diversity receiver and transmission

Antenna Diversity

We sometimes are told a quad antenna, by virtue of vertical side wires and horizontal top and bottom wires, is "diversity". This just isn't true at all! The quad is normally a single polarization antenna, although some feed systems can create a rotating wave.  If for example the feed system of the quad antenna was circularly polarized, the quad would transmit and receive a rotating (circularly polarized) wave. A standard quad antenna is just a single polarization antenna.

Other antennas sometimes claimed to be non-fading by virtue of polarization diversity are not diversity either, and are certainly not "non-fading". An intentional mix of vertical and horizontal reception (or transmission) from one antenna, like a Windom with radiating vertical feedline, actually are linearly polarized antennas.  Even two separate horizontal and vertical antennas, combined into one receiver or transmitter, are not polarization diversity. They are not non-fading, and offer no fading reduction.

Summing or combining two different polarization antennas will not decrease fading on a skywave path. If we directly sum two sources, one vertical and one horizontal, the result is a single tilted polarization. Some might think they obtain isolated vertical and horizontal responses when they look at a computer model, but that's a necessary result of the display! Traditional modeling programs display the results exclusively in V and H without regard for phase. This means a tilted pattern of a single polarization has to show as a partial mix of V and H.  The antenna's radiation actually is a skew or tilt of a single polarization. If we had perfectly equal V and H fields at some directional location on a plot or display, and if a receiving antenna was tilted 90 degrees from the maximum sensitivity, the result would be nearly zero response.

If we mix or combine a vertical with a dipole of equal field strength, and if we look at the polarization from broadside to the dipole, we would see a single tilted wave of only one linear polarization. Any skywave signal would be just as likely to hit the null of that system as it would a pure vertical or a pure horizontal dipole. There is no reduction in fading, and fading can actually increase. This is because skywave normally has multiple paths of varying polarizations and phase delays, and these multiple signals are as likely to subtract as add at any given angle.  Combining multiple polarizations, or multiple wide-spaced antennas, actually increases long term fading by increasing response to secondary paths.

There is a way to have an infinite number of  polarizations at some angle and direction, circular polarization.  Circular polarization is a  time-varying polarization. At any instant of time, we have a single polarization but the polarization angle actually rotates over time. It passes through vertical and horizontal and everything between, rotating like a spinning dipole. It takes one millionth of a second for a 1 MHz wave to rotate 360 degrees, because the rotation time is the reciprocal of the operating frequency.

Circular polarization unfortunately does not work to reduce fading on skywave paths. Circular polarization not only costs 3 dB of gain when mixed into a linear polarization circuit, circular polarization also severely increases fading when transmitting or receiving via skywave. This is because HF skywave has significant polarization rotation over time, and also has multiple simultaneous paths between the transmitter and receiver with varying phase delays for different polarizations. When an antenna launches a rotating wave into the ionospheric soup, circular polarization increases the likelihood of multiple paths with different phase delays reaching the receiver at the same instant of time! The transmitting signal's rotation actually increases fading, just as multiple simultaneous paths with random phase and polarization angle increase fading in the receiving system.

The lowest fading transmitting antenna (and receiving antenna) actually is one with a single polarization that is focused as narrowly and cleanly as possible at the best wave angle and bearing, with the fewest side lobes that might respond to unwanted multipath.
 

Diversity Receivers

True diversity for skywave is just as effective with two single polarization antennas of the same polarization, spaced a few wavelengths apart, as with V and H antenna combinations. True diversity always requires some form of intelligent summing of the signals. To be effective, signals can't just be directly mixed either at audio, IF, or radio frequencies.

Early systems used a voting system that followed the AVC (automatic volume control) and muted the lower level antenna, or had a noise detector and voting system that routed the better S/N path to the audio line.  The operator or printer only "heard" one signal at a time, and the system electronically selected the best signal.

After unsuccessfully attempting to build a voting system in the 1970's, and unsuccessfully attempting to combine various receiving antennas, I experimented with R4C's. I "tricked" one receiver into thinking it was feeding a T4XC, and instead fed it into a second R4C. The second R4C assumed a T4XC was feeding it. This gave me one receiver as a master oscillator for both carrier and injection oscillators. The receivers were phase locked and one tuning knob tuned both. With identical receiver channels the results were startling. After some learning curve for my ears, I could copy signals that were buried in the noise in mono.

