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Magnetic Loop Antennas for HF

 Magnetic Loop Antenna FAQ

Please read: Considering the controversy any ham antenna conversation can generate, I thought it was important for me to write down some of my opinions on and about Magnetic Loops. People are going to ask these questions over and over again, so I might as well save them the trouble upfront. Some of these observations are based on real world testing, building, and daily use of many magnetic loop antennas. So please find this FAQ a resource for your exploration and in no way a dogmatic listing of ideas. Some of the information you find here is simply what I have found to be the case when tested by me. I welcome questions, and I'll be happy to add information to the FAQ as it is relevant and meaningful. This is a start.
 
1. What is a Magnetic Loop? 

A Magnetic loop or STL (small transmitting loop) is a large coil with a capacitor added to bring the system to resonance. It is unlike a large resonant loop (more common) in that the loop size is smaller than 1/4 wavelength and does not resonate on its own. The loop is brought to resonance by attaching a capacitor to the two open ends of the loop. The loop is tuned by a changing the value of the capacitor. An increase in capacitance will bring the resonance of the loop lower in frequency, a decrease in capacitance will make the loop resonate higher. An excellent resource for more in depth information is the ARRL Antenna Book, section 5 (there is a lot of good info, but don't believe everything you read!). 

2. How well does it perform compared to other antennas? 

A Magnetic Loop's performance seems to be dependent on construction, placement above ground, and other factors. A generalization of performance would place it a bit below that of a dipole, and in some instances exceeding  the performance of a dipole. It usually will far surpass the performance of most mobile antennas, and other compromise configurations (however, there are always exceptions). 

3. Why do you build your loops as squares instead of circles or octagons? Aren't circles more efficient?

Yes, circles are more efficient, by about 10%! But they are harder to make when your working with one or two inch copper. I would rather just build the loop a little bigger to gain back the loss. For instance, according to KI6GD loop modeling software, it only takes a extra foot and half of circumference or an extra half inch of diameter of pipe to gain back the lost efficiency. Also, squares are easier to make and have less solder joints than octagons. 

4. Is copper better than aluminum for loops? 

Absolutely and for the most part. It has over a 20% difference in efficiency according to some modeling software. More efficiency equals more performance. It's also a lot easier to solder, and can be found at your local hardware store. However, aluminum is far lighter. It's a trade-off. I would hate to know what my 12 ft x 12 ft loop with 2 inch diameter pipe would weigh if it was made out of copper instead of aluminum. Please remember, my aluminum loop works great. 

5. How important is loop circumference? How important is the size of the conductor diameter?

Loop circumference relates directly to efficiency and performance (also, loop image: see below). The larger the circumference of the loop, the better your loop will work -- however, if you exceed 1/4 wavelength, things seem to get mysterious. It's a balance, I try to get about 85-90% of the wavelength in circumference, and that seems to be the magic number. However, I'm not making any definitive statements because I have found loops above 1/4 wavelength that seem to work well. There are other factors that effect the total loop size, and that is loop pipe diameter. I have built loops that were 16 feet in circumference and made out of 2 inch copper. The loop worked poorly on 20 meters (where 16.5 feet would be 1/4 wavelength), and worked better on 40 meters. What I assume is that if you use pipe that exceeds a certain size, the total loop size (not circumference), but total loop image, in other words the electrical size is too big for the intended frequency -- maybe. You end up exceeding the wavelength limit and the loop starts to self resonate (according to some people). I have built half inch copper 16 foot circumference loops that worked much better on 20 meters than the 2 inch copper version. So be careful when building your loop and understand that pipe diameter plays a part in its performance. What I would propose is that my 80 meter loop that is 12 feet by 12 feet is in fact larger electrically (or in image) larger than its 48 feet circumference because of the 2 inch pipe size. I believe that if I would have used smaller pipe, the loop would not have worked so well on 80 meters. This is conjecture. I would compare this to the effect of a fat dipole or cage dipole. The thicker the diameter of the wire the shorter the length of the total dipole. It seems to make sense that the thicker the diameter of the conductor, the larger the total loop image will appear electrically. However, one thing I and others have noticed is that if you build a loop for the low bands, make sure the pipe is thicker rather than thinner. I would suggest two inches and above, this seems to deliver the best results. 

