I tried emailing QRP Guys directly but got an answer that was completely unrelated to what I asked and sort of smart ass. I dunno what it is about this hobby and responders answers being completely unrelated to the question but it's horrible.

Anyway. On an EFHW, what part in the transformer makes it a single versus multi band? I ask because I've built some that were resonant across multiple bands. 49:1 for instance with a 133' element. But with the QRP EFHW, it's only resonant on the band in which the element is cut for. So is it the toroid? The winding of the toroid? The capacitor? It obviously isn't the element, so I know it's the transformer but what part.

Funny you should ask that. Mine is wound with 21 total turns, and 3 "twisted". Doing the math, 21 turns divided by 3 "twisted" =7. square 7 and you have a 49-1 transformer.
If you put a 67' wire on it, it should be close at 40, 20, 15, and 10 with some trimming. I didn't measure the cap, but if it is around 100 picofarad it should be good to go.
I was out trimming a 20m wire for mine, and took a break for a drink and read this.

It's the fact you're doing a large impedance transformation, either 49:1 or some use a 64:1, to get the very high impedance on the end of a half wave element down to close to 50 ohms. If you took a center fed dipole for say 80 meters, on that band it is about 50 ohms at the center and also 50 ohms on the odd multiples of that band (3.500MHz X3, x5, x7, etc.) But on the even multiples the center dipole impedance is very high so it won't work there. But at the end of a half wave element, it will always be similarly high impedance at the ends of the element on all odd and even multiples, so if you can feed it there you get that advantage.

It just so happens that a lot of the ham HF bands are lined up as multiples of each other (harmonics) and it works out nicely.

3.500MHz x2 is 7.000MHz (40m)
3.500MHz x3 is 10.500MHz (close to 30m)
3.500MHz x4 is 14.000MHz (20m)
3.500MHz x6 is 21.000MHz (15m)
3.500MHz x8 is 28.000MHz (10m)

From there the minutiae of the transformer design will dictate what frequency range the transformation is relatively efficient. Most of the common designs using the 49:1 ratio with 43 mix cores is best between 4 and 20MHz or so, and gets more lossy as you expand outside that range.  The capacitor helps pull down the SWR on the higher bands like 10M to keep the radio happy but it's still suffering from more core losses there.

ETA: there is a different method for making a mono band EFHW, which uses a tuned tank circuit at the feed point with a specific value inductor and capacitor to achieve the match. This is like what a tuner would do but it uses fixed values instead of variable components.  I've seen some of those products out there as well.

Has anybody done an efficiency test using a field strength meter for the various end-feds on the market? I would guess that a tuner might be more efficient, but the tuner is, of course, probably more bulky. When it comes to efficiency in tuners and matching networks, all other things being equal, larger parts are better at current handling.
73,
Rob

Thanks that clears it up. So it could be any number of things in the transformer on why QRP Guys is mono band versus multi band. I kinda figured it was the case, but wasn't sure if it was something specific that changes it.

The one they make that is a basis for a mono-band EFHW fed with a tuned circuit is the one with the little tuning capacitor on the feedpoint board.  It has an fixed value inductor on an iron toroid on the back and then you tune the trim cap to suit the band/length of wire you use:  https://qrpguys.com/end-fed-half-wave-sota-antenna-tuner

Their more popular "no-tune" model should be capable of being multibanded with a single wire, it's construction is a classic 49:1 transformer on a 43 mix toroid (though the size toroid used is pretty crummy for efficiency unfortunately).  The literature doesn't really advertise it that way but maybe they just wanted to keep the explanation simple since it's a kit and people would chose what length wire to use, obviously the shorter the wire you do use, you'll have much fewer or even no additional harmonic resonance points of use.

Thanks for info. I really appreciate it.

This is the one I bought. It is very definitely a mono band.

https://qrpguys.com/qrpguys-end-fed-wire-antenna

We tried and tried to get it dialed in on the harmonic frequencies in addition to 40m and it will not do it. Tunes up nice on 40m though. Got to reading and it only briefly mentioned it's a single band antenna. Realistically I only need 40 for my QCX Mini anyway.

Keep in mind how capacitors and inductors work.  Inductive reactance is (2 * pi * f * l) so as the frequency goes up so does the reactance.  That's a physical law and you can't break it.  You ca get around it though.  Capacitive reactance is 1 / (2 * pi * f * c) so as the frequency goes up the reactance goes down, the exact opposite of an inductor.  When you put a capacitor and an inductor in parallel you basically have a rudimentary band pass filter since the inductor blocks the higher frequencies and the capacitor blocks the lower frequencies.  For an antenna the capacitor and inductor are chosen to provide a 50 Ohm net reactance at the desired frequency.

