TX Magloop for 11m - terrible SWR - Page 1
Forum section: electronics / rf-microwave

author: TinkeringSteve
author_rank: Frequent Contributor
timestamp: July 27, 2025, 01:53:17 am
content: I used that affordable material called "PEXb/Al/PEXb", 16mm outer tube diameter, actual aluminium pipe inside must be about 14mm, to bend the outer loop.I bent it around a 28" bicycle tire, so the outer diameter is around 64cm. The coupling loop as around 12.5cm. I tried coax, and 5mm diameter copper pipe as well.The coax one I also tried to squash, to have more of it touch the outer loop.I tried different positions and orientations of the coupling loop, or with and without a 1:1 choke balun (5 turns RG58 on 11cm diameter cardboard cylinder, according to some chart I found, that looked like the best option for 27 MHz).The drawing shows twin wire a the small loop - it's RG58 there now, too, going to the choke, the cylinder base of which is concentrical around the vertical PVC pipe which holds everything together.The capacitor is made of 2 alu tubes with shrinkwrap on the inner one.Its wires are connected to the left and right gap ends of the loop with screws going through the tube material. So their threading touches, through the tighly drilled holes, the thin aluminum material only with small contact surfaces on its sides. But the tension that is in the loop, which wants to get slightly bigger in diameter when the screws at the top are out, at least puts some pressure on those contacts.With the NanoVNA, it shows a clear SWR minimum in the curve at the desired 26.85MHz - so the antenna seems to be tuned, but...I never got a better SWR than 3.2or so, and that was 0.1mm, 1 degree angle work , mostly it's more between 6 and 4.The best SWR (measured with NanoVNA while fiddling, ~ confirmed later with SWR meter) I got with the small loop reaching slightly below the big loop (maybe 1 coax width), touching it at the front of that pipe, roughly being in a parallel plane of the big loop's, but, by necessity when reaching below, slightly outside to the front.Are any of these things a giveaway for my bad SWR?What could I try to improve things? (if making a loop from a better contactable material is among them, I have some bit of 10mm diameter coax that may be long enough. I am reluctant to saw open the mentioned bicycle wheel and use it directly:D The, according to online calculator, optimal diameter is a bit bigger than this, but it's less unwieldy that way now))About the pipe capacitor reaching vertically into the loop - I have seen reportedly succesful builds on this band doing it very similarly, just with thick coax.

author: ME
author_rank: Frequent Contributor
timestamp: July 27, 2025, 08:03:39 am
content: I made a big loop for 80m i could get a 1.2-1 swr until i tried to use ssb,the ssb bw was more than the loops, The swr went wild when i spoke so gave up ond scrapped the idea!, it was 30ft circumferunce and used 15mm copper pipe.

author: TinkeringSteve
author_rank: Frequent Contributor
timestamp: July 27, 2025, 04:21:06 pm
content: Heh, I was wondering about that aspect, too.But what dies the NanoVNA do when showing SWR? I assume it uses close to 0 bandwidth, i.e. not (deliberately) modulated per point on the swept curve.If that already is horrible, it's not that problem (yet)?

author: TinkeringSteve
author_rank: Frequent Contributor
timestamp: July 27, 2025, 07:32:05 pm
content: Btw. if you look here, below "coat hanger" (though the dimensions are described rather a bit above), have you tried that coupling method?He claims that the SWR was flat over a broader range than usual circular coupling loops.https://web.archive.org/web/20250209232011/https://www.nonstopsystems.com/radio/frank_radio_antenna_magloop.htm(I use the archived version because the original site seems tobe currently disintegrating, and the author of that very extensive experience collection is reported as "silent key")I will try that, too, if the simpler thing I'll try first fails: using 1/8 instead 1/5 coupling loop (vs outer loop), which is also mentioneed there that some people had more success with it.It is not explained why... but unless someone here suggests something concrete with good reasons behind it, I'll just wildry try things that seem to remotely pertain to this

