The Directional Loop DX Xtal Set

The Design Project

The Directional Loop DX Xtal Set did not come about due to a systematic or scientific attempt at design (though those methods where utilised at different times). It was defiantly a case of an ‘Ah Ha !’ occurring after several months of reading a variety of topics about radio while having the need for a new crystal set design lurking in the back of my mind. The reason for this need had come about due to a growing sense of personal frustration when working with traditional crystal radio set designs.

Like many others I have wound my coils and then combined them together with a variable capacitor in various configurations. When the result is connected to the proper antenna and ground, the magic of radio is experienced as each station is ‘pulled in’. Still, there are problems found with these designs, particularly in the area of sensitivity and selectivity . These problems are greatly magnified if you are living in a large urban area where one finds that strong local stations tend to dominate the reception and later impose themselves on weaker signals through inter-modulation. The traditional method that has been used to overcome these problems has been to increase selectivity and sensitivity in crystal sets through focusing on the three main elements of its design; The Antenna, the L/C Circuit and the reproduction of the Audio Signal.

In each case in the improvement of the crystal set design, the primary way this has been done has bee through the addition of ‘something’ to the basic circuit. In general what has been most often undertaken is to increase the length or height of the antenna, add more coils and capacitors, and use signal amplification, particularly of the audio. These are all fine in themselves and can produce the required results, but often at the expense of the basic simplicity of the original design. The question then is; Is there a ‘better way’ to design a crystal set in order to produce the level of performance desired without making the set complicated to operate or require a large antenna array?

The Propagation and Reception of Radio Signals

The search for the new design, as said before, did not come out of a systematic plan, but rather out of the influence of several concurrent ideas that were being perused. The main areas that were part of this investigation were; 1) broadcast radio transmission propagation, 2) loop antenna designs for the broadcast band, 3) traditional crystal set design and 4) current techniques in BCB (broadcast band) DX for conventional radios. Through this research there came this basic realisation; you can develop a good design that assists you in the reception of weak signals by making their initial reception easier, or you can use ‘brute force’ to amplify those few signals you have managed to captured with an initial poor design. It was found that by focusing on understanding how radio waves arrive at the site of reception, and then finding out how to ‘captured’ these signals with greater efficiency, was more important than trying to amplify or filter them after they had been placed into the radio circuit by the antenna.

In organising the information that was being encountered during the projects research, it was found that the topics grouped themselves in the following way ; 1) how radio waves are polarised and directed at the point of transmission by the antenna systems used, 2) how radio waves become ‘tilted’ due to the effect of air ground and water on their transmission paths, 3) that a directional loop antenna is needed for the successful reception of medium wave transmissions, 4) that the improvement of the ‘Q’ of an L/C circuits produces circuit efficiency and 5) ‘low noise’ design is important for either RF (radio frequency) or AF (audio frequency signals) amplification. The connecting idea that was derived from study in each of these areas, and later incorporated into the final design of the crystal radio presented here is this; if the circuit design (including the antenna) allows for the maximum amount of radio signal to be received, then only a minimum amount of amplification will be needed to comprehend the signal at the level of human hearing.

What was often found in this research as well was that all too often radio designs attempt to do the opposite of the statement given above. Many ‘traditional’ designees used complex (and expensive) components to amplify weak signals. Often these signals were in a such condition because they have been poorly captured to begin with. Worse, one finds with these circuits that the results at the speaker or headphone is often not very satisfactory because of the compromises that must be used (narrow bandwidths, intermediate frequencies, audio filtering, to name a few). The main experience here is that the ‘Fidelity’ of the original signal is lost, and the resulting sound is ‘tinny’ or distorted, thus removing much of the pleasure in the listening.

Another unfortunate side effect that is experienced when using signal amplification is an increase in the ambient noise that is produced by nature and the radio itself during this amplification. To reduce this ‘signal and noise’ product, further technology must be introduced, increasing the complexity (and cost) of the radio set. Working with the assumption that a signal that is produced at the antenna of a broadcaster is close to ‘perfect’, what makes it ‘less-so’ at the point of reception, such that it requires the amount of reworking performed by some radios in order to make it satisfactory for use.

With all of these points in mind, the design of an improved crystal set was embarked upon. It was found in reading that the following points were important clues for such and improvement and these were that;

A) Radio waves in the frequency range of the broadcast band (530-1710 kHz) are affected not only by the reflective property of the ionosphere, but also the ground, water, and air surfaces over which it travels.

B) BCB (broadcast band) radio wave are generated by vertical antennas and are thus vertically polarised when they leave those antenna, but this will be modified by the effects of ‘A)’.

C) In general, when BCB radio wave arrive at a ‘distant’ reception site they are no longer vertical, but are tilted in several directions away from the vertical, and this is generally pointing forward and to either the left or right side.

