In Search of the Optimum Magnetic Loop Antenna with John Portune, W6NBC
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- Опубликовано: 6 окт 2024
- John Portune has outdone himself with this magnetic loop design made from PVC parts covered with aluminum foil tape to come up with one of the most efficient yet inexpensive mag loop antennas I’ve ever seen. I have built it and can testify that it works extremely well. I think you will want to build one, too.
I wanted to thank you for your presentation. I learned more about loop antennas from you than anyone else. I was getting frustrated with my commercial small loop that tuned a low SWR at 40 meters but nobody could hear me. I replaced my coax radiator with an all aluminum radiator and conductor to increase the antenna efficiency this evening and tonight made made a DX contact on 40 meters with Italy with 25 watts. I am grinning ear to ear that I watched this, learned something, made a change, and it works! I am keeping this loop configured for 40 meters and my second coax loop for 10 and 20 meter bands. It has me thinking about getting a square loop put together and tune that specifically for 20 meters. I primarily enjoy QRP work because it is so challenging and everything has to be set up perfectly. Thanks again…. KD2ZTA
Thank you, that was so enjoyable and educational, Learning is exciting at any age, with wonderful teachers.
What a wonderful presentation! You guys did a great great job. To the hardware store Robin! Theres PVC pipe just waiting to be whacked and drilled!
Excellent thank you! I got my ham licence in 1957 too! I was VE6KX then, now VE7YJ.
I have learned new stuff, thanks for sharing!
Outstanding material, Thanks so much!
I've get started to build one 🤗 Thank you for sharing!
How did it go for your construction?
Very good. I started building a similar loop. However, I used 50 mm PVC pipe and aluminium flashing on the outside. The aluminium flashing comes in different widths but in long roles. It is possible to cover the whole loop with no joins. The flashing can be held in place with zip ties or something similar. The flashing can be bent around the corners with care and some practice. The loop is potentially more durable and should handle high power. Drew VK4ZXI
Great talk and experience, thank you
Really interesting presentation. With the conductive tape on pipe, how do you ensure a good conductive join at the corners? Especially with aluminium I'd be concerned about just a mechanical contact.
Forgive me for pointing the following out, but John used the word circumference as well as diameter (3 times for diameter) when describing how a loop can be more than 1/10 of a wavelength, which did he really mean? Loop diameter is diameter, circumference is the amount of conductor end to end which the loop is made from, the two terms cannot be used interchangeably. I suspect he simply misspoke on that point.
I was under the impression that the 1/10λ came about because you want a constant current around the loop thus maximizing the field. He also glanced over the coupling used in the original Army Loop (capacitive coupling). Fernando N2FQ/6
At 16 min, having a bigger conductor for a bigger loop means < 24:1 I think, not > 24:1. If the diameter grows you need a bigger pipe. So the formula should read > 1/24 ..
Thank you. This is some good, common sense info. But I do have a couple questions and a comment.
What elevation above the ground did you use for your loop's null models?
What is the bandwidth without retuning, meaning what is the Q, since the PVC dielectric will reduce it compared to an air- or vacuum-variable cap?
And the comment: A round loop has a lower resistance for its cross-sectional area and thus a slightly higher efficiency since a circle has the maximum possible area for its circumference.
I have mine at least one diameter width above the ground usually the top is 6 feet off the ground. Anything more than that doesn't bring any benefits. The Q seems to be band dependent. I don't generally have to retune much as I stick to certain parts of the band. But tuning isn't at all difficult. Yes, it appears the efficiency drops a little being a square rather than a circle but the square is much easier and gives you a larger radiating surface compared to a circle with a smaller diameter material such as coax so that becomes a wash. Actually it works better than the typical circular mag loops. I have made 2 of these now, one for 10 - 17 meters(3 foot diameter) and one for 6-10 meters (2 foot diameter). I love them! And they're a heck of a lot cheaper than buying one. I have $70 and $30 respectively in mine.
@@DigiCommCafe, thanks for the info. I have 3 pieces of 1" hardline each about 10 feet long, some big air-variable caps, and some 12 V geared motors, so with just a bit of work and without spending a dime I figure I can make 2 loops to overlap their coverage and cover 80 through 6 meters. So I'm watching a bunch of videos and reading some technical articles in order to figure out how to build the best loops with the best nulls and highest Q's (for maximum xmit efficiency and best off-frequency rejection on receive,) that I can manage to build out of the materials at hand. So far it seems like even the commercial loop designers all make some design errors or cut some corners on their designs, and I don't want to. If I'm taking the time to build it I might as well build it the best I can.
@@DigiCommCafe The antenna is designed in regard to total loop length, so whether a loop or a square, the lengths are calculated to give the same aperture anyway, correct? I suppose with tuning, aperture could be increased to provide a lager area inside the loop and therefore more signal, up to a limit, and the diameter of the conductor increased to keep the loop resistance low, if concerned about that aspect.
Can you ask John to provide a written and more detailed account of the building process? Is there any actual comparison between loops, done in real life and not in a FEM simulator?