author: shabaz
author_rank: Frequent Contributor
timestamp: July 27, 2025, 07:44:46 pm
content: I don't know the answer (I've never built such an antenna) but dusting off my personal notes from back when I was considering it, it would appear that it may be worth trying to rotate that coupling loop, see if you can reach a minimum SWR by doing that.Also, personally I would not have looped the cable to make the balun, I'd rather just use several ferrite cores around the cable instead, right at the coupling loop. I appreciate you've done the sums and feel the five air turns are sufficient, so feel free to ignore what I've written, I'm just stating what I would have done.I didn't entirely understand the construction (I had to google PEXb/Al/PEXb and I cannot tell how much of that is actually aluminium having never seen such pipe) but more rigorous material selection would be worth considering, since it's not every day one takes the time to assemble such a thing. Worth trying thick (maybe double-braided) coax, or using plain copper pipe/tube since then there's no doubt about the material. Online resources suggest LMR400 as a viable option.Regarding the coupling loop, RG58 can be pretty poor quality, so it may be worth replacing that too, but I don't know if that will make any substantial difference. You could use a double braided coax. Alternatively, how about 1/4" copper pipe for that coupling loop, that's not too expensive and easily bendable and drillable or solderable.That link you mention is a resource I looked at in the past too; sorry to hear that the author may not be around any more.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: July 27, 2025, 09:34:56 pm
content: I built and used a Mag Loop for years. They work very well if they are built correctly. The SWR is controlled by the matching elements. The typical small loop match is the most popular. You have to be able to move the loop up and down on a diameter line of the large loop to tune the SWR. The small loop should be opposite the capacitor trim of the large loop. It is VERY difficult to get a good match but the antenna will work anyway, You can also use different matching circuits such as a delta match or a gamma match. If you can get less than 2:1 SWR with a Mag Loop, you are doing quite well. There are even people who make devices to change the small loops position remotely so it works better on different bands. This group has a lot of information on these loops:https://groups.io/g/MagLoop/topics

author: TinkeringSteve
author_rank: Frequent Contributor
timestamp: July 27, 2025, 11:23:24 pm
content: Quote from: shabaz on July 27, 2025, 07:44:46 pmRotating the loop out of plane alignment only worsens the SWR for me.The alu thickness in the material seemed pretty thin, maybe 0.5mm. Copper pipes are rather expensive these days, though. For experimenting, 10€/meter is a lot less nice than the 2€/m the Pex/Al/Pex stuff costs. Of course, if that *actually* doesn't work, it would not be a benefitBut I don't know that, if it's only about skin depth, this material should cover it?The 5 windings of coax for the choke I took from here - the best option, when using air core, for 27MHz. Some of the ferrite ones can be more broadband.http://www.karinya.net/g3txq/chokes/

author: TinkeringSteve
author_rank: Frequent Contributor
timestamp: July 28, 2025, 01:49:50 am
content: I made more experiments.Different coupling loop sizes and more oval shapes did not do drastic changes.Making the "spiral" version of the coathanger coupling, changing the turn direction in the middle, as referenced earlier, looks promising, though.Whereas I had difficulty before to get an SWR < 5, and it was extremely finicky to ever see below 4,the current setup as seen in the photo gets me a 2.75 SWR on the VNA, connected via a 1m RG316 BNC cable.Though the Albrecht 11m/10m SWR meter disagrees and shows something like 3.3. And if I use the (ok, Chinese,...?) RG58 cable of 2m length that came with BNC connectors (BNC I use for everything as it's practical for experimenting - it's ok for 4W CB radio I guess?), then the SWR gets quite a bit worse still.But this is great compared to before. The SWR was a bit over 3 on the VNA when I had fewer turns of the black thick wire, so I made more turns and somewhat wider stretch to the sides, and soldered on the thinner wires that were laying around - no special purpose to that.So next weekend or so, I might try to get even more wire around, making the density more like in the referenced photo.I initially started going by the description "total wire length of a bit longer than the diameter of the loop", which seems way off. But the photo in the article shows much thinner outer loop material, so there it may be a different story.After this change, the resonant frequency changed from the 26.9 MHz I had for days, very stably, to 27.3 MHz, so I had to retune, to be on my test channel.Why might that be?So what does the overall picture look like now? Problem = "not enough coupling"? Why would that be, though, when 1/5 outer loop circle is the go-to thing for coupling.I am still wondering why the VNA shows a markedly better SWR than the analog SWR meter on the CB radio, connected by a 30cm or so readily bought PL cable, wheras the ANT output of the meter has a PL-to-BNC adapter on it. I am now also connecting the short 1m RG316 there like I used for measuring, as that there also gets better SWR than with the 2m BNC cable (yes it's 50 ohm - or so the label says, while nothing is printed on the cableThey're really skimping. Wait, I can measure that with the NanoVNA, too, right?).EDIT:One quick thiing I still tried was getting it in the center of the room instead of close to the window corner it was in (large windows, plastic frames, but there certainly must be some metal in there for the mechanics of German tilting windows to work). I.e. a bit more "free standing".The SWR on the NanoVNA went down from 2.75 to 2.32.Just too bad, as that window corner seemed like that chance to get actually out there