Makes sense. This is stuff I don't know about antenna building. I remember reading about a lot of this when studying but it never went very in-depth.

But that makes sense. Lots of little go into whether it'll be multi banded or mono banded. Interesting.

The more I learn the more I learn that I know even less than I thought I knew.

I am picking up little things like winding on the toroid that crossover the center........is only to reposition the end of the wire versus just winding around the circle continuously and the wire ends up near the other wire end.  

Insulated wire versus enameled magnetic wire.....the enamel gets closer to the core for a better effect but at the risk of scratching it and shorting it.  I will use it up but likely not bother with enameled magnetic wire again.

I got the 20m antenna tuned, and after running some errands and installing a tonneau cover, I fired up the radio.
I got russia, Bulgaria, WW1USA special event, Italy, and a few Canadian stations in a contest. I have it set up in the front yard in a shitty spot, and it seems to work pretty well hanging off a 7.2m fishing pole.

This is probably one of the most humbling hobbies I've ever been a part of in my life.

Hang in there you guys, lots of us enjoy helping out.
73,
Rob

Running low power, it is easier and cheaper to make antennas and mistakes. I am getting the hang of what to expect when I build something. Like in real life, an antenna adds in all the things that calculations do not consider.

I appreciate it too. I like to learn so while it's very humbling, it's also very satisfying. Keeps my brain engaged. Plus I learn a lot of stuff I didn't know or probably would've never learned too. Morse code? Had no use for it before amateur radio. Building antennas and learning electrical theory, same thing.

I could see that. Lot less damage to be done with lower power. I've built a number of antennas from plans. But in the several months started trying to learn what makes them tick so to speak.

I learned a lot from this guy, he had a bunch of videos, but his channel disappeared for some reason. I hope he gets them back or makes new ones.

I have not looked at this software besides watching him use in in his videos. Looks really cool, and gives an idea if you have a cloud warmer or not.

Imagine if someone could take Tinkerkad 3D modelling and meld it to RF? Now that would be an antenna modelling software platform!

That program in the video above does 3d rendering of the waveforms of any antenna you build.
He had awesome videos showing gains of various antennas and the db at various take off angles. That is a very cool program.

Callum is still on youtube, with a different channel. His got nuked for a a bizarre issue, I think something about how his files were stored?

Search around, he is there somewhere.

I was trying to make a contact on station that just started calling cq,   no takers so I tried and couldn’t make it through the noise,.   I looked up the call, it was Callum.  It would have been nice to make the QSO and thank him for his videos.

Kind of like when Bob Heil popped into the arfcom net one night.  They’re real hams and are on the air.

with a tuner, you should be able to tune that EFHW from QRPGuys to other bands.

I have one with 80 and 40 wires and next time I set it up ( when it isn't raining ) I will use a tuner and see what happens.

My 40 meter QRPGuys single band endfed is a perfect match to 50 ohms if I string the wire about 20 feet up in the middle with ends about 4 feet or so.

So the transformer ( and cap ) is taking that high impedance end fed feed point and making it 50 ohms, so with a tuner, it should work on 20, and 10, and maybe 6 and 15 too.

I will check it out and report back

Lol, I am so dragging my but and getting my end fed actually built.  I bought the last bits needed yesterday, satainless cap head bolts, washers, nuts and a couple wing nuts.  

I want the Elecraft T1 tuner but none available.  I am hoping this works on multiple bands without the tuner.  If not then i just keep using my single band dipoles until I get a tuner.  Lots of choices for QRP tuners, some kits, some prebuilt manual tuners, a few autos.  I thought about one of the LDG ones but they have kind of big and heavier than I want for SOTA type minimalization.   10w on a single band dipole and I am not even using a choke.  Seems to work ok.

True. But I do my best to avoid tuners. I'd rather be limited on bands and resonant with what I have than tricking the radio. About he only time I use a tuner is if I string up my G5RV while activating.

Lots of people wrap the toroids with Teflon tape to help protect the enamel.  I wrapped with vinyl electrical tape and it's thinner than plastic insulation and stays put better than teflon tape.  I suppose you could use kapton tape too if you were worried about heat.

I added a tap to this one for a lower ratio, but I think I want to wind it again and add more turns for a higher ratio with the tap at the 49:1 and another at 64:1, or at least I think that's how it would shake out.

Might just get another enclosure and build a second one.  This one is too pretty to tear up.  To get a nice even twist on the first 2 turns, I put the wire in a cordless drill chuck.  Makes a perfectly even twist.  The other end you clamp in a vise padded with leather.

Attached File

Nice job.  Looks very well done.

FWIW, I didn't bother to wrap mine with anything, mainly because I didn't have anything to wrap it with.

