How to wind an End Fed Half Wave Autotransformer

Demonstration on how to wind an End Fed Half Wave Autotransformer.
Check out the Autotransformer in action -
• Build Yourself a Cheap...

[@ko4nrbs]

I have been trying to use my 127 foot long Inverted L antenna as a Random Wire antenna. Currently it doesn't tune up on 80 or 40 meters but does well 20 thru 10 meters using a 9:1 UNUN. I built a 49:1 UNUN using 3 stacked FT240-43 Toroid cores with your instructions, and it solved my problem. Now the antenna tunes up 80 thru 10 meters very well with no added counterpoise wire. 40 meters doesn't need a tuner with the 49:1 UNUN which surprised me. So, what a difference!! Thanks for the information. Bill KO4NR

[@MM0OPXFieldRadio]

Glad to hear you have a working solution Bill. I've yet to experiment with 9:1's

[@TheSmokinApe]

Nice work Colin 👍

[@MM0OPXFieldRadio]

Thanks Ape 👍

[@KD9ZHF]

He always does great work. One of my faves.

[@pgramsey1]

I solder the tap on before winding. Wind a few turns so you get an idea of where the tap needs to be, take the wire off, solder, heat shrink, then wind the coil. Keeps the heat away from to ferrite and allows a more secure and insulated joint. My problem is that the efficiency is crap on 10 m. I built a 3:21 autotransformer with a 2643625002. Colin's testing showed good performance all the way through 10 m. Mine are fine at 15 m but about 7.5 dB down at 10 m for a pair back to back with 100 pF capacitors. I've tried re-building them with different wires and they are all about the same. Can't figure that one out.

[@MM0OPXFieldRadio]

Agree, Thats how i do my taps now. Strange that your readings don't tie in with mine on QRP core. I will double check it.

[@tjwalkr3]

Thank you for this amazing video! I was apprehensive about building an efhw, but this guide was just what I needed!

[@MM0OPXFieldRadio]

Your welcome, glad it helped. Ive another to make with some hints and tips.

[@coherent3237]

Thanks for the great videos. I'm building my first EFHW and just received a couple of the 26432511002 toriods and TDK caps from Mouser. Looking forward to getting the project completed and see how it all tunes and operates. Thanks again very informative!

[@MM0OPXFieldRadio]

Best of luck, sure it will turn out great.

[@alanb76]

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.

[@MM0OPXFieldRadio]

This wire isnt too bad. Its 1.18mm. I have 1.12mm on order as I'm about out.

[@colinmcdonald7591]

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.

[@MM0OPXFieldRadio]

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.

[@g0fvt]

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

[@MM0OPXFieldRadio]

Top tips 👌

[@leapofshed]

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?

[@bazzaar1869]

@@leapofshed nope its not, people keep making the mistake that ferrite is conductive.

[@jackK5FIT]

I've built a number of 49:1 transformers for EFHW using the pass through method. I ran across your videos about the autotransformer winding and just finished winding a 3 stack of 240-43 FairRite toroids. One difference I made was to put a tap at the 3rd wind by twisting the wire out a couple of inches instead of scraping and soldering a tap. I'm curious to see how well it performs. Thanks for all the great videos! 73

[@MM0OPXFieldRadio]

Thats a better mechanical solution than I have. That's what I will do too.

[@75ohmHAM]

You're the man Colin!!!

[@MM0OPXFieldRadio]

Thanks 👍

[@se2702]

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]

Thanks for letting me know as I'm not an ARRL member.

[@se2702]

@@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).

[@sv2dsy]

Thanks a lot Colin. Nice video.

[@MM0OPXFieldRadio]

Thank you 😊

[@David-ik9yn]

Rather than soldering on taps, pull out a short tap and fold it together onto itself. You can twist it. No soldering.

[@MM0OPXFieldRadio]

Good idea butbI would likely still solder when running QRO.