D) The ‘strength’ of the signal will always be greatest from the point of the broadcast.

and finally,

E) ‘A)’ though ‘D)’ are complicated by the fact that with BCB radio wave propagation there will always be some mixture of ‘groundwave’ (signals that travel along the surface of the earth) and ‘skywave’ (signals that bounce off the reflective ionosphere layer of the atmosphere) signals.

The Loop Antenna and its design features

The solution for the above conditions that is applied in "conventional" radio design for BCB and Low Frequency use has been to use the loop antenna. This type of antenna has many virtues that make it ideal; it is small and compact, while being able "capture" radio signals with close to the same strength as strung wire antennas such as the dipole or long wire. A further advantage with the loop is that it actually reduces natural radio noise, rather than increasing it as is found in the case of the vertical antenna. The most notable and desirable characteristic for the loop, and which it is most often utilised for, is its ability to be highly directional, which also comes with the added ability to be able to "null" or neutralise signals that may be on or near the same frequency being tuned.

Despite all of these advantages, the loop antenna has never been favoured in the classic and contemporary literature on crystal sets, though at least one design has been found that did incorporate its use. Indeed many writers expressly state that the loop should not be used at all, and that only a large external wire antenna was suitable. Yet for this writer the obvious advantage of the loop begged further investigation, and to be certain, it was found that a simple wire loop made up of several turns of wire cannot supply the necessary signal to the tuner in order to "fire" the detector, except with only very strong local signals. Even then, the loop did not exhibit the directional features that were desired. It was obvious that further investigation and experiment were in order.

The full range of issues, topics and ideas that are related to the design of the loop antenna is vast, and it would be impossible to summarise even the main points in this article. Suffice to say that for this writer, by being introduced to one particular loop design that is popular amongst BCB DXers in Canada and Britain the foundation for this project was made. This loop design uses a simple L/C device that "couples" to an existing ferrite loop antenna as is found in many of today’s transistor BCB radios. By placing the external loop near such a radio when it is tuned to a weak station, and by using a 365 pf tuning capacitor that is attached to the external loop, the coupled circuit will increase the effectiveness of the radio’s own antenna circuit. By pointing both directly at the weak station, the signal strength will be further increased, while nulling any other stations that are on the same frequency, but not directly in line with the antennas.

The loop antenna design presented here uses the basic L/C circuit and dimensions of the BCB DX antenna, with the primary addition of the "lazy Susan" turning base and Xtal set circuit. The loop design itself is simply two 18" lengths of wood around which approximately 84 feet of wire (roughly 16 turns) have been wound. The two free ends of the wire are then attached across a 365 pf tuning capacitor. The centre frequency of the loop is roughly 940 kHz, and will tune across the entire broadcast band. By adding a simple diode, which is then attached to either a high impedance headset, or a suitable audio amplifier, you then have a completed radio set. By attaching the turning base you are then able to "point" the antenna at a desired station. By having the antenna cross arms form into the "diamond" form used, a further advantage is gained by being able to capture both the groundwave and the skywave of a transmitted signal at an optimal angle. Surprisingly, the design works very well without a ground attached, and no particular advantage has been found when it has been used. This lends a feature not often found in Xtal radio sets, which is portability.

Building the Loop Crystal Set

It should be stated that the design of the loop crystal set presented here is not absolute, but can be the foundation for much experimentation. In general the aim here is to show the most simplest design, and one that needs only a minimum of tools, if any.

A project could be constructed entirely of pre-cut wood that is available from many hobby, hardware or building supply locations, and which is glued together. Likewise it is possible to simply build the loop using either the wood cross arms, or even a cardboard box, and then purchasing a plastic "Lazy Susan" tray to place is upon. The key here is to use your imagination and improve upon the design as need dictates.

To otherwise assist, as set of step-by-step instructions are provided here.

A). Begin with the construction of the wooden "cross arms" for the loop.

Note: Pre-cut and sanded wood of the lengths described are available for a reasonable price

at many hobby and hardware stores. A parts and source list is provided at the end of this article.

1) if tools are available, cut two notches into the centre of two pieces of wood board, what

measure 18 inches long, by 1 1/2 inches wide, by 1/4 to 1/2 inch deep. The notches should be equal to 1/2 the width, and accommodate the depth. The result should be a snug fit that can be glued for further strength.

2) if tools are not available, use a long single piece with two short pieces. Try and have the lengths of the short pieces be such as to keep the symmetry of the cross arms, so as to keep the loop square. If it is not possible then use two pieces of nine inch length.

B) Wrap the wire onto the loop.

1) if tools are available you should drill two holes in one of the arms in order to anchor the beginning and the end of the wire. Likewise is helps to have the ends of the arms notched so as to keep the loops from "falling off" of the arms as it is being wound.

