AM Antennas -- Build The AM Superloop Antenna (Jan. 1982)

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author: Richard Modafferi


Usable long- and medium-wave radio signals travel further than you would believe.

Cheap transistor radios and the poor AM sections of modern hi-fi tuners have conditioned us to accept only local reception. In the early days of AM radio, people happily listened to radio sets which would offer reliable daytime reception of signals as far away as 500 miles from the transmitter! Large, powerful console radios by Scott, RCA, Zenith, Philco, and others brought good and varied programming from surrounding metropolitan stations to the distant listener.

Today, fine AM radio programming still exists. Every large city and even some lucky smaller ones have at least one worthwhile AM radio station. If some of these stations exist within 500 miles from you, the powerful antenna I'll discuss here can be used with any AM radio to make possible a choice from a variety of interesting and listenable programming.

The Superloop tunes continuously from 140 to 1800 kHz, which covers both the long- and medium-wave bands. North American low-frequency radio broadcasting is confined to the medium-wave band only, 540 to 1600 kHz, but in Europe the long-wave band (150 to 350 kHz) is also used for radio broadcasts.

Large loop antennas make long-distance low-frequency reception easy. Big loops provide much greater signal pickup than the tiny ferrite rod antennas used in modern radios. Loops work better than long-wire antennas at long- and medium-wave band frequencies, because loops tend to reject the common types of "electric field" static interference from power lines which often plague long-wire antenna systems.

Within reason, the bigger the loop, the better it will work. Some readers may remember the "Wavemagnet" loops hidden inside. The large cabinets of the Zenith consoles of the 1930s. The Super loop is bigger and better. Ambitious DXers can even pack the Superloop and a good transistor portable radio to any convenient mountaintop for some superb long-distance reception.

DESCRIPTION

The Superloop is easy to build I put mine together in an afternoon with the coil winding being the only tricky part of the construction. I'll cover detailed assembly plans later, but first a description of what the loop antenna is and what it can do is in order.

The Superloop has two large coils, each wound on separate parts of a wood frame which is mounted on a base. Tuning controls and small parts are grouped together at the center of the frame. Terminals are provided near the base for connection to radio sets requiring an external antenna.


Table I-Frequency tuning ranges with various positions of the two Superloop switches.

Referring to Fig. 1, the Superloop schematic, L1, the tunable primary, is the larger outer winding. The smaller inner winding, L2, is the secondary coupling coil which connects to output terminals T-1, T2 and T3.

C1(a) and 01(b) is a two-section radio variable capacitor of the "TRF" type, having two identical 365-pF sections.

See Fig. 2, a close-up photo of the central control section of the Superloop, showing C1, C3, L3, 51, and S2.

Switches S1 and S2 are used in various combinations for control of the frequency tuning range of the Superloop. This action is summarized in Table I. The other possible switch combination, with 51 and S2 both open, is not useful since it yields a tuning range already covered.

The capacitor C2, connected between output terminals T2 and T3, is part of an impedance matching circuit on the output terminals. Radios having high-impedance external antenna inputs are connected between terminals T1 and T2; radios having low-impedance external antenna inputs are connected to terminals T1 and T3. You should experiment if you're not certain about your radio's antenna impedance.

C3 is a padder capacitor on C1(b) which causes the two tuning ranges determined by S1 to just barely overlap.

The long-wave loading coil, L3, is a common "loopstick" AM radio antenna coil which is switched into and out of the circuit by S2. Hand capacity will detune the loop slightly, so a large knob should be used on the tuning capacitor C1 .

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Table II--Parts list.

L1 .. Approx. 1/4 lb. No. 18 magnet wire.

L2 .. Approx. 1/8 lb. No. 22 magnet wire.

L3 Loopstick antenna coil, 600 µH. C1(a),(b) ... Two-gang tuning capacitor, 365 pF per section.

C2...180-pF mica or ceramic capacitor.

C3...390-pF mica or ceramic capacitor.

S1, S2 SPST toggle switches.

T1, T2, T3 Five-way binding posts.

Lumber, screws, glue, paint, a large knob, etc. as needed.

