Actually they can cover quite a wide range of freqs, depending on how large and the construction. There is a loop antennas Yahoo group, and we do have a loop antennas wiki, but for the most part, it concentrates on commercial units and although some cover HF, most cover the MW band. where they are very popular.
A very basic loop would be what is sometimes referred to as an attic skyloop. It's built indoors, as the name implies, and it consists of wrapping wire around the perimeter of the attic, then taking the 2 ends down to a 9:1 transformer (sometimes called a magnetic longwire balun) and feeding that down with coax. The Shortwave SWL antenna yahoo group has pictorials, as well as some fairly technical information on how to wind your own transformers. Our antennas wiki also has some links to commerical units.
Another one that I can recommend, having fooled around with one for awhile, is the Carpet Loop. The plans are also linked in the wiki. It consists of a switchbox you can build, and using 4 or 5 conductor cable as the antenna wire. You can make it as big (or small) as you like. In fact, it appears that it's possible to use a small 9:1 transformer in the switchbox (utilizing the switch position that feeds the antenna as a random wire). I haven't tried that yet, but it looks promising.
In addition, there are outdoor good sized loops called Delta Loops. More on that as I turn that stuff up with a good search....73 Mike
If so, is the goal to save space or to null noise?
Most transmit-type small vertical loops need a VERY large capacitor to deal with the extremely huge voltages generated. For RX-only, you may get away with a simple low-voltage capacitor - or even a simple untuned-loop to get you started improving your S/N ratio just by rotating it.
If the goal is to null noise, the basic design criteria is to make the circumference (be it circular, square, triangular, etc) it has to be about no larger than 1/10th of a full wavelength in circumference to get the deep nulls.
I could go further, but need to know if something like the small vertical loop is the real goal..
Not with a high-Q loop. Every few khz or so you have to retune it. If this is in the attic or out in the backyard, that may make it impractical.
I actually favor a lower-Q loop that is either untuned, or where the antenna system as a whole is tuned in the shack.
In your case for 5.680 mhz, about 17.5 feet of a wire fashioned into a loop, square, triangle (the circular loop being the most efficient), attached to nothing but coax would start you off. For this simple calculation, try 100 / f Mhz to get the foot length. We can go slightly smaller to keep it simple.
Try 16 feet total. If made into a square, that's 4 feet each side. Try to keep the wires in as flat a plane as possible. Attach directly to coax. Rotate through 360 degrees to find the best null.
NOTE: This is FAR from optimal, but it is a start. There are many things you can do to optimize it from here, but for some this might be all you need.
The problem is that loop fanatics such as myself might make one think that unless you immediately start out with huge-tubing, gigantic capacitors, perfect antenna balance etc, that building a simple loop isn't worth the effort. It is. Once you get some sort of null, we can optimize from there to make it even better.
Tip #1. Use the largest wire diameter you can find *provided* you can make a good electrical contact with it. In other words, a well-soldered #10 wire would definitely be superior to 1-inch tubing that uses alligator clips for a connection.
Each time you double the OD of the wire or tubing, you can expect about a 3db increase in efficiency. (1/2 of an S-unit) To make a really appreciable difference, (6db, or 1 S-unit), quadruple the size. For wire, this is pretty easy. For tubing, going from 1/4-inch to 1/2-inch to get a 3db improvement may not be worth it from a weight/cost standpoint. Going from 1/4-inch to 1-inch tubing definitely would provided you could mechanically support all that weight.
Normally an electrically well-balanced vertical small loop will produce null depths that are equal on each side of the loop - hence you only need to rotate it 180 degrees to find a null. In this simple case with nothing but a direct connection to coax, you should rotate it 360 degrees to find the side with the better null depth. Why?
In this instance, we have connected a balanced antenna to an unbalanced feedline. This virtually guarantees that your coax braid is functioning as a random wire connected to one side of the loop unbalancing it electrically. So the null depth won't be extremely deep, and the direction of the null will be skewed and not exactly perpendicular to the plane of the loop.
So we need to reduce the common-mode current coming from the braid.
One way to do this if mounted outdoors is to ground the shield of the coax to earth very near the feedpoint with perhaps a simple ground rod.
Or, you can wind a coaxial choke-balun near the feedpoint. There are many references to winding your own coaxial choke-balun, and in this case I'd think that about 8 turns of RG-213 wrapped neatly on top of each other in about a 6 to 6.5 inch diameter would do for a start.