2) if tools are not being wired, it is suggested that double sided tape be attached to the ends of each arm in order to secure the wire as it is being wrapped on. If this is not available, you may place some "white" or carpenters glue on the ends, which is then allowed to become tacky, over which the wire is then wrapped. When the glue is dry it will assist the wire to remain anchored to the arms.

C). Attach the tuning capacitor to the loop

Note: Before beginning this step it may be necessary (depending on the design of the capacitor) to remove a set of screws that hold components used for "fine tuning"(which should also be completely removed). This step will allow you to be able to tune through the entire frequency range of the BCB, but is optional if you do not wish to make these changes.

In general it is simple to attach the loop to one of the arms by using epoxy glue. Use rubber bands to hold the capacitor to the arm while the epoxy is curing, but be certain to have all of the plates closed and protected during the procedure. You must also remember to place it near the centre of the axis, otherwise the loop will tend to by "tipsy", even when it is attached to the base.

Give the epoxy 24 hours to cure (even for "5 minute" type) and then solder the two wires of the

loop to the capacitor. Remember to attach one wire to a "lug" of the stationary plates, and the other to the lug (generally the "grounding" case) of the rotating plates.

D). Testing the loop

It is wise at this point to see if the loop actually works before proceeding to add the crystal diode and the base. At this point it is easier to re-wire the loop, or fix the capacitor, than at a later stage.

The test requires that you have a simple transistor type BCB radio that has a built in ferrite antenna (not a telescoping type). Simply turn on the radio and tune in a station at the "low end" of the dial (around 550 kHz) and place the loop beside the set). Turn the tuning capacitor rotator until you hear a change in the signal strength of the station heard. This will indicate that the loop is working. You should then tune to a station in the "high" end of the band (above 1500 kHz) and tune the loops capacitor again. You should have the same results.

If you do not find that any effect is taking place, check the wire, capacitor and solder joints to see if any problems can be found, then fix. Likewise use a different radio if one is available in order to see if the problem lays there.

E) Making the base

Note: It is suggested that an easy to find source of pre-made bases for those who do not have access to tools are "trophy bases" or "wood burning plaques". These are inexpensive, come in different size, and are available at many hobby or hardware stores.

1) Begin by attaching the "Lazy Susan" mechanism to the block of wood that will be the "stationary base". You can either use epoxy to "glue" the mechanism to the block or screws if desired. The important consideration here is ensuring that the turning mechanism is centred properly so that the antenna portion will turn through its axis with ease.

2) When the base is completed, attach the upper "turning base" to the upper rotation mechanism by either using epoxy or screws. Take your time to ensure that the upper unit turns true and that the axis is centred properly.

F). Attaching the loop to the base

Note: to perform the following steps properly, ensure that all positions are correctly laid out first. Take your time and follow the rule of "measure twice, and attach once". What is required is very simple; attach the two support legs to the loop and then glue the two legs to the rotate-able base. If it is not done well, e.g. with the loop being centred over the turning axis of the base correctly, the unit may tend to tip over or be difficult to "track" through the turning radius.

For the first steps have a ruler and a soft lead pencil available, and for the later have glue and optional screws available.

1) take the two support legs and by hand hold them against the "bottom" leg of the loops support arms.

2) place the legs on a flat surface, or on the rotating base, and after ensuring that the loop and the legs are even, mark the upper location of the legs on the arm with the pencil.

3) glue the legs to the arm, and use elastic bands (or any other firm clamping device) to hold them until the glue is dry. You may also use screws if an even firmer hold is desired.

Note: as an alternative, if you have the tools available, you may wish to not glue or permanently

secure the legs to the arm, but to use removable bolts and wingnuts. The legs would be secured to the base only, and this would allow the loop and the base to be separated for storage or easier transportation.)

4) while the glue is drying, you must lay out the position for the legs on the rotate-able base. What is needed here is to establish where the axis of the turn is (which may not be in the exact centre of the base due to construction flaws and "design problems" in the Lazy Susan mechanism.) Take some time to simply observe the turning character of the base and work out the approximate axis. From that point begin to measure out the outer and inner distance from that centre to the approximate location of the two support legs. Mark these locations lightly using the pencil.

5) when the glue is dry, place the un-glued legs on the base in the locations where they are to be attached, and see how the unit balances. Try a few turns of the unit in this way as well in order to

see how the unit performs. When all appears satisfactory, then mark final location of the legs using the pencil. Remove all other marks at this time.

6) Glue the legs to their permanent location, and use elastic bands to hold the loop and legs against the based. Allow to dry for at least 24 hours.

G. Final Touches

At this point you now attach the crystal diode to the tuning capacitor, but before you do be certain to;

1) attach a wooden or plastic tuning knob to the rotor shaft of the capacitor. This is needed in order to isolate the capacitor from the small but noticeable capacitor effect that your hand may have and which can affect tuning.

