Small loop antennas

In this Section you will learn about small loop receiving antennas. The small loop antenna is almost ideal for DXing the crowded AM broadcast band and low frequency 'tropical' bands. These antennas are fundamentally different from the large loop types previously discussed and are very often the antenna of choice for low frequency work.

Large loop antennas are 0.5 lambda or larger and respond to the electrical field component of the electromagnetic wave. Small loop antennas, on the other hand, are <0.1 lambda (some sources say 0.17 lambda and <0.22 lambda) and respond to the magnetic field component of the electromagnetic wave. One principal difference between the large loop and the small loop is found when examining the radio frequency current induced in the loop when a radio signal inter sects it. In a large loop, the dimensions in each section are an appreciable portion of one wavelength, so the current will vary from one point in the conductor to another. But in a small loop, the current is the same through out the entire loop.

The differences between small loops and large loops show up in some interesting ways, but perhaps the most striking are the directions of maximum response - the main lobes - and the directions of the nulls. Both types of loop produce figure-of-eight patterns, but in directions at right angles with respect to each other. The large loop antenna produces main lobes orthogonal, at right angles or 'broadside' to, the plane of the loop. Nulls are off the sides of the loop. The small loop, however, is exactly the opposite: the main lobes are off the sides of the loop (in the direction of the loop plane), and the nulls are broadside to the loop plane (see FIG. 1A).

Don't confuse small loop behavior with the behavior of the loopstick antenna. Loopstick antennas are made of coils of wire wound on a ferrite or powdered iron rod. The direction of maximum response for the loopstick antenna is broadside to the rod with deep nulls off the ends ( FIG. 1B).

Both loopsticks and small wire loops are used for radio direction-finding and for shortwave, low frequency medium wave, AM broadcast band, and VLF DXing.

FIG. 1A

The nulls of a loop antenna are very sharp and very deep. Small changes of pointing direction can make a profound difference in the response of the antenna. If you point a loop antenna so that its null is aimed at a strong station, the signal strength of the station appears to drop dramatically at the center of the notch. But turn the antenna only a few degrees one way or the other, and the signal strength increases sharply. The depth of the null can reach 10 to 15 dB on sloppy loops and 30 to 40 dB on well-built loops (20 dB is a very common value). I've seen claims of 60 dB nulls for some commercially available loop antennas. The construction and uniformity of the loop are primary factors in the sharpness and depth of the null.

One of the characteristics of the low frequency bands in which small loops are effective is the possibility of strong local interference smothering weaker ground wave and sky wave stations. As a result, you can't hear co channel signals when one of them is very strong and the other is weak.

Similarly, if a co-channel station has a signal strength that is an appreciable fraction of the desired signal, and is slightly different in frequency, then the two signals will heterodyne together and form a whistling sound in the receiver output. The frequency of the whistle is an audio tone equal to the difference in frequency between the two signals. This is often the case when trying to hear foreign broadcast band signals on frequencies (called split frequencies) between the standard 10 kHz spacing used in North and South America. The directional characteristics of the loop can help if the loop null is placed in the direction of the undesired signal.

Loops are used mainly in the low frequency bands even though such loops are either physically larger than high frequency loops or require more turns of wire. Loops have been used as high as VHF and are commonly used in the 10 meter ham band for such activities as hidden transmitter hunts.

FIG. 1B

Let's examine the basic theory of small loop antennas, and then take a look at some practical construction methods.

AIR CORE FRAME LOOPS ('BOX' LOOPS)

A wire loop antenna is made by winding a large coil of wire, consisting of one or more turns, on some sort of frame. The shape of the loop can be circular, square, triangular, hexagonal or octagonal. For practical reasons, the square loop seems to be most popular. With one exception, the loops considered in this section will be square so you can easily duplicate them.

The basic form of the simplest loop is shown in FIG. 2. This loop is square, with sides the same length 'A' all around. The width of the loop ('B') is the distance from the first turn to the last turn in the loop, or the diameter of the wire if only one turn is used. The turns of the loop in FIG. 2 are depth wound, meaning each turn of the loop is spaced in a slightly different parallel plane. The turns are spaced evenly across distance 'B'. Alternatively, the loop can be wound such that the turns are in the same plane (this is called planar winding). In either case, the sides of the loop ('A') should be not less than five times the width ('B'). There seems to be little difference between depth and planar wound loops. The far-field patterns of the different shape loops are nearly the same if the respective cross sectional areas (_r 2 for circular loops and A2 for square loops) are <_2 =100.