author: A.Z.
author_rank: Super Contributor
timestamp: July 28, 2025, 02:55:50 pm
content: Steve, please be patient with this poor guy; I can't understand the utility of such a narrowband antenna on the 11m band, I can understand its use on 160 meters, where antennas take a lot of real estate, but for 11 meters a dipole or an efhw would be viable, so .. why the loop ?That being said, check the "army loop" tuning approach and see if it may be of help with your loophttps://www.qsl.net/zl1bpu/DOCS/The%20Army%20Loop%20Illustrated.pdfand then, also check this design from Harryhttp://sm0vpo.altervista.org/antennas/20m_ant_00.htm

author: iMo
author_rank: Super Contributor
timestamp: July 28, 2025, 03:21:41 pm
content: The capacitor there has to be of high quality and for High Voltage (like 6-8kV minimum for example when talking 100W into it). The entire antenna is a high Q resonator, with pretty large currents in the loop and high voltages at the capacitor. So all the solder joints has to be made of a thick copper. Your cap does not look high Q.I have a 35cm dia loop made of a copper tube, and variable air capacitor with perhaps 2mm gaps there (for 6m band).Your capacitor may even catch fire.. Mine 5cm long piece of RG58 did immediately (I tried to add a small capacitance in parallel to the variable capacitor). Even touching the variable capacitor while TXing may bite you pretty well.The tuning into the band is difficult as you would need pretty fine adjustment of the capacitor (and it is narrow bandish antenna).I would not mess with that type of antenna unless you get motorized tuning of the capacitor..

author: TinkeringSteve
author_rank: Frequent Contributor
timestamp: July 28, 2025, 03:26:33 pm
content: The utility is that it's theoretically a lot more efficient than a rubberduck and has some chance of me actually reaching someone else, even if it's useless for talking within a "round" that manifests itself also geographically. I don't expect much, due to interference from close by conductive objects (e.g. grounded steel concrete in floors/ceilings)If there is no "real state", an antenna using any is too much. Anything markedly longer than 1m is too much.In short, the utility is that it's even remotely possible at all.

author: TinkeringSteve
author_rank: Frequent Contributor
timestamp: July 28, 2025, 03:52:16 pm
content: Quote from: iMo on July 28, 2025, 03:21:41 pmMy alu pipe capacitor is basically a sturdier version of the "piece of coax" (thicker than RG58) capacitor that seems to be fairly common for this, including one on a site of a pro RF engineer.The several layers of heatshrink tube hold the alu tubes at a distance where even an air dielectricum should be sufficient for that voltage at 20W, whereasI have max 4W, and the heatshrink should provide better insulation than air. At least according to this calculator:https://hamnut.com/calculators/magnetic-loop-antennaI can't judge how accurate it is, but it seems more detailed & practical than a number of others I've seen, and their calculations match a series of successfull 11m loop builds I have witnessed from somebody else, with dimilar dimensioning.Good tip about touching the thing!I luckily already knew that.I do have a 15 RPM DC motor on a 433 MHz remote ctrl reciver module that shoves the inner tube in or out by rotating an M3 threaded rod inside a long plastic nut which is wirestrapped & hotglued to the pipe - my orig post sketch is a bit outdated and not as detailed.The tuning seems to be fairly fine grained like that, almost annoyingly so

