A MAGNETIC LOOP ANTENNA FOR 600 KHz-4MHz by Glen E. Gardner, Jr.AA8C

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  http://mysite.verizon.net/~vze24qhw/loop.html       A MAGNETIC LOOP ANTENNA FOR 600 KHz-4MHz The finer points explained by

Glen E. Gardner, Jr.

AA8C

Introduction

The classic diamond shape of the antenna creates a nostalgic look that many people find appealing. The simplicity , ease of use and effectiveness of this antenna offer a combination of estetics and performance that are hard to come by in expensive commercially made antennas, yet this antenna costs only about $45 to construct and can be built in just a few hours using simple hand tools. The best part is that the performance on 160M and 80M is excellent!

Features

The antenna is built on a cross-shaped frame with spar width (and height) of 48 inches, providing a height of 52 inches and a width of 49 inches. The loop supports are 12 inches deep. It is constructed of PVC plastic tubing that is reinforced (for stiffness) with wooden dowel rods. The mounting pedestal is approximately 21 inches high, and has a swivel mount that allows a full 360 degrees of rotation. The tuning box incorporates a 3 section 45-540 pf air variable capacitor with a switch that can be used to progressively engage the two extra sections. A custom built ELF-HF autotransformer is incorporated with switched taps for 1:1, 4:1, 9:1, 16:1 and 25:1 impedance ratios. The main loop is four turns of #22 insulated wire with two inches spacing between adjacent turns. The pickup loop is a single turn of #24 enameled wire wound about an inch inside of the main loop on holes in the central vertical and horizontal spars. At resonance, the antenna has a Q of approximately 160 at 3.9MHz, and a characteristic impedance of about 800 ohms at the feedpoint when used without the autotransformer. The antenna is highly bidirectional with deep (>30db) nulls on both side lobes (normal to the plane of the antenna). The use of the autotransformer provides a good match (<1.5:1 VSWR) across the tuning range of the antenna. If available, 40 to 50 strand litz wire would be superior to using 22ga hookup wire for the resonator and yield an even better Q while enhancing the upper frequency tuning limit.

Theory and Construction

The idea behind a magnetic loop antenna is simple. Construct a very high Q resonant circuit and it will develop an RF potential across itself as the magnetic lines of force from incoming signals move across the conductors of the resonant circuit. The higher the Q of the tuned circuit, the higher the potential developed. In this way, the loop appears to develop some gain at resonance. Signals arriving from the side present a similar phase relationship at both ends of the coil and are cancelled because the opposite ends of the resonator have a 180 degree phase relationship. Signals arriving along the plane of the loop are not in phase at all points on the loop and are not cancelled , so signals can be heard in two directions only. The end result is that the loop is bi-directional in the plane of the main loop. The directivity of the loop will appear to vary from signal to signal, depending on the elevation angle of arriving signals. High angle signals will appear to be coming from somewhat omnidirectional sources , while the aizmuth of low-angle signals is well defined and can be easily determined. Normal operation requires that the loop be operated in the vertical position. Orienting the loop horizontal with respect to the earth will result in a null condition wherein all but the strongest signals are effectively cancelled out when the antenna is near the surface of the earth.

Problems arise when attempting to construct a loop antenna that is large and of high Q. The larger the antenna is, the more sensitive it will be by virtue of the total cross sectional area of the wire. However, the series resistance of the wire soon increases to the point where Q is adversely affected and the sensitivity of the antenna becomes less than optimal. Also, skin effects on the inductor will limit Q. Lastly, the distribuited capacitance of the antenna as a whole can become quite large and will ultimately be the controlling factor that limits the usable upper frequency limit for the antenna.

Impedance matching and the method of coupling to the main loop are also important to maintaining a high Q. There are a number of schemes, but I have developed a preference for inductive coupling via a single turn loop spaced about one inch inside the main loop. Inductive coupling allows for easier impedance transformation, and avoids having to connect the feedline directly to the resonator, thus allowing the designer to to control coupling and minimize the amount of undesired loading the resonator experiences. In this way , a managable impedance matching (in this case by means of an autotransformer) scheme can be readily implimented without loading the resonator excessively and spoiling the Q. Bear in mind that the ideal resonator will have a self-impedance that approaches infinity , so a good , real-life resonator will have a self impedance that is quite high. With this in mind, it becomes readily apparent that any impedance mismatch is likely to degrade Q, and thus performance will be limited by the quality of the resonator and the fidelity of the impedance matching scheme.

The secrets to high Q and efficiency in magnetic loop antennas.

1) Use a small diameter wire. A smaller surface area significantly reduces distribuited capacitance and allows for a higher usable upper-frequency limit.