If you have snap-on ferrites that work down in the lower portions of the HF bands (ie, Amidon, MFJ, or other suppliers) snap on at least 4 of them. The lower in frequency you go, the more you will need. Or you could wind a choke balun and add a few of these as well.
You may never get to a perfect state of isolating the feedline from the loop, but everything you can do here helps. Readjust your loop and you should find that your nulls are much more sharp and deep.
Tuning a loop with a capacitor right at the feedpoint is great - although it may be impractical.
One option is to tune the system as a whole in the shack near the radio. That is, you can buy or build your own so-called antenna-tuner, and tune out the complex impedance presented by the whole structure. Common L-type or T-types work. We just acknowledge that the feedpoint impedance match is poor and just tune the whole system as it stands.
Trying to match the very low impedance of the loop's feedpoint to the coax can be done in a number of ways such as using 1:10 step-up transformers, independent coupling-loops, parallelling 2 - 4 lengths of coax in parallel, etc. Some might even use preamps here. For now, I'm just keeping it simple with a tuner at the rig.
Since we are in an Rx-only mode, we have an option when it comes to adjusting the tuner's coils and caps. Normally one strives for the most capacitance when using a tuner if you find that several combinations work. One option here is to find the highest-Q combination instead! That is, a combination that is very narrowband and peaky. Having a high-q combination tuned circuit may really help with the more inexpensive receivers.
A small vertical loop need not be mounted very high - the rule of thumb is to mount it above ground at least half the diameter of the loop. Ideally place it away from other metallic conductors too. On the lower frequencies, this may not be very practical, so just do what you can. On a few occasions, I've had the lower wires of mine only a foot above ground. Just do the best you can.
If you mount it high, your vertical elevation angle rises turning it into a cloud-burner! So keeping it low is a nice side-benefit.
A few things can throw you off here if you mount it high:
* Mounting it high raises the elevation angle and turns it into a an NVIS cloud-burner. However, this can just get you further away from noise below tricking you into thinking the loop is doing the job, when it might just be a matter of physical separation from the noise itself, or the elevation angle is so high that it is also ignoring the noise from below at the cost of lower angles for DX.
* Mounting it high WILL allow you to hear the locals better from direct-wave signals. But in most cases we are concerned with sky-wave signals, and there is no need to mount it high unless you need local direct-wave coverage.
* If you don't have your common-mode current from the braid choked off, mounting it high means that you have a nice random vertical attached to the loop as well. Thus one might assume that the loop is rockin' when in fact the braid is doing most of the job as a common-mode vertical. AND, if this is not under control, noise from the shack can travel right up the braid, into the loop, and then back to the shack.
Thus I prefer a low-mounted loop with the coax on or buried slightly in the ground, with either the braid grounded near the feedpoint, using common-mode choke baluns or some other measures to attenuate the braid current.
The reasons for building a loop that is 1/10th of a full wavelength, or even shorter, is to get the directional pattern that nulls noise from the horizon perpendicular to the loop, yet still allow skwave signals to pass as it comes down from above. Briefly, signals arriving on both sides of the loop wire at the same time do not create a differential voltage at the feedpoint. Signals that traverse the sides of the loop wire in slightly different time and phase as it passes through will.
Making the loop as large as you can without surpassing the 1/10th wavelength limit, will produce the largest signal. If you make the loop much longer than 1/10th of a full wavelength, it turns into a different type of antenna (specifically one that has both voltage and current nodes) and you will start to lose your nulls and the directivity starts to change.
What this means is that this 16-foot total circumference loop designed around 5.680 mhz, will still perform as a small vertical loop at say 2.5 mhz with some great nulls. Unfortunately, the overall signal level will drop since the area that the loop surrounds at this lower frequency is much smaller electrically. To optimize for 2.5 mhz, it might be best to build another larger loop - or just live with the reduced efficiency. At even lower frequencies you still have great nulls, but the efficiency drops even more.
In the end, most accept that a small vertical loop can cover about a 2:1 ratio below the target frequency before things get too inneficient.
But what if you tune higher in frequency? Say at 13.5 mhz or so with this 16-foot loop? Now the loop is larger than 1/10th of a wavelength, and you null directions will change, may not be as deep, etc. BUT, if you don't have any noise issues to contend with this high up, the loop designed for attenuating noise at 5.68 might make an interesting "middle-sized loop" at 13.5 mhz because it is now a different animal so to speak.
