A MAGNETIC LOOP ANTENNA FOR 600 KHz-4MHz
The finer points explained
by
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 aesthetics 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 loaded Q of approximately 160 at 3.9MHz, and a characteristic impedance of about 800 ohms at the feed point 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 magnet 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 canceled 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 canceled , 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 azimuth 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 canceled 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 area of the antenna. Also, more turns on the main loop increases pickup efficiency as well. 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. Proximity effect will interact with skin effect to increase the effective series resistance of the loop, thus reducing efficiency and spoiling the Q even more.
The maximum usable upper frequency limit is commonly believed to be controlled primarily by the first self-resonant frequency of the loop antenna and to a lesser extent by the minimum capacitance value of the tuning capacitor resonating with the inductance of the loop. This self-resonance is ultimately determined by the length of wire in the main loop. It can be measured by shorting the tuning capacitor with a piece of wire and checking the antenna for the lowest resonant frequency with a dip meter or an antenna analyzer. There are likely to be multiple resonances of varying magnitude above the first self-resonant frequency. In general, the length of wire in the main loop should be significantly less than ¼ wavelength at the desired upper frequency limit. At frequencies at, or above the first self-resonant frequency, the antenna will begin to act as an E-field device instead of H-field, and the radiation pattern will shift from inline, to a broadside pattern relative to the plane of the loop antenna. In some antennas attempting to tune the antenna at or beyond the first self-resonant frequency may prove to be difficult, and tuning will appear to be erratic. Lastly, the distributed capacitance of a large multiple turn magnetic loop antenna can become quite large and will further affect tuning range and self-resonant frequencies. Operating the antenna in this mode is undesirable as performance will suffer greatly. So, while bigger is better, and more turns is better, there are limits that need to be considered when building a multiple turn magnetic loop antenna.
A note about self resonance in magnetic loop antennas
I’m finding more and more indications that self resonance does not typically limit the upper usable frequency in most small HF loop antennas. It CAN occur, as everything does have self resonant frequencies. However, in a well built HF magnetic loop antenna the first self resonant frequency is usually well above the upper tuning limit frequency. As always, your mileage may vary, but this is what I’m finding and I thought I’d share it, as the myth of needing the circumference to be less than a resonant length at the highest frequency in use is one that has persisted and might just be completely wrong in most cases. In any case I do recommend checking the first self-resonant frequency just to be sure.
Impedance Matching
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 manageable impedance matching (in this case by means of an autotransformer) scheme can be readily implemented 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 multiple turn magnetic loop antennas.
1) Use a small diameter wire. Experiments performed by the US navy suggest that for multiple turn magnetic loop antennas, a conductor diameter of less than 1/8 inch is desirable in order to minimize reduced efficiency due to proximity effect. A smaller diameter wire also helps reduce distributed capacitance, thus raising the usable maximum frequency.
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 distributed 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. Lately, 40/44 litz wire seems to be available from several online sources. This is the good stuff, use it if you can get it. For the higher HF bands (above 75M), litz becomes impractical, so simply use a small gauge solid copper wire for those bands.
3) If you can't use litz wire, use a small gauge (20-24Ga) solid copper wire. Insulated wire is fine. Stranded wire that does not have the individual strands insulated is subject to hysteresis losses and effectively 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" deficits will ultimately limit the antenna's performance.
5) Generously space the wire in order to minimize distributed capacitance and proximity effect. About two inches between the conductors of the resonator seems to be good for multiturn loops for HF.
6) Large antennas have a better pick up efficiency by virtue of having a larger 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 manageable combination for MF and low HF. If you are a VLF-ELF fan, plan on a larger, deeper frame and a lot more wire. Accordingly, for the higher HF bands and VHF, a smaller loop with less wire ought to be used.
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 less than 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 dilemma; 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 effectiveness. Bear in mind that the ideal resonator will have an infinite self-impedance at resonance and the loaded Q would be determined by the load placed on the input. Our problem is much more complex than that. DC resistance, proximity effect, self-resonance, excessively low impedance loads, and impedance mismatches all conspire to spoil the resonator, thus limiting the performance and efficiency of the antenna. 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-metallic materials for fasteners where possible. This is a magnetic loop antenna, and materials that exhibit ferromagnetic behavior (particularly iron, steel, and nickel) or are strongly diamagnetic (aluminum, brass) can add to your noise figure and mess with your antennas’ resonant frequency and radiation pattern in ways that are both weird and magical. Large masses of any metal near the antenna will have a negative effect on antenna performance and directivity. If you must use metallic fasteners, use nonferrous materials such as aluminum, nonmagnetic stainless, brass, or titanium as these will be the lesser evil as compared to using ordinary steel fasteners.
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 between turns, 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 1620 pF tuning capacitor, 1:5 turns ratio tapped rf 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. Fixed capacitors used for padding ought to be silver-mica or polystyrene (polystyrene caps work really, really well!). Ceramic capacitors will work, but they tend to have a poor Q and drift around due to temperature variations enough that you end up needing to adjust the tuning a lot.
Transmitting
This type of antenna can 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 s few 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.
In general, the single turn magnetic loop antennas are easier to optimize for transmitting than the multiple turn magnetic loop antenna. Most magnetic loop antennas used for transmitting are of the single turn variety. However it is possible to optimize a multiple turn magnetic loop antenna for transmitting, albeit a technical challenge, and there remains a lot of work to be done in terms of learning to properly understanding the problems involved. In 1971, the US Navy published a paper on optimizing magnetic loop antennas. In the second chapter of the paper the author discusses optimization of multiple turn magnetic loop antennas in painful detail. Click HERE to download the PDF file of this paper. Borrowing knowledge from their work, I have recently built a single turn HF magnetic loop that is optimized for transmitting. If you are interested in this, click HERE.
Contrary to what some people believe, multiple turn magnetic loops can be used for transmitting, and can be rather efficient. Because of the necessity for small diameter conductors, it is difficult to obtain high efficiency and the ability to withstand high power levels as easily as is the case for the single turn loop, where a single , large diameter, low resistance conductor is typically used. I have used a 4 foot box loop wound with 4 turns 24 ga wire to make contacts on 75M with 5 watts, with good signal reports. Build the antenna well and you can easily use it for QRP. Those wishing to use a multiple turn loop for higher power levels are going to have to strike a compromise between usable tuning range, efficiency and the ability to withstand higher currents/voltages. You may wish to delve deeper into the theory behind multiple turn magnetic loop antennas in order to develop a better understanding of how to optimize the antenna appropriately for your intended application.
There is a lot of information on the internet regarding single turn magnetic loop antennas. Some of it is very good, and some of it is not so accurate.... In any case, much of it simply does not apply to multiple turn magnetic loop antennas. Avoid the tendency to apply contemporary theories that apply for single turn magnetic loop antennas to multiple turn magnetic loop antennas. They are quite different from each other and require different numeric models to properly understand them.
Parts 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 disassembly 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
