HAM RADIO - Magnetic Loop Antenna 20m-40m

15-240PF 2000V Single-unit Air Dielectric Variable Capacitor
www.ebay.com/itm/254836361298
99.9% Copper Tube 1/2" OD x 7/16" ID 10 Ft, Tube Wall: 1/32"
www.amazon.com/
www.qrz.com/db/VE3DAL

[@johnnorth9355]

The vageries of magnetic loop antennas take a lot of learning . They are very effective in conditions where normal antennas struggle due to noise and poor propogation and the directionality of their gain means that you can pull signals out where none seem to be. I love mine ,especially for QRP , and have had some very surprising results . Keep at it - you will learn to love it as you get to know it better. (I now adjust mine instantly entirely by ear !). It's a bit like learning a musical instrument. 73 M7BLC

[@VE3DAL]

@@johnnorth9355 Hi John, thank you for your feedback. For me definitely still a mystery everything about it, and that's what I liked. It will keep me going for future modifications. My next goal is to test at the cottage can't wait for that! Best 73's my friend.

[@danimac3953]

Nice project antenna. You may want to consider putting a 1:1 balun line insulation in the feed line coax at the antenna. MLAs are prone to common mode currents which will limit the amount of power you can apply to the antenna and limit the ability to get a good low swr. MLAs have very low radiation resistance so there can be 15 amps on the loop say at 30 watts plus thousands of volts on the capacitor. Soldering the loop to the capacitor will greatly improve efficiency and reduce mechanical resistance losses which is the biggest problem in loop design. I really like your project antenna. Great job and well engineered! Best 73s. Dan K1YPB - On the air since 1962

[@VE3DAL]

@@danimac3953 Hello Dani, thank you so much for your feedback! This was my first MLA , my skills on electronics are very limited and I have a lot of questions. You answer on of them that soldering the components are better than tie them with bolts and nuts. I been getting very good SWR 1:1 but I think the efficiency of this antenna is maybe 50% or less. I have some better ideas as the project still going. At the feed point I have some ferrite beads on the coax I'm not sure if helps. Wow you have been operating for a very long time that's awesome! Best 73's my friend!

[@danielzdanowicz6899]

@@VE3DAL Thanks for the reply. Your efforts are commendable for the first attempt. You are doing the right things. The diameter of a meter is about optimum for 40 through 10. Any longer and you would start getting lumped constants of rf current which should be avoided. A loop like yours with soldered connections and minimized mechanical IR losses can be up to 88% efficient on 20 meters even with as much as a 1.5:1 SWR. With 50% efficiency I might suspect higher than normal mechanical losses. This would usually result in "heat loss" due to the high currents normally flowing through the loop components. This can be tested quickly with extended key down time and say 35 to 40 watts, while watching the SWR meter slowing creeping up as the elements heat up. With the size of your variable cap, I would suspect it could easily handle 25 watts digital, 45 watts cw and 75 watts of SSB, and maybe even 90 watts of SSB with a balun in the feed line. However at those power levels the voltage on the cap would be in the thousands and many amps of RF on the loop. Needless to say... never contact any component of the loop under power, as the RF burns would be extremely severe. Even at 5 watts it will render a "bite" that you will not forget for a long time. RF exposure is also a concern with these types of antennae, so it is recommended to stay perpendicular to the loop plane, where the deepest null (approximately 20dB) is for safety. IF the loop is hard to tune, or the SWR changes when you touch or get near the feed line coax, and / or changes with rotating the loop you would most like have common mode currents (CCMCs) on the coax and should use the balun I mentioned earlier. The MLA magnetic "H" field is effective for a distance of about three or so loop diameters from the loop making it less bothered by surrounding (unless those surroundings are metallic), after which it starts performing like a regular antenna "E" field there out to its destination. If the "H" field is vertical, the "E" field will be horizontal. Being a Mag Loop everything is backwards from an ordinary antenna. Such as... ordinary short antennas are capacitive and need a coil to load (ie a short coil loaded vertical such as a Buddistick) where as... a small MLA is inductive and needs a capacitor to load it to resonance. The vertical mounted MLA sends out a vertical "H" field, but then dissipates and the horizontal "E" field takes over giving the small vertical ground mounted loop the equality of performance of a dipole "at height". In some cases the MLA will actually out perform a dipole, which makes the MLA a big favorite for portable ops. The MLA some times gets a bad rap from the grouchy old timers out there because they don't understand anything other than a dipole and are not willing to explore the technology of this type antenna. (Some OTs will never change... hi hi) If you can put up with the high Q of a MLA, it can be your best antenna solution for many circumstances, such as stealth ops in an antenna restrictive HOA or POTA ops where trees are limited or don't accommodate an EFHW wire. I have personally worked 7800 miles on an Alexloop one loop diameter above the ground with 10 watts on FT8... Not bad in my book. Best of luck on your continued experimentation. Have fun! 73s, Dan K1YPB