author: iMo
author_rank: Super Contributor
timestamp: July 28, 2025, 04:00:33 pm
content: 1m (or even smaller) diameter loop may work well for 11m band for Rx and for Tx, but it must be built properly. Perhaps you want do CB, there is the power limited, afaik. Make it from a copper pipe, like 6-8mm dia, or a coax like RG58 or RG213 (any thick coax - the outer shield is the "pipe"). It must be made of copper.The capacitor must be quality one (high Q from RF point of view), air variable with larger spacing, like 2mm, perhaps 20-50pF. You may even create your own, but always with air gaps.The capacitor must be joined with the pipe with thick massive low ohmic joints (high currents). Isolation materials have to be of quality as well.The coupling coil is not that critical (I would keep it isolated from the main loop). Inband SWR below 1.5 is then to be expected.Mind when TXing with the antenna even with 4-10W you have to be at least 2m off the mag loop as there is High Voltage and strong RF field at that antenna. When used in a small room mind the strong field may damage your electronics around.

author: TheSteve
author_rank: Unknown
timestamp: July 28, 2025, 04:05:43 pm
content: I made a mag loop recently. It works between 6.9 and 30 MHz. The SWR is the highest at the upper frequencies, the lowest I can get is 1.8:1 on the 10 meter band. I started with a design from this website -https://k1fm.us/2020/07/k1fm-magnetic-loop-2020-edition/I bought a variable cap from aliexpress and modified the case k1fm used to fit it, I then tweaked a few other items. So far it seems to work ok. I've made voice and digital contacts with it running 5 watts.Moving the smaller coil up/down may help, the diameter can make quite the difference. Any metal anywhere near it will also change the tuning a lot. The feed coax also has to be kept away from the tuning portion.

author: iMo
author_rank: Super Contributor
timestamp: July 28, 2025, 05:22:47 pm
content: When using variable capacitors from older radios - like 2x 250pF packets (for example), always wire the packets such there is none movable "wipe contact" in series with the RF current.So wire 2 packets in series - such the rotor will be the common part for both packets. The total capacity will be 125pF then (from the example), and the isolation will improve 2x as well (the air gap will be "twice" the single packet).

author: Geoff-AU
author_rank: Frequent Contributor
timestamp: July 29, 2025, 06:37:36 am
content: Quote from: TinkeringSteve on July 27, 2025, 11:23:24 pmThis tells me the coupling loop wants to be bigger. Make a bigger coupling loop and repeat the same test.

author: A.Z.
author_rank: Super Contributor
timestamp: July 29, 2025, 06:58:33 am
content: Quote from: TinkeringSteve on July 28, 2025, 03:26:33 pmwell, almost anything is more efficient than a rubber duck, in your case (11m), using a fiberglass pole (or fishing rod) you may consider setting up a vertical EFHW like this onehttps://www.aa5tb.com/efha.htmlso that the tuning circuit will be at bottom and will ease matching the antenna once installedagain a fiberglass pole or fishing rod or either a bamboo cane will be ok to support the radiator wire and the efficiency and ease of use will be much better than using the tuned loop

author: iMo
author_rank: Super Contributor
timestamp: July 29, 2025, 07:08:54 am
content: He would need ~5.5m long wire for the EFHW. There are "mobile antennas" for 11m CB band, a quarter wave shorted with a magnet base - put it in your window on a metallic base and that is the shortest wire antenna you may get (and it works).

author: A.Z.
author_rank: Super Contributor
timestamp: July 29, 2025, 09:11:21 am
content: Quote from: iMo on July 29, 2025, 07:08:54 amwhere did he ask for a mobile antenna ? Also, do you really think your "magnetic mount stick" has some real efficiency or will your next suggestion be about a linear amplifier?

