Tiny Magnetic Loop - Big DX 

Yes, I'd like to see the inside of that little box!

Thank you Toly, I appreciate your feedback 😀👍

Yes, definitely coming... Thanks Steve!

Mooore please! Lotttts more!

@@K6ARK can't wait

Did Adam ever do a build video for this loop ?

I’ve built a few loops, from simple ones made from reclaimed RG6 and a toroid core ripped from a switch mode power supply, through to one made from 1” x 1/16” aluminium strap with a motorised tuning capacitor that handled 50-60W but caught fire when I gave it the full dollar :-) They are excellent and, elevated enough seemed to out perform my back yard verticals I have made from aluminium tube The narrow bandwidth keeps the noise levels down, and certainly removes broadcast band interference on the Pixie my son built :-)

That big loop was 3m wide as well. Killer on 40m

Thanks Ed. Sometimes you can do some impressive stuff even with a compromise antenna. It's always an adventure.

They are fun, unique antennas. One like this is no replacement for a full size antenna, but as you can see, they do work. And they are a lot of fun to build too.

Thanks Anton, it's a quirky antenna that actually works. A fun build, for sure.

I'm certain good propagation conditions were in my favor and it was largely a matter of luck, but the antenna does radiate. It will never be as efficient as a dipole or EFHW, but sometimes the compromise of a very small antenna is worth it.

The high current exists everywhere in the loop; the actual feed is much lower in current. Of greater concern is the capacitor that sees the high current and thus also high voltage, but not so much at 5 watts. Also, "skin effect" is such that the current only flows on the surface of the conductor; that's why I and other use hollow copper tubing. it wouldn't use the inside anyway.

@@thomasmaughan4798It's hard to fold up copper tubing.

@@ahbushnell1 "It's hard to fold up copper tubing." That it is which makes portability an issue particularly at lower frequencies (larger loops). RG-8 and similar coax works pretty well to provide large diameter outer conductor and some flexibility. Naturally the inner conductor is completely unused in that configuration. I might try some tinned copper braid to achieve the same result at less weight and increased flexibility.

I plan to, at least for the key elements of the matching capacitor and feed. Just need to find the free time to do it! 😜

Yes! For SOTA and POTA activating, most operating involves just selecting a frequency and calling CQ. I don't often use a mag loop on summits, but they are very convenient when space is limited for setting up a more efficient and broad-banded wire antenna.

The aluminum is much lighter. Measure the weight of about 10 feet of coax, and a frame to hold it up. I used stainless hardware to connect the aluminum bars and a little wiggling back and forth as I straighten the segments seems to make the contacts clean enough to prevent issues. Even after not using the antenna for an extended amount of time. I suspect you could also use some dielectric grease but I didn't want the sticky dirt collection involved with that. Minimizing resistance is key in magnetic loops, and I have no illusions that this loop is very efficient. But it's light, packs small, and it certainly seems to work well enough. There seems to be a good amount of interest in a build video, so it sounds like I need to order up some more material and make another. 😁

@@K6ARK YV5FCK ARMANDO We will wait for the building video I will see in Lowes or ome Depot to find the Aluminum bar, please put in the video the mesures and the part for folding the elements Tks Adam

Here you go! content://com.amazon.cloud9.FileProvider/images/screenshot/1620769332965-980462730.jpg

Wheels in head spin round and round

Ran the calculator and it confirms my thoughts about the size. Going to add another 4 feet. something like 95% efficient on 10 meters. Pretty good on 20. Not real happy on 30 and 40 but it looks like it will work at a low efficiency

Awesome, John! Would love to see pictures when you're done.

It's how much power is coming out of the antenna compared to how much your radio is putting into it. Different antennas have different efficiencies depending on what kind of metal it's made of, the way the metal is shaped, how much metal you're using, and a whole bunch of different factors. This antenna is a lot smaller than most antennas but what you gain from it being so small you lose in how well it transmits that power. This answer is not the most technical but I hope it at least answers your question well enough to understand.

Yes, thanks Greg, that's a good simple description... The links in the description will give some approximations of efficiency for different size and dimensions of loops. Additional losses are incurred in points of electrical resistance like the joints in my accordion folds. With antennas everything is a trade-off. Size, bandwidth, and efficiency are 3 primary competing factors. Small transmitting loops (aka mag loops) are very small for the frequency of operation, and are actually relatively efficient radiators, but as a trade-off, they have very narrow bandwidth (high Q). And despite that relative efficiency, there comes a point of pushing too small of a loop too far, and losses grow.

Yes, I just need the free time to make a build video. Been working hard on antenna kits, winders, and new 3d printed paddles. Hopefully soon?

@@K6ARK Isn’t it great to be busy!