[@VE3DAL]

@@danielzdanowicz6899 Hey Dan, thank you so much for the info very helpful! I love to explore new technology and the challenges that comes with it, and MLA is one of them. This afternoon I changed the capacitor to be on top and the coupling at the bottom and I didn't noticed much of difference. I'm aware of the danger from this antenna, but is always great to know even more information about it. Thank you again I loved your comment. Best 73's my friend!

[@larrypicard5969]

I have quite a lot of experience with magnetic loops. You can see the two loops I have in regular service on my QRZ page. MFJ made a loop tuning box that was rated for 100+ watts. I have used one to make portable field antennas on many occasions with excellent results. It must be kept in mind that high voltages can develop across the capacitor. It is essential that you insulate the cap from the tuning knob or tuning motor particularly if you are tuning under power even a few watts. Ohmic resistances are a critical source of efficiency loss. You might do better by soldering a copper washer to the copper tubing. Alternatively, just using a screw with a large head and the smallest possible hole to connect the capacitor might be helpful. you can purchase a venier drive used to help with the small increments that tuning requires. It is generally recommended that you use a BUTTERFLY capacitor or a vacuum variable capacitor. Otherwise you will suffer ohmic losses at the wiper contacts. The effects of a very small angular change on the peak frequency is remarkable. With air variables you wiould have to use a reversible DC motor with gear reduction or even a stepper motor with gear reducion for romote tuning. My vaccum variable loop tunes remotely using a homebrew PWM motor control and a NanoVNA. Magnetic loops are surprisingly sensitive to ambient conditions; temperature changes and/or humidity changes will shift the position of the peak.

[@VE3DAL]

@@larrypicard5969 Thank you so much for the excellent feedback. I'm very new on MLA and any information is very helpful. My next goal is to figure out to make tuning remotely. I have a very small 12V motor with reduction gears with a controller forward and reverse that I used for my timelapse photography on rails, I'm thinking to give a shot with that. The gears that will be touching the capacitor I will make on my 3d printer. Also I will figure out how to do a better connection on the components by soldering. I will check your QRZ page. Best 73's my friend!

[@geirha75]

Great information. I was thinking about a monoband loop antenna for 20m CW portion. Would it work with smaller capacitor or a RG213 as a capcitor? Is there a way to find how much capaticance is needed?

[@VE3DAL]

@@geirha75 Thanks for the feedback! I very new on magloop antennas, yes it does work with a smaller capacitor it will be limited on output power. I'm still try to learn what band needs how much capacitance.

[@DK5ONV]

Nice MagLoop Almir, good Job on that. The capacitor should be on top and the small loop on the bottom. This reduces the receiver noise.Or Am I mistaken? Happy Weekend & 73 de Uncle Günter from across the Pond 💯🙋‍♂

[@VE3DAL]

@@DK5ONV Hey Gunter! Absolutely I will try that because one thing I was getting a lot of noise as it is. At first I thought it was my neighborhood. My next goal is to take to the park but this type of antenna some logistics good sense has to happen. I hope you are doing better and good luck my friend 73! Gunter buddy