I haven't found that yet either.
I made a flat plate capacitor using PVC as the dielectric. I found that the capacitor got very hot after operating for a while. To me this indicates that PVC is lossy at RF frequencies. Does your loop get hot in the area of your capacitor ?
At higher voltages it must. I ran 100 watts on my 6 meter loop and it started melting the pvc! haha. I couldn't move the inner tube past the gap until I knocked off the drip of pvc. So I only run up to 25 watts now.
I'm sure it will work, however it is durable having it out in all types of weather? Wouldn't that covered foil paper peel off over time when exposed to the elements and also applying heat. In theory you may be correct, however I would look at durability. Pretty interesting video. Best regards. 73.
Hi Danny. I just use mine for portable ops. It's very convenient and quick to setup in the field, so the tape holds up nicely. But then I've only started. Ask me again in a couple of years. hihi
@@DigiCommCafe Oh OK very good. Have a good day Sir. 73.
The foil tape they use is just foil, no paper.
outstanding
Thank you for this very clear description. Still I have a few heretic questions. When you say that diameter matters, I understand that there is the skin effect. But does this effect not also matter for a normal dipole? So, why using wire for dipoles, but tubes for magnetic loop antennas? Next: how to connect the copper coax cable to aluminium? Is this possible at all? Finally: Because current is high in a loop, all connections are critical. When you make the loop out of one aluminium foil, it is ok, but connect them just by pressing, is this enough? Best 73, Roland AB7SN / DL9NBX
Since a dipole is high current at the feed point for 1/2 wave, I imagine so but how much current compared to a loop?
Thank you! This a rare gem of a video. Lots of great practical advice backed by data. One question... for those of us who are receive-only, how much of this is relevant? For instance, the MLA 30+ mag-loop antenna ships with thin wire. Would it benefit from a 2 inch conductor or is that for tx only?
What about multiple-turn loops? It gets the current up by repeated low current turns, so I^2R goes as low as you want it.
Fantastic info!
How about feeding the loop with window line or open wire?
I’ve been told that this construction (aluminium foil on pvc pipe) would be too lossy on 10w. I’d like to know for sure.
Can multi-turn loop antennas have a 200kHz bandwidth?
Have these components ever been used successfully?
2 in. x 3 in. x 10 ft. White Aluminum .0145" Downspout $12.98 each
2 in. x 3 in. White Aluminum A-Style Downspout Elbow $4.28 each
Wirebush the paint and use Pop-Rivets on the elbow connections?
It wouldn't at all surprise me concerning the things that will work. Give it a try!
This sounds like a great experiment. Have you done it yet? My first thought as I was trying to mentally assemble it in my head was that, even with rivets holding the segments together, I could picture the loop having difficulty holding it's shape and maintaining the necessary gap at the top (12 o'clock). Those gutter elbows tend to allow a bit of play - wiggle. But with experimentation, I bet they could be made more snug. Please let me know if you have given this a try. It really is an intriguing idea. 73- Dave K1DBD
when I measured the resonant frequency of a loop with no capacitor I found it resonated where its length was 1/2 wavelength. It was like taking a dipole and wrapping it into a loop. I put 100 watts into it and could light a 4 ft long fluorescent tube with it. The loop was about 9 feet long and resonated in the 6 meter band. 6m was dead at the time so I did not try making any contacts with it. So what does this actually tell me about the frequency that it would work on as a mag loop?
Skeptic- Thank you for sharing your test result. In reply to your question- To behave as intended, hence optimize the H field and suppress E field, I believe that a mag loop antenna must be no more than 1/10th wavelength of a 1/2 wave dipole.
@@Supernumerary Believe or know?
Wouldn't it have been a lot easier to use a piece of plastic tubing bent into a circle then just wrap a strip of the foil tape around it?
Hello. If I understand correctly, a loop antenna does not work at lower frequencies. I want to use a loop to receive channels at the top of the medium wave band. Between 1.6 Mhz and 1.7 Mhz. I bought an MLA-30+. Will that work? Do I need to change anything for a better result? Greeting. Cees
You are correct. This particular magloop would not be well suited for very low bands. With a 5 foot square loop you might get it to work on 40 meters fairly easily.
I am not clear on the way your feed loop is connected. Is the feedpoint connector connector connected to the loop, and the other side, the u bolt, inductive? or is it conneccted by a bolt to the aluminium? or just an independant loop altogether?
John says it's a "tapped transformer" and is inductive. It uses a portion of the larger loop. Moving the u-bolt up and down the side adjusts the swr.
@@DigiCommCafe thanks. The connector on the tube would then be the tap? So a part of the big loop would be shared by the small loop. Am I getting this right?
@@denislacroix654 Exactly. This is all new to me. I'm learning a ton from John meet with him regularly to learn more. I've been making modifications to mine as I go making small improvements. I can tell you from using it that it works very well. I have 2 of them now. A three foot square one for 10-17 meters and a 2 foot square for 6 meters. They both work very well. (And they're very inexpensive and easy to build!)
