Compact loops, often called magnetic loops, have been around for years.
They're tiny open-ended ring radiators, usually less than 1/10th wavelength
in circumference. Recently we've seen them mostly on HF where their small
size is of great appeal. But they work very well on other bands too, here
on 2 Meters. Remarkably, this VHF version achieves an efficiency of 93%,
yet radiates as if it were a full-sized dipole or a J-pole, but from a
space only 7 inches square. What's best, it is disguised as a nice-looking
weather vane. Your neighbors won't guess that your rooster has a callsign.
Figure 1: The completed antenna on a rooftop
What makes a magnetic loop so easy to camouflage besides its size is its radiation pattern. It may surprise you, but a horizontal compact loop radiates something like it were a full-sized half-wave dipole on the vertical axis of the loop. Yet here it looks like nothing more than the support for the weather vane's direction letters (Figure 1).
There is a restriction, however. Compact loops require low conductor resistance as compared to full-sized dipoles. (See the technical discussion later in this article.) This requires that we use copper water pipe for construction. Also, compact loops have relatively narrow bandwidth, in this case 600-700 KHz theoretically. Practically, however, the bandwidth is wider. Figure 1a shows an actual SWR plot of my loop.
Figure 1a: SWR plot of my loop, measured at the transmitter end of 50 ft. of mini RG-8. By compressing or expanding the feed loop a 1:1 match is possible at the resonant frequency (here roughly 145.7 MHz).
On the positive side, a magnetic loop's narrow bandwidth increases the received signal to noise ratio. In a noisy environment a compact loop will better discriminate against received noise than a dipole or J-pole. They also tend to work better close to the ground or near other objects.
Construction
All materials (Figure 2a, 2b) are common hardware store items. I recommend a tubing cutter for cutting the pipe, copper and PVC. Also, a different brand of copper fittings may require slight adjustments to the cutting dimensions shown. But don't worry, high precision isn't required. The loop will tune up easily if you are within roughly 1/8th inch.
The only part of this design that requires special attention is the >-= in. reducer that acts as a tuning capacitor across the open ends of the loop. Drill or file the inside so that it will slide freely on the pipe (21/32 in. roughly). Otherwise you will not be able to tune the antenna easily. As purchased, stock pipe fittings are too tight. Also cut the slot shown for a securing screw.
Figure 2a: Top view of antenna (only)
Figure 2b: Side view of antenna, mast and Vane
IMPORTANT NOTE: Do not cut the two end pieces at first. Instead, cut just one piece the same size as the opposite side. Later, after soldering, you'll cut a gap in center. This assures good alignment of the tuning capacitor.
For soldering, clean all joints thoroughly and apply a little flux. Use a propane torch and common solder, not silver solder. (Figure 2c)
Figure 2e: Detail of copper, coupling loop and PVC support
Also, solder sparingly. It isn't necessary to make water-tight connections tack soldering is fine. At RF frequencies, skin effect makes low DC resistance at the joints completely unnecessary. Keep the loop flat on a heat-resistant surface while soldering.
IMPORTANT NOTE: Do not have the tuning capacitor installed while soldering. You will put it on later. Also, be very careful not to solder the one joint indicated (Figure 2a). Otherwise you will not be able to assemble the antenna.
After soldering, cut a = in. gap in the center of the copper pipe on the tuning capacitor side of the loop. Also make a hole for a sheet metal screw to secure the tuning capacitor. Then cut the two short PVC center support arms. Note that the copper T-fittings are of the reducer type. The arms of the both = in but the neck 3/8 in. Also add two more sheet metal screws (Figure 2a) to keep the loop from rotating.
Next, install the tuning capacitor, glue the PVC support arms to the center PVC cross, put all the parts together and then finally solder the one remaining copper joint, again keeping the loop flat. For the final solder joint, wrap wet rags around the T-fittings and PVC pipe to prevent damage to the PVC.
The Feed Loop
For matching to the coax, I prefer an inductive loop as shown. To me this is the easiest technique for compact loops. Others methods also work, such as a gamma match, but are generally more difficult to fabricate. I only had to make a couple of trial loops during the prototype stage to find the correct size. Loops seem to normally need a coupling loop roughly 1/3 the size of the main loop. The dimensions will yield a good match without any further experimenting.