author: CaptDon
author_rank: Super Contributor
timestamp: July 29, 2025, 12:36:31 pm
content: And nobody mentioned the most important thing about these loops which are similar in construction and theory to the MFJ SuperLoop. You tune the loop for best reception or highest background noise level at the frequency you intend to transmit on and ignore the V.S.W.R. as it may be nearly 2.2:1 If you tune for lowest V.S.W.R. you will be very disappointed with the overall results!!! Extremely high Q resonant loops have so many tradeoffs that nearly anything else is a better antenna unless you have some sort of physical space limitation. I used the MFJ loop with good results WHEN you understand how to properly use it. Lowest V.S.W.R. is NOT where the loop is working best for you.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: July 29, 2025, 02:32:17 pm
content: Capt Don is correct. With these loops typically it is very hard to get a good SWR. But they work. Since I built one, I can sympathize with the difficulties of design and assembly. His point about tuning the antenna to the proper freq rather than lowest VSWR is well taken. I have had a very tough time trying to get the VSWR down, but even a high VSWR does not mean the Mag Loop antenna is bad. As I recall, sometimes I could get a low SWR as TheSteve reports, but this did not last long with tuning the antenna for the next freq, even though the next freq was very close. I think that even inside a dipole is still a good antenna. It is just 5.2 meters long and can be wrapped around a room. If you bend back the ends of the dipole, you can get a shorter antenna with good VSWR.https://www.qsl.net/kk4obi/Center-fed%20Folded-end%20Horizontal.htmlMobile CB antennas use a big coil to make the thing resonant. They also need a big ground plane, like a big piece of metal to mount it on. Magnetic mount CB antennas need a big piece of magnetic metal to work well. There are special CB antennas for motor homes and such that include both sides of a dipole and are quite short and work OK. Mobile antennas rated for higher power are usually much better especially at low power. Better components and lower real resistance, but more expensive. Anything you do to actually make the antenna short causes losses however. It does decrease your radiation resistance. Magnetic Loop antennas have very bad radiation resistance and need to have very low real resistance to work well. This means things like welding vs soldering joints. A real problem for most of us. Current construction of truck cabs with fiberglass confounds many truckers who use CB.EDIT: A.Z. has a post about folding dipole:https://www.eevblog.com/forum/rf-microwave/an-efficient-shortened-dipole/

author: TinkeringSteve
author_rank: Frequent Contributor
timestamp: July 29, 2025, 08:34:16 pm
content: ..unless you have space constraints" - yeah, pretty hard space constraints. I don't even have a balcony. This is indoors. 2,4... 2,5m or so room height, and a corner where two walls meet where that half of the wall going into the corner is windows top to bottom. Plastic frames, but some metal inside for mechanics.It is all very suboptimal.I did make contact yesterday within my town, one was barely hearing me, the other, about 4km away, got S8.So it somewhat works.Note that narrow bandwidth of this may not be a big issue because CB radio is almost dead, and I found here from listening now a bit locally,that most action concentrates around 2 neighboring channels here.I.e. likely I won't have to tune that much.Less often, something is going on further away, but that's on SSB (where I, with a DSP radio, heard quite a number of French CBers who did - I did not know this was a thig on CB - contest work when there was a big amateur radio contest weekend also, a whle ago. My CB does not support SSB and those are too pricey for what I can do here

author: A.Z.
author_rank: Super Contributor
timestamp: July 30, 2025, 02:49:31 pm
content: OK I See, fear nothttps://hflink.com/antenna/#BBTDROOFand then, not only HOA won't barf for an invisible antenna, but you may also sustain that the coax is for an FM band antennano stellar performances but better than a rubber duck and widebanded ... heck you may use it on 40 m as well

author: TheSteve
author_rank: Unknown
timestamp: July 30, 2025, 04:00:16 pm
content: Here is a pic of my magloop. The large adjustment knob is very useful for the fine tuning required as I have no vernier reduction on the capacitor. It also keeps my hand as far away from the capacitor as possible while performing tuning.The whole thing easily fits in a laptop bag. The main loop support is a selfie stick. The light stand/tripod extends to over 2 meters, it is the biggest/heaviest part and not very ideal for a true portable solution - it is very solid though.The main box, mounts, knobs, and loop box are all 3D printed.

author: TinkeringSteve
author_rank: Frequent Contributor
timestamp: July 30, 2025, 04:43:29 pm
content: I noticed some people have the coupling loop on the top, with the feed coax crossing the entireloop, passing the capacitor at the bottom, some have the coupling loop at the bottom and the feed coax barely crossing the loop.Does it make any difference?

author: TheSteve
author_rank: Unknown
timestamp: July 30, 2025, 05:05:17 pm
content: You do need to keep the feeding coax away from the tuning capacitor/main loop - as soon as it is more than 3-4 inches away it seems to have very little interaction.The feed loop at the bottom is ideal in terms of the feed coax but not ideal for manual tuning. In my case if the tuning capacitor was at the top I couldn't reach it with the tripod extended and having my body so close to the loop during tuning wouldn't be ideal.