Ham Radio - Magnetic loop transmitting antenna overview and details. 

Keith, did you short circuited the braid with the center conductor of the coax? 73 from py2top

@@fabiomeneghim4149 : Yes I did. Effectively, the coax became a fat "wire". The coax I used was the cable-TV version of RG-11/U, so the braid was stainless steel. I had to mechanically crimp a bullet splice to the center and braid, then solder to that. I used high-temp oven crimp splices for the transition... de WB2VUO

+rod byam Thanks for the kind words Rod. I'm no expert! My experiments are about 45% intuition and guesswork. But I'm always happy to share what I've learned that works. Check out my other video on my field day vertical antenna. It's the best performer I've ever used in the field.

Yeah, I'm almost embarrassed by the lack of quality on those old vids.

budleyca1 Thanks. I was kind of proud of that little box. I used a tricky bit of wiring with two relays as reversing switches, and each relay needs the other one to be off in order to operate. That way you can't press both buttons and short out your supply! I suppose someone using more modern approaches would just use a microcontroller.

Nice and simple hey? If he had a really good capacitor to put on it, that thing would be even more surprising.

Actually, no. Its good but there are a few others that are at least as good. Vk3ye has many videos on mag loops and Andybythesea has a few very good videos on his loop antenna with efficiency figures etc. on different bands given. Have a look, you'll learn a bit more about these interesting antennas. Fantastic if you don't have much space!

Sorry that should have been Andy Davies by the sea

So, how did it go?

@@ERICtheLATE Very well, I built one with copper pipe and PVC tubing

Thanks Paul, they're fun antennas. If you haven't seen it yet, check out my junk box magloop. Build: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-b9KSDvjycyw.html And the final design: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-ZTsYRyEkCkQ.html

🤣 75% is a joke I think. That antenna needs a ground plane. I would put it right under the driven loop.

Gwendolyn H. Thanks for the kind words Gwen. And if I tune to just the right frequency, the big loop starts to glow in the center. Maybe I should step though. My emails on my QRZ page, maybe we can set up a SKED on 40 or 20 and talk antennas.

Kevin Loughin /When I get my 40 meter loop built and the band conditions are right, I hope to hear you on the coax. I'm still working on my CW for Field Day next year. 73, Gwen - K6GRH

Thanks for watching Michael. I'm surprised more people don't do the FS meter approach. It's much easier to fine tune with it. Check out my junk box magloop video. ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-ZTsYRyEkCkQ.html

Michael, I've ran 5 to 100W with 1" diameter wire, wound ~8x to drive an LED without problems.

I saw a video of yours that flattened the small interior coil. Are you able to do that with this loop?

It's being limited by his capacitors minimum capacitance. If the loop was made smaller, that would also fix that, but efficiency would suffer more down lower. Another possibility would be to make it only a fraction less in circumference and use larger diameter tubing. These will both simultaneously raise its resonant frequency by lowering the loop inductance but the larger diameter will improve the efficiency. Sure it will get heavier, but you will get 10 meters

It was easier to bend, no other reason. ;-)

Wow, thanks for the interest. The larger the diameter, the lower the radiation resistance and greater the efficiency. Bandwidth will also increase just slightly. You'd probably need to go to a larger diameter loop to really get down to 80 meters with any efficiency though. I've driven mine to 40 watts, it arced across the cap around 45, so I keep it to no more than 30. Caps, well, I haven't looked online, I picked mine up at hamfests. There's usually plenty around there. They can be built weatherproof, there are several vids I've seen where people have done so and put them outside. The field directly around the loop is pretty strong. What I've read indicates you want to be at least 1.5 times the loop diameter away from it. But we are talking HF here, and the wavelength is far too long to have any effect on cells or dna. Plus, it's non-ionizing radiation, so it won't affect you physically anyway. There's 90 year old guys out there that worked in broadcast transmitters for decades in higher fields, and some of them still have hair even. ;-) Thanks for watching.

