The idea of s circularly-polarized omnidirectional antenna is a bit hard for some folks to wrap their heads around, but they are quite common: Antennas used on GPS receivers are circularly-polarized and have a more-or-less hemispherical response so that they "see" the sky, but not as much of the ground.
What is circular polarization, anyway?
If you are familiar with the propagation of radio waves you are likely familiar with "linear" polarization - that is, just like the "waves" that you can produce in a piece of rope that you move up and down, their peaks and troughs "move" only one direction. While any direction may be used for this movement it is quite typical for an antenna to be oriented such that this wave is oriented vertically - as in the case of a so-called "vertical" antenna (such as a whip) or horizontally, as is the case of many TV antennas. In each case the selection of vertical or horizontal is mostly a matter of being able to conveniently mount that particular type of antenna.
As it turns out, if you have a signal emitted by a vertical antenna and try to intercept it with a horizontal antenna, very little of it is likely to be intercepted - particularly if the antennas are exactly 90 degrees apart from each other. Quite like the darkening of an LCD watch or LCD computer monitor when looking at it through polarized sunglasses and one tilts one's head just right, the "cross-polarized" antennas just won't intercept each other's energy. One oft-quoted figure with cross-polarization is that the signal is reduced by 20dB (e.g. 99%), but the amount of diminution can be much greater (e.g. lower signal) that this if things are carefully adjusted.
In addition to this "linear" polarization (e.g. horizontal, vertical) there is "circular" polarization in which the signal (radio frequencies or even light) "screws" through the air as if it were a threaded bolt and as with nuts and bolts, one can cut the threads to be either "right handed" (as are most nuts and bolts) or "left handed". Just like with nuts and bolts where you cannot thread a right-handed nut onto a left-handed bolt (or vice-versa) a "right-handed" antenna cannot efficiently receive a signal that was emitted via a "left-handed" antenna on transmit.
For an interesting, graphical illustration of both linearly and circularly-polarized wave fronts see this YouTube Video:
http://www.youtube.com/watch?v=Fu-aYnRkUgg
Why did we need a circularly-polarized antenna?
In 1999 I helped Glen, WA7X, put together some propagation beacons for the 6 and 2 meter amateur bands. Initially using simple "J-Pole" antennas - since they were cheap and easy to construct - we knew that we had a problem: These beacons were intended to be used as an aid to discern propagation (e.g. band openings) and were likely to be used by stations equipped for weak signal operation, it was likely that the fact that we were using a vertically-polarized antenna would be of detriment to those for which the signals were intended as those equipped to receive weak CW and SSB stations have typically used
horizontally-polarized antennas!
What this meant was that an already-weak signal could seem to be even weaker with this cross-polarization - a definite problem! Practically speaking, in conditions such as ionospheric reflection and meteor-scatter - just to name two possibilities - it was likely that the polarization would be effectively randomized from the viewpoint of the receive station, but this also meant that even if you
were using the same polarization of antenna as the transmitter you could, under some conditions, experience fading as the polarization of the signal was randomly changed along the path to be opposite of your antenna.
While we initially considered the use of a horizontally-polarized omnidirectional antenna, it turns out that there are relatively few practical designs for these for 6 and 2 meter operation - and that would still leave the problem of random polarization changes with propagation, so we considered circular polarization as a novel solution.
If a circularly-polarized signal is received via a linearly-polarized antenna, half the signal is lost, but it doesn't matter whether that receive antenna was vertical or horizontal. While always losing half of the signal
isn't inconsequential, it is certainly far preferred over losing
most of the signal - which is what would happen if one received a horizontally-polarized signal via a vertically-polarized antenna and vice-versa!
In
our research for candidates of omnidirectional antennas
that were also circularly-polarized we considered several
possibilities such as the
Turnstile - link and the
Quadrafilar Helix - link, but we
were intrigued by a type of antenna that had long been used in the FM
broadcast industry - the "Cycloid Dipole."
The "Cycloid Dipole"
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Figure 1:
A typical "Ring and Stub" Cycloid Dipole
as viewed from slightly "below" the antenna. |
In
Figure 1 one may see a typical "Ring and stub" antenna of the sort that has been used by FM broadcasters in the past. Consisting of two vertical stubs fed by semicircular rings, it's an odd-looking antenna and while it may be clear that it contains both horizontal and vertical components to radiate a signal, it may not be obvious how this combination of elements imparts a "spin" on the transmitted signal.
Note: The "Ring and Stub" antenna isn't widely used for very high-power FM broadcasting these days but occasionally shows up in older installations or with low-to-medium power transmitters.
In extremely simplified terms, one can "spin" a signal by feeding vertical and horizontal sections slightly out of phase (e.g. a delay) - a trick applied to "crossed Yagi" antennas that are often seen for satellite communications on the 2 meter and 70cm bands and a property demonstrated nicely in the video linked above. In those antennas, the "vertical" and "horizontal" elements themselves are spaced apart from each other, but there is also a bit of extra delay provided by carefully-measured feedlines that split the signals that are delivered to the crossed Yagis' driven elements and the combination of these two signals effectively imparts a spin on transmitted signals and "de-spins" received signals.
