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Making a Receive Loop Antenna


Contents

Element Diameter

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.

Electrical Balance

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 Low Q Loop

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.

Mounting the Loop

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 the author prefers 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.

Frequency coverage

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 inefficient.

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.

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. 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.

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.

If you are not constrained by space or noise issues, or just want to build one for kicks, there are better alternatives!

Practical Loop Shapes and Indoor Loops

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 you should 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. It might be best to forego the circular loop on the big ones, and use a square on it's side, or triangle (either normal or inverted). For rectangular loops, the backs of non-metallic doors can be used and swing 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. The backs of doors can be used, 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. The 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.

Thanks to hertzian for the above. Any transcription errors are entirely mine


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