An easy-to-build general purpose receive only small wire loop antenna
As much as I like my coax loops, I am also quite satisfied with small loops made with wire or tubing. They have the same or better performance as the coax loops, but might require that you invest in a balun to help maintain directivity and avoid common-mode noise ingress from the feedline. If you need to null local noise yet still be able to listen to most skywave signals, these loops really perform.
The antennas described below bridge the gap between operating as a constant-current small loop (0.10 wavelength or less circumference), and intermediate-sized loop a bit larger than 0.17 wavelengths long in circumference.
If you are interested in building loops made entirely from coax cable you may want to check out my earlier project pages on that subject. It has many operational notes and other items of interest that pertain to small plain wire loops as well as to coax types.
The voltage balun was essential to help me fight common-mode noise and maintain directivity. If you don’t use a balun and have good results, you may not have much noise to deal with in the first place, or the skewed directional pattern has a null that works for you – even if it isn’t textbook. See my balun notes below.
I initially chose 14 feet since my noise problem extends up into the 40 meter band; I didn’t want the antenna to be longer than 1/10th wavelength because you start to lose your nulls with larger wavelengths of wire. I just did a quick calculation: (1005 / 7.150 * 0.10)
Note that I have since opted to use 28 feet overall, because I wanted better sensitivity on 160 and 80 meters, and now at 40 meters the 28 feet of wire still gives me a slight null – adequate enough for me to null my local noise on 40. Unfortunately I don’t have the room for a full-sized loop, so I had to wind it with two turns. See the EZNEC® antenna modeling plots below.
Here are some quick construction tips to get you up and running quickly. I’m still studying the antenna and will improve the page as time goes on.
I am running the loop right at the operating position. Here is what I’m using:
- 14 feet of #12 gauge wire formed into a loop. Coax braid is an option.
- W2AU 1:1 voltage balun by Unadilla.
- 10 foot coax jumper from loop balun to tuner input.
- Common tee-type C-L-C antenna tuner.
The loop seems nearly omnidirectional for medium to high-angle skywave signals, yet has great noise-nulling directivity at very low angles from 160 – 20 meters. These two qualities make it a great general purpose antenna especially indoors.
Let’s take a look at the elevation angle for 20 meters. It shows good medium to high-angle skywave directionality. The other bands have much the same elevation pattern:
Look at the azimuth angle for 80 meters. 160 and 40 meters are similar. Notice the deep null; great for nulling noise by rotating the loop:
At 20 through 10 meters, the circumference of the loop is becoming progressively larger than 0.10 wavelength, and starts to have a nearly omnidirectional horizontal plane no matter how you rotate it. Fortunately I don’t have to null out any local noise on 15 and 10. On 20 meters I have a very minor noise problem, and the smaller null on 20 meters takes care of it.
This means that this 14-foot circumferential loop is performing as a small directional loop on frequencies of 40 meters and lower, and as an omnidirectional intermediate-sized loop on bands higher than 40.
The most efficient small HF loops are single-turn affairs. Multi-turn loops of this type are less efficient, but you may have no choice to wind a smaller loop with multi-turns if you can’t find the space for a single-turn, such as with indoor applications.
If you are really space constricted, you could cut the dimensions down and run a 7-foot circumference loop. Just don’t expect great performance on 160 or 80 meters. Rectangular loops might also be considered if you have a lot of vertical or horizontal space, but not much of both at the same point. Perhaps you have very high vaulted ceilings in which you can make long vertical runs for the sides of the loop whereas the horizontal runs would be much smaller.
Just remember that the key to small loop success is to enclose as much AREA as possible; keeping in mind that when your antenna starts to appear longer than 0.1 to 0.25 wavelengths in circumference, you’ll start to lose the deep nulls.
Strive for a single-turn loop, but if you must, you can wind multi-turns if you have to. To help reduce the proximity effect of the turns, (one of the elements of loss resistance) try to keep the multi-turn loop wires spaced one or two wire-diameters apart.
Skin-depth rf currents on closely spaced coil conductors have a tendency to reject each other and “pool up” on opposite sides of their respective wires, thus effectively reducing their own conductor area. I have had success by winding one turn on one side of my pvc mast, and the other turn on the back of the mast. However, I am not so sure that this is very critical for a receive-only application. More study required …
I have had the best results using a VOLTAGE balun at loop feedpoint. I tried a hefty commercial 1:1 CURRENT balun, and it turned my small loop into a noisy non-directional random wire.
(I’m not condemning the use of choke or current-type baluns, it’s just that the voltage-type balun seems to work better for me in this application.)
Since I wanted my loop to be general-purpose and work across several bands, I made no attempt to match the loop impedance to the feedline. This may be affecting balun performance somewhat, but so far it is performing adequately with the loads presented by the loop in this rx-only application.
I really didn’t want to use a balun, but found that I had to. I experimented with a direct connection to the coax without a balun, and got some directivity on 160 and 80 meters, but the common-mode cable ingress noise on 40 meters and higher was pretty bad. It also changed my tuner settings radically. Putting some clamp-on RF cable chokes on the feedline reduced the noise a bit. I reinstalled the 1:1 voltage balun, took off the chokes since they were no longer necessary, and got my deep nulls and quiet reception back.
Keep your balun connections neat and symetrical. For example, the 1:1 voltage balun I use is a W2AU type and it has small jumper wires behind the strain-relief eyelets. Connect the loop wires to the jumpers close to the eyelets, and then symetrically dress the remainder of the balun leads neatly. I made the mistake of letting the balun wires hang in a hay-wire fashion, and attached the loop leads to random points along the balun jumpers. Although the antenna worked well, tighter nulls and better overall balance was achieved just by being a bit neater with my connections. It’s worth the effort.
