How to build an "Optimum" Magnetic Loop Antenna - John Portune W6NBC M0GCK
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- Опубліковано 19 гру 2022
- Mag loop have been wildly popular since their appeared in ham radio in 1968. Yet working from “guess and golly” internet designs, disappointment has not been uncommon. Here are the six main factors needed to optimize all mag loops, plus a quick look at two low cost full-limit high-efficiency optimized PVC and foil tape designs: 10 through 40-meters and a compact version for 6 & 10-meters. Also, no expensive vacuum variable or heavy-duty rotary capacitors required.
This presentation was made at the QSO Today Virtual Ham Expo. For more information go to: qsotodayhamexpo.com Also visit the QSO Today Podcast for 1 hour interviews of ham radio operators who make these presentations: www.qsotoday.com - Наука та технологія
Great thorough explanation of the mag loop antenna!
I used 4 inch diameter aluminum dryer vent tubing. Mine is 4 foot square with a square made of 3/4 inch copper water pipe for the sense or inner loop. I eventually found by trial and error I could get a good impedance when the inner or sense loop had a 7 foot circumference.
(Note, it was very easy to build my design using dryer vent tubing and the elbows did not require being split. I suggest using it the next time if you don't mind $8 for each fitting and each two foot length of tubing. 3 inch dryer vent is also available and slightly cheaper)
I am very, very happy with the antenna.
This is mine experience:
1. Antenna blade must be whole.
2. Material - copper or aluminum, but aluminum gives losses in transmission about 10% more at the same size than copper (according to various programs for calculating magnetic antennas).
3. Antenna shape - better round.
4. The area of the antenna web should be as large as possible. If it is a pipe, the diameter of the pipe should be as large as possible (as a consequence, the external area of the pipe will be greater), if it is - a strip, the width of the strip should be as large as possible.
5. Antenna blade (pipe or strip) should approach directly to the variable capacitor without any intermediate inserts of wires or strips soldered to the antenna blade and the capacitor. In other words, it is necessary to avoid packs and "twists" in the antenna web where possible. If it is necessary to soldered something, it is better to use welding, for copper it is copper welding, for aluminum it is aluminum welding to avoid metal inhomogeneities in the antenna web.
6. The antenna web should be rigid so that there is no deformation, for example, from wind loads.
7. The capacitor must be air dielectric and have a large gap between the plates, even better is vacuum.
8. I have a capacitor with an electric motor closed in a plastic box. At the bottom of the box there are two small holes to drain the condensate.
9. There should be no slip on the condenser, so you need to use a condenser type "butterfly" in which the stator plates are connected to different ends of the antenna web, and the rotor is not connected to anything.
10. Coupling loop has a diameter of 1:5 from the diameter of the antenna, It should be taken into account that with a decrease in the diameter of the coupling loop increases the quality of the antenna, and thus its effectiveness, however, narrowing the bandwidth of the antenna. On the Internet, I found information that you can use a coupling loop diameter of 1:5 to 1:10 from the diameter of the antenna frame.
11. Gamma matching is much more effective in comparison with the coupling loop. Gamma matching with the power of the coax cable wins the coupling loop on the strength of the received signal. A double gamma matching, symmetrical with respect to the zero point, and the feeding of the antenna by a ladder line is much more effective than a simple gamma matching with the coax cable. In this variant, a symmetrical antenna is obtained in which the in-phase interference is additionally suppressed.
12. In case of power supply by the coax cable in close proximity to the antenna I use a choke from the cable 6-7 turns of the same cable, wrapped on a ferrite ring from the deflecting system of the TV. In case the antenna is feeding with a ladder line, the choke is not used.
13. Antenna "does not like" near itself metal objects, long wires, etc. - this may affect the CWR and the directional diagram.
14. The height of the magnetic antenna above the ground should be no less than 0.1 of the wavelength of the lowest frequency band of this antenna in order to maximize its performance.