[@DK5ONV]

Lokks fantastic and surely works well. Good job, Master Colin 💯💥👊

[@MM0OPXFieldRadio]

Thank you UG 😊

[@leonardmiller76]

I wound the autotransformer as you've shown. I then checked it using resistors as you've shown in one of your videos. Everything looked good, but when I actually put a real antenna on it, the SWR was very high (almost 3:1). I may try the 56:1 winding to see if that works better. I'm using the 26432511002 core that you recommend. Do you use a counterpoise on your end feds? I'm pretty sure I did something wrong, and so far I'm not sure what that would be? Thanks for everything you've done!!

[@MM0OPXFieldRadio]

Its frustrating when we get these issues but sure you will get through it. Ive found the counterpoise to make no difference in my use case. Try the 56:1 as you suggest. Ive found wire location/orientation critical to the SWR.

[@leonardmiller76]

​@@MM0OPXFieldRadio SWR problem is now fixed!! My NanoVna had a broken trace. I had to resolder the board, and now it works like it should. I did a quick scan before it got dark, and the SWR seems close, I'll fine tune it tomorrow. SO HAPPY!!! Thanks for all the hard work Colin.

[@danazlin3409]

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.🙂

[@MM0OPXFieldRadio]

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 👍

[@epikmero]

have you thought about getting into this meshtastic craze everyone is playing with at the moment ?

[@MM0OPXFieldRadio]

Yeah its not got my interest at the moment. In time who knows.

[@pulponair]

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?

[@MM0OPXFieldRadio]

Definitely. I haven't tried that. I should 👍

[@SM3TEK-Lars]

Tnx. I do the tap befpre I wind the autotransformer and isolate with heatshrink tubing. Cheers

[@MM0OPXFieldRadio]

Thats a better way Lasse if know your lengths. I seal with liquid tape.

[@TheArtofEngineering]

Wow…. Love the fishing style spindle at the end of the video on your UNUN box… great idea for deployment in the field!

[@K1ZEK]

Thank you for sharing this with us. I will try one out this coming week. 73 Leo. K 🙂

[@micron00]

Great stuff, thanks a lot for the experimenting and charts comparison, thats awesome! What wire awg would you recomend for the antenna for 500 watts?

[@MM0OPXFieldRadio]

Without the science, if I was to build for 500w I would feel more than safe using 1.5mm diameter wire.

[@micron00]

Thanks a lot

[@JayN4GO]

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.

[@MM0OPXFieldRadio]

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.

[@R50_J0]

Excellent. Thanks.

[@MM0OPXFieldRadio]

Your welcome 🙏

[@paulmadsen51]

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]

I did try this too briefly but there was no noticeable difference. Big benefit of this is you can bleed off static.

[@paulmadsen51]

@@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.

[@dadasrider]

Nice jop. Ty very much.

[@MM0OPXFieldRadio]

Your welcome.

[@rogerhampson9592]

I have built a "Standard impeadance blaum, but the frequency is on the low side ,6.46MHz, now i am quite sure the wire is not 70 odd feet long, i am wondering if the impeadance transformer is suseptable to the aluminium plate used to mount it???

[@MM0OPXFieldRadio]

Quite possibly. I dont know what you mean by "standard" a 56:1 will have a shorter wire than a 49:1. Get it to resonance then measure the length.

[@KD9ZHF]

Thank you for the video. What is the power rating? 1500 w PEP?

[@R50_J0]

IMO, a three-stack of FT240-52 cores can handle at or above 1KW but the enclosure needs some convection ventilation at that power level.

[@KD9ZHF]

Understood, thank you.@@R50_J0

[@MM0OPXFieldRadio]

Agreed 👍

[@x_CrossHair_x]

I'm Just wondering if you ever did any "R.X." tests with the Various "Recommended Toroid Materials (43 /52 /61) ??" Plus, Comparing (Winding Methods) the "UnUn Cross Over Winding Style" compared against the "Autotransformer Style" as shown in this Video.. (Reason is..) I am trying to Discover what Core Material and Winding Style.. Would perform the Best (R.X.) in the Frequency Range of 500 KHz to 30 MHz.. (Fore Shortwave Listening).. NO T.X. is required.. So a Smaller Single core would do. Like a T130-43 (Example)

[@MM0OPXFieldRadio]

I think your after something different. If I was you I would be looking at a wellgood loop or similar.