Dennis , I built a loop with 3/4 copper at 12.5 feet circumference . Its great on 40 meters . But the size of the small loop is a question and my cap was too low . So I am going to upgrade the variable cap to 1000uf as the loop only work on 40 and a little 20m . but very good.. And i am going to upgrade the small copper loop to 3/8 copper. So I enjoyed your post ..thanks for your post as mag loops are fun to build and try .

If you go with a larger cap, it will lower the frequency range. I think you need to go to a smaller cap.

I messed around with several different variable capacitors and a 1,000 pf is a little too much for 80 meters. A 750 pf might be good and it should get you down to 40 meters. The increase in capacitance will mess you up on the low bands though. The increase in circumference of the loop also would need to be decreased which then messes up the coupling loop which also has to be reduced in size. I am thinking with what I have a 600 pf capacitor would get me into the upper end of the 75 meter band and work good across 40 meters. I want to experiment with some flat bar aluminum stock 2 inches or more in width to see what it does. The aluminum is cheaper and easier to work with. Good luck on your mag-loop.

+Skip Pickens The tuning cap was in a box of a few of them at a hamfest. I think it goes from 20-150pf or something like that. It's not terribly critical, but you do want decent spacing between the plates to prevent arcing due to the high RF voltages at the cap.

It's been awhile. I think somewhere in the area of 40 bucks.

Kevin, Regardless, it's still a great video and cool idea using PVC pipe. I worked for the Navy for 24 years in Avionics and did some Doppler Radar antennas. You're video reminded me of one of the antenna for the AWG-9 Weapons Radar for the F-14 Tomcats. Thanks for the cost too.

Hello Leonardo, yes, you can use any capacitor you like for receive only. The high RF voltages are only created by the power coming from a transmitter.

At 6ft. diameter with a cap that large, it's probably going to tune from below 3 MHz and maybe just get up to 7. The killer on magloops is the resistance of the outer loop. I've seen some use copper foil wrapped around plastic pipe. It will work with aluminum foil, but not as well.

Thank you, I’m having nothing but trouble with it, I think I’m going to make another, 9ft in circumference, which seems to be more standard.

Any transmitter will. Receivers will not.

Thank you !

The controller probably uses some kind of micro-controller and pulse width modulation to control motor speed. That would make noise for sure. Put a switch on it for the battery. Turn it on when you're tuning, and turn it off when you're done tuning? My controller is just two relays and push buttons with the relays wired as a reversing switch. I have a switchable resistor to run the motor slower for fine tuning. Since it's passive, there's no noise.

Tnx for the reply and info, Kevin. Yes, the controller is a PWM DC motor controller. A switch would be a simple solution... hi.... Will definitely throw one in until I come up with a controller without the micro-controller and PWM speed control..... 72 de wa4chq

Hi Robert. I found it in a box of them at a hamfest. There's always some guy there with boxes of old parts.

what is meant by 5/15th? Are you referring to size of the copper tube? 1/2 , 3/8, 1/4 , 5/8, they also sell copper wire , is the key to this project using the tube?

It's a simple circuit. The diodes with a short pickup wire are near the main loop, two wires bring the rectified DC back to the control box where the pot and meter are. Have a look at my vid on building a field strength meter, it will be clear. ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-8Dd0oEzDepA.html

Much appreciated.

As long as it's a circle, it should work. Just make sure the coupling loop is 1/5th the diameter of the outer loop.

Hell Fernando, Figure the number of kinks needed and flat sides, divide the circumference of the loop to determine the length of each flat side. Then kink the pipe at those intervals. Make final adjustments to get a roughly circular shape. 73, Kevin.

Thanks Kevin for the prompt reply and the blurb. Take care. Fernando N2FQ

Daniel Block good tip!

Daniel Block you can also buy the tube in 10 and 20 foot lengths already rolled up in a box. You just make it larger by unrolling it and you are done.