In the case of the Cycloid dipole this "delay" is provided, at least in part, by the length of the horizontal ring section. By carefully selecting the lengths and dimensions, one may not only provide the necessary amount of delay, but also control how much "horizontal" signal is being radiated by different portions of the ring - and the vertical by the vertical stubs - so that not only are the "vertical" and "horizontal" aspects of the emitted signal equal, but also omnidirectional
and with circularity rather simply radiating both vertical and horizontal signals at the same time.
As it turns out this sort of antenna isn't
really omnidirectional - a term that would imply equal radiation in all directions, both horizontally and vertically. In the case of this antenna it is "omnidirectional" and circularly-polarized only at low elevations rather than at high angles above or below the plane of the antenna: In these "other" directions (above and below antenna) the amount of radiated signal drops off and the tendency for circularity falls apart, too. Since we are generally interested in emitting signals (more or less) at or near the horizon rather than overhead, this limitation actually affords us a slight amount of gain since we aren't radiating as much signal in those directions needlessly
(e.g. skyward and groundward) and power may be directed more along the horizontal plane where it will actually do some good! For beacon use where ionospheric reflections may be the cause of long-distance propagation, the low angle of radiation of this antenna is still a reasonable match for the incident angle of the signal's path as well.
An interesting property of circular polarization is that any reflection of the signal will flip the sense of the rotation. Specifically, any odd number of reflections (1, 3, 5 etc.) will flip a Left-Hand signal into a Right-Hand signal while an even number of reflections (2, 4, etc.) will produce the same as the original when the signal is "re-flipped." Why this happens can be demonstrated if one looks at the threads of a bolt in a mirror: The careful observer will note that in the reflection, the bolt suddenly becomes left-hand threaded if it had previously been right-hand threaded! What this means is that odd-order ionospheric hops will flip the rotational sense as will odd-ordered reflections from mountains and buildings.
If you are receiving a circularly-polarized signal on a linear antenna this will be of little importance
(other than the 3 dB loss intrinsic to the circular-linear "mismatch") as it won't "care" which way the signal spins and in this way, almost nothing that propagation can throw at the signal emitted by this antenna will result its diminution at the receive antenna because of a polarity mismatch.
If, however, you are receiving the signal with a circularly-polarized antenna, the rotational sense
will matter, but since very few of those interested in weak signal CW/SSB work on the VHF
bands routinely use circularly-polarized antenna for such activity, most operators will not experience a problem! If you wish to
minimize the signals from reflections, receiving with the same rotational sense as the transmit antenna will help attenuate them, but if you receive with the opposite sense, you will likely be more sensitive to signals received via reflection!
(Note: This trick is often used in RADAR when you want to discriminate all but the first reflection as much as possible. By using a transmit antenna that is opposite the receive antenna in rotation, signals from even-ordered bounces are suppressed.)
Replicating the Cycloid Dipole:
When it comes to replicating the "Ring and Stub" configuration the difficulties involved in trying to bend a piece of metal tubing to a precise radius preclude its being constructed by the average antenna builder so it was decided to attempt a modeling of the antenna built with orthogonal components such as straight pieces of copper tubing and 90 degree elbows. Using the NEC2 program, the original circular "Ring and Stub" configuration was first modeled and verified (in software) before it was converted to square dimensions.
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Figure 2:
Cycloid Dipole using "square" design elements. |
After some trial and error the simulations began to reveal that it was, in fact, possible to design an antenna that exhibited good properties of omnidirectionality and circularity using "square" elements in the form depicted in
Figure 2. As can be seen, it follows the general form of the "Ring and Stub" in
Figure 1 but is
much easier to construct!
At this point it should be noted that an antenna of this sort is
not resonant anywhere 50 ohms resistive at the operating frequency so it was necessary to feed it with a matching network that was both low loss and capable of matching awkward resistance and reactance values to 50 ohm coax. While several possible configurations were considered, we settled on a balanced 1/2 wave stub using the two pieces of copper water pipe as a balanced line - a configuration that is capable of matching nearly anything!
Comment:
There are online descriptions of this sort of antenna using other types of matches, such as a "Gamma" or other "shunt" type match. In simulations, while these sorts of matches will provide a 50 ohm impedance, they appear to upset the symmetry and current distribution along the elements and if applied to an antenna built to the dimensions given below they will likely result in it having something other than omnidirectional, circularly-polarized signal radiation! If such a feed technique is used, the geometry of the antenna must be reworked to assure that it provides the desired radiation properties!
Since the symmetry of the antenna's radiation was considered to be important, and since it was considered important that the feedline itself should not radiate, the matching network itself is
balanced using a coaxial delay line. Most importantly, this stub matching network is about as low-loss a matching network as can possibly be built using common materials and it is fairly forgiving in its operation provided that it be constructed rigidly enough that the spacings of the various components be maintained and, as can be seen in
Figure 2 and
Figure 3, it is protected somewhat from the elements - namely, the buildup of ice and snow. Also note that this matching network is "folded" on itself to reduce its overall size and to partially counter-balance the weight of the antenna on its mounting.