What about a 4:1 voltage balun? It works well! On a lark I thought I’d try a 4:1 balun and see how much worse it would be. To my surprise, I still have my low-angle bidirectional directivity, my nulls are sharp all the way from 160 to 20 meters, and my tuner settings require about half the inductance! (except on 160 where my inductor settings stayed the same). It seems that as long as I can tune out the reactance of the antenna system, the loop-to-feedline mismatch isn’t as much of a concern as I once thought. I can’t even begin to explain what’s going on with all the variables. I’d sure like to learn how to model small loops with differing balun ratios … until then I’m enjoying the loop with the 4:1 balun.
I had a 50-foot piece of coax left over from my earlier coax loop experiments, so I thought I’d experiment with it by using just the braid as the antenna element (continuous braid loop – no gaps). I had to take into consideration noise-nulling vs sensitivity for my location. I prefer single-turn loops, but in some cases I had to wind them into multi-turns (with one wire-diameter spacing) to fit indoors. In all cases I used my tuner to resonate the whole system. The lengths listed below are not super-critical.
The first band listed is operating as a 0.05 wavelength loop, and the second listing is operating as a 0.10 wavelength loop.
160 – 80 meters optimized: 52 feet of wire
This is the best 160 meter loop I have used to date. To fit indoors, I had to wrap it into 4 turns. Deep nulls on 160, medium nulls on 80, everything higher in frequency turns omnidirectional. I really wish I had the space to open this up as a single-turn loop, but I have to make do with the space I have.
80 – 40 meters optimized: 26 feet of wire
I still have a noise problem on 40 meters, so I had to cut the length of the wire down to get my deeper nulls back. I still had to fit it indoors by wrapping it with 2 turns. Deep nulls on 80, medium nulls on 40, everything higher in freq omnidirectional. (I can still copy the locals on 160 ok, but since the loop is smaller than 0.05 wavelength on 160, it’s very inefficient. Considering that the loop is best for medium-to-high angle skywave reception anyway, this isn’t as bad as I thought on 160. So what if the locals on 160 are a bit weaker – the SNR of the loop makes it usable anyway.) I’ve also reduced the top-heavy weight of the loop by using less turns. This is now the favored loop size for my situation. In this case, less is more!
40 – 20 meters optimized: 14 feet of wire
This wire length now allows me to use a single loop of wire. Great nulls on 40, medium nulls on 20. Everything higher in freq omnidirectional. Since I don’t have a big noise problem on 20, I use the larger loop in the previous experiment. The loop is so light with only one turn that I’d probably use a much bigger conductor diameter for the loop if I wanted to maximize sensitivity on 40 meters.
Velocity Factor issues with outer coax braid (basically none!)
Since I’m using the outer braid skin of the coax as the antenna element, I can ignore cable velocity factors. In other words, the velocity factor is only applicable to the inner differential-mode currents, and not to the common-mode current that exists on the outer braid skin.
I’d like to offer my apologies to earlier readers where I indicated that the velocity factor of a loop using coax braid as the antenna element should taken into affect. I was wrong.
Wire diameters for 0.10 wavelength or smaller loops
I recommend using 1/4 to 1/2-inch diameter or larger conductor diameters for the loop if you desire them to be self-supporting. You can also use just the braid of RG-58 or RG-8 coax and affix it to a mast.
Small-gauges of wire don’t perform as well as tubing does with loops under 0.10 circumferential wavelength, and larger conductor sizes, while offering greater performance and a larger bandwidth, may present a problem when considering the cost, weight, and general hassle of construction. The general rule of thumb would be to use the largest diameter conductor that you find practical so that you can lower the loss resistance.
Since small loops are high-q antennas, it is very easy to mis-tune or mistake a peak in your tuner settings for an optimal match of the system. If you have a noise-bridge, or antenna analyzer handy, this can make finding the right tuner settings much less of a chore. I’m going to describe doing it “by ear”.
After building a new loop I usually get impatient and madly start adjusting the caps and inductor settings hoping that I can hear the peak quickly. Sometimes I get lucky, but more often than not, I dont’ find any peaks, or I end up on a very inefficient one. Sadly, I resign myself to the fact that I’m going to have to do it in a more methodical fashion and maybe eat up an entire afternoon to find the settings for most of the bands.
Let’s assume you have a typical C-L-C type tuner; a cap for the receiver side, an inductor, and a cap for the antenna side. You’ll want to do this when the band is open, or perhaps tune to a local noise source. Here is my generic method going from the lowest to highest freqs (tedious to be sure, but I don’t want to accidentally skip over a great match):
1. Set L for for maximum inductance.
2. Set both caps to zero, either fully meshed or fully open.
3. Set the antenna cap to 1
4. Rotate the receiver cap all the way through it’s range.
5. Set the antenna cap to 2
6. Rotate the receiver cap all the way through it’s range.
7. Continue with the above steps advancing the antenna cap by one.
8. If no peak is found, lower the inductor value, and try the caps again.
On the lower freqs, you may even want to advance the antenna cap settings by only half-steps until you find the peak. Ugh.
Even if you do find a nice peak, don’t give up just yet! Make a note of the settings, and try it again with the next inductance value. You might be surprised at how well the NEW match point works. This is pretty tedious stuff, but the result are worth it – don’t be tempted to be satisfied with the first peak you find!
After you’ve found the major match settings, you’ll probably need to make slight capacitor repeak adjustments when you tune the receiver from one band edge to the other, especially on the lower 160 and 80 meter bands.
originally hosted at www.greentech.com