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