Richard, that is the soft copper and is meant to be bendable whereas the straight tubing is meant to have greater rigidity for straight, strong, rigid runs. KC0GMF

Don Riedel You can also put irrigation tubing into the copper tubing to give it more strengh and prevent smashing or cracking. Irrigation tubing is usually sold coiled up and is made from very strong and hard plastic but it does flex well. This will allow you to use the bendable copper and get a better loop shape. I'm currently ordering 1.5 inch OD and 2 inch ID plastic tubing and copper foil to make a completely flexible loop to mount on a vehicle. The foil wraps the smaller tubing which slips into the larger tubing, then a strong platic rope goes through the small tubing and is pulled tight and tied. The goal is to survive tree branch assaults at highway speeds.

richard vaughn That’s a great idea 💡

I have seen them both ways. I also recall that current lags voltage by 90 degrees on a radiator, so I suspect that current is higher around the coupled loop area. I have taken a field strength meter and found that radiation does seem to be slightly higher towards that area. My gut instinct is that with the cap on top, you might be radiating a bit more of your power downward toward the ground. When I built my junk box mini loop, I tired some experiments with it. Broadcasting WSPR on 20 meters, I looked at the foot print on wsprnet with the capacitor down for 15 minutes and then with it up for 15 minutes. The footprint was noticeably further out with the cap down at the bottom. I repeated the experiment three times over almost two hours with the same results each time.

Current and voltage are in-phase in a resonant circuit. That's the definition of resonance. Phase has nothing to do with it anyway. The electric field is highest across the capacitor gradually decreasing to a null opposite the capacitor. Current is for all purposes uniform around the circumference of the loop, assuming the loop is a small fraction of a wavelength. The coupling loop has only the current of the 50 ohm system and does not need to be large, except for mechanical support and stability. The main electrical problem is voltage from each end of the capacitor end of the loop to ground is half of the voltage across the open ends of the loop. The advantage of using a split stator capacitor is shaft or frame voltage is closer to zero, instead of half of the open end voltage. A split stator capacitor mitigates that problem, reducing common mode on the feedline and mast. Most articles miss this point, and probably have the feedline doing a lot of the radiating. This is why vacuum caps and other caps with the shaft tied to one connection point are not the best electrical choice for loop balance, which is critical for divorcing the feeder from the loop so the loop alone is the antenna.

No they are not, Jerry. A correctly loaded antenna is 90 degrees out of phase in a resonant circuit. Current drives the RF and the electric field is the unwanted component. This is why mag. loops work so well relative to their size.

That's totally incorrect. Current and voltage are in phase across a parallel resonant system. You better get out the textbooks. hyperphysics.phy-astr.gsu.edu/hbase/electric/serres.html

+Jerrythenerdful that vid was a few years ago, I've learned a few things since then.

Yes, copper tubing will work a bit better than coax. You want as low of an electrical resistance as possible in the outer loop. It's a pretty simple circuit, not a lot that can go wrong in building it. There are lots of types of noise that could mean different things. A constant whooshing could be noise from an internet terminal. A buzz could be florescent lighting or power lines. Static is just atmospheric. You should hear very little until you tune the antenna to your frequency. When tuned, you'll hear static come up quite a bit. If you tune the radio 20kHz in either direction, it should get quite again until you tune the antenna to your new frequency. They are that narrow. Double check your wiring. If you're using coax for the outer loop, make sure the center and shield are tied together, making it one long single conductor. Make sure you're connected to the right terminals on the variable cap. I don't know what else could go wrong.

Ok, thank you again.

Nope. As long as the overall shape is symmetrical.

@@loughkb Ok, good to know. Thanks very much.

I've tried four methods. Coupling loop, gamma, wrapped loop, and torroidal transformer. I didn't see any major difference between them.

