From out of the past – antennas
with a new twist
By
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 George’s help, unloaded them onto some saw-horses in my side
yard.
During
the next few months I’d 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 wasn’t 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 didn’t 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 George’s 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 didn’t 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 couldn’t 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 don’t 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.
What's next?
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.
KN
Reprinted
from WORLDRADIO