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Fig. 1 Schematic of the Superloop, which has specifications as follows: Outer coil, L1, has an inductance of 180 µH at 1 kHz, self-resonance at 1775 kHz (includes stray capacity in C1), Q = 143 at 550 kHz and 57 at 1300 kHz. The inner coil, L2, has an inductance of 160 µH at 1 kHz, self-resonance at 2300 kHz, while Q = 115 at 790 kHz.

Coil L3 is a loopstick radio antenna type with 600-µH inductance and Q of 75 at 790 kHz.


Fig. 2 Component parts placement at the crossbar of the author's Superloop.

Clockwise from 4 o'clock are the large-knobbed C1 beside which is C3, switch 51, switch S2, and finally the loopstick, L3.


Fig. 3--Superloop frame and wire supports. Four 4-inch crosspieces are placed halfway from the end of each arm to the frame center. The loop arms are made of 2 1/2 by 1/2 inch material. The two braces should be set at a 90° angle and attached firmly to the base, which should have at least an 18inch diameter.


Fig. 4 Detail of wire support notches which are cut into the ends of the loop arms.


Fig. 5 Schematic of the outer or L 1 winding, which consists of 6% turns of No. 18 magnet wire.

All the components used in the Superloop came from my well-stocked parts junkbox. A parts list is given in Table II; you may substitute electrical equivalents as necessary. The 6004H loopstick coil is vintage Radio Shack and may be the only component which would be hard to find new. Any loopstick coil can be brought up to 600-µH by adding turns. Coils of lesser inductance will not tune to the bottom of the long wave band (140 kHz). Note that if you are not interested in using the Superloop to tune the long waves, then merely omit S2 and L3. You will then have a tuning range of 380 to 1 800 kHz.

The secret of the Superloop's high performance lies in the somewhat complex low distributed-capacity coil windings. Radio loops were wound in the same manner back in the 1920s old technology is merely being reused here!


Fig. 6 The spacer for winding L1 is inserted so that wires are evenly spaced around the outside. One of the four used is shown.

ASSEMBLY INSTRUCTIONS

Building the Superloop is not difficult, but the coil winding is a bit tricky. I made my loop frame and supports from odd bits of scrap lumber. Yours need not look exactly like mine, so use your ingenuity and available materials. The only thing to preserve in building your loop is the physical size and winding schemes for the two loop windings, L1 and L2; follow mine closely.

Begin by making the two loop arms. My vertical one is 80 inches long and the horizontal one 61 inches. Fasten the four crosspieces (for support of the L2 winding) to the loop arms as in Fig. 3.

With a saw, cut notches about 1/4 inch deep and evenly spaced over 1 1/4 inches into each end of the two loop arms (Fig. 4). These notches will secure the windings of the large coil, L1, later. Fasten the two loop arms together at their centers, forming a large cross. Use a proper woodworking joint, that is, notch the arms halfway through, so the result will be neat and strong. Refer to Figs. 3 and 4 as you go along, as well as the overall photo, so you get things looking right.



Fig. 8 The inner winding, L2 showing ONLY the first turn and the last quarter of the final turn, along with the connections to the terminals T1 and T2.

The outer coil, L1, can now be wound. This MUST be done BEFORE the base is attached! Using #18 magnet wire, begin L1 at the center of the cross formed by the loop arms, leaving several inches slack for connections later. Run the wire along one of the long arms to the notched end, and bend the wire into one of the outside notches. Proceed clockwise around the loop arms, placing the wires into the notches in the ends of the arms until you have wound 61/2 turns. As you wind, pull the wires taut but not excessively tight. At the end of 6'h turns, bring the wire down the length of the long arm (you are on the opposite end of the arm on which you started the winding), returning to the center of the cross (see Fig. 5). A few preparatory steps are required before winding L2, which begins and ends at the loop output terminals that I mounted on one of the base braces. The location of terminals T1 , T2, and T3 is not critical but they should be somewhere near the base of the Superloop.

After mounting the terminals, drill two small holes (1/32-inch dia.) about 1 inch apart through the loop arm near the base and output terminals.