You can, however, after final tuning, make minor adjustment to SWR by squeezing or stretching the feed loop. Fabricate the feed loop from bare AWG #10 or 12 copper wire. Drill holes in the PVC pipe, then thread the coupling loop through, progressively bending the corners as you go.
For weather protection, solder the coax to the coupling loop inside the PVC cross fitting. Ideally the loop could have a balun. In practice, however, I have experience no discernable difference with or without. But if you prefer, heatshrink several small VHF RF cores over the coax inside the PVC cross to form a balun.
The Weather Vane
I cut the decorative parts of the vane (Figure 3) with tin snips from an inexpensive heavy-duty plastic storage tub. The Web contains a wealth of wonderful weather vanes designs, or you may use the one shown.
Figure 3: The decorative parts. Cut from plastic, not metal.
You are obviously free to innovative here. Do NOT, however, use metal for any of the pieces. This will detune the antenna.
For the rotating arm, keep two things in mind. First, the tail end must have more wind surface than the head end so that the vane will point into the wind. Therefore, the head end must be weighted. I used six one ounce fishing sinkers silicone glued inside the boom. To hold the head and tail pieces on the arm, cut slots in the ends and use a little more silicone glue. Second, the pivot hole must be drilled at the balance point. Otherwise the arm will not rotate easily. The length of the vane is not critical. Mine is roughly 16 inches.
Attach the rooster (Figure 4), or whatever "critter" you choose, with plastic cable clamps and
Figure 4: My rooster. Look on the Web for other designs
some additional glue. Punch two small appropriately-located holes in the direction letters and attached them with wire ties. Finally, give the entire structure a coat of black outdoor spray paint. This will give the impression that your weather vane is made of wrought iron, a final touch for the disguise.
Tuning
For tuning, you'll need no special tools, only a VHF SWR bridge and a 2 Meter transceiver. In the design phase, I did use a FET dip oscillator to get the prototype in the ballpark. Subsequently I've only needed an SWR bridge and an HT to tune up a new vane.
The basic tuning technique is simple. Begin with the tuning capacitor as far away from the = in. end gap as the securing screw will permit. Apply low power with your HT and measure the SWR at the top and again at the bottom of the 2M band. Write down the difference. With the capacitor all the way out, the SWR should be worse at the low end. That's because the loop is tuned too high in frequency. Remember, always keep the securing screw tight when making the SWR measurements. Also, at first, both SWR readings will be poor. Only the difference in high and low matters.
Then, in small increments, progressively move the tuning capacitor inward, again taking high/low readings. As you reach the desired operating frequency, high and low readings should become equal. Still they won't necessarily be low. If you go too far, the SWR will become better at the low end, the reverse of above. Your objective is to find the position where both readings are equal and lowest. To obtain a final 1:1 match you may need to squeeze or stretch the feed loop a little. Generally, I have not had to bother with the dimensions given. Adjusting the tuning capacitor alone has yielded an adequate SWR.
Technical Discussion
Theoretically a compact/magnetic loop antenna is a parallel-tuned LC "tank" circuit. The variable capacitor across the ends (= in. - > in. reducer) resonates the inductance of the loop to the frequency of operation. A multi-turn inductor can't radiate because of its small size. But with a large single-turn inductor, such as a compact loop, good radiation can be achieved.
A compact loop must, however, MUST be smaller than roughly 0.1 wavelength in circumference. This causes it to radiate in a unique way. The classical RF bible, Terman, p. 907, Electronic and Radio Engineering, McGraw-Hill, 1955, states that "The directional pattern of a small loop is identical with that of an elementary doublet (dipole). The only difference is that the electric and magnetic fields are interchanged." Hence a horizontal loop radiates as if it were a vertical dipole (Figure 5).
Figure 5. Radiation pattern of a horizontal magnetic loop (Terman)
If we make the loop larger than 0.1 wavelength in circumferencer the radiation pattern will slowly take on the more familiar form of a dipole
On the other hand, within limits, a compact loop may be made much smaller. A loop tiny compared to wavelength is easily implemented by merely increasing the capacitor across the loop's ends. In this way, lower frequencies or even multiple bands can be achieved. This technique is often used on HF, for example, and several good small multi-band HF compact loops are currently on the market for hams with limited space.