The actual voltage across the output terminals of an untuned loop is a function of the angle of arrival of the signal a, as well as the strength of the signal and the design of the loop. The voltage Vo is given by:

Vo = 2_ANEf cos (_)

FIG. 2

where:

Vo is the output voltage of the loop A is the area of the loop in square meters (m^2 ) N is the number of turns of wire in the loop Ef is the strength of the signal in volts per meter (V/m) a is the angle of arrival of the signal _ is the wavelength of the arriving signal.

Loops are sometimes specified in terms of the effective height of the antenna. This number is a theoretical construct that compares the output voltage of a small loop with a vertical piece of the same kind of wire that has a height of:

Heff = 2_NA

If a capacitor (such as C1 in FIG. 2) is used to tune the loop, then the output voltage Vo will rise substantially. The output voltage found using the first equation is multiplied by the loaded Q of the tuned circuit, which can be from 10 to 100 (if the antenna is well constructed):

Vo = 2_ANEf Qcos (_)

Even though the output signal voltage of tuned loops is higher than that of untuned loops, it’s nonetheless low compared with other forms of antenna. As a result, a loop preamplifier is usually needed for best performance.

TRANSFORMER LOOPS

It’s common practice to make a small loop antenna with two loops rather than just one. FIG. 3 shows such a transformer loop antenna. The main loop is built exactly as discussed above: several turns of wire on a large frame, with a tuning capacitor to resonate it to the frequency of choice. The other loop is a one or two turn coupling loop. This loop is installed in very close proximity to the main loop, usually (but not necessarily) on the inside edge not more than a couple of centimeters away. The purpose of this loop is to couple signal induced from the main loop to the receiver at a more reasonable impedance match.

The coupling loop is usually untuned, but in some designs a tuning capacitor (C2) is placed in series with the coupling loop. Because there are many fewer turns on the coupling loop than the main loop, its inductance is considerably smaller. As a result, the capacitance to resonate is usually much larger. In several loop antennas constructed for purposes of researching this Section, I found that a 15-turn main loop resonated in the AM broadcast band with a standard 365 pF capacitor, but the two turn coupling loop required three sections of a ganged 3 _ 365 pF capacitor connected in parallel to resonate at the same frequencies.

In several experiments, I used computer ribbon cable to make the loop turns. That type of cable consists of anywhere from eight to 64 parallel insulated conductors arranged in a flat ribbon shape. Properly interconnected (of which more later), the conductors of the ribbon cable form a continuous loop. It’s no problem to take the outermost one or two conductors on one side of the wire array and use it for a coupling loop.

FIG. 3

TUNING SCHEMES FOR LOOP ANTENNAS

Loop performance is greatly enhanced by tuning the inductance of the loop to the desired frequency. The bandwidth of the loop is reduced, which reduces front-end overload. Tuning also increases the signal level available to the receiver by a factor of 10 to 100 times. Although tuning can be a bother if the loop is installed remotely from the receiver, the benefits are well worth it in most cases.

FIG. 4

There are several different schemes available for tuning, and these are detailed in FIG. 4. The parallel tuning scheme, which is by far the most popular, is shown in FIG. 4A. In this type of circuit, the capacitor (C1) is connected in parallel with the inductor, which in this case is the loop.

Parallel resonant circuits have a very high impedance to signals on their resonant frequency, and a very low impedance to other frequencies. As a result, the voltage level of resonant signals is very much larger than the voltage level of off-frequency signals.

The series resonant scheme is shown in FIG. 4B. In this circuit, the loop is connected in series with the capacitor. A property of series resonant circuits is that it offers a high impedance to all frequencies except the resonant frequency (exactly the opposite of the case of parallel resonant circuits). As a result, current from the signal will pass through the series resonant circuit at the resonant frequency, but off-frequency signals are blocked by the high impedance.

