Types of Magnetic Loop Antennas (
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- Опубліковано 30 вер 2021
- Inspired by Andrew, Dave will explain how Magnetic Loop Antennas work, how they differ between companies, and the Advantages and Disadvantages of Magnetic Loop Antennas.
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A note: At 14:06 in the video Dave said putting multiple loops on the loop antenna will give you 'higher' frequency when he meant 'lower' frequency.
The other way to get lower frequency, (for 80 m, 160 m, the AM broadcast band, and LF) is to add capacitance. That means a higher capacitance air-variable capacitor, or perhaps paralleling the tuning cap you have with a second air-variable capacitor, or adding a fixed capacitor. A fixed capacitor is handy for just listening, and can be selected for the frequency range of interest, although it will broaden your tuning somewhat, picking up more noise due to broader bandwidth being received. (Air-variable capacitors have the absolute highest Q, narrowest bandwidth, while fixed capacitors have lower Q's that are a function of the design of each type capacitor, so try a few different types.)
Remember that for transmitting, a fixed capacitor in parallel with the tuning cap will need to be rated for high voltage, and the higher power you transmit with the higher voltage that fixed cap will need to be rated at. Roughly speaking, for hundreds of watts you are looking at thousands of volts across the capacitor, so you may need to series connect fixed capacitors of lower voltage rating, further reducing the Q.
Fantastic video - I have gone through a few, and your explanation - your understanding and your presentation are very effective. I truly appreciate your guidance and training on the mag loop. This will be a classic on UA-cam for decades.
In addition to the common inductive match shown, you can also use a Gamma Match or a capacitance match. The MFJ Portable Loop Tuners use a capacitance match and the benefit of that is NOT having a capacitor at one end and a small loop at the other which you cannot really do much with it to improve the match. So on the MFJ tuner, the loop is just a loop and the feed point is through a variable capacitor to one side of a grounded butterfly capacitor. I easily achieve nearly a 1:1 match at any frequency that the loop can actually tune and it is easy to swap out different loops.
Liked the video very much. Always wondered what the story was about these antennas. I'm an OT and saw some designs back in the 60s' and was fascinated by them. One thing I'd need to understand though. MFJ sells you a nearly $600 antenna, it comes defective, and your grateful they let you fix it yourself. Just seems a company making a $600 anything should have good enough quality control that it doesn't leave the factory in a condition that the buyer needs to fix. I think Mr. Jue should come to my house and fix it for me. 73.
Dave, Thank you for the nice lesson on how the mag loop antennas work.
Another great video Dave. Thank you!
Thanks for explaining, I never really understood magnetic loops.
Really great explanation on basic mag loop theory. Thanks!
Man, you're active. Please be, I'm watching all of that.
Fantastic video, David. Very organized, very full of great info. Thanks a bunch!
73,
Kip
WV5F
really appreciate all this excellent information, thank you! Ever tried the MFJ-936C or the various other versions?
always a pleasure, great work, thanks you vk3
I enjoyed this video, Dave.
Perhaps in the future, it would be good to mention vacuum variable capacitors. I have a mag loop project in the works for 20m and purchased a VVC for use in that project. I have been drooling over the Baby and MIDI loops listed on DXE, though.
73, KJ7RDV
I've never been this early before...This is a brand new video...
Great stuff
I will attempt building one set for 10 meter which what my license allows in HF. Hopefully it will be OK. Thanks for the video!
I have built one of these and have an air gap capacitor to use at low power so it does not arc but I can't seem to get it to tune.
I hooked it up to my nano VNA and calibrated the VNA for the band (say 40M) and turn the capacitor very slowly and never see a dip on the VNA. I have done this to all the bands the loop is supposed to work on with the same results except 15M. Through this band I get good SWR but when I turn the capacitor the SWR never changes. I have check all connections to the loop and proper configuration but it just seems like the capacitor does nothing.
Any suggestions as to where I am going wrong?
can I use loop antenna measures 3inches to gain vlf or elf sir?
As always, excellent explanation of mag loops Dave!! I have the older Alpha Loop and use it primary for portable operation. It works quite well and gets me on the air in minutes...../k6sdw
Is there an advantage or disadvantage to mounting the loop horizontally of vertically?
Precise Rf sell a full legal power magnetic loop. It uses a vacuum capacitor. What do you thing in regards of such type of antenna at high power? It is really a benefit? I know reception should be less noise, but in gain or more power? Thanks 73.