The loop cross may now be attached to the base. Make something which looks roughly like the scheme in Fig. 7. I used No. 6 wood screws and panel cement to hold everything together, and the result was a sturdy construction. Be careful to make things reasonably square so your Superloop will stand upright on the base.

With the Superloop now standing on its base, construction can now continue.

Insert the four spacers into the wires of L1 (Fig. 6). Mine are of stiff cardboard; thin plastic or fiberboard would be more elegant. Slip the spacers into the wires, adjust the wires evenly around both sides of the spacer, and apply a bit of cement to each wire at the spacer to hold things in place.


Fig. 7--Output terminals for the Superloop are mounted on one of the frame supports near the base.


Table III--Winding table for the inner coil, L2. All turns are clockwise, F means put the wire on the front of the crosspiece, and B means put the wire on the back of the crosspiece.

The numbering system for the crosspieces refers to Fig. 8.

The inner coil, L2, will be eight turns of No. 22 magnet wire. Begin by passing the wire through one of the holes drilled in the loop arm, and solder the end to T1 . Pass the wire upwards along the vertical loop arm to the bottom crosspiece (see Fig. 8). You are now ready to begin winding L2. Stand facing the loop, with the plane of the winding L1 at right angles to your line of sight. Refer to Fig. 8 and note that the windings of L2 will pass over the front (nearest to you) or rear (farthest from you) of the crosspieces, as determined in Table III.

Begin winding L2 with two complete clockwise turns passing over the front of all four crosspieces. Begin turn three on the front of the bottom crosspiece, No. 1 , but now alternate front and back wind positions on the crosspieces as you go around, following the positions as indicated in Table III. For example, you will be at the back of crosspiece two at 3 1/4 turns, at the front of crosspiece three at 3 1/2 turns, and so forth. At the beginning of turn five, you should be at the front of crosspiece one. Begin turn five by first looping the wire from front to back of crosspiece one so that turn five will begin at the BACK of crosspiece one. Continue winding turns five and six, alternating back and front wind positions on the crosspieces as you go around. At the end of turn six, you should be at the back of crosspiece one. Wind turns seven and eight remain on the back of the four crosspieces, as indicated in the table.

Upon completion of turn eight, pass the wire downwards on the vertical loop arm, pass it through the remaining hole in the loop arm, and solder to T2. Keep the wires to T1 and T2 separated by at least 1 inch in order to minimize stray capacity. Attach C2 between T2 and T3, and this completes the inner loop, L2.

Finish your Superloop by mounting C1, S1, S2, L3, and C3. Group these components near the center of the loop frame, making things look roughly like Fig. 2. Connect the components as in the schematic (Fig. 1), and your Superloop is finished and ready for testing.

Check the wiring carefully, then go find an AM radio set.

OPERATION

Two modes of operation of the Super loop are possible, each determined by the type of antenna system in the radio set to be used with it. For radios with built-in small loops or ferrite rods, merely place the radio near the Superloop and tune both radio and loop to the same frequency. Be sure that the axis of the radio's loop coil and the Superloop windings are parallel. Rotate radio and loop together for best reception. Radios not having built-in loop antennas are connected to the Superloop secondary-winding output terminals. Tap the radio's antenna lead to T2 or T3 on the Superloop, whichever gives the best results.

Connect the ground lead of the radio to T1. Both antenna and ground must be connected for best results. If the radio and the Superloop are located physically far apart, you may use shielded 50- or 75-ohm coax to connect the Superloop to the radio. Ordinary "zip" cord, 300ohm twin lead, or simple twisted wires will also work to connect the radio to the loop, but may pick up some noise.

All my radios work on the Superloop, with surprising and amazing success.

Summer thunderstorm activity will be the only restriction on DX reception with the Superloop, although not as much as you might think. The loop does seem to pick up less static than a long-wire antenna.

You will be able to detect the approach of a storm front from great distances.

With a bit of practice, you might even try using the Superloop to forecast the weather! Bigger loops can be built. I am plan ping a 20-foot tall monster set in concrete and rotatable with remote tuning via varactor diodes. If this thing is built and works, I may report on it. Good DXing!

(adapted from Audio magazine, Jan 1982)

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