There is a wide margin for error in the inductance of loop antennas, and even the precise-looking equations to determine the required values of capacitance and inductance for proper tuning are actually only estimations. The exact geometry of the loop 'as built' determines the actual inductance in each particular case. As a result, it’s often the case that the tuning provided by the capacitor is not as exact as desired, so some form of compensation is needed. In some cases, the capacitance required for resonance is not easily available in a standard variable capacitor and some means must be provided for changing the capacitance. FIG. 4C shows how this is done. The main tuning capacitor can be connected in either series or parallel with other capacitors to change the value. If the capacitors are connected in parallel, then the total capacitance is increased (all capacitances are added together).

But if the extra capacitor is connected in series then the total capacitance is reduced. The extra capacitors can be switched in and out of a circuit to change frequency bands.

Tuning of a remote loop can be a bother if done by hand, so some means must be found to do it from the receiver location (unless you enjoy climbing into the attic or onto the roof). Traditional means of tuning called for using a low speed DC motor, or stepper motor, to turn the tuning capacitor. A very popular combination was the little 1 to 12 RPM motors used to drive rotating displays in retail store show windows. But that approach is not really needed today. We can use varactor voltage variable capacitance diodes to tune the circuit.

A varactor works because the junction capacitance of the diode is a function of the applied reverse bias voltage. A high voltage (such as 30 volts) drops the capacitance while a low voltage increases it. Varactors are available with maximum capacitances of 22, 33, 60, 100, and 400 pF. The latter are of most interest to us because they have the same range as the tuning capacitors normally used with loops. Look for service shop replacement diodes intended for use in AM broadcast band radios. A good selection, which I have used, is the NTE-618 device. It produces a high capacitance >400 pF, and a low of only a few picofarads over a range of 0 to 15 volts.

FIG. 5 shows how a remote tuning scheme can work with loop antennas. The tuning capacitor is a combination of a varactor diode and two optional capacitors: a fixed capacitor (C1) and a trimmer (C2). The DC tuning voltage (Vt is provided from the receiver end from a fixed DC power supply (V)). A potentiometer (R1) is used to set the voltage to the varactor, hence also to tune the loop. A DC blocking capacitor (C3) keeps the DC tuning voltage from being shorted out by the receiver input circuitry.

FIG. 6

FIG. 7

THE SQUARE HOBBY BOARD LOOP

A very common way to build a square loop antenna is to take two pieces of thin lumber, place them in a cross shape, and then wind the wire around the ends of the wooden arms. This type of antenna is shown in FIG. 6. The wooden supports can be made of 100 _ 200 lumber, or some other stock. A test loop made while researching this guide was made with two:

3=1600

_ 300

_ 2400

Bass wood 'hobby board' stock acquired from a local hobby shop. Model builders use this wood as a stronger alternative to balsa wood.

The electrical circuit of the hobby board loop shown in FIG. 6 is a transformer loop design. The main tuned loop is on the outside and consists of ten turns of #26 enameled wire. It’s tuned by a 365 pF capacitor. The inner loop is used for coupling to the receiver and consists of a single turn of

#22 insulated solid hook-up wire.

Details for the boards are shown in FIG. 7. Each board is 24 inches (61 cm) long. At the mid-point (1200 or 30.5 cm), there is a 1 4 inch (0.635 cm) wide, 1.5 inch (3.8 cm) long slot cut. These slots are used to mate the two boards together.

At each end there are ten tiny slits made by a jeweler's saw (also called a 'jig saw' in hobbyist circles) with a thin blade. These slits are just wide enough to allow a single #26 wire to be inserted without slipping. The slits are about 1/4 inch (0.635 cm) long, and are 1/4 inch (0.635 cm) apart.

There are ten slits on both ends of the horizontal piece while the vertical piece has ten slits on the top end and 11 slits on the bottom end. The reason for offsetting the wire slits is to allow room on the other side of the 3 inch width of the vertical member for a mounting stick.

When assembling the antenna, use wood glue on the mating surfaces, square them to be at right angles to each other, and clamp the two pieces in a vice or with C-clamps for 30 minutes (or longer if the glue maker specifies).

Next, glue the support blocks into place and clamp them for a similar period.

