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CIRCULAR POLARIZATION ON HF
Bob "Bacon" Bruhns, WA3WDR
A few years back, I read an interesting article, "The Enhancement of HF Signals by Polarization Control" by B. Sykes, G2HCG, Communications Quarterly, November, 1990 (reprinted from Practical Wireless).
Sykes found that most of the short-term fading seen on 10-meter beacons is actually due to polarization rotation of the incoming signal causing occasional cross-polarization nulls with ordinary receive antennas, and not due to real fading of the signal itself.
Sykes's findings agreed with some lower-band antenna stories I had heard over the years, so I set up a circularly-polarized antenna on 75 meters to see how it would work. I found that some big improvements were possible from some relatively simple antenna hardware.
A CIRCULARLY POLARIZED ANTENNA FOR 75 METERS
Basically, what I have is a 75-meter turnstile antenna, but because of space restrictions it actually is two vertical half-wave loops, top-fed, at right angles to each other. In fact, space is so restricted that the ends of the dipoles don't just meet, they overlap. This antenna configuration may be a factor in the results I got.
Both antennas are coax-fed with W2DU ferrite-sleeve baluns. The coax lines (RG-8X) are both 100 feet long, just about 1/2 wave electrically. The feed ends of these feedlines are connected together with about 40 feet of RG-8 (1/4 wave electrical). By connecting the transmitter or the receiver to the junction of the 1/4 wave phasing section and one of the antenna feedlines, I am able to feed the two antennas 90 degrees out of phase. The circular "sense' is determined by which junction I select.
I built a special T-R switchbox which allowed me to select which junction to use as the feedpoint. After a little use, I modified the switchbox to allow independent TX and RX sense selection, because I found that optimum TX was usually the opposite connection from optimum RX, at least for local TX. (I haven't tried DX yet.)
In RX tests, circular polarization was noticeably better than either linear polarization most of the time, and one circular sense was clearly better than the other. The optimum circular sense always reversed at sunrise and again a couple of hours after sunset, and while it was reversing, there was no clear advantage to either sense. On nights when 75 meters "went long" (when the high-angle MUF dipped below 3.9 MHz), the optimum sense for local signals would reverse again as the signals began to fade out.
When the optimum circular sense was selected, receive signals were significantly stronger and suffered less fading and less selective fading. Receive signals often had substantially better audio quality, as if a "phasing" sound had been removed. Unfortunately, severely disturbed ionospheric conditions would still mangle receive signals, regardless of the antenna sense.
The optimum receive sense for European DX was the reverse of the optimum receive sense for signals within about a 500 mile radius, except when the band went long, and the optimum local sense reversed.
In transmit tests, I was received much better on one circular sense than the other, the difference being about 10 dB. During the day, optimum TX was always on the other end of the 1/4 wave phasing line from optimum RX. At night, this was usually true, but there were occasions when optimum TX was obtained with the same connection which produced optimum RX.
While other stations were exhibiting short-term fading of 20 dB, my TX signal had almost no short-term fading. I was fading slowly, with little selective fade effect. Reports indicated better quality audio in the optimum TX sense; evidently, the reduced "phasing" effect I heard on RX was evident on TX as well. Signal reports were good, too, especially considering my 80 watts output and tiny antenna.
It would be interesting to run tests with another station using circular polarization. As far as I know, mine was the only station using circular polarization in these tests. All of the other stations used linear polarization.
CIRCULAR POLARIZATION AND SENSE
Circular polarization consists of an equal amount of electromagnetic field polarized in two directions separated by 90 degrees, such as vertical and horizontal, with a 1/4 cycle time difference between them.
The concept of polarization becomes more complex when a signal is coming from somewhere in the sky instead of somewhere on the horizon. Polarization will still be at right angle to the direction of propagation, but it is no longer a simple matter of vertical versus horizontal. For example, the polarization of a very high angle skywave signal is essentially horizontal - but the wave might be north-south polarized, or east-west, or something in between; it could also be circularly or elliptically polarized, in either sense.
The electromagnetic field of a circularly polarized wave rotates once per cycle. A 75 meter signal might rotate 3,885,000 revolutions per second! As the cycle progresses, positive would be up, then around to one side, then down, then around to the other side, then up, etc. This rotation can be either clockwise or counter-clockwise, and this is what is meant by the term "sense".
When a circularly polarized antenna is used to receive a circularly polarized signal, the antenna sense must match the sense of the incoming signal, or very poor reception results, very much like the situation of a horizontal antenna receiving a vertically polarized signal. Such cross-polarization results in large signal losses, theoretically infinite, but typically around 20dB in practice.