There has been some attempt to copy what I started years ago with two R4C receivers, stereo diversity. Despite what we are sometimes told, we can't simply adjust two receivers to the same basic frequency, pipe one output into each ear, and have diversity. This is especially true with unlocked receivers that are even a fraction of a Hz different in frequency. Your brain will not be able to phase lock the signals unless the lock is closer than a small fraction of a Hz, perhaps 1/25th Hz or less. The more rapid the phase rotation, the less effective the brain will be at adding signals and subtracting noise.  Phase rotation between receivers has to be significantly less than phase rotation between the stereo antennas.

There are articles describing locking the dials of the FT1000D so the sub-receiver tracks the main receiver. This does somewhat work in the FT1000, because the oscillators share a common time base. The main problem with the FT1000 is the second receiver isn't a "good" receiver.

Testing For Diversity Capability

You can test your system for correct phase-lock by tuning in a carrier (like WWV) and mixing the two outputs together. When audio levels are equal, you should hear no warble or vibrato in the tone. You should hear something between a full or peak that stays perfectly locked. If your system fails this test, it will deduct greatly from possible diversity advantages. Click here for a failed test. Click here for a passed test.

Remember any warble or slow fade variation indicates the receiver is unacceptable.

True vs. Stereo Diversity 

I use a loose form of diversity reception on 160 and 80 meters I call "stereo diversity". It really isn't true diversity where the receivers vote and the best S/N ratio captures the audio output, it is a system that allows your "brain" to do signal summing and noise subtraction.

I've found this system very good for substantial improvements in readability of noise-floor signals. The difference can be worth as much as a signal being nearly readability 5 (perfect) in stereo to readability 2 without. When a signal is marginal, stereo diversity is all the difference in the world.

I accomplish stereo diversity by phase locking two separate receivers together so audio outputs are exactly in phase. The receivers are virtually identical, even to the point where I hand select crystal filters for equal group delay change over the filter passband. Every oscillator in the receiver is common to both receivers. The only commercially available amateur receiver I'm aware of that does true stereo diversity is the Elecraft K3. I currently use an Elecraft K3, because it is the only thing close to my heavily modified R4C's.

The stereo method, used with the proper receivers, allows me to successfully combine two antennas or antenna arrays that are several wavelengths apart into one system, a task otherwise impossible. Stereo reception using two very quiet wide-spaced antennas causes noise to appear smoother and more "hollow sounding", while the signal actually stands out more. Some of the recordings on my DX Sound Files page are in stereo. You can listen to this effect with stereo headphones by changing your computer's volume control settings to stereo or mono.

Transmitting

One common thought or claim  is by transmitting both vertical and horizontal via skywave, we would have the best of two worlds.  Worse yet, there is a common idea or belief we can build antennas producing two independent polarizations, and that resulting dual polarization will provide the best of two worlds and reduce transmitted signal fading.  Several obvious flaws with this concept are outlined below. 

Generating Two Polarizations

When speaking of polarization, we are talking about the direction of the imaginary flux lines in the electric field. When dealing with the far-field effect called EM radiation, the imaginary electric and magnetic flux lines are conveniently at right angles to each other. While either could have been used for the reference standard of polarization, the electric field became the polarization reference.

The imaginary flux lines represent the force caused by any and all electric fields. They "exist" only at one angle in one small portion of space at any instant of time. We can not generate two polarizations at the same time at any reference point in space when broadcasting our signals, not with any antenna! The idea we can have dual-polarization transmissions probably comes from misunderstanding what antenna modeling programs are showing us, or a flawed or limited imagination causing an incorrect mental picture of what actually occurs with antennas.

Modeling programs only show two perfectly filtered views of the actual field. They do this out of necessity, because the actual electric field is far too complex to display in its entirety. While we could see a slice of the field showing true polarization at any given angle and distance, I'm not aware of any commonly available programs that provide such useful information. A typical pattern display generally shows the response that would be observed through perfect vertical and horizontal filters. Modeling programs generally do not tell us the phase relationship between the intensities we see displayed, so we have no idea what the actual polarization is.

The bottom line is this....we don't know, when looking at the display, what the actual polarization is unless it is 100% vertical or 100% horizontal.

A Simple Tilted Dipole

Visualizing an actual antenna might help us picture an antenna pattern correctly, and understand what we commonly perceive incorrectly. 