VE9LZ ex: VE3LL, VA3LLZ
D0G is not a German callsign! Haha! Yes, you done really well. I am experimenting with loops for quite a while now. You did great work, I will destroy all what I have and start from scratch. This really gave me some new ideas. And I wasted money on material, that never really worked. Thank you!
de dl7hh dDdDd
Regarding the question if Litz wire can be used for a MLA, I think it can, but a huge bundle of it would need to be used to have an equivalent amount of surface area compared to a large diameter round PVC with tape conductor of 2 inches or more (taking skin effect into account). I've been looking into this myself, but for transmit, Litz wires are not spaced far enough from one another in a bundle and due to this have heavy proximity effect which causes the same kind of radiation problem similar to skin effect, but for another reason. To have an efficient transmission using Litz wire, the insulation on the wire would need to be about three times the diameter of the copper of each wire to assure the wires are spaced at least 6X the diameter of each wire inside a bundle to prevent too much proximity effect from wire to wire or between them. I tried to find 40 gauge magnet wire with that amount of insulation or coating on it to make my own Litz wire for use above 10 Mhz, I can't find it. There is a type III magnet wire with extra heavy insulation for higher voltages, but not nearly enough to space the wires far enough apart to mostly negate the proximity effects. I found a place which could make it for me, but not in a small batch and very expensive for one 10 kg reel. Far better to just use large diameter copper or aluminum foil.
Minute: 51:42
Regarding his statement about loop polarization, my google findings are quite different than what he stated indicating a vertically oriented loop is vertically polarized, instead I found the opposite: A vertically oriented magnetic loop (perhaps any shape) will be horizontally polarized if fed from the bottom, vertically polarized if fed from the side of the loop.
That said, if a square shaped loop is fed from the bottom or the side, is it equally polarized in both polarities, or leans towards one or the other predominately? I have not found an answer to this when it comes to square shaped loops. I suspect the shape does not matter much yet I've seen John show RF radiation patterns for his square shaped 4 meter loop with both horizontal and vertical polarization which appeared to be equal in both. DE A75HF KL7FB ex WN8RZU, WD8DVV.
I've been working on a magloop for 80,40 and 20 meters. I was thinking of telescopic tubing being used on either side of a square loop. I have 2inch and 1and 7/8 inch aluminum tubing. The loop would be an 8 foot diameter loop when collapsed and 12 foot diameter when extended. The capacitor is vacuum variable 75 to 250pf.
Question is, will it have stable reception (no static noise from telescopic edges) and what kind of radiation pattern will develop from a rectangle?
Thanks again for sharing your knowledge 👍
Hi from NSW Australia just subscribed 73s 🤠
So for 27mhz single band use, what would the optimum circumference be? The antenna will be a circle.(When you say 4ft or 6ft, I'm not sure what you're referring to. Diameter maybe, area??)
So would the optimum length/circumference be just below a quarter wavelength? Or do you want to keep it a fair bit less than that. I was thinking 2.5 metres for, say, 27.355mhz. Half inch Heliax will be used.
These are wildly informative and fascinating and I'm thankful for Mr. Portune and those who host him for taking the time to share such antenna insights. I haven't yet gathered enough knowledge to intuit what's viable and what's not but I can't help but make [possibly naive] attempts to further simplify this really cool concept. For example, moving up to 3" PVC for a larger antenna is straightforward enough save for finding wider foil tapes that are affordable (even considering less workable flashing). I believe it was mentioned that tapes could be layered to get the desired thickness so would there likewise be an optimal way to use the more affordable narrower foil tapes to cover larger diameter PVC?
Alternatively, I've stumbled across a great price on some 3" square aluminum bar though my antenna-fu is insufficient for determining whether that geometry would affect an antenna negatively. The major appeal is that it would cost less and be far simpler to assemble since the foil and elbows could be eliminated altogether. I would greatly appreciate anyone's insight regarding this.
Tim - What we are discovering with the magnetic loop antennas is that the radiation comes from the surface of the loop and not the inside. Don't make your antenna too heavy using bar instead of tubing. There are a number of videos on UA-cam showing that wrapping PVC with conductive material can make a great antenna. Good luck on your project and please report back here on its success.
In regards to 51:45 "If a vertically oriented loop is fed at the bottom, its radiation will be horizontally polarized; feeding it from the side will make it vertically polarized."
Is this antenna horizontally or vertically polarized?
Where is the explanation of the metal arm at the bottom? What is it for? Is that where you hooked the coax?
Grt tyvm now i hv confirmed with ur presentation i can use aluminum tape wrapped aroungd a hula hoop of at least .5 minimum to around at least 5/8 diameter tubed hula hoop
What if I wanted to use 4in pvc pipe?
This is a video on optimising a small loop antenna, and you can't get any more optimised than for single frequency use.
Can you please specify for me what fraction of a wavelength is most ideal for performance and efficiency?
Probably 1/5 to 1/4 wavelength. Any larger and it gets difficult to get the capacitance low enough.
Will this loop tolerate 100 Watts?
John answers this question at ua-cam.com/video/ZmUQgyQ2uSg/v-deo.html . Short answer is that this loop will handle 1 KW.
BTW, My call sign, ke4nc 👍