@@PatAutrey I need to retire! This dang work thing is cutting into my fun time.

@@K6ARK I was wondering if you have ever used an auto tuner to tune a magnetic loop directly ,- I have the MFJ 939 auto tuner and was wondering if it would work on a magnetic loop that I made mounted to a hula hoop, would not be using any other tuning circuitry other than this automatic container what are your thoughts o

Jeff, it's a homebrew key made out of a sheet of acrylic with a slot cut in it. The paddles are pieces of shim stock material, and a small screw provides the ground contact.

photos.app.goo.gl/D7EqdfsHLjLyBmGTA

@@K6ARK very nice... thanks

@@K6ARK yv5fck Armando very difficult to see the electrical connections and the contact points, I think there are in the acrylic parts that form the box

The idea is relatively simple. The capacitor tunes the loop BUT at that tuning, impedance increases as you work your way up either way from the feed point. So you want a 50 ohm point and it varies by frequency. That's why the alligator clip. Tune the loop for minimum SWR (maximum received audio noise) then move the clip for 1:1 SWR. Once you have found the sweet spot, put a mark on the antenna with a Sharpie or similar so you can find it again.

@@thomasmaughan4798 Thanks. Yeah, I get the alligator clip concept. Where I run into a wall is the "box". As I mentioned, I'm a good instruction follower, but my electronics technical skills are deficient, hence my plea for detailed construction directions. I like the ,"The idea is relatively simple." So am I ;-)

I found this thread on another method of marching that looks enticing - Thomas mentioned this in another comment thread as well. forums.qrz.com/index.php?threads/want-to-homebrew-a-loop-tuner-box.522931/ Might have to try that out as well.

@@Brewerbill Ah, careful reading, I see. Yes, the exact nature of the tuning capacitor is crucial. It is common to use a "butterfly capacitor" because the relatively high RF current does not then cross a "wiper" or rotating contact. What happens is each side of the loop has its own stator plates and *between* them pivots the rotor plates and acts as the capacitance between the stators. Consequently a high RF current never crosses a wiping contact. There's a hidden benefit to butterfly capacitor; the rotor can be grounded! That means it can be used as part of the feed circuit. The shield of the coax goes to the rotor of the butterfly, and the center conductor goes *through* a smaller variable capacitor to one of the stators. Electrically it resembles a gamma match; but instead of an auto-transformer action, this works on the difference in capacitance which thus divides the capacitive reactance between the main butterfly and the matching capacitor and the whole thing can be at the low end of the loop. This is how the MFJ portable loop tuners work. You can see the schematic online for the MFJ 936B for instance. Conventional magnetic loops with a little induction coil can be difficult to achieve a perfect match. You can tune the loop easily enough but you cannot ensure a 50 ohm impedance. To achieve the 50 ohm impedance you have to tinker with the spacing and size of the induction or exciter loop and if it is high in the air, a bit difficult to reach! Capacitive matching eliminates that complexity, but at the cost of another variable capacitor.

@@K6ARK The purpose of the matching capacitor is not immediately obvious since it *seems* to be in parallel with one of the main tuning capacitor sections, but in fact is is in series with the radio transmitter. When everything is in tune, the radio presents a 50 ohm resistive source without reactance; the matching capacitor is effectively in series with one of the sections of the main butterfly tuning capacitor. This creates a "voltage divider" exactly the function of a gamma match; but the gamma match relies upon inductive reactance and the capacitor match relies upon capacitive reactance. AT resonance it doesn't really matter! Initial tuning is NOT entirely obvious. You start with LESS matching capacitor and then find the strongest receive signal by tuning the main butterfly capacitor. When you have it, transmit a very low power continuous wave (I use "FM" mode on my transmitter) and adjust match for lowest SWR. Doing that changes the main tuning slightly, so you have to retune the main. With a bit of practice the procedure takes only a few seconds and the matching capacitor usually does not require adjusting within the band; only the main tuning capacitor. Being able to have the entire electronics of a loop antenna in a single box is hugely convenient. It means you can have as large a loop as you like and it can be highly irregular in shape if that's what it takes to make it fit. It makes it almost convenient to have several loops each optimized for a band, which is to say, the largest that you can make and still tune it. 20 feet of 3/8 or 1/2 inch copper tubing makes a great 40 meter antenna. Occasionally you'll encounter an amateur radio operator that thinks it isn't a magnetic loop antenna unless it is magnetically (inductively) coupled; but that is not the reason for the name. Magnetic loop antennas are named because they couple to the magnetic field of an incoming radio wave (or excite the magnetic field of an outgoing radio wave). Gamma matching and capacitor matching do create a DC electrical connection to the loop from the radio which in rare circumstances might be a safety consideration.

