even tuning it with my MFJ-998 1k5KW auto tuner. I through the project in the junk drawer for another time. I tried again this time using 67.5 feet of wire to make the before mentioned portable end fed but again the antenna won't tune no way no how. The 49/1 transformer appears to be built properly. Are thes transformers wire dependent? Mine is wound with number 12 stranded copper house wire complete with the capacitor. I have since disassembled it and have ordered 14AUG magnet wire, some new capacitors and have the cores taken apart as they were taped together. These are 242/43 mixes I believe. Again I've tried winding my own but couldn't get an antenna to work. I have some friends with purchased end feds from My antenna in the states and they love them. Is there some magic ingredient I'm missing here? TNX and 73: Mike DE VO1OP
One idea that may help is to test your transformer before you connect the antenna. At least you then know when the transformer is good. I have tested them by connecting a 6W 3300 Ohm resistor between the antenna and ground terminals. You can then connect an antenna analyser or NanoVNA to the transciever socket to check for low VSWR. If you don't have an analyser, just use your transciever set to low power (5W or less). You should get a low VSWR. For these types of broadband transformers, I am usually happy if VSWR is below about 1.5:1.
I can only agree with Steve. Something definitely not right. Get it right on the bench first. I had a similar issue and the issue was there was no continuity between my twisted primary.
Fantastic tutorial. I did one with the bifilar wind…. This looks easier. Has anybody nano VNA’ed the 2 types and compared? My guess is at HF would be much of a muchness. 73 de VK2AOE
I have done some tests, long time ago, as I recall the auto transformer slightly favoured the higher bands 14 to 29 Mhz, bifilar for 7 to 14 but there was only 1dB in it. But there are a lot of variables here, type of core, size, wire thickness so it's a bit of a suck it and see process.
I find looping and twisting the wire at the tapping point is far less bother than a soldered tap. Just one continuous length of wire with a twisted section for the tap. I normally build the autotransformer design like yours with the turns spread out over most of the core. The capacitor choice is interesting. 'Conventional wisdom' as in 'copying what others do... lol!' suggests a 3KV rating, but since the capacitor is across the low voltage 50 ohm winding and your radio is likewise also connected across it,the use of a 3KV rated part is total overkill. There are no high voltages present on the primary, Wrapping the cores with Plumbers PTFE tape or even PVC electrician's tape can be useful if they have sharp edges. Good luck with your online presentations. Al GM1SXX.
Thanks Al. Totally agree on the Capacitor. These are nice quality, really small and cheap so I've just stuck with them. I don't understand the folks with the huge doorknob capacitors. Im going to do a bit more experimental with the 1002 core as an Autotransformer. Cheers.
Here’s another way to make the tap Colin. Measure out an extra 10 centimeters to allow for the tap. Calculate the amount of wire that 2 turns requires. Add 10cm to that length. Fold and twist for 5 centimeters (this was the extra 10 centimeters we added first, plus 5cm for the tail). Scrape the enamel off and solder at high temp with flux to burn off all the enamel at the crotch of the twist. Butt the crotch up against the toroid/s and wind 2 turns that way, 12 turns this way. Trim as you like. Been winding tapped cores like this since for ever 73 John
