From out of the past Ė antennas with a new twist
Bill Petlowany, K6NO
If you have any interest in antennas at all, fasten your seat belts and hang on to your hats, because what you are about to read here is going to blow you away. Conventional wisdom concerning antenna matching and resonating is about to be shattered and the principles revealed here might just be the start of a new chapter in the field of antenna design.
The path leading to my discovery started with the four-element 40-meter antenna given to me by K6SG in 1995 after it had been damaged in a severe storm. I loaded the pieces into the back of my Chevy pickup, drove two houses down our street and, with Georgeís help, unloaded them onto some saw-horses in my side yard.
During the next few months Iíd occasionally go out and look at the huge pile of aluminum and wonder if my Rohn 25 tower would tolerate the additional weight of such an antenna if I were somehow able to put it together again. I think I realized subconsciously that adding that much more weight to my tower was not a good idea.
On one such occasion, as I looked at the linear loading on one of the elements, I was struck by the complexity of it all and how much weight was added to the antenna as a result. I clearly remember thinking at that moment, "There must be a better way to do this." It wasnít until several weeks later, however, that I was able to work on the problem of simplifying the antenna.
Experiments with 2M antennas
At that time I borrowed an MFJ-259 SWR analyzer from K6SG and started to build some test antennas on 2 Meters. I fashioned the antennas from eight-gauge aluminum wire and proceeded to test the methods most commonly used to resonate them when they were too short to be self-resonant.
I experimented with inductors placed at various places along the elements, end-loading capacitors, wires hanging from the ends of the elements, folded-back elements and, yes, linear loading too, but I didnít feel that I had made any progress toward "a better way to do this." In frustration, I returned the SWR analyzer to K6SG.
After a few weeks of not giving the idea much more thought, I borrowed Georgeís analyzer again because I had the uneasy feeling that I had missed something in my earlier experiments. As I reviewed the results of the various things that I had tried, I noted that hanging wires from the ends of the elements had proved to be not only simple, but effective as well.
In an attempt to make the hanging wires more compact I wound them into coils and re-attached them to the ends of the elements. The coils of wire then had little effect on the resonant frequency of the short antenna. In theory, it would take infinitely large inductance placed at the ends of the short dipole elements to tune the antenna to resonance, so the results were not at all surprising.
At this point in my experimenting I thought about my late father-in-law, W8TS. He was into Amateur Radio before 1920† so early, in fact, that he didnít need a license to operate. I recalled that in the past he had built antenna tuners using some very unusual coils.
Like many other Hams, I never throw anything away, so I still had one of his home-made coils in my junk box. I had looked at the coil many times and had no real use for it, but for sentimental reasons I just couldnít throw the coil away. I decided to try winding coils similar to his by using the lengths of the hanging wires.
I wound the coils in a spiral fashion by starting a turn with a very small diameter and winding each successive turn with a slightly larger diameter until the wire lengths were used up. The completed coils then had a pancake shape with all of the turns in the same plane.
I did not expect these coils to react any differently than the previous ones. Much to my surprise, when I attached them to the ends of the short dipole the resonant frequency was lowered somewhat, although not nearly as much as the hanging wires themselves.
The unexpected results of this test prompted me to many more experiments with spiral-wound coils and caused me to formulate what I like to call (due to my overly-modest nature, no doubt) "The Petlowany Principle."
It states that "if a length of wire is wound into a spiral-shaped coil and excited by a radio frequency current connected to the innermost portion of the coil, it will then, and only then, exhibit RF characteristics that closely approximate those of a resonant linear wire of the same length."
The shortest self-resonant linear length of wire is not the half-wave dipole as one might mistakenly assume, but instead, a wire one quarter of a wavelength long. Vertical antennas of that length are commonly used by many amateurs. I used wires 1/4 wavelength long in each of the spiral coils that I tested in an effort to keep the size and weight of the coils to a minimum. However, spiral coils wound with wires with resonant lengths greater than 1/4 wave-length also exhibit RF characteristics similar to the linear lengths used.
To further test the spiral coils, I built a full size half-wave dipole and also a 1/4 wave dipole for 2 Meters. I tuned the short antenna to resonance on 2 Meters with two spiral coils. Each coil was made from a length of wire about 1/4 wavelength long. They were then connected to each end of the short dipole. I trimmed off equal lengths of wire from both coils to tune the short antenna to the same frequency as the half-wave dipole.
On-the-air tests on 2 Meters with KI6O indicated that the transmitted signal strengths of the short dipole were equal to or better than the full half-wave antenna. Because the "on-the-air" tests were crude at best, I donít make the claim that the short antenna had any gain, but in any case, it was no worse than the full-size antenna.
To test the spiral coils on an antenna for use in the HF Ham bands, I then constructed a full-size 20-meter dipole from aluminum tubing and by adjusting the lengths of the elements resonated it to 14 MHz. I then took two lengths of wire, each slightly longer than 1/4 wavelength on 40 Meters, wound them into spiral coils and attached them to the ends of the antenna.