Radiation Resistance
There is, however, a practical limit to how small a loop can be made, due to another important characteristic of all antennas radiation resistance. Radiation resistance is not resistance in the usual sense. It is a kind of virtual resistance created by the actual loading of space on an antenna. Said as a simplification, radiation resistance is a measure of how well an antenna couples to space.
But as far as your transmitter is concerned radiation resistance looks just like an ordinary resistor at the end of the coax. But instead of converting transmitter power into heat, which an actual resistor would do, radiation resistance does what we want and converts RF power into a radio signal. Therefore, radiation resistance is the kind of resistance we want in an antenna. And here is why.
In free space, a full-sized dipole has moderately high radiation resistance, roughly 72 Ohms. Smaller antennas like compact loops have a radiation resistances that is much lower. The bigger the antenna, the larger a piece of space it captures and the higher the radiation resistance. Specifically, as the size of an antenna decreases, the radiation resistance decreases roughly as the square of size Our weather vane has an actual radiation resistance of only 1.7 Ohms.
But as long as we correctly match the transmitter's output impedance (normally 50 Ohms) to the radiation resistance of the antenna, efficient radiation will take place. Hence a compact loop with a very low radiation resistance can theoretically radiate as well as dipole or J-pole with a much higher radiation resistance.
There is a fly in the ointment, however conductor resistance. That's because conductor resistance is in series with radiation resistance. And both forms of resistance must share the power from the transmitter. The power the conductor resistance, a real resistance, receives is wasted as heat. The power the radiation resistance, a virtual resistance, receives makes useful radio waves.
Therefore, in a dipole, with a relatively high radiation resistance of 72 Ohms conductor resistance isn't of much concern. Even for thin wire of poor conductance, conductor resistance is only a small fraction of the radiation resistance. Hence it shares very little of the transmitter's power. But for a compact loop, where the radiation resistance may be very low, conductor resistance can easily waste significant power. That's why we should construct small antennas with large diameter conductors, not thin wire. This is equally one of the reasons why mobile HF whip antennas are characteristically low in efficiency.
Metal Conductivity
Also for small antenna, the kind of metal can be important. Figure 6 illustrates how poor some common metals are and why copper is really the only good choice for a compact loop. Two interesting points to note are that gold is a poorer conductor than either silver or copper. Gold is used on electronic connecters not because it is the best conductor but because it does not corrode. Also notice that stainless steel is thirty times poorer than copper and that aluminum is barely half as good.
|
Silver |
6.8 |
|
Copper |
6.0 |
|
Gold |
4.3 |
|
Aluminum |
3.8 |
|
Steel |
0.6 |
|
Stainless Steel |
0.2 |
Figure 6: Room-Temperature Electrical
Conductivity of Common Metals (Ohm-m-1)
Bandwidth
The final theoretical limitation of a small antenna, as mentioned earlier, is its low bandwidth. Bandwidth is a function of an antenna's natural LC ratio. In a dipole, this ratio is low. That is, the native self inductance of a straight wire is relatively small compared to its self capacitance. Q, therefore, is low and bandwidth high. For a compact loop the opposite is true. A one-turn coil has high L compared to C. Hence, Q is high and bandwidth low. This is equally true for a loaded mobile HF whip. The large loading coil is very inductive, giving the antenna high Q and low bandwidth.
So as a overall guideline if we will use large copper conductors and can live with a small bandwidth, a compact loop is a high efficiency choice for a small space. It is also easier to camouflage. My 40 Meter rose trellis loop is also rarely noticed.
Designing Your Own Loop
And in case you would like to try your hand at a compact loop on another band, I have placed on http://www.w6nbc.com/loop.html the loop design equations and an excellent little Basic program that quickly calculates all parameters, including efficiency, of compact loops of almost any size and frequency. It is one of the many freeware programs for compact loop design available of the Web. It is a simple DOS program, but emulates well on a modern computer operating on XP.
My Operating Experience
I have had two of the weather vane loops in service for several months now, one on my mobile home and the other at a friend's house. At my location I also have a comparison J-pole (basically a full-sized dipole) at the same height, and I can detect little difference. Also, the weather vanes have experienced significant wind and weather exposure during that time and show no tendency to detune. Give one a try. They've been an "undercover" winner for me.