Circular polarization is a special case of elliptical, in which the fields are exactly equal and exactly 90 degrees out of phase.
If the horizontal field and the vertical field are unequal in strength, or if the phase difference between the horizontal and the vertical field is not exactly 90 degrees, the resulting polarization is called elliptical. Elliptical polarization lies between linear polarization and circular polarization, something like an oval. Elliptical polarization also has a sense, but it tends to favor one angle of linear polarization.
Linear polarization, such as vertical or horizontal, is a special case of elliptical, in which all the power is in one polarization direction.
Although groundwave can contribute to selective fading on amateur frequencies, usually it is too weak to make much difference unless the path is very short, or over salt water. Selective fading in the amateur bands is primarily ionospheric.
There are usually several ionospheric paths a signal can take which will deliver usable power to a receive location. Portions of the signal which follow these various paths arrive with different delays. Selective fading results when these variously delayed signals recombine.
Generally, the signal from one of these paths will be the strongest, and normally it will dominate. As long as this signal does not fade, reception is good. But when this strongest signal fades, it no longer dominates. Weaker signals from one or more secondary paths interfere significantly, and selective fading results.
Under normal ionospheric conditions, most of this apparent short-term fading of the strongest signal is actually due to random polarization changes. Except for the hours around sunrise and a little after sunset, this predominant signal seems to have a generally consistent sense, varying in ellipticity from linear to circular. The use of circular polarization of the correct sense will result in optimum transmission and reception, with minimum selective fading.
MAGNETIC EFFECTS IN THE IONOSPHERE
It has long been known that the earth's magnetic field causes the ionosphere to twist the polarization of radio waves it returns to the earth. This can have strange effects.
Imagine that the polarization twist on some signal path holds steady for a while at 45 degrees clockwise, and that the antennas of two stations also differ in direction by 45 degrees. Both stations are readable to an observer with circular polarization. Station one will hear station two just fine (perfect polarization alignment), yet station two will hardly hear station one at all (crossed polarization). This is an example of non-reciprocal propagation caused by magnetic effects in the ionosphere.
There is a great deal of theory about exactly how the ionosphere acts in the earth's magnetic field, but for most of us poor hams, the most valuable information is that ordinary fading can be reduced, and improved communication can be achieved, by using a circularly polarized antenna with switchable sense.
ABOUT THAT ANTENNA...
My crossed loops don't produce perfect circular polarization at all angles of radiation. Offhand, the only type of antenna I know of which comes close is the quadrifilar helix. I considered building one of these for 75 meters, but I can't install structural supports for one here, and reversing its sense would be tricky. This is a field for further study.
If I ever get enough room, I would really like to try a pair of full-wave "CCD" antennas in a 90-degree-apex inverted-vee turnstile configuration. "CCD" antennas are full-wave antennas with approximately constant current distribution along their length, due to numerous series "unloading" capacitors along their length. A pair of half-wave inverted-vee dipoles in turnstile configuration may be a good solution for those with smaller lots.
But since my yard is small and uncooperative, I pretty much have to settle for bending my turnstile antenna into the dual half-wave folded loop form. The impedance of each folded loop appears to be around 17 ohms, which results in mismatches, odd TX loading, and unequal power fed to the two loops. Even so, this antenna provided improved reception and produced a strong and steady signal on the air.
I have considered using 6:1 step-up baluns at the antennas instead of 1:1 baluns; this would step the feed impedance up to around 100 ohms per loop, and then I could use 95 ohm cable for the feed and delay lines. The impedances would match, TX power would split evenly, and the TX load would be right around 50 ohms.
I always thought fading was an overall ionospheric effect, independent of the antenna; I thought the only possible improvement would come from complex, multi-receiver diversity systems. Not any more!
It seems that circular polarization is a simple and natural method of improving signal propagation on HF. This is not new knowledge, but I sure didn't know about it, and I don't see it in the popular books on amateur antennas for HF.
I've already noted the work of B. Sykes, G2HCG. There is coverage of ionospheric polarization twisting in Terman's Radio Engineer's Handbook (McGraw-Hill, 1943) and Radio Engineering (McGraw-Hill, 1947). Several people told me about improved HF communications when two antennas of different polarization were used together. More than twenty years ago, I heard about one ham with a 75 meter turnstile antenna, who noticed reduced selective fading. While I was testing my antenna in early 1991, people broke in and told me details about circular polarization and the ionosphere. Thanks to all who participated and offered reports, suggestions and ideas.
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