Nice video dave! I'm getting ready to start working on my mag loop see if I can put a different capacitor in there and change the feed at the bottom. I think that having a good low resistance capacitor is as important at low power as it is at high power. If you don't have a really low resistance path then you lose energy of course then your capacitor burns up haha!
If the power is too high for the cap, the cap will arc over. That can ruin the cap.
The coupling loop changes the gain? Did you mean gain or coupling?
Happy 1st of October & a great weekend to everyone! 73's de Your Friend Uncle Guenter
Thanks for the lo-down on these fascinating antennas. I guess buying one is an option.....but build it yourself, is so much fun and you get to experiment. Hams are supposed to experiment. $500 will buy materials for 3 or more antennas. Or building it yourself leaves money for the QRP kit or two kits!
Thanks, Dave for the best intro on TX mag loops out there. I see you suggested the 15 to 40 meter MFJ over the 10 to 30 meter version. Can you elaborate on your choice? Does the higher Q at 40 get difficult to manage? What would be an optimum type of 50' feed cable to minimize loss? For example, would LMR-400 give any worthwhile advantage over RG-8X?
Jerry
KK4GMU
Thanks again for a great video. I bought a qrp magloop that has a coupling ferite loop (feed wire to the big coax goes through the loop) near the cap that has a few turns of wire from the drive coax so the cap is at the low voltage side of the loop with all components the box. The mail loop can also be switched so the inside wire of the coax can be used as a second loop at 80 meters. it all seems to work fine but I do not know how well it compares to other antennas. I do have a working long wire with a remote tuner that seems to have about one s meter higher on any frequency received. I have kept it for field testing someday.
A mag loop out in the open performs about as well as a dipole.
"the cap is at the low voltage side of the loop with all components the box"
The capacitor is ALWAYS the high voltage component of a loop. That is where the current in the loop stops, piles up, and creates a high voltage. You can do the math easily enough, say 20 picofarads at 14 mhz Xc=1/(2pi f c) is 568 ohms. You can proceed from power and Q to figure out the current but for simplicity, say 20 amps of RF is circulating. That would be 11 kilovolts on the capacitor. It is also on the inductor, but where are the endpoints of the inductor? Tied to the capacitor.
Interestingly, the same current exists around the loop, but not the same voltage.
That means the impedance isn't the same around the loop.
That means you can do a Gamma Match or a capacitance match (used by the MFJ portable loop tuners).
Thank you, Dave. Have a great weekend and week. N0QFT
As has been reported elsewhere, because of skin effect only a paper thin layer of the loop actually conducts. Therefore, making the loop out of pipe is rather wasteful (IE: expensive!) and a plastic pipe antenna covered with metal foil will be just as effective as a copper pipe of the same diameter. Futhermore since only a thin layer is conductive, the diameter of the loop material is what determines the RF resistance of the loop, and it is this resistance that determines how 'lossy' the antenna will be. For a 4 foot diameter loop, the minimum diameter of the loop material should actually be about 2"! Also shape of the loop doesn't matter, it is the circumference and the square area of the loop that is important. Therefore, a square loop is the most effective, as it takes up the smallest shape while providing the required overall length and area size. NO commercially available magnetic loop antenna for ham radio follows these specifications! Therefore, the claims made for their performance are like what snake oil salesmen would provide.
I don't see any reason not to use an external antenna tuner with a loop once the the has been adjusted for frequency. IE: put your tuner on bypass, adjust the antenna and LEAVE IT as is, THEN put your tuner in line and adjust for lowest SWR. This should work if the tuner is AT THE RIG on the far end of the coax from the antenna! The tuner isn't tuning the antenna, it's matching the impedance of the antenna to the rig. When re-adjusting the antenna after QSYing, you again bypass the tuner, adjust the antenna, and ONLY THEN adjust the tuner for lowest SWR, WITHOUT touching the antenna setting!!
When I consider an attic or trellis loop installation for my condo and check the ARRL rf exposure calculator for 7-14 MHz it looks like min safe distance uncontrolled is less than one foot SSB. Does that seem right? In this talk you were concerned about rf entering the house from horizontal installation on your roof at +5' - was that a health concern?
Take a look at the fine print in the ARRL RF description about the "near" field. The calculator talks about "far" field in which the electromagnetic waves are well-formed. You may find that you want to be further away from the antenna.