2) Use 40 to 50 conductor litz wire. Multiple insulated strands and the braided design reduces series resistance, reduces skin effect, and the braided lay of the wire further reduces distribuited capacitance. At higher frequencies, skin effect is more pronounced and more strands are required in the litz to overcome the effect. For low HF bands, use 40-50 strands. For LF, 15-20 strands will probably be adequate, and for VLF and ELF, 10-15 strands should be acceptable. This is the good stuff, use it if you can get it.

3) If you can't use litz wire, use a small gauge (20-24Ga) solid copper wire. Use silver plated, solid wire if you have it. Insulated wire is fine. Stranded wire that does not have the individual strands insulated is subject to hysteresis losses and effectily reduces the Q of the resonator, so avoid using stranded wire.

4) If all else fails, use what you have. About any wire will "work" , but the accumulation of "little" defecits will ultimately limit the antenna's performance.

5) Generously space the wire in order to minimize distribuited capacitance. About two inches between the conductors of the resonator seems to work well for my purposes.

6) Large antennas have a better pick up efficiency by virtue of a larger surface area, but large antennas have a lot of wire and more problems with self-resonance, and low Q. In general, using less than 1/4 wavelength of wire on a 3-6 foot frame , 8-12 inches deep results in a managable combination. If you are a VLF-ELF fan, plan on a larger, deeper frame and a lot more wire.

7) A high Q will manifest itself in the form of sharp tuning of the variable capacitor. If all is well, the tuning will exhibit a definite, sharp, narrow peak. The typical 6DB bandwidth of this antenna on the 75 meter amateur band is about 25 KHz. If the input impedance to the autotransformer is given as 50 ohms with the 16:1 impedance tap used , the result is an estimated load of 12,800 ohms on the resonator. Here is the dilema; If the Q of the resonator drops, you might be forced to use a lower impedance tap on the autotransformer to obtain an impedance match and the resonator will suffer from an even lower loaded Q, thus reducing it's efficiency. Bear in mind that the ideal resonator will have an infinite self-impedance and the loaded Q will be determined solely by the load you place on the input. Our problem is much more complex than that. Resistance, self-resonance, excessively low self-impedances, and impedance mismatches all conspire to spoil the Q of the resonator and limit the performance. At this level, the small details matter, so pay careful attention to how your resonator is behaving so you can tweak it to near perfection.

8) Avoid using large masses of metal in, and around the antenna. Use non-magnetic metals for fasteners where possible. This is a magnetic loop antenna, and materials that exhibit ferromagnetic behavior (particularly iron, steel, and nickel) will add to your noise figure and mess with your antennas' resonant frequency and radiation pattern in ways that are both wierd and magical. Large masses of any metal near the antenna will have a negative effect on antenna performance.

Construction

The main loop requires about 50 feet of wire. The single turn pickup loop will require about 18 feet of wire. The frame is a diamond shape that is approximately 4 feet between opposite vertices. The main loop is up to one foot deep and wound on small pvc cross spars with 1/8 inch holes drilled in them every inch so that I can test different configurations. The one that I finally settled on as optimal is 4 turns, spaced two inches per turn, 8 inches deep, with the single turn, 47" wide pickup loop wound inside the main loop on holes in the horizontal and vertical central spars.

The tuning capacitor connects to, and resonates the main loop, and the pickup loop is connected to the autotransformer. The autotransformer is fed with 50 ohm coax via a bnc connector.

The tuning box houses the tuning capacitor, autotransformer, and the essential switching. Use good quality components here. The chances are that the box will outlive several loop antennas, so you want it to be rugged and nicely made. I use my tuning box for little experiments ranging from about 100KHz up to about 15 MHz. In theory, it's usable down to 10KHz, but the tuning capacitor value is a bit silly for ELF, honest.

Transmitting

This antenna ought to work well for transmitting, however, the ferrite core used in my version is quite small and probably would not be suitable for power levels much over a watt or two. If you want to use the antenna for higher power levels, you should invest in a more massive autotransformer. Those wishing to use high power levels (in excess of 25 watts) with this type of antenna need to consider the fact that an RF potential of many thousands of volts can be readily developed across the resonator. Choose your components accordingly and build the antenna carefully so that it does not start throwing lightning bolts around and otherwise self-destruct the first time you try to transmit.

Partls List (tuning box)

1) plastic weathertight electrical enclosure 4" X 4" X 4.5" (I got mine in the electrical dept of a hardware store).

1) 2-pole, 3-position ceramic rotary switch, progressive, shorting type (for the tuning capacitor sections).

1) 2-pole, 5-position ceramic rotary switch, non-shorting (for the RF autotransformer).

2) dual 5-way binding post (available at Radio Shack)

1) 3-section 45-540pf air variable capacitor (purchased from Fair Radio surplus sales)

1) Ferronics 11-720-B ferrite toroid (used for winding the RF autotransformer).