Thanks a lot. I learned a great deal from that video, thanks a lot for bringing that to RUclips.
@@DigiCommCafe Just trying to make sure I have this 100% correct. The Feedpoint connector center pin connects to the Feed Arm and the shield of the Feedpoint connector connects to the bolt that connects to the foil on tube? Thank you for your time. Pretty new ham here.
Question: What power was used in the efficiency measurements at 7:20 and 9:50? If you are running QRP, won't those curves look very different since you are far less likely to saturate the surface of the conductor with charge?
I run mine as QRP (5-10 watts) and it works great!
@@DigiCommCafe Thanks for the quick reply! I'm sure the loop runs great on QRP, but I'm wondering what power the simulation assumed to determine whether a smaller diameter loop would work just fine if it's only going to be used for QRP. Cheers!
That I couldn't tell you. Sorry. I just built it and use it.
@@DigiCommCafe My thinking is that since power goes as R*I^2, a 10X reduction in power results in a 100X reduction in current, so you can get away with a far thinner piece of foil for your antenna.
An interesting test would be to make one antenna with thick tape, and one (same band) with thin tape, then hook your radio up to one and a power meter up to the other. Then do a series of transmission tests at increasing power output of the radio and record the power meter readout on the other antenna. Then switch the antenna on the radio with the antenna on the power meter and compare the curves to see at which power level the transmit efficiency of the thin foil antenna starts to drop...
Funny how non ferrous metals become magnetic though never understood how aluminium can resonate and also copper surly Loop Antenna is enough magnetic seems a breach of the trades description act no one calls a di pole a magnetic di pole?
what are you guys thoughts/opinions on: instead of using a cylinder shaped conductor, you use flat stock shaped conductors ?
Hey! I like the idea, Jeremy! I'm gonna try it. Use Foil tape and a large piece of cardboard in a square. That should work! I love it when people think outside the box. You could build a "picture-frame loop".
So Jeremy...today I built my flat cardboard 6-meter mag loop antenna using scrap box material and aluminum tape. It was cheap, easy and tuned up beautifully. Here is the link to the steps and results. t.me/s/digicommcafe
r John Portune, W6NBC ,
Thank you for this very informative lecture ;
"Looking for the optimal magnetic loop antenna".
We live on an apartment with and small balcony on S-W of 7 m2 , so an ideal antenna !
I built a MLA the classic circle shaped diameter 1 meter from 1,5cm plumber pipe. At the bottom of the circle a variable air capacitor 20-250 pF and at the top of the pickup loop 20% of the circumference of the main loop. This MLA works on the HF 10 - 20 M
My favorite HF bands.
My question , I would like to recreate your design of the optimal magnetic loop antenna for Hf 10-20 M bands, but due to my poor eyesight your drawings are not easily visible.
Do you have a printable version of the construction description ?
Or possibly where are these for sale , Amazon ?
That would be very nice !
2nd question, my built classic MLA where is the biggest improvement factor ?
Best regards from the Netherlands, Theo PA0HTY , 73
Translated with www.DeepL.com/Translator (free version)
how do i connect the wires from the 239 to the anntena? I'm not an engineer,
help me out here.
So are multiple loops a way to compensate for the conductor size?
It appears to me the increased conductor diameter is simply to reduce the resistance caused by skin effect, if multiple loops, then having a minimum conductor diameter would be even more important, correct? I've watched several of John's videos, and videos produced by others too and never any mention of that. Seems to me, if two loops the greater than 1/24 rule for the size of the conductor to the diameter of the loop should then be greater than 1/12 due to having twice the length (in reference to diameter).
So how many watts can the loop take?
I generally run 25-50 watts.
CAN WE USE THE BELOW ?
Not sure what you mean, but yes...build away!
very nice presentation. excellent information. 73 N 9 I Z X
The more turns you have on a magnetic loop, the more efficiency you get. Ad infinitum? This is rubbish. There is no such things as a free lunch. What about the area? A lot of what you said about magnetic loops is true, but this is not at all. Stop quoting Maxwell, if you don't understand it.
Wow! Who rained on your parade today.
@@DigiCommCafe Is that your only answer to my comment?
@@tomtwist1081 First of all, I'm not an expert. I interviewed John Portune and have built two antennas based on his design and they work great! Secondly, I thought your comment was arrogant and extremely rude and I have no desire to continue this conversation any further with you. I'm sure that most who have watched this video and know John and have left thoughtful kind comments would agree. Have a nice life.
@@DigiCommCafe As I also wrote, he has a lot of correct information on magnetic loop antennas.
But the claims he makes are dead wrong. Anyone with a little knowledge on electromagnetics will
realize this. I hate when misinformation is spread, especially by a guy many regards as a guru.
He also claims that his flimsy PVC/aluminium tape antennas will take any power you throw at it.
This is also wrong. A thick walled aluminimum tube will have the ability to transport the heat
generated by the resistive losses in the skin effect layer. The aluminium tape and the PVC tube will not.