|
I've always liked the idea of a small-loop antenna, but never had the time or energy to build one. Now that I've got some time, let's find some energy...
For those of you unaware of the beauty of the Small Loop Antenna, allow me to take a little of your time as I explain a few important pieces of information related to the SLA. Maybe this information will sway you to give the Small Loop a try! What is a Small Loop Antenna? The small-loop antenna, sometimes referred to as a magnetic-loop antenna in some circles, is an antenna where the primary conductor is less than 1/10th (some prefer < 30%) the wavelength of the lowest operating frequency. In addition, the loop is "balanced" - the loop is symetrical with a tuning capacitor located opposite the feed-point, which is inductively coupled to the primary loop. Some Benefits of the Small-Loop Antenna Size: If you're limited in your selection of an HF antenna, consider the Small Loop Antenna. To qualify as a Small Loop, the length of the conductor for the antenna MUST be less than 1/10th the wavelength of the lowest signal you wish to cover for transmit or receive. High-Q: The nature of the antenna is that of a high-Q, tuned circuit. As an example, my current design for an antenna should be in excess of 1100 - this means that much of the signals adjacent to your primary operating frequency are rejected by the antenna, resulting in a MUCH cleaner signal coming into the receiver. Height Requirements: Because the antenna operates in a balanced mode*, the antenna is relatively immune from the effects of earth ground with respect to SWR. Some Detriments of the Small-Loop Antenna Efficiency: At the lowest intended transmit frequency, the efficiency of the built antenna is quite good. However, as you move away from the lowest intended frequency, the efficiency is reduced. It still operates well, just not as well as one would prefer! Tuning Capacitor: The small-loop antenna requires the use of a capacitor to properly tune the antenna to resonance at the target frequency. It is exactly because of this capacitor that we have an extremely high-Q antenna that performs as well as it does. While the capacitor can be anything from a dedicated stub capacitor to a full-blown variable vacuum capacitor (my personal choice), the choice is yours. There are a variety of options to choose from, but they all will be required to withstand high voltages... Useful Links AA5TB - Transmitting Loop Antennas This is an excellent resource describing several build options for Loop Antennas. Felix Meyer, HB9ABX - ABXKOPPEL A very good discussion of Small Loop Antenna options and a good source of useful information on the topic. Bob's Approach to a Small Loop Antenna... After reviewing all the bits and pieces of information related to small loop antennas, I decided that the idea was "nice" but the engineering seemed to be lacking. With my own ideas milling around restlessly in the back of my head, I took out on a new design approach. After ordering a couple of very nice variable vacuum capacitors from my friends in the former Soviet bloc, picking up a little 3/4 inch copper tubing pre-formed in a spiral, and locating a number of joints to put it all together, I'm able to start work on the project. In short, I wanted to try a new approach to the small-loop: Rather than one huge hunk of copper or aluminum floating around in the air, I wanted to see what happened when I reduced the diameter of the loop -- while maintaining the circumference. How, exactly, would one accomplish that goal? Of course, I'm applying an approach to this problem from the standpoint of "balance." If you've worked in the audio industry, you know that running an XLR cable for a microphone will carry you MUCH farther than a 2-wire setup. Why? Because you're using a BALANCED circuit, which reduces NOISE in the signal path. Extraneous signals are introduced to both out-of-phase legs of your circut simultaneously, resulting in phase-cancellation of the extraneous signal. I believe the same approach to the consideration of the small-loop antenna is in order... Instead of a single loop of the secondary of this transformer, I opted for a multi-turn loop. AA5TB and Mr. Meyer both note that a multi-turn of the secondary can be had, and the minute references to multi-turn loops didn't indicate significant losses in the circut, so I'm taking this path. However, keeping the antenna balanced seems to be of significan importance, so how am I to accomplish this goal? I've come up with a workable solution which will form the foundation of this experiment:
Archtectural Drawing of the compressed loop, side view. This picture shows the overall design approach for the (now named) WA7ARC Compressed Small-Loop Antenna. In short, this is a coil of 3/4" copper which has been drawn out to form a circular loop of X-length by Y-radius. Viewed from end-on, it looks like this:
End-on Architectural drawing of the WA7ARC Compressed small-loop antenna. Note that although this APPEARS to be helical in nature, it is not - the circumference of the loop remains the same throughout the length of the antenna. You'll note that every other design I've found has the tuning capacitor located at the top or bottom of the loop, 180 degrees apart from the feed-point. After careful review of the formulae and several shots at modeling the antenna, I've arrived at a conclusion - the feedpoint needs to be 180 degrees ELECTRICALLY opposite the tuning capacitor, which isn't, necessarily, the physical opposite of the capacitor. You'll note, in the first drawing, the rectangular box in the middle of the transformer secondary. This is located in the MIDDLE of the antenna, equidistant from each return-end of the antenna. I've made a small model from a length of solder and I'll try to get a picture of the model to give you a better idea where I'm going in my logic... This box will hold the variable vacuum capacitor at the electrical center of the antenna, along with the drive motor and associated drive electronics for the stepper motor that will control the variable capacitor. What about that feedpoint? There are options (there are ALWAYS options!) and I've decided that the most electrically efficient solution is a gamma-match, instead of a air-coupled transformer solution. While the air-coupled transformer solution seems simple, I'm concerned about losses -- and the consistency of coupling between the primary and secondary in an outdoor environment. Of course, there's always that issue of power - what happens if I want to dump 200W into the primary of an air-coupled transformer loop? Good chance we'll have smoke and flames long before we have lift-off! Of course, I'm thinking about some new-fangled solutions to the gamma-match as well, although I haven't finished my research in this regard. No matter, the antenna won't be built until I have my ducks lined up! Stay tuned for further developments - I think you'll enjoy this experiment, especially if you're a loop-antenna-type-of-guy! |