@@loughkb Thanks Kevin, I have seen that gamma matching can change the directionality of small loops favoring the direction of the match side of the loop. Upsides and downsides to this I suppose, if it is true, you then need to rotate 360 opposed to 180 :) I didn't know if there were any other advantages/compromises though. Thanks again!

Thanks Gail, I used the smaller pipe for the smaller loop because it was easier to bend smoothly. By making it as uniform and circular as possible, the loop's very linear across it's whole range. The shorter the wires to the condenser, seems to also help with linearity across the whole range. It will come right down to 1:1 easily at the top end as well as the bottom end.

Kevin, early this summer I threw together for grins, a FM broadcast magloop. The capacitor had to be real small value like from 9 pf to 2 pf. The loop does work and tuning is real sharp and directional. I need to have time to refine it and haven't done anything for a few months since we have been busy and now we are in the harvest season. I want to try something different on the HF loop and would want to increase the diameter of the tubing to at least to 2 inches for a broader bandwidth plus more gain. I might op-out and use flatstock aluminum bar at least 2 plus inches and a longer length than what I currently have. A good butterfly capacitor is what should be used with these loops.

Hello Maurice, I can't remember where I found that one, it was years ago. Search for 1 or 2 rpm geared down DC motor on amazon.com and you'll probably find a few choices.

I'm sorry, I don't understand what you mean by "approve". If you want to know how I tune it, maybe this will help. ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE--fVhYwqYci4.html

Sorry about my bad English. I mean antenna matching with transceiver. Because input resistence of magnetic loop and output resistence of feeder are different.

When tuned to the operating frequency, my radio see's an SWR of 1:1. The coax from the smaller loop goes directly into the radio.

It is interesting. Ok, thank you. I will continue to experiment with my magnetic loop.

Дима Еременок .. Ahh, the land of the vacuum capacitor ;) Try this site. Ignore the big words, the pictures say plenty. www.nonstopsystems.com/radio/frank_radio_antenna_magloop.htm

I don't know what to say about that capacitor. I'm not sure what you're asking. If you're talking about how sensitive it is to tune, then, yes, it is very sensitive. I use a DC motor that is gear reduced to 4 RPM and that's really too fast also. A series of plastic gears would be needed if you're going to tune it by hand. Or a large and really small pullly and belt.

"What is our neighbour doing inside his living with that copper doom device? Is he going to irradiate us?"

I can run 40 watts.

Their radiation pattern is toroidal, like a big doughnut. I've worked from the US to europe with the loop in my living room, inside! If you turn it on it's side, it radiates the same as a vertical antenna and become omni-directional. It's always a surprise when I use it.

How did it go?

Sorry, I don't know. I didn't use a mathematical approach to this build. I copied the very common dimensions and it worked out fine.

Thank you, your work is fascinating and has inspired me to build my own. 🙂

It's the ratio. To match 50 ohm co-ax its about 5:1 (fifth diameter/circumference for the Faraday loop). A ferrite ring through the main loop, about 9 turns for the co-ax. Or maybe if you use a twin feeder to the open ends where the capacitor is.... Around 250 ohms or so(?) but I've only played receive here.

I 3D printed it, it's custom.

It goes up to around 160pf. I picked it out of a box at a hamfest. There's almost always one guy at those with a box of variable caps.

I found it on amazon.com, I think. It was a long time ago. I think I searched for 1 RPM gear reduced dc motor, and found several options from 12 up to 24 volts.

Hello Mike, I thought I'd mentioned that in the video. I know I have on my other magloop vids. If you bounce around a lot in frequency, magloops need retuning constantly. If you're doing a POTA activation, or something else where you tend to sit on a frequency, they are fine.

That guy's intense! Ha. Good vid.

I picked it from a box of variable caps at a hamfest. There's always a guy selling them. I think it's around 350 pf when fully meshed.

+Randy S Indeed. Were it that I had a camera with such capability, I would set the focus and exposure manually.