THE SPORTS FAN'S LOOP

OK, sports fans, what do you do when the best game of the week is broad cast only on a low-powered AM station - and you live at the outer edge of their service area where the signal strength leaves much to be desired? You use the sports fan's loop antenna, that's what! I first learned of this antenna from a friend of mine, a professional broadcast engineer, who worked at a religious radio station that had a pipsqueak signal but lots of fans. It really works - one might say it's a miracle.

The basic idea is to build a 16-turn, 24 inch (61 cm) square tuned loop ( FIG. 8) and then place the AM portable radio at the center ( FIG. 9) so that its loopstick is aimed so that its null end is broadside of the loop.

When you do so, the nulls of both the loop and the loopstick are in the same direction. The signal will be picked up by the loop and then coupled to the radio's loopstick antenna. Sixteen-conductor ribbon cable can be used for making the loop. For an extra touch of class, place the antenna and radio assembly on a dining room table 'Lazy Susan' to make rotation easier. A 365 pF tuning capacitor is used to resonate the loop. If you listen to only one station, then this capacitor can be a trimmer type.

SHIELDED LOOP ANTENNAS

The loop antennas discussed thus far in this Section have all been unshielded types. Unshielded loops work well under most circumstances, but in some cases their pattern is distorted by interaction with the ground and nearby structures (trees, buildings, etc.). In my own tests, trips to a nearby field proved necessary to measure the depth of the null because of interaction with the aluminum siding on my house. FIG. 10 shows two situations. In FIG. 10A, we see the pattern of the normal 'free-space' loop, i.e., a perfect figure-of-eight pattern. But when the loop interacts with the nearby environment, the pattern distorts. In FIG. 10B we see some filling of the notch for a moderately distorted pattern. Some interactions are so severe that the pattern is distorted beyond all recognition.

FIG. 9

FIG. 10

FIG. 11

The solution to the problem is to reduce interaction by shielding the loop, as in FIG. 11. Loop antennas operate on the magnetic component of the electromagnetic wave, so the loop can be shielded against voltage signals and electrostatic interactions. In order to prevent harming the ability to pick up the magnetic field, a gap is left in the shield at one point.

There are several ways to shield a loop. You can, for example, wrap the loop in adhesive-backed copper foil tape. Alternatively, you can wrap the loop in aluminum foil and hold it together with tape. Another method is to insert the loop inside a copper or aluminum tubing frame. The list seems endless. Perhaps one of the most popular methods is to use coaxial cable to make a large single turn loop. FIG. 12 shows this type of loop made with RG-8/U or RG-11/U coaxial cable. The cable is normally supported by wooden cross arms, as in the other forms of loop, but they are not shown here for sake of simplicity. Note that, at the upper end, the coaxial cable shields are not connected.

FIG. 13

Another example is the antenna shown in FIG. 13. This antenna is made of wide metal conductors. Examples include the same type of hobbyist's brass stock as used above. It can also be copper foil or some other stock that can be soldered. Some electronic parts stores sell adhesive backed foil stock used for making printed circuit boards. The foil can be glued to some flat insulating surface. Although K-inch plywood springs to mind immediately, another alternative is found in artists' supplies stores.

Ordinary poster board is too floppy to stand up, but poster board glued to a Styrofoam backing can be used. It’s extremely easy to work with using X-acto knives and other common household tools.

Two controls are used on this antenna. Capacitor C1 tunes the loop to the resonant frequency of the desired station. Potentiometer R1 is used as a phasing control. The dimensions of the antenna are not terribly critical, although some guidelines are in order. In the Villard article, he recommended a 40 cm (15.75-inch) square loop ('A'). If the loop is 7.62 cm (three inches) wide, the antenna will resonate at 15 MHz with around 33 pF of capacitance. If the dimensions are increased to A = 91 cm (36 inches) and B = 10:16 cm (four inches), then the inductance increases and only 28 pF are needed at 15 MHz. The larger size loop can be used at lower frequencies as well. For example, the 91 cm loop will resonate at 6 MHz with 177 pF.

To use this antenna, position the radio's telescopic antenna close and adjacent to the loop but not touching it. The loop antenna can be rotated to find the best position to either null or enhance a particular station. The 'Lazy Susan' idea will work well in this case.

TESTING YOUR LOOP ANTENNA

When each loop prototype was completed, I tested it on the AM broadcast band over several evenings. The same procedure can be used with any loop. A strong local signal at 1310 kHz served to check the pattern.

