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Jump to diversity
receivers
Some
Phase measurements of antennas
Antenna
Diversity
Single Antenna with
Multiple Polarization
for Diversity
There are claims a
quad antenna,
by virtue of
vertical side wires
and horizontal top
and bottom wires,
provides
polarization
“diversity”.
This
just isn’t true at
all! The quad is
normally a single
polarization
antenna, although
some very
specialized feed systems
can create a
rotating (circular
polarized) wave.
If for example the
feed system of the
quad antenna was
circularly
polarized, feeding
the sides and flats
in phase-quadrature, the
cubical quad
would transmit and
receive a rotating wave
(circular
polarization) of no particular
polarization. A
standard quad
antenna, however, 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 antenna with
a radiating vertical
feed line, are generally linearly
polarized antennas.
Even two separate horizontal and
vertical antennas, combined into one receiver
or transmitter, are
not normally polarization
diversity. Such a mix of antennas are
not non-fading, they
offer no fade
reduction.
Polarization of
antennas such as
these are primarily a single
polarization that is
tilted at different
angles in various
directions. There can be a small portion of the radiated field that is rotating
polarization with time (circular polarization), but is not helpful in reducing
fading.
Because Eznec and
other programs only
plot two
polarizations of
patterns, perfectly
vertical and
perfectly
horizontal, skewed
or tilted patterns
appear as a “mixture”
of two
polarizations. The radiated field is really a single
uniform polarization
that is tilted at an
angle.
This
is the pattern at 90
degrees azimuth.
This
is the same antenna
at 135 degrees
azimuth. Notice V
and H are now
approximately equal at some elevation points.
This is because
polarization is
tilted at 45 degrees
at those particular elevations and at this azimuth
angle. If the display included phase, we could calculate the angle of the
polarization tilt.
EZNEC does have a way to look at circular polarization content. Go to the
Desc Options tab (at the bottom of the window list). When you open the “Description Options”
window select one of the Circular “Fields to Plot”. This will tell you the ratio
of linear polarization to circular (rotating) polarization. Remember for
skywave, circular polarization will generally at best not improve things…and
more often make fading worse!
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.
The casual observer
might think he has
obtained isolated vertical
and horizontal
responses when he
looks at a computer
model, but the dual
polarizations
displayed are a
necessary result of
display limitations!
Traditional modeling
programs can only display
polarization exclusively
in V (vertical) and H
(horizontal) without
regard for phase.
This means a tilted
pattern of a single
polarization
displays as a
mixture of V and H
of differing levels.
The antenna’s
radiation actually
is a skew or tilt of
a single
polarization.
If the pattern
displayed had perfectly
equal V and H fields
at some directional
angle and elevation 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.
Circular
Polarization
If we mix or
combine a vertical
with a dipole of
equal field
strength, and if we
look at the
true 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.
As a matter of
fact, a perfectly
horizontal dipole is
only perfectly
horizontally
polarized directly
broadside to the
dipole. As we look
at the dipole moving
around towards
the ends, the
polarization tilts
more and more
vertical!
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
compass heading, with the
fewest side lobes
that might respond
to unwanted
multipath. Circular
polarization
increases fading on
skywave paths, when
either receiving or
transmitting.
True diversity
for skywave is just
as effective with
two same-polarization
antennas as with V and H antenna combinations. Using similar or same
polarizations, the
antennas must be spaced
a few wavelengths or further
apart. True
diversity always requires
some form of
intelligent summing
of the signals. To
be effective,
signals cannot just
be directly mixed
either at audio, IF,
or
radio frequencies.
Early diversity systems used a
voting system that
followed the AVC
(automatic volume
control). Since both RF and IF systems shared the same AVC voltage, the channel
with the strongest signals “muted”
the lower strength
channel. Of course this created a problem if the audio outputs were directly
summed and signal strengths were similar, because phase errors from the antennas
would translated directly into phase errors at combining point in the receiver. Other
later systems used a
noise detector and
noise-controlled voting system. These systems
routed the better
S/N channel to the
audio system. This produced a much better system, since signals were never
combined into one channel. The operator (or
printer) only “heard”
one monaural signal at a
time, with the noise controlled voting system
electronically
selecting the best
signal-to-noise channel.
True vs. Stereo Diversity
I use a loose form of diversity reception on 160, 80, and 40 meters I call
“stereo diversity”. This really isn’t true diversity where the receivers
vote and the best S/N ratio captures the audio output, it is a system that
requires our “brains”
manage signal summing
and/or noise
subtraction. I’ve found this technique 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 signal-to-noise
ratio
is marginal, stereo
diversity can be make all the difference in the world in readability.
I implement stereo diversity by phase locking two separate receivers (heavily
modified R4C’s) together so audio outputs are exactly locked 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 system is common to both receivers. I’m aware of only
one commercially
available amateur
receiver that does
true stereo
diversity, the
Elecraft K3. I
currently use an
Elecraft K3 since
it is the only system
that phase-locks two identical receivers, it interfaces a good transmitter with
the receivers, and is a modern stable design with accurate frequency readout.
The stereo
method, used with
the proper
receivers, allows successful combining of two antennas or antenna arrays that
are several wavelengths apart. Normally this would be impossible, since the
phase of signals from two wide-spaced antennas continuously changes from one
moment of time to the next. If we simply summed the signals in phase, in a matter of
seconds or minutes the phase would rotate 180-degrees out and cancel.