Imagine we have moved back a considerable distance from the center of an extremely high dipole that was installed tilted at 45-degrees. We move back from the dipole center without changing height, and observe the electric flux lines near us. To us, the distant antenna appears tilted at a 45-degree angle from our lower left to upper right. Each end of the antenna is exactly the same distance from us. In other words, this is a "side view" of a perfect sloping dipole.

If we could actually see flux lines near us representing the electric field, the lines would appear to parallel the distant antenna. Yet a view on a modeling program would show the field intensity of the electric field to be an exactly equal mix of vertical and horizontal fields!

Many of us would (incorrectly) describe this antenna as producing equal vertical and horizontal polarization in the direction where we view the antenna. The logical conclusion would probably be a vertical or a horizontal would respond equally well to that field, and that is correct for a perfect vertical or horizontal. 

What we probably would fail to understand or visualize is the actual polarization. The peak response would be to a dipole antenna tilted 45-degrees in the same slope direction as the radiator, from lower left to upper right. Most important and most often missed is the simple fact that another dipole tilted 45-degrees opposite, from lower right to upper left (even though broadside towards the distant source) would have no response! It would be cross polarized, and response would be minimal.      

This idea we have both polarizations is the root of the misunderstanding, and misunderstanding always seems to breed voodoo antenna claims and snake oil solutions. The most common false conclusion would be thinking sloped antennas reduce polarization related fading. The "equal-V-and-H-antenna" would be assumed to provide the best of two worlds, transmitting or receiving when either vertical or horizontal polarization or anything in between is required for an optimum signal.

Ionosphere Propagated Signals

Any distant signal arriving via the ionosphere is constantly changing in polarization. The ionosphere is a poorly aligned soup of ions, and that soup is constantly being stirred. It is not a flat perfectly aligned mirror. The ionosphere also provides multiple modes and paths for signals, particularly on frequencies well below the maximum usable frequency. The phase and level of the same signal arriving from each path constantly changes. Because of this, arriving signals tilt and rotate. 

While there are some statistical odds that more time will be spent centered around one effective polarization than another, the fact remains that very little time is spent at one distinct polarization. The same effect holds true when transmitting.

Because of the random nature of polarization, the signal just as likely would be tilted 45-degrees left on sloper as 45-degrees right. It is just as likely to fall into a cross-polarization null with the sloper as with any other angle of radiator, except one centered at the optimum tilt. We can easily see the idea sloped wires, Inverted L's, and even Windom antennas with "leaky" baluns reduce fading by providing "diversity" is pure rubbish. The same holds true for intentionally mixing two polarizations from two separate antennas, even if each is fed from separate amplifiers.

Statistically, we are actually MORE likely to have deep fades when we transmit with two very different systems than with one! The reason is simple, we excite the multiple paths better and increase multipath propagation. Since the phase delay is random and constantly changing, any attempt at circular or dual polarization would greatly increase fading when more than one path makes it to the receiver.

This is actually the reason 5/8th wavelength verticals fell out of use in broadcast work. The small high-angle lobe of the 5/8th wavelength antenna created severe deep fading and phase distortion at receivers in the fringe areas. A mix of horizontal and vertical antennas, even if properly phased to provide a rotating wave, would be even worse.      

Reducing Fading

The best solution is to have two separate antennas, selecting the best antenna at any given instant of time for the path. This is true for both receiving and transmitting. Commercial sites do this by employing some form of voting, based either on signal-to-noise or absolute signal level.

Dealing with weak signals provides special problems. Good CW operators can copy code that is actually below noise floor. Because S/N ratio is near zero, noise detectors in a voting system would become overly complex and unreliable. Perhaps someone can develop a DSP system that allows voting, but my attempts have been largely unsuccessful.  

My solution is to use a stereo system with phase-locked receivers, and process the audio in my head. With antenna separations over a few wavelengths, the background "white noise" takes on a distinct hollow sound. Signals are easier to pick out, and the ability to copy CW below the noise floor is greatly enhanced. The end effect of this is reduced fading.

For transmitting, the only useful approach is having a variety of antennas available and picking the antenna generally more optimum for the particular distance, direction, and time of day. Without feedback from the receiver, it is all a guessing game. One thing I do know is that mixing my antennas directly never resulted in improved signal strengths or reduced fading in many dozens of blind tests.