I can't remember whose instructions I followed, but I used two T240-43's superglued together.  That is supposed to make it capable of handling higher power (500 watts, IIRC?). I overbuilt mine a bit because I wanted to be sure it would handle 100 watts without worrying about it getting too hot (not that I ever talk long enough for that to happen anyway! ).




This is the one I used for FD last year.  I had it tied to a tree about 7' off the ground with the other end of the 132'-ish wire up in the trees about 30-35'.  I trimmed it for resonance on the 80-meter band.  


The 7300 was able to tune all bands from 80-10 with no external tuner (except 30, but IDGAF about 30). I made 50 FD contacts and a few non-FD, including DX to Russia and Slovenia (S51DX! )

I just ordered the 20-watt K6ARK antenna kit.  I'm not going to be able to do Field Day from the location I'd planned this year (stupid gas prices!) so I'm going to set up somewhere closer to home this time.  It would be nice to get similar results with the K6ARK antenna.

Since everyone is posting transformer pics, here is one of mine.
Attached File

I have one like it on a fence post for my main antenna.
Here it is next to a qrpguys single band cut for 20m.
Attached File

Depending on wire length, they will do 80-10 no problem. The one in the box should handle 100w. I only have 20, and it does great.

I don't have a pic of my current go-to without breaking open the printed enclosure, but I've become more partial to the autotransformer winding method with tight spacing between turns, it's easier to make and in my testing seems very slightly better than the common twisted pair design.  I also use the Fair-Rite 2643251002 toroid core, it's a bit of an unorthodox choice but I like it because it has much more cross sectional density and material.  It's similar in size to two stacked FT140 toroids, but much chunkier.  For a fairly small size it should handle 100W no problem even though I use it for 10W portable.

I used two 43 mix toroids as well. But I didn't glue them, just used the tape to keep them together.  Still the insulation on the magnet wire is actually way tougher than you think and I wouldn't feel uncomfortable wrapping bare cores with it.

I need to go back through my old computer backup and see if I saved the instructions I used.  This was a fun build and I might want to make a spare.  I'd also like to make a smaller QRP version

I will need to order more cores though.  I have some 31 mix I used for a line isolator but thats not ideal for these.

That would be interesting....


I just got done building a multiband end fed half wave and my trim to length ended up being 64 feet. (i started at 74’)

The only band with a good looking antenna field is 40 M (theoretical I didn’t/ cannot measure), the 20, 15 and 10m add more lobes and nulls as you go higher.   So if one is willing to pull down and rehang to change bands then individual tuned wires would in theory be a better way to go.  

I knowingly went with the multiband out of convenience for field use.  I will just have to wonder if my failure to be heard is from qrp low power or conditions or that station is simply located in one of my null zones.

From some reviews on field end feds the KM4ACK design seems to out perform the QRP guys version.  Pretty sure the former uses the bigger 140-43 mix toroid.  

You guys are certainly great online Elmers.

Some damned fine-lookin' baluns in this thread!

I just finished a 20w K6ARK kit, but I still want to hot glue the toroid in place.  It just seems way too fragile to me.  I have to wait until the UHF/BNC adaptors I ordered get here before I can test it.

I've got a couple of T130-2s so I might give another try at building a 9:1 unun for an EARCHI end fed, just need a little box to put it in.

Yeah I really like the little EARCHI antennas.  Supposed to handle 100W, but I would turn the power down for digital modes.  If you just want a quick, easy to deploy antenna that will work, it's a great lightweight option.  

I am not sure how wire length relates to efficiency.  I chose the shortest wire that would work on the bands I wanted using this chart.

https://udel.edu/~mm/ham/randomWire/

If you scroll down to the chart it lists non-resonant wire lengths for the various bands so that you never have to match too high of an impedance.  The 9:1 transformer should help with that and get the impedance down within the capabilities of an internal tuner.  With the transformer and my little travel tuner, it matches well enough that I don't need to re tune much across the entire general class SSB portion of 20m, which seems to be the best band for the 6 hour stretch centered around sunset right now.

Shortest because I was aiming for portability over efficiency, but I've never tried a longer wire on it.

I used it to make a contact on 20m 1400 miles away with an FT817ND set to 5 watts.  The conditions were right that day and I got a 5/7 signal report.  So there's a totally unscientific, anecdotal report to consider.

So I checked the QRPGuys EFHW antenna with a 40m 1/2 wave wire according to the wire length listed on the device, 61.5 feet.

With the little MFJ-9201 QRP manual tuner it tunes to better than 1.5 SWR on all bands 80-6 meters including WARC

And since there is a grand total of 18 inches of coax from the tuner to the antenna, I am guessing it is fairly efficient.