1) BNC jack (use an SO-239 if you still like obsolete stuff that breaks all the time)

3) knobs for the switches and tuning capacitor.

1) 3" X 5" G-10 fiberglass board (for mounting the tuning box to the mast of the tuning pedestal).

1) 1 foot of insulated hookup wire for connecting things inside the tuning box.

2) 2-1/2" long , 1/4" brass bolts with flat washers and wing nuts (for mounting the tuning box to the pedestal mast). The box is mounted to the 3X5 fiberglass board , and the two brass bolts go through the mounting feet of the box, through the board, and through the lower pedestal mast , securing the dowel rod and the box. The other two mounting feet for the box are secured to the board with #6 brass machine screws, flatwashers, lockwashers and nuts.

The frame is nothing special. I used PVC pipe and wooden dowel rods to build it. If you are waxing nostalgic, you might consider all-wood construction. In short, the antenna loop is 4-feet wide, 4 feet high, one foot deep, and sits on a 19" pedestal that is made to swivel. Build yours as you see fit, with what you have on hand. The important thing is not so much the parts that I used, but putting the materials which you have to proper use. The parts that I used for the frame and pedestal are listed below. You might want to peruse it to get an idea of how I did it, but it has been my experience that one is generally limited to what the hardware store has in stock, so be prepared to adjust your design accordingly. The frame spars, the vertical pedestal mast pipe, and pedestal legs were carefully cemented with pvc pipe cement. The remaining PVC pipe connections were press-fit for ease of dissassembly later.

Parts List (frame)

3) 1/2 CPVC pipe, 24 inchels long. (vertical and horizontal spreaders) Small pipe was used to keep the mass down, and the fliminess was reduced by forcing a dowel rod inside the horizontal spreaders. The vertical mast was stiffened by using 3/4" pvc for the lower half, fit to the 1/2" pvc "X" with a reducer and a short bit of 1/2" pvc pipe.

1) "X" fitting , 1/2 inch CPVC (central point for spreaders)

6) 6" length of 1/2 inch CPVC (top and side cross supports for individual turns of resonator)

3) "T" fittings , 1/2 inch CPVC. Cross supports for the 6" long 1/2" pipes used to support the wire loop (top, and two sides. the bottom support is made with 3/4" pipe)

1) 2" long 1/2 inch CPVC pipe for use as a coupling between 1/2" "X" fitting and 3/4" -1/2" reducer.

1) 3/4" to 1/2" CPVC reducer Adapts the lower 3/4" vertical mast to the upper mast 1/2" "X" fitting when used with the 3/4" coupler and a short bit of 1/2" pipe.

1) 3/4" CPVC coupler

1) 3/4" T fitting, CPVC. This is a point of attachment for the 6" cross supports for the loop wires.

2) 3/4" CPVC pipe, 6" long (bottom cross supports for resonator wires)

1) 3/4" CPVC pipe, 24" long. This is the lower half of the vertical mast.

1) 48" long 1/2" wooden dowel rod, force-fit into horizontal spars, through the 1/2" "X" fitting, for stiffness.

6) 1/2" CPVC pipe caps. These are for appearance only. Leave these out of you want to save a couple of dollars.

2) 3/4" CPVC pipe caps. These are for appearance only. Leave these out of you want to save a couple of dollars.

Parts List (Pedestal)

1) 19" long , 1" wodden dowel rod. Shimmed with tape to fit into pedestal support for stifffness and also operates as the swivel bushing for the antenna.

2) 1" CPVC coupler This is used as a swivel bearing between the 9" long pipe connected to the antenna and the 9" pipe mounted to the pedestal. The 1" dowel rod fits inside the pipe to support the antenna.

1) 3/4" to 1" CPVC coupler This adapts the antenna mount to the upper (the rotating part) pedestal mast.

2) 9" long , 1" CPVC pipe One of these is the upper mast (the rotating part) that fits to the antenna mount, and the other one is the fixed mast that is attached to the pvc floor flange.

1) 1" CPVC to 1-1/4" threaded adapter. This adapts the pedestal mast to the floor flange.

1) 1-1/4" threaded CPVC floor flange This attaches the pedestal mast assembly to the legs.

4) 1/4" X 2-1/2" brass bolts with nuts and flat washers as needed (for mounting the floor flange to the feet).

4) 1-1/4" CPVC pipe, 12 inches long.

4) 1-1/4" CPVC pipe caps These just make the legs pretty. If you are on a budget, leave these out.

4) screw-on rubber feet (so we don't wobble around or skate across the floor madly)

1) "X" fitting, 1-1/4" CPVC. The four 12-inch legs fit into this, with end caps, and rubber feet. Attach the pipe flange to the "X" with four brass bolts, flatwashers and wingnuts.

73

Glen Gardner

AA8C

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