The MFJ Portable loop tuner requires that it be close enough to tune it. It can be a bit concerning at higher power levels. A magloop at close distances will have negligible electric field, which is the principle worry if I remember right, and a more pronounced magnetic field, which the human body ignores. Getting an MRI exposes you to incredibly strong magnetic fields without apparent adverse effecfs.
Isn't the hi Q also an advantage for rejecting adjacent QRM QRN?
Dave can you please tell me (if you know) what is the minimum size for a square shaped loop for 7MHZ, please?
A square shaped loop has a little lower Q for the same overall size antenna since it has more material length (higher resistance,) so a circular loop is slightly better for a given size. As for the overall size, there is no minimum size at which a loop no longer works. The signal simply continues to get weaker and less effective as the antenna size decreases. Three to 4 foot loops are most common because they are usually the largest size that is convenient for the user to build or use. These loops continue to work better (more efficiently on xmit, more sensitive on receive,) as they get larger, up to about 1/10th of a wavelength in circumference for the highest frequency of interest. However, as they get larger they also pick up more noise, so a larger loop is better on a mast in a rural, low noise environment, while a smaller one is best in a noisy apartment environment. It's always a matter of trade-offs and designing or buying what's right for the location.
Dave, I have the MFJ-936C Loop Tuner. You attach a loop of wire based on the band you desire and then tune accordingly. How does this antenna system compare to the Mag Loop described in your video. My instinct-reaction is they are both "the same" except the 936C control box is NOT weather proof. Your comments please... de W4WMM
Howdy. Nice.
Kevin Loughin has a clip where he achieves much better SWR using an oval driver loop.
I can get SWR = 1 using a gamma match on my 27 MHz CB loop.
In regards.
What is the angle of radation of a magnetic loop in the vertical plane?
You know, I've never heard a satisfactory answer to this. MFJ claims that if the antenna is vertical, the polarization is vertical, but I think it's really horizontal because it has nulls. And if you mount the antenna horizontally, there are no nulls.
Directional discontinuity ring radiator? Nobody seems to have covered these.
Gamma match can be used as well yes
Great video Dave. How well would an MFJ loop work in close proximity to the metal roof on my house. Cheers from Moose Jaw. Tim VE5THF
If it is perpendicular to the metal roof, it should work fine.
@@davecasler
Thanks Dave
I was just asking myself yesterday what the difference between a F loop and a p loop and now I know. Moved to an apartment and had to get rid of my radio power supply and Chameleon F loop plus 2.0 (which I loved) Gave them to the local Ham club put now I have the itch again so I bought a small IC-705 and am looking for a Loop. I see that Chameleon has a F loop 3.0 coming out for shipment at the end of October which sounds interesting. 73's KA1FQM.
Lots of loops to choose from. Give DX Engineering a call and see what they suggest.
I have that Casler cryptic handwriting
Is tuning so critical on RX, please?
Brian
M6 BAQ
Somehow it just clicked, it’s in essence just a LC circuit. 👌🏻
Question: Are magnetic field antennas safer than electric field antennas? I ask following all the discussions in the community wrt EMR and its' effect on the human body, particularly the eyes and brain.
Great video as always.
Many thanks,, J.
The electromagnetic field contains both electrical waves, and perpendicular to that, magnetic fields. As it turns out, you can't have RF fields that are only electrical or only magnetic. However, your antenna can excite the electric field or the magnetic field, and then the companion field also radiates.
Dave, I'm confused why these are "magnetic" when none of the loop material is magnetic, only conductive??
Thank you for the video.
It's not about the material the loop is made of, it's the way a loop responds to RF. Radio waves are electro-magnetic. The electrical and magnetic fields are perpendicular. A loop only responds to the magnetic field but not the electric field. That's why they are directional with a "null". Better than other antennas in environments with man-made electrical noise (QRM) for receiving.
@@Anon-fv9ee Thank you.
For 100W or so, people often use a vacuum variable capacitor, they can handle very high voltages, but are fairly expensive.
There are also other ways to couple the antenna - I've seen a gamma match used. The smaller loop is easiest though.
For portable operations, Peter Parker has home made loops he uses portable on his UA-cam channel.
👍👍👍👍👍
Vacuum capacitor
Right! A vacuum variable capacitor can handle the high voltages. Expensive, but worth it for the size and voltage handling.