It is helpful to think of summing, even at audio, as system with both
antennas directly combined into one very large array. There is actually no
difference at all if we summed at RF before one receiver, or summed at audio
after processing through two separate receivers! With the wide spacings useful
for diversity, the resulting pattern would be very sharp. There would also be
many deep nulls. With such a sharp pattern (from the wide
spacing) that the desired signal would continually slip from a pattern peak to a deep
pattern null.
If antenna spacing is less than 1 wavelength and we directly sum the audio,
the resulting pattern is just like phasing the antennas directly together! The
only difference is we cannot control the phase very well unless both channels
share common oscillators. This is not the same as using multiple
VCO’s with a common reference oscillator. The Elecraft K3 actually rotates phase
as the VFO is tuned, so even if we (through some sort of luck) had a good
pattern on one frequency the pattern would change as the VFO changed
frequencies. I actually control the phase in my diversity R4C’s by changing the
phase in the 50 kHz BFO. Since the PTO and all other oscillators are
common from the master receiver, the phase difference remains set by the BFO
phase difference. The controlled predictable stable phase allows the R4C’s to be
used for direction finding as well as signal nulling when the audio channels are
mixed into mono.
By allowing our brains to process the signals, we can mentally extract
coherent tones from random noise.
Stereo reception
using two very quiet wide-spaced antennas
causes background noise to appear smoother and more “hollow sounding”.
The coherent signal appears to stand out from the noise, even as phase is
rotating. 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.
Stereo Diversity
As with many
things, it’s
difficult to say who
actually started
stereo diversity. I
had one of the
earliest systems I’m
aware of.
After
unsuccessfully
attempting to build
a voting system in
the early 1970’s, and
after unsuccessfully
attempting to
directly combine various
receiving antennas,
I experimented with
R4C’s. I
initially tried
running one VFO to
both receivers via
the PTO jacks, by “tricking” one
receiver into
thinking it was
feeding a T4XC PTO
in transceive. Instead
of feeding the PTO
into a T4X
transmitter, I fed
it to
a second R4C. This gave me one
receiver as a master
oscillator for both
carrier the carrier oscillator,
one tuning knob
tuned both receivers.
While each receiver
sounded OK by itself
in mono, in stereo
this produced an odd
sounding signal.
I discovered I
also needed to use a
common carrier
oscillator. Once I
shared the carrier
oscillator, the
receivers were phase
locked. If I
combined the audio
outputs in mono, I
could peak or null
any tones by moving
the passband tuning
knobs. In stereo,
with the identical
R4C receivers, results
were amazing.
After some learning
curve for my ears, I
could copy signals
that were buried in
the noise in mono.
After proper configuration,
background noise
took on a different
hiss or “color”. If I
ran one common
antenna into both
receivers, it
sounded just like
one receiver. If I
split antennas and
used some loop
arrays in one ear
and the Beverages in
another ear, the
noise got a distinct
hollow sound and
signals stood out.
Another interesting effect of combining channels using common oscillators is
a signal can be peaked or nulled by varying phase shift between the two
channels. This only works when common oscillators are used for each channel, and
where a method of altering phase shift is available. In a similar manner, by
watching phase error between independent channels fed from widely spaced
antennas, direction of signal arrival can be determined.
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 unlocked or dissimilar receivers to the
same basic
frequency, pipe one
output into each
ear, and have
effective diversity. This is
especially true with
unlocked receivers
that are even a
fraction of one
hertz
different in frequency. Our
brains are unable to phase-lock
or phase-compensate the signals unless
the lock frequency 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
becomes when adding
desired signals and
subtracting noise.
Phase rotation
between receivers
has to be
significantly less
than the natural
atmospheric-caused phase rotation
between the stereo
antennas.
There are articles describing
how to
electronically lock the
receiver tuning of the FT1000D
(and other
receivers) 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
very good receiver,
the filters do not
match, the AGC does
not match, and phase
shift through the
receivers
(especially as the
signal moves off the
filter center) is
not at all similar.
I had similar disappointment with the Orion system.
To the best of my
knowledge the K3
Elecraft is the only
receiver that has
absolutely identical
receivers that share
one common reference
system. That is what
I have switched to
now. The K3, unfortunately, cannot be used to measure phase differences because
the oscillators are not perfectly phase synced. It does work OK for diversity.
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 null or peak
in level, a
steady tone that
varies only in
level. as the
receivers are tuned
across the signal. If your system fails this test, it
will deduct greatly from possible
stereo 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.
Best diversity Receiving Antennas
For best performance, diversity antennas should be spaced more than 2
wavelengths apart. Both antennas should have similar directive patterns and
similar signal-to-noise ratios. Polarization diversity generally only works on
higher bands. It is not practical to have a low wave angle horizontally
polarized antenna for 160 or 80 meters. Remember a Beverage antenna is
vertically polarized, it is not horizontally polarized!
Diversity Transmitting
One common thought or
advertising claim is by transmitting both vertical and horizontal via
skywave, we have the best of two worlds.
There is an idea or belief we can build
antennas producing two independent polarizations, and
the resulting “dual
polarization” will provide the best of
both 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 instant of 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
generally show two perfectly
“filtered” views of the actual field. They do this
out of necessity, because the actual electric field
tilt or angle is far too complex to
display in its entirety
on a flat screen. 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.
We know if patterns
show both vertical
and horizontal
component the wave
is tilted, but we
don’t know the tilt
angle.
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. For minimal fading on transmitting, we want ONE path and
polarization only! Multipath is not helpful, it is actually one of the the
root causes of ionospheric short-term fading.
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.
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