If you still had noise at 13.5 mhz, perhaps just build a smaller loop to get your deep nulls back. Perhaps a 7.5 foot total circumference loop would be in order to null things at 13.5 mhz.
Again, this is all assuming that the feedline braid hasn't become part of your antenna.
Tip #6 - Playing the S/N game rather than bending the S-meter
A untuned low-q loop is about 30dB lower in signal strength than a well placed dipole or vertical. Even with tuning, you can expect about 20dB less. The best solution is to WAIT if you don't hear anything at first. You could overcome this with a preamp, but you haven't changed the S/N ratio, only affecting the overall level of signal AND noise. Only the directivity of the small loop is creating the ratio.
(If you want to get into the magnetic-vs-electrical aspects of small loops - great. If I was a betting man, 99.99% of your noise nulling ability is in the far-field electrical signals - loop fanatics can go here if they want, but I won't start that now. You can solve the question if you build the loops out of coax by placing a snap-on ferrite on one of the arms of your coax loop and discovering that it won't work anymore. Some other time perhaps.
Here's how it goes: You spend an afternoon building this thing and rush into the shack to hopefully see the s-meter bent. Drat. Band isn't open. Local signals are way down since you have this mounted low, and your feedline braid is well choked. About all you can do is null the noise that you do hear with rotation. Hey, let's at least peak up on that with the tuner, and null it again. Wow, even the tuner is hard to peak up with by ear.
Before you ball the whole thing up and sign it off as the most complicated dummy load you've ever built - just WAIT.
You may have never heard your receiver's actual noise floor before! There isn't enough noise to actually tune with, although you may get lucky by purposely trying to peak the man-made noise before the band opens. Forget the locals - that isn't what you are interested in anyway probably.
When the band does open, do yourself a favor and tune by ear, and not by eye! Put a piece of tape over your s-meter if you have to. Unfortunately, we don't have a S/N meter on our rigs, but only an S-meter.
What I'm getting at is that your neighbor with his perfectly high dipole might receive an S9 on the same signal you are listening to. Yet you are only peaking at about S5. Then you hear his noise level is about S7 when the transmitter drops. With your loop, the S-meter drops to the bottom peg and would go further if it had the room.
Bottom line: Your neighbor will listen to about a half-hour of that poor S/N ratio before throwing the headphones down in disgust. You on the other hand will be able to listen the entire night with armchair-copy, even though you aren't giving your S-meter a workout. So don't judge your loop by S-meter alone. Listen to the S/N *ratio* instead.
Don't be afraid to crank the volume. It will be a LOT cleaner than before.
Here is the ironic part: With a good s/n ratio from your loop, you just may start to hear the weaker noisemakers that were formerly "in the grass" so to speak that were undetectable from your formerly high noise level! At least they are much easier to deal with.
In the end, I really like small vertical loops. However if you are not constrained by space or noise issues, or just want to build one for kicks, there are better alternatives!
Remember that the circle is the most efficient, followed by the hexagon, then the square, then the triangle. Yes, even a rectangle would work - although I try to limit the width / height difference to no worse than a factor of two with rectangles.
The smaller loops are nearly self-supporting with stiff wire, but what about the larger loops for the lower bands, especially indoors?
Sure, you can make a rotatable cross brace if you have the room. Normally I forego the circular loop on the big ones, and typically use a square on it's side, or triangle (either normal or inverted). For rectangular loops, I've used the backs of non-metallic doors even and swung them to get a null (doesn't always work to find the null).
The triangles are perhaps the easiest to deal with without a cross-brace indoors. For normal triangles, one can hang it from a cup hook or other hook on the ceiling, and bring the wires down to a horizontal stick, pvc, fishing pole, etc. The feedpoint is in the middle of the horizontal wire near the floor. Reach down and rotate the stick. If you find a null, anchor it to a tennis shoe. After your dx session, toss the antenna in the closet so the family won't think you have gone crazy.
For the inverted triangle, you can attach the horizontal support stick near the ceiling, and bring the point down into the room. Reach up and use some painter's prep tape for a temporary anchor. This might be the most convenient in cramped spaces and at least allows you to walk around the antenna. The feedpoint is at the point near the floor. Remove all evidence of loop after session like above.
This can even apply to squares tilted on their sides if you support them in the middle with a horizontal stick and just hang it from something.