Here you go... photos.app.goo.gl/vyJRVBGFBP7FMgbR8

The primarily limiting factor for loops is the capacitor which experiences very high voltages. Because mine is built with 500v fixed value caps and a small air variable cap, 5 W is pretty much the limit. To push it up to 100 W, I would need a much larger, more expensive vacuum variable capacitor or a very large air variable with big spacing between the plates.

No, I've never seen one. It's not a particularly efficient antenna, but as you could see, it does work. I generally prefer an EFHW with very small wire which packs down much smaller, but obviously requires a larger footprint when set up. Your best bet if you don't want to build your own is probably one of the mag loop models that uses coax as the loop. I still plan to make a build video for one of these antennas, just haven't had time. Work and life are taking up a lot of that lately

Yes, but as I understand, the feed point needs to be opposite the capacitors unless you have a double loop. Placing the capacitor at the bottom would require the feed point to be at the top. That definitely works, it's just a trade-off.

Thank you Adam, good point, love your projects and yt channel, hope for qso 😃be safe 73!

The impedance of the loop varies with highest impedance at the capacitor. it's a tuned circuit so the net reactance is theoretically zero BUT the feed from the radio isn't part of the tuned circuit and will see the same current everywhere in the loop but a much different voltage. This produces a smoothly varying resistance that will be very high at the capacitor and very low, nearly zero, at the far side from the capacitor. The alligator clip on the feed allows to clip onto the loop at the point where it makes 50 ohms. Not shown here is that you can make a two-capacitor feed like the MFJ loop tuners, in which case the capacitors are at the bottom.

Yes, it will do SSB as well, particularly on higher bands. It's a little less efficient of a radiator than larger antennas, but certainly works well enough.

Indeed... photos.app.goo.gl/vvxfF9YWXS3D2kKK8 Fixed capacitors on a 3 position switch in parallel with a small air variable cap. The switch swaps in the fixed capacitance required to get the antenna resonant within each band and the variable cap tunes across each band.

@@K6ARK TNX - I worked on a similar project some months ago and had trouble getting suitable results with switching the capacitors so I've abandoned it. Perhaps I'll go back to it and try reworking it. I purchased a high voltage capacitor kit from Amazon for about $9 and have been using them with good luck on other antenna projects. They're very tiny and should work on this loop project.

@@glevideo cool. Feel free to email me direct for any questions/discussion on how I make it work. Also, if I remember correctly, some of the ceramic caps I used drifted too much with 5w of RF going through them, so I switched to silver mica. Class 1 caps should work too though (C0G or NP0). I believe I was using class 2 not knowing what impact that might have, and the result was drifting tuning in a very high Q circuit.

@@glevideo Sounds interesting, I must leave in my home in Caracas Venezuela all my junk boxes of many years as a ham, and here in USA the pandemic situation doesn't permit me to assist the hamfest usually happens here. Could you share the specs of the cap on Amazon? Many tks from YV5FCK Armando in MI.

Chris, I'm using this one now. amzn.to/3eCEXks If you prefer AAA batteries instead of the rechargeable LiIon, consider this one amzn.to/3oa5f0H

@@K6ARK thanks. I’m still hard at work on improving my cw, and at times I’d like to go back and listen through my QSOs.

Yes, I have used them for SSB. ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE--5x8n_NDD1M.html They definitely work. Not as efficient as an EFHW or a dipole, but in the right direction (off the ends), they perform well, especially on the higher bands where they are more efficient.

Pierre, to identify the needed fixed capacitors, I set up the loop and attached a larger air variable capacitor across the tuning point using short wire leads and alligator clips soldered to them. I tuned the cap until I found resonance, then removed the capacitor and measured the capacitance with a LC meter. Knowing the capacitance range of my small air variable, I determined how much fixed capacitance I would need to have the same value I measured from the larger air variable. I did the same for each band. I determined the gamma match length and configuration experimentally by arbitrarily choosing a starting length and then testing. I would move the contact point closer or further from the feed until finding the match closest to 50 ohms with the capacitor set to resonance. Hope that helps!

@@K6ARK It does help! you did experiment and came to specific value for your specific antenna. can I ask you what are the value you got? And confirm that the small capacitor is 1 to 7 pF??

@@K6ARK this will give me some starting point as I dont have larger air variable to test.

@@pierremartel3552 yes, the small capacitor is roughly 1-7 pF. Total capacitance needed will vary by the size of the loop and the frequency. Smaller loops with lower frequencies require more capacitance. For a 20-40m antenna, a 2m diameter loop is ideal. Check the calculators in the description for capacitance requirement estimates for a given loop size and frequency. They should be decently accurate.