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The Sony SRF-59, one of the more popular "Ultralight" radios for DX, available for about $14-22, street price ( My Atwater-Kent model 76, made in 1930, provides a nice backdrop ). |
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The loop is wound with insulated wire in a spiderweb pattern, on a plastic binder cover. The number of turns was experimentally determined; about 18 in this case. An odd number of slits results in the spiderweb pattern, note that only 9 turns are visible on each side. A notch was cut in a big plastic peanut jar, stopping about a centimeter from the jar bottom. The loop assembly is inserted. A hole in the notebook cover permits re-attaching the jar cover, which provides a convenient surface on which to attach the SRF-59. Adhesive Velcro strips attach the SRF-59 to the yellow jar cover. There is no electrical connection to the radio, the SRF-59 is positioned so that its internal ferrite rod, which is across the top of the unit, is approximately centered in the loop. The turns of the internal ferrite coil and the turns of the external loop are in the same plane, resulting in inductive coupling. The whole assembly is easily rotated to null out undesired signals. |
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A junkbox variable capacitor is connected across the loop to tune it. A switch shunts a 100 pf capacitor across the variable capacitor. The extra 100 pf extends downward the frequency range of the loop. This is often necessary in home brew loop antennas. The reason is because the loop inherently has more capacitance and less inductance than the ferrite loop the variable capacitor was designed for. With the inherent capacitance of the loop already in the equation, a larger swing in capacitance is needed to cover the entire MW band with this antenna. |
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Eventually, I decided I needed a larger tuning knob, so I committed a SRF-59 permanently to this antenna. Here it is, mounted "on the half-shell", with a larger tuning knob (made of plastic jar covers). |
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More construction details... (the fine print) Building this tuned loop is easy. For seasoned radio experimenters, a quick glance at my photos and the schematic is all that is needed to fully understand this simple device. But the following notes may be helpful if this is a new area for you. ( Click on image to left for schematic.) First, you will see that the schematic diagram is very simple. The wire forms a multi-turn loop which has inductance, forming a tuned circuit with the tuning capacitor (variable capacitor) and optionally, the switched fixed capacitance. The relatively large size of this inductor enables it to capture MW radio signals as an antenna. There is no electrical connection between the loop and the radio. The loop you are making couples the signals it receives inductively to the internal ferrite coil antenna in the radio. |
I assume you have access to a soldering iron and suitable electronic solder (rosin core), and very basic soldering skills, or someone who can help you with this aspect. Electrically, what you need is about 12.5 meters (40 feet) of 18-gauge insulated wire, a variable capacitor designed for the MW (AM) broadcast band, a fixed capacitor having a value of anything between 80 - 200 pf, and a small SPST switch of some kind. Structurally, I recommend a sheet of plastic from an 8.5 by eleven inch (or A4) binder cover, or something similar, and some heavy scissors, shears, or a knife (careful!) to cut slots and make holes in it and in the plastic jar used for a base. By cutting an odd number of slots, about 2 - 3 cm deep, a "spiderweb" coil winding pattern can be achieved, similar to antenna coils in some radios from the 1920s and '30s. I used a large empty plastic peanut jar with cover, to serve as a base, and its cover as a convenient platform to hold the radio. Adhesive Velcro on the cover and back of the radio help hold it in place. I cut slits from the top down to about 2 cm from the base, on each side, to hold the loop assembly vertically. A big opening in the plastic binder cover enables inserting the cover, and of course, the radio. A drop or two of glue helps secure the cover to the threads. This is needed because the slits reduced the integrity of the jar's structure. I have little patience or skill in mechanical construction, so you can probably come up with a more elegant and attractive design than this. The important thing is to support the loop in a vertical plane, and be able to rotate the entire assembly on a tabletop. Also, you need to support the radio such that its internal ferrite antenna coil is approximately centered in the loop. In the case of the Sony SRF-59 and SRF-49 Walkman radios, the ferrite loop is mounted across the top of the radio, and is properly aligned in the loop when mounted as shown in the photo. The biggest problem may be locating a tuning capacitor. These devices are rarely used in modern electronics, as tuning is usually done digitally or with a varactor diode. An exception is some inexpensive analog tuned radios, like the SRF-59 itself! Some electronics supply shops geared to hobbyists might still stock tuning capacitors, but the best bet is to salvage one from an old dead analog radio of 1970-1990 vintage. The cheaper and junkier the radio, the better chance of finding the right thing! Air-gap, rigid plate variable capacitors, from old tube-type radios, will work fine, but you will lose the lightweight, portable simplicity of this design, so save those high-demand items for other projects. Haunt yardsales, and you'll find what you need in a junk clock radio, or similar device for a couple dollars. Ideally, the capacitance of the tuning capacitor should adjust from 10 to about 350 or 450 pf. You won't find this info printed on most units. If your tuning capacitor is from an AM-FM radio, be sure you are using the terminals for the AM section. Some AM tuning capacitors have a somewhat lower maximum capacitance than ideal. That's still OK, you will be able to compensate with the switched fixed capacitor. If you've never worked with a tuning capacitor, it may be a mystery which two terminals you should connect to. You may have to experiment a bit. Sometimes two or more terminals will be connected together internally. If you have an ohmmeter or continuity checker, you can identify these. There should be no electrical continuity between the plates of a capacitor. Experiment. Another problem is determining the correct number of windings. This is done entirely experimentally. There are too many variables to be able to predict the correct number. My unit ended up with 18 turns. Start with this number, then connect the tuning capacitor. Tune your radio to the top of the band, 1700 kHZ. By adjusting the tuning capacitor, you should be able to peak up the signal (or noise) at 1700. This should be at or very near the upper end of the tuning capacitor's range. If 1700 resonates near the middle or bottom of the tuning cap's range, you will need to add one or more turns and try again, to shift the range. If you can't peak up 1700, it may be that you have too many windings to be able to tune the top of the band. To find out if this is the case, tune a frequency near about 800 on the radio, and see if you can peak that. If so, you need to remove one or more turns. The switched fixed capacitor will add a lot of range near the bottom, so the important point to get right is to be sure that 1700 resonates very near the top of the dial, without the added capacitance switched in. When you think you have the turns correct, add the switch and fixed capacitor. You're done! Tune the loop to maximize the signal at the interested frequency. This loop is bidirectional, with nodes (greatest sensitivity) in the plane of the antenna, and nulls (minimum sensitivity) broadside to the loop. By rotating the assembly, you can peak up desirable signals, and null out undesirable ones. While my project is practical and effective, its a bit ugly. To see a truly beautiful project involving an ultralight radio, check out John Bryant's project to turn his ultralight into a classic wooden radio with a vernier dial. Go to http://www.dxer.ca Find the "ultralight file" section, and look for the file, Ultralight-Building_and_Operating_the_National_SRF-39.pdf. Also, there is a wealth of information, with more added regularly, at http://www.dxer.ca regarding ultralight receivers and tuned loops. Check it out! Finally, it should be noted that while tuned loops are great accessories for small portables, like the SRF-59, antenna design for MW DX-ing goes far beyond tuned loops. If you are serious about MW DX-ing, search the web for EWE, Kaz, Flag, Pennant, K9AY, SuperLoop, and other forms of larger untuned, unidirectional loop antennas. This is currently a very hot area for MW antenna design by amateurs. But tuned loops still have the advantage of strongest signal pickup for their size. A slightly larger, amplified tuned loop for MW reception is described at http://thorburn.org/gary/loop This feeds the 50-ohm input of a communications receiver. |
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