By trimming off equal lengths of wire from the outside turns of each coil I was able to resonate the antenna to 7040 kHz. Amazingly, the antenna was also still tuned to the 20-meter band, although the resonant frequency was lowered somewhat by the capacitive end loading that resulted from attaching the coils.
As amazing as the resonating capabilities of spiral coils appeared to be, I found its matching abilities even more remarkable. When the 20-meter dipole was tuned to 14 MHz, it presented a fairly good match to the 50-ohm line feeding it. The SWR was somewhat greater than 1 to 1. On 40 Meters, however, the match was much better than on 20 Meters and was about 1 to 1.
The 1/4 wavelength 40-meter dipole antenna would normally have a radiation resistance of about 14 Ohms. The radiation resistance of the short 40-meter dipole was increased to 50 Ohms by the use of the spiral coils and resulted in a much better match to the 50-ohm transmission line. The RF current on the antenna "sees the spiral coils" as simply more linear wire and the additional radiation resistance presented by that wire contributes to the overall radiation resistance of the system.
In the process of checking the SWR on 7040 kHz, I had reduced my power output to about 10 Watts so as not to cause any unnecessary interference. When I sent my call to identify, a station in southern California called and we had a short QSO. He surprised me by giving me a 569 signal report. At the height of the antenna (about 30 feet), the power level, and the time of day (mid-afternoon), I was not expecting to be heard at all. Apparently, in spite of its unconventional method of tuning, the short 40-meter dipole could also radiate quite well.
The bandwidth of the 40-meter antenna over a 2-to-1 SWR range was about 80 kHz. The coils were wound with bare aluminum wire that measured .061 inches in diameter and were built with a spacing between turns of about one wire diameter. Subsequent tests with other wire diameters and spacing indicate that the bandwidth can be improved significantly by using larger wire diameters and greater spacing between turns. It is also important to wind the coils with the diameter for the innermost starting turn to be as small as possible if the maximum bandwidth is to be realized.
I have not made any tests to measure the improvement in efficiency to be gained by using the spiral coils, but since they are not connected in series with the high current portions of the antenna, their use can help to reduce the losses normally associated with matching networks, loading coils and linear loading schemes.
During my testing of the spiral coils, I found that their resonant frequency was little affected by the length of the linear portion of the short dipole. The antenna length can literally be from inches long to just short of full half-wave resonant size with only small adjustments to the wire lengths in the coils necessary to achieve resonance. I also found that the radiation resistance was always very nearly 50 Ohms, regardless of the length of the linear portion of the antenna.
I have given much thought to the spiral coils and their behavior in an attempt to better understand how they function. I have concluded that, due to the unique physical and electrical characteristics of the coils, they act as low impedance series-resonant circuits connected to the ends of the antenna. The linear portions of the dipole are simply extensions of the transmission line which is delivering current to the coils. Due to the low impedance nature of the coils the linear portions of the antenna are carrying large RF currents. If the linear portions are long enough in terms of the wavelength of the applied RF current, an appreciable amount of radiation takes place resulting in an efficient antenna.
How can the amateur take advantage of the spiral coils with their unique characteristics to improve his antenna systems?
He will now be able to resonate a short antenna using an inductor placed at the ends of the elements which, according to conventional wisdom, would not have been possible with anything other than an infinitely large inductor. It is now possible to build very short resonant antennas using coils that do not introduce major losses and that are not impossible to build.
Short dipoles or short monopoles resonated in this way are resonant at two frequencies. One frequency is essentially that of the linear portion of the radiator, the other is that set by the end resonating coils.
Multiband antennas are possible by using multiple coils to resonate the short linear portion of the antenna at the desired frequencies provided that sufficient spacing between coils is allowed to prevent detuning of the individual coils. The desired frequencies need not be harmonically related.
Broadbanding of an antenna for a particular frequency range is possible by the use of multiple coils that are all tuned within the desired range of frequencies. Again, to prevent detuning, adequate spacing between coils must be provided.
The driven element of a parasitic array can be resonated and matched to the transmission line simply by the use of such coils. In fact, the parasitic elements of such an array can also be tuned as directors and reflectors in this manner.
Short vertical antennas (such as a short tower one might wish to use as a radiator on 160 Meters) can be resonated to the desired frequency simply by adding the appropriate spiral coil consisting of a wire length of approximately 1/4 wave attached to the uppermost portion of the tower or its mast. Doing so will increase the radiation resistance at the base of the tower resulting in improved efficiency.
I believe that there is much more to be learned about spiral coils and their RF characteristics and I hope that my work with the coils has proved to be thought-provoking. If only a few of you have been inspired to further experiment with the concept, writing this article will have been worthwhile.
Oh, I almost forgot! You might be wondering what became of the 40-meter antenna which precipitated all of the experiments with the spiral coils. Well, the antenna is still patiently waiting for me, but these spiral coils have proved to be such a fascinating distraction that I must further explore some or all of the possibilities I have suggested before I can get back to modifying it.
I would like to acknowledge the help and encouragement of the following radio amateurs: My late father-in-law Fritz, W8TS, George, K6SG, Jay, W6GO, Peter, W6QEU, Derek, K7FF and my wife Carolyn, K8TFR.
Reprinted from WORLDRADIO