6. Do I really need to solder or weld all the joints on a loop?

According to almost all the literature I have seen on loops, all connections must be soldered or welded. Please read on because I do not believe it's that simple. It appears that mechanical connections create resistance in the conductor which cause major losses in efficiency. It is interesting to note that all the loop articles I have seen and read do not discuss mechanically built loops, only welded versions. I myself have built both mechanical and soldered loops and have not found major variations in performance if the mechanical connections are well made. This is purely observational. My 12 foot by 12 foot loop for 80 meters has no welded or soldered connections and works well from my daily observations and anecdotal evidence. Keep in mind, however, that each segment of pipe has a 3 inch taper that slides into the next pipe (and is then bolted together) -- so there is plenty of overlap for each connection. I have also built loops mechanically, tested them for some time, and then soldered them together to see if there would be a measurable change. As of yet, as long as the mechanical connections are well made, there seems to be no measurable difference. These conclusions do not seem to correspond to the general literature of loop making, but I feel it is important to put this information out there for debate. 

7. Can I use regular air variables from an old transmatch for tuning? 

Most loop literature would say that the wiper connections on commonly found air variables are too lossy. Also voltage capacity is an issue. Any capacitor used should be able to withstand high voltages in the 3-4KV range if you are running 50-100 watts. If you are using welded stator or butterfly capacitors with a high enough voltage rating  there should be no problem or significant losses. However, high voltage stator or butterfly capacitors with a large capacitive range usually cost about the same as a nice used Jennings Vacuum Variable. I personally use the vacuum variables. A cheap alternative to expensive high voltage variables is the use of coax for monoband utility. Take a good piece of coax, separate the braid from the conductor for a couple of inches and attach each side to each open end of the loop. Use varying lengths of coax to create different capacitance values for tuning. Trim the coax till you reach your desired resonance. This works very well if you don't mind being stuck on one band. You can also change the coaxial stubs when you want to change frequency pretty easily. I have heard opinions put forth that braid in coax is too lossy for good loop performance for capacitors, but I have not found this to be the case at all. 

8. How well does a loop made of coax work?

When using coax, either the ground braid or the center conductor can be used. Unless you using one inch hardline the diameter of the braid is simply too small for high efficiency. Have I tried it on RG-8U and other types of coax? Yes. Does it work well? In my opinion the answer is no. One inch or even half inch copper pipe will work much much better. I would not recommend this strategy at all since every experiment of this type yielded poorly performing loops. Also, some opinions believe that the braid in coax is too lossy because it is not one piece of metal but strands of copper with minute gaps between the pieces introducing excessive losses. I cannot comment on this point. 

9. How do you compare copper or aluminum strap to pipe?

Modeling software claims strap is equal to one half the size of full diameter pipe. 

10. Does a loop have a gain pattern? What does it look like? Does it matter?

A vertically positioned magnetic loop has a figure eight gain pattern along the plane of the loop itself. In other words, think of two donut shaped patterns radiating from the the vertical portion of each vertical side. This pattern creates a null where the two donuts meet in the center of the loop. So a loop with its vertical sides placed north and south, will radiate best north and south. The same is true if  you rotate the loop east and west. Loops can be placed on rotors and the loop can be turned for best signal strength or it can be used to null out offensive interference. Magnetic Loops also radiate RF at all angles, so they are well suited for both high and low angle reception and transmission. This is advantageous for both high angle NVIS work, and DX. Loops that are mounted horizontally radiate in an omnidirectional pattern and do not share the benefit of a vertical polarization. I would recommend vertical placement. 

11. Why are commercial loops so expensive?

Magnetic Loops require high voltage capacitors for more than QRP power level operation. These capacitors tend to be very expensive. A brand new Jennings vacuum variable can cost at least five hundred dollars (they are a lot cheaper used). High voltage stator and butterfly capacitors also cost a lot of money. So the final product including the capacitor can be quite expensive. Also, commercial manufacturers desire to build the best possible loop, so they employ all welded construction on loop joints. This also adds to the cost. 

13. Do radials help?

Radials added to the ground feed of a loop is supposed to lower Q and raise efficiency (see the ARRL Antenna Book, section 5). I know this sounds anti intuitive, but I have tested this many times and it seems to be the case. The bandwidth of the loop opens up by about 10%, the efficiency seems to be raised (this is indicated by observations I have made in antenna feed arrangement in relation to loss), but I have not noticed improved performance. It would seem to make sense that with higher efficiency there be better performance, but I have not seen a measurable difference. This needs more testing. When possible, I do add radials. Radials do not need to be cut to operating frequency, but can simply be twice the diameter of the loop. 

14. What if I exceed 1/4 wavelength circumference will the loop still work? 

Yes. I once believed that it does not seem to work as well as a loop of 1/4 wavelength circumference. Testing this theory now leads me to other conclusions, as larger loops seem to work better in some instances. 

15. Are loops quieter than other antennas?

A lot of  literature on loops make this claim. I have noticed that I do hear less noise than hams I am talking on many occasions. They complain about it and I don't hear what they hear . But remember, they may be using better antennas which have more gain and usually have more noise as well. Another factor could be the high Q of my loop acting as a filter for more broad band noise. 
 
 

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Copyright 2002 Brian Levy