[@x_CrossHair_x]

@@MM0OPXFieldRadio Nice but OUCH Price WOW

[@MM0OPXFieldRadio]

Make sure it's the Wellgood and not the Wellbrook. The latter is 4 or 5 times the price of the former. I have a Wellgood kit not build yet.

[@x_CrossHair_x]

@@MM0OPXFieldRadio Ahhh... OK .. Thanks.. I Will look around.. and hope to find a trusted supplier / Source 👍👍

[@FFdo.]

is it the same wiring for the longer, smaller od youve used in the past the 2643251002?

[@MM0OPXFieldRadio]

No, I've not measured an autotransformer on a 1002 core yet. The twisted core is 90%+ efficient already. May be marginal gains.

[@johnmckinney6829]

Great video, thank you. Would 17AWG , 1.15mm wire be ok for QRO? I'm finding the h eavier gauge wire a bit difficult to wind properly.

[@MM0OPXFieldRadio]

Maybe, hard to say as I havent tried. Personally I would go bigger but you never know.

[@Aimsport-video]

Solder it first. Place the soldered leg right where you want it and wrap outward from that.

[@MM0OPXFieldRadio]

Yep, that can be an easiet way.

[@BRJ2]

Could this be placed in a metal box if properly insulated, or would that affect something?

[@MM0OPXFieldRadio]

Potentially yes but it creates more problems as you then need to isolate all the connections so its not just one massive short.

[@BRJ2]

Ultimately, I would be looking for heat dissipation.

[@MM0OPXFieldRadio]

I get that. I would install a mini PC fan and draw the heat away.

[@revbikerbigd8664]

Why is it called an Autotransformer???? And not a balum or unum??

[@MM0OPXFieldRadio]

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.

[@lampshadesneeded]

wouldnt spreading the turns out reduce saturation?

[@MM0OPXFieldRadio]

Yes but you lose efficiency. Whats your priority?

[@ErikTheVikingMechanic]

Great thank you.

[@MM0OPXFieldRadio]

Your welcome.

[@billgatewood9578]

How many watts is that good for?

[@MM0OPXFieldRadio]

If its 52 material 3 stack then easily 1K plus.

[@allancopland1768]

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.

[@MM0OPXFieldRadio]

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.

[@kariolavi]

Good idea for winding! What does ”auto” mean in autotransformer?

[@MM0OPXFieldRadio]

Thats 1 question I dont know. I need to research that.

[@allancopland1768]

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.

[@Tokyo1991.JL1AJE]

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

[@MM0OPXFieldRadio]

Yep, that a much easier way. Thanks.

[@Michael-DE-VO1OP]

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

[@Steve-GM0HUU]

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.

[@MM0OPXFieldRadio]

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.

[@dleecann5795]

Anyway you could video a winding of the 56:1 transformer, using toroid 26432511002, w/15:2 windings..so appreciate..KN4CKP

[@TheArtofEngineering]

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

[@MM0OPXFieldRadio]

I need to do this and measure. Its piqued my interest again.

[@g4lmn-ron401]

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.

[@TheArtofEngineering]

@@g4lmn-ron401 Thanks for the reply. Yes RF is the black arts and gets messier the higher we dare venture! 😂

[@PD5RM_Robin]

is there a reason not to wind 56:1?

[@MM0OPXFieldRadio]

The reason for the extra turn on the secondary is so I could get an acceptable match on 40m. The SWR wouldn't go below 2:1 on 40m when wound as a 49:1.

[@PD5RM_Robin]

@@MM0OPXFieldRadioso 56:1 should be beter for 40m?

[@MM0OPXFieldRadio]

@PD5RM_Robin Yes.