Note that rectangles can be horizontal or vertical. Indoors, it is usually more convenient to swing a vertical rectangle. I've mentioned backs of doors, but if you have a tall space, you can hang the rectangle from a horizontal stick at the top AND bottom. Anchor to tennis shoe. You don't want to make the rectangle too thin - in fact if you do you'll see a folded-dipole quickly taking shape. My best luck with rectangles has been with no more than a 2:1 width/height ratio difference. The feedpoint is just in the middle of the bottom horizontal wire. Won't beat a circular loop for sure, but you have to do what you have to do.
Like any antenna indoors, you have to use your imagination.
In fact, I just took a Grundig Satellit 750 portable out in the back, and didn't feel like dragging out the external loops. Normally I throw a small radial connected to the black ground clamp on the ground when using the whip. This time, I tried something different for an impromptu vertical loop.
All I did this time was to connect the end of the 10-foot radial wire back to the top of the whip with an alligator clip, so it formed a triangle loop fed at the bottom corner. I just held down the far end of the horizontal part of the loop with a book on the picnic table.
(note - on the Grundig, clip to the whip element itself, NOT the cap, as the cap seems insulated from the whip)
Neat - I nice little loop, although I had to be careful I didn't bend the whip with too much stress from the rest of the loop wire. It provided a very interesting change in directivity and was much quieter than just the vertical whip alone. I moved the loop wire around a bit for some fun, but it was hard to predict, so I just took what I got and went for comfort instead.
Ideally this should have been #10 or copper tubing, but I'm not going to drag that around while portable, so #24 speaker wire was used instead.
Indoor note: rather than soldering coax directly to the wires, I've found the LDG RBA-1:1 current balun most convenient. For 90 meters and below, I'll snap on a few additional ferrites just after it if necessary.
First - my apologies for totally dominating this thread. I kind of went off the deep end for sure.
Hopefully this will be my last tip. For those of us with severe noise issues, the joy that comes from nulling out the noise that had your receiver sitting on the shelf for years is indescribable.
Earlier I mentioned that although it isn't absolutely necessary, striving for electrical balance by using an RF-choke balun, ferrites, or some other form of divorcing the coax braid from the loop at the feedpoint will get you the sharpest and deepest nulls.
You'll know if you are reaching a nice balance by only having to rotate the loop 180 degrees instead of 360 degrees as the nulls are equal now on both sides. If not, just pick the best side.
Here's another catch - even if you do build it perfectly, you may need another choke at the rig or shack entrance if you are lucky enough to already have an antenna farm and hook this loop to a coax switch. If all your other antennas don't have a great rf-ground, those other feedlines may still present enough braid-current to electrically unbalance the loop.
So, when initially building / evaluating a small vertical loop, test it on it's own first. That is, try it out as the only antenna connected to your receiver without going through an antenna switch. You may even want to run the radio with batteries instead of from AC to give the loop a fair shake - at least temporarily.
If you notice a big change when using the loop directly vs going through your antenna switch, huge changes in tuning etc, you may have an rf-ground issue and need another rf-choke at the receiver end as well.
It pays to revisit those links every once in awhile - while some drop away, some new ones appear that are full of great ideas.
In almost every instance, each of the small vertical loop links are high-q that require a tuning capacitor at the feedpoint, independent coupling loops, gamma matches, or other errata that might overcomplicate matters to a newcomer.
In my case, a simpler direct-connect low-q loop is described. As long as one knows how to play the S/N game, you might not need as much time scrounging around in the junkbox. At the very least, the low-q loop is more practical with maybe nothing more than a store-bought tuner at the listening position to make it more convenient - but it isn't absolutely necessary in many rx-only cases. The tuner at the rig end won't cure the low loop-impedance to feedline match, but overall nets about 10db of overall signal gain.
With coax serving as the loop elements, you won't see anything much larger than about 20 feet in circumference. This is because there is so much stray capacitance in the coax itself that it may become hard to tune. If this 20-foot coax loop were designed for say 2 mhz operations, the total circumference is only about .05 wavelength, so the gain is much less than one designed for .10 wavelength. Switching to plain wire here if you have the space for 50 feet in circumference would make a great direct-connect loop at 2 mhz or so. This is the one that only had the lower wires of the square about a foot off the ground for me.
Just keep in mind that if a loop exceeds .10 wavelength in circumference, it is no longer a small vertical loop with the great broadside nulls, but something else in the loop family with different characteristics.