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Figure 3:
The as-built prototype 6-meter (upper) and 2 meter Cycloid Dipole (lower) that has been in service
since 2001 on the WA7X 2 meter beacon. Most of the "antenna" isn't really the antenna at all, but
the 1/2 wave matching network!
Click on the image for a larger version. |
Figure 3 shows this antenna, constructed using 1/2" copper water pipe and fittings. As can be seen it has been mounted atop a piece of large-diameter PVC pipe and a piece of acrylic plastic has been attached
(with UV resistant wire ties and RTV adhesive) to cover the matching section to protect it from the buildup of ice and snow which would detune it. Barely visible in the pictures are copper clamps that are soldered to the feed portion of the antenna as it passes over the open top of the PVC support pipe that are screwed in place to hold the antenna in position.
Originally, this antenna was considered to be "temporary" but it has continued to function perfectly and remain intact despite the fact that it has been in use for well over a decade
(since 2001) at this remote, 8500 foot (2600 meter) elevation mountain site, exposed to high winds and heavy snow!
Note: Initially tuned before installation of the show shield and atop the PVC mast, a slight readjustment of tuning was required afterwards to restore a <1.2:1 VSWR due to the slight changes in dielectric loading by the proximity of the the mast and acrylic cover.
"Testing" the antenna:
Without a proper antenna range for testing and evaluation we have had to satisfy ourselves with computer simulations and field observations of how it seemed to behave under various circumstances. From our in-field observations, the antenna appears to be quite omnidirectional and when using switchable left/right antennas on receive it also seemed to have reasonably good isolation between the two polarity senses.
The original modelling was done using NEC2 - a rather awkward and user-unfriendly program, but subsequent modeling was also done using other programs: I have also used MMANA
(which doesn't "know" about circular polarization, but will still show horizontal and vertical radiation components) as well as by others using variants of NEC4. The upshot of all of these simulations is that they all agree that this antenna appears to be fairly well-behaved and works more or less like it says "on the tin."
One fact with any antenna is that a feedline and mounting hardware is going to be required
and the antenna will also be mounted within a finite distance of the ground. Since these imply some sort of conductive medium within fairly close proximity to the antenna it is inevitable that the pattern will be altered from that of the ideal antenna in free space and as such it is likely that an
in-situ analysis of this antenna, along with its matching network and feedline and location above an imperfect ground, will yield a rather skewed pattern. Since we can only do "the best we can" we often just live with the alterations in performance and patterns that inevitably result in these real-world installations!
The two major effects of real-world installations are the fact that the feedline and metallic mast (if used) will inevitably skew the omnidirectionality of the pattern somewhat while placing any antenna over a real ground at a finite distance will not only cause the pattern of RF radiation to be shifted upwards by a few degrees, but also introduce many vertical lobes in the response. Again, these really can't be avoided in the real world!
Dimensions of the antenna:
As of the time of this writing, only the 2 meter version has been extensively tested, but a 50 MHz version was also derived at the time that the original NEC2 simulations were performed and details of the construction of both the 2 meter and 6 meter versions may be seen in
Figure 4, below:
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Figure 4:
Dimensions of the 6 and 2 meter versions of the antenna.
Click on the image for a larger version. |
Comment: Not shown in Figure 4 are the details of the 1/2 wave balanced matching network, but this information may be found in any version of the ARRL Antenna Book and in other literature.
It is worth noting that if built as depicted in
Figures 3 and 4 the antenna will exhibit LHCP (Left-Hand Circular Polarity) but this may be changed simply by switching which vertical element
(e.g. elements "D" in Figure 4) is up and which is down.
Comments:
- This particular antenna design does not lend itself well to wideband signals such as TV owing to its rather limited bandwidth. If it were used for FM broadcast, it would have to be modeled and constructed for the specific frequency to be used.
- Because the antenna's dimensions do not easily scale with frequency, producing a design that will work properly at other frequencies (such as FM broadcast) will likely require careful modeling with the appropriate antenna design software - which would take a fair amount of time. Because of this, I CANNOT honor requests for"custom" frequencies! Since these tools are readily available and free it should be possible for you to extrapolate this design for your needs.
- The use of a stub-type matching network (e.g. using the same copper pipe as that of which the antenna is constructed) is recommended over using lumped-constant components such as individual capacitors and inductors, particularly if more than a few watts of power are to be used. The losses, currents and voltages involved - not to mention the susceptibility of such components to weather - can create significant challenges in terms of both losses and durability!
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Both the 6 and 2 meter Cycloid Dipoles, installed and in use - read more about the new(er) 6-meter Cycloid dipole at the link below.
(Are there any other 6-meter, circularly-polarized beacons in the world?)
Click on the image for a larger version. |
In the fall of 2015 a 6 meter Cycloid dipole dipole was installed - read about the "new" antenna here - link
For more information about the antenna visit the WA7X Cycloid Dipole page - link which contains a bit more information about these antennaa, their construction, and the results of various simulations.
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This page stolen from ka7oei.blogspot.com