2) attach either rubber feet or a non-slip material to the lower base.

To connect the diode circuit to the tuning capacitor.

1) if available, attach a two unit soldering lug to the arm of the loop near the tuning capacitor. This is useful, though not completely necessary for attaching the wires used for the headset/audio amplifier to the crystal.

2) solder or firmly attach one wire from the base (which is attached to the adjustable plates) of the tuning capacitor. This will be the "ground" or negative side of the circuit.

3) solder or firmly attach the crystal diode to "free" lug of the stationary rotors that is on the opposite side of the lug that is attached to the loop of the antenna. (I mention this so that if you have a "dual ganged" capacitor, that you do not use the "unused" side.) Be certain that the wire from the black polarity indication band side of the crystal set is not used as well, and to note that this is the "positive" side of the circuit.

4) attach (but do not solder) the negative wire to one soldering lug and the positive wire from the crystal to the other. Then attach the wires for either the socket or the plug for the headphones/audio amplifier to the solder lug, being certain to observe the proper polarity as well, and again do not solder.

5) connect a .001 microfarad (m fd) capacitor across the soldering lugs and solder all joints.

Using the Loop Antenna Crystal Set

The operation of the loop antenna crystal set is very easy; simply connect headphones/audio amplifier to the unit and then "aim" in the direction you wish to receive signals from while tuning. You should quickly hear a number of local stations.

To test the directional ability of the loop, simply tune in a station that is about "medium strength" and turn the loop. You should hear the station drop in strength when the loop is broadside to the stations location. You should also find that as you continue to turn the loop that the strength will increase as the "back" of the loop is pointed at the station.

To fully take advantage of the directional ability of the loop, work out by using a compass and a map where the points of the compass are at your receiving location, and where key cities or stations are located. One word of note here; don’t use the compass with the loop ! As an interesting experiment, set-up the loop and compass together and tune the loop with the capacitor. You should see the compass needle move as the loop creates its own magnetic field.

To dramatically demonstrate to yourself the DX capability and directional effect of the loop, begin listening with the set just before sunset and point it to the south. Then move the direction of the loop to the west if you live near the east coast or central states, or to the east If you live in the west, as the sunset progresses. You should be able to "track" the opening of the DX as new stations begin to "appear" at tuning spots where none where hear before.

What is the most exciting part of using the loop is being able to "separate" several stations on the same frequency by turning the loop. Loud stations that once "covered up" weaker ones can be "nulled out" and weak stations can be strengthened by pointing the loop at them. You may be able to hear many "clear channel" stations that have a frequency all to themselves which are hundreds of miles away.

The portability of the set also allows you to go to "quiet" locations in the country, or even in different parts of a city. It would be interesting to try Trans-Atlantic or Trans-Pacific DX by setting up the unit right beside the ocean. By not having to string a long wire antenna, or even using a ground, you are no longer limited to a permanent location.

Other loop designs and applications are available. Larger "Box" type designs using longer lengths of wire can be used for even better amplification of signals and improved nulls. You can try building sets that allow you to rotate the arms of the loop along its own axis so that you can experiment with changing the polarity of the loop from vertical to horizontal. Improvement in signal reception can also be found in tilting the loop to the right and left, as well as using two loops in tandem.

Do not forget that the loop can be used as a passive coupled antenna with an existing BCB radio that has a ferrite loop antenna. The use of the loop will significantly improve reception of both local and DX stations. Just tune in a desired station, place the loop near to the radio, and then tune the loop itself. You will hear the signal strength improve if the loop is coupling properly.

Bibliography

Cooke, B.W "Radio Wave Radiation and Antennas" Applied Practical Radio-Television , Coyne Radio School 1947

Gebhardt, Philip "Here’s How to design the best beginners’ MW Loop" in The Canadian Amateur Magazine,

Vol.24 No.9 October 1996 pp. 41-43

Gebhardt, Philip "January Loop Party" in DX Ontario, Vol. 22 Edition 246 March 1996 pp. 11-12

Kendall, Jr. Lewis F. and Koehler, Robert Philip "Amplifiers, Speakers and Loops" Radio Simplified : What it is - How to Build and Operate the Apparatus The John C. Winston Company 1925

Jordan, Edward C. "Electromagnetic Waves" Fundamentals of Radio Prentice-Hall Inc. 1942

Lankford, Dallas "Loop Antennas: Theory and Practice" NRC Loop Antennas - Design and Theory Book (date unknown)

Staff, "Looking at Loops" Radio Netherlands Wereldomroep 1987

Trauffer, Arthur "Loop Crystal Set" Radio and TV Experimenter, Volume 2 1952

Parts List

Illustrations and Diagrams