There's a small half inch bit of wire near the main loop. The diodes rectify the RF picked up by the wire and send a DC voltage back along the control line to a meter in the control box. This gives a nice relative radiated power indication for fine tuning. I can tweak the loop for peak radiated power. A bit finer adjustment than what is capable with most radios built in SWR meter.

Good Idea Thanks

+Cristian Anastasoaie They were germanium diodes, glass body types low forward resistance. The meter movement can be any type, non-shunted. I used a 10K pot for sensitivity. Oh, and you want a .01 capacitor across the wires at the diodes to prevent RF going along the wire back to the control box.

The capacitor to be in series with the diods? :D tks for answering 73

No, not series, you wouldn't get any DC voltage to drive the meter! At the antenna end of the control cable, you put the capacitor across the two wires from the diodes. It will pass RF like a short, leaving the rectified DC to travel down the cable to your meter.

+Kevin Loughin If si not to much to ask...can you make a schematics with all the values of the circuit?

Cristian Anastasoaie It's almost identical to this one, with the diodes and 470pf cap at the antenna and the rest in the control box. The 'antenna' is a 1 inch bit of wire, near the big loop. home.alphalink.com.au/~parkerp/gateway/fsm.gif

Well, in this application, that's not actually indicating Standing Wave Ratio. It's indicating a relative power reading and is non-calibrated. We only want to see maximum deflection. That meter movement was originally in an old CB radio SWR meter enclosure, but I'm not sure of it's full scale current rating. It might be 100ua or 1ma. Shouldn't really matter though. You're going to get more than enough energy from the pickup wire near the loop.

Perfect. Thank you very much for the response.

Yes. That's correct.

Ok, thanks.

If I wanted to make one of these antennas just for receiving only, would you tell me what kind of variable capacitor would be needed? I mean, I guess it wouldn't need to be able to handle much voltage. But I would guess a capacitor with no 'stop' would still be a better option.

Oh, and does the small loop actually make physical contact with the big loop? Or is it just in 'proximity' only?

To answer the first question, no, it doesn't need to handle voltage if it's just receiving. So any variable cap should work. It should have a range from 10 to 300pf at least. Second, no, they're not touching, just close to each other.

+ke0nc That sounds right, But you also have to consider the greater surface area can lead to an increase in bandwidth. In an earlier experiment, I used 12 gauge wire and later changed the outer loop to RG8 coaxial braid and saw an increase in bandwidth. I think that are other factors affecting it. The lower resistance does increase the efficiency though, due to higher RF current in the loop. I'm pretty sure of that.

+Kevin Loughin Run the numbers through the 66pacific calculator, and change only the tubing diameter. You will see the correlation in the results. I remember reading about a Japanese man, who built a VERY large diameter loop, that was so sharp in Q, it was only good for CW. I really like your RF diode sensing circuit. I wouldn't mind doing that myself, but I would have to run more wires again. 73

ke0nc Yep, like I said, that sounds right. My experiments are looser, I think I did say there's lots of math out there to use to design these. Thanks for the correction. And thanks for the kind words on the feedback strength meter. It really makes fine tuning quick. One guy emailed me to tell me that he's adapting that method to his automatic tuning project using an arduino for control.

The motor does create some brush noise. But that doesn't really matter since your generally transmitting when fine tuning. It's not enough noise to interfere with the rough tuning during RX mode.

It's called magnetic because the inductive coupling between the small and larger loop is magnetic in nature. The near field around the antenna is a magnetic field. A short distance from the antenna, those magnetic waves create an electrical field which propagates away from the antenna as an electro-magnetic wave.

@@loughkb Thank you for your answer. Did I get right; Nature is in a magnetic position that calls electromagnetic waves towards it. This magnetic medium attracts the electromagnetic waves from outside. However, my question was this; Why magnetic? that is, why isn't it called "electrik loop antenna" instead of "magnetic loop antenna"? After all, EM wave, which is a combination of two components, is produced by the antenna.