The station and my home were located on US Geological Survey 7.5 minute quadrangle maps of my area (or the equivalent Ordnance Survey maps in the UK). The maps had adjacent coverage, so the compass bearing from my location to the station could be determined. Checking the antenna showed an S7/S8 signal when the loop was endwise to the station -- that is, the station was in one of its lobes. Rotating the loop so that its broadside faced the direction of the station dropped the signal strength to less than S1, and frequently bottomed out the meter. Because my receiver has a 3 dB/S-unit calibration on the S-meter, I figured the null to be more than 20 dB, although it will take a bit more experimentation to find the actual depth. This test is best done during daylight hours, I found out, because there is always a residual sky wave cacophony on the AM band that raises the S-meter 'floor' in the null.

USING A LOOP ANTENNA

Most readers will use a loop for DXing rather than hidden transmitter hunting, navigation, or other RDF purposes. For the DXer, there are actually two uses for the loop. One is when you are a renter or live in a community that has routine covenants against outdoor antennas. In this situation, the loop will serve as an active antenna for receiving AM broad cast band and other low frequency signals without the neighbors or landlord becoming annoyed.

The other use is illustrated by the case of a friend of mine. He regularly tunes in clear channel WSM (650 kHz, Nashville) in the wee hours between Saturday evening ('Grand Ole Opry' time) and dawn. However, that 'clear' channel of WSM isn't really so clear, especially without a narrow filter in the receiver. He uses a loop antenna to null out a nearby 630 kHz signal that made listening a bit dicey, and can now tape his 1940s/1950s vintage country music.

It isn't necessary to place the desired station directly in the main lobes off the ends of the antenna, but rather place the nulls (broadside) in the direction of the offending station that you want to eliminate. So what happens if the offending station and the desired station are in a direct line with each other with your receiving location in the middle between them? Both nulls and lobes on a loop antenna are bidirectional, so a null on the offending station will also null the desired station in the opposite direction.

One method is to use a sense antenna to spoil the pattern of the loop to a cardioid shape. Another method is to use a spoiler loop to null the undesired signal. The spoiler loop is a large box loop placed one to three feet (found experimentally) behind the reception loop in the direction of the offending signal. This method is detailed in FIG. 14. The small loopstick may be the antenna inside the receiver, while the large loop is a box loop such as the sports fan's loop. The large box loop is placed about one to three feet behind the loopstick and in the direction of the offending station. The angle with respect to the line of centers should be 60- to 90-degree, which is also found experimentally. It's also possible to use two air core loops to produce an asymmetrical receiving pattern.

FIG. 14

LOOP PREAMPLIFIERS

All small loop antennas produce a weak output signal, so a loop preamplifier is indicated for all but the most sensitive receivers. The preamplifier can be mounted either at the receiver or the antenna, but it’s most effective when mounted at the antenna (unless the coax to the receiver is short).

SHARPENING THE LOOP

Many years ago the Q-multiplier was a popular add-on accessory for a communications receiver. These devices were sold as Heathkits and many construction projects were seen in magazines and amateur radio books. The Q-multiplier has the effect of seeming to greatly increase the sensitivity of a receiver, as well as greatly reducing the bandwidth of the front end. Thus, it allows better reception of some stations because of increased sensitivity and narrowed bandwidth.

FIG. 15

A Q-multiplier is an active electronic circuit placed at the antenna input of a receiver. It’s essentially an Armstrong oscillator, as shown in FIG. 15, that doesn't quite oscillate. These circuits have a tuned circuit (L1/C1) at the input of an amplifier stage, and a feedback coupling loop (L3). The degree of feedback is controlled by the coupling between L1 and L3. The coupling is varied both by varying how close the two coils are, and their relative orientation with respect to each other. Certain other circuits use a series potentiometer in the L3 side that controls the amount of feedback.

The Q-multiplier is adjusted to the point that the circuit is just on the verge of oscillating, but not quite. As the feedback is backed away from the threshold of oscillation, but not too far, the narrowing of bandwidth occurs as does the increase in sensitivity. It takes some skill to operate a Q-multi plier, but it’s easy to use once you get the hang of it and is a terrific accessory for any loop antenna.

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