Introduction
This
Quad antenna project covers a practical Polar Diversity QUAD antenna design
and related experiments with Polarization Diversity to enhance reception
of fading signals in the Amateur Radio Service, specifically on the TEN
meter amateur band. The selective fading when using the AM mode can be so
severe as to cause total intelligibility loss of the received signal. Deep
selective fading of AM signals can be reduced and possibly eliminated during
a QSO using one form or another of diversity reception. For other modes
such as SSB and CW selective fading is also a problem, but to a lesser degree
due to the narrower occupied bandwidth of those signals. Many Amateur radio
operators have noticed, DX signals appear to “shift polarity”
during signal fading. Operators have used two antennas and switched between
them to see this effect. The problem is that one cannot predict the best
polarity to use at the transmitter to compensate for the polarity shift
at the receiving station. For this reason, I have decided to investigate
and design an antenna that would be effective in providing the standard
Vertical and, Horizontal polarities in addition to a Circular polarization
mode using the standard full-wave quad loop, to reduce severe distortion
of AM signals caused by deep selective fading. It is widely known that two
separate antennas of the Yagi type can be easily set up to affect a dual
polarity antenna, and use of the phase relationship of 90 Degrees so that
a circular “cork-screw” wave front could be generated. However,
no data or practical antennas have been published that make use of the Single
Quad Loop as a functioning multi-polarity radiator that can be Circularly
Polarized. The design details for such an antenna is described herein. For
additional reading, an interesting link to attenuation values as a function
of “cross polarization” can be found at:
http://www.kyes.com/antenna/navy/polarization/polariza.htm
Without
polarization diversity, a station has one receiving antenna, typically either
horizontal or vertical. Over the long term, ionospheric fading mechanisms
impact both polarizations equally so the choice of polarization of this
single antenna can be made for other considerations (mobiles / DX). However,
over the short term measured in fractions of a second to seconds, ionospheric
fading can be substantially different between the two polarizations. An
interesting link to a paper concerning “Faraday rotations through the
ionesphere”, can be found at:
http://www.tuc.nrao.edu/~demerson/ionosphere/ionopol.html
In
addition to the theoretical information obtained at the above stated links,
Amateur radio station operator W0RPK collected polar diversity data using
two separate antennas at right angles to each other, and two separate receivers.
To see the study in detail, go to his website via QRZ.com. and use the associated
links he provides. The W0RPK investigation provided propagation data as
plotted on a graph. Dual Receiver AGC data points were logged and plotted
on a computer at his station. The brief but important differences in polarized
fading can be seen in the graph of a constant transmitter amplitude 10m
signal (each horizontal division represents 1 second). The radical differences
in horizontal vs. vertical signal strength as ionospheric fading mechanisms
impacted one polarization and then the other is evident.
In
as much as a the receiving station benefits from diversity receiving antennas,
the transmitting station reciprocally benefits in providing a more diverse
signal to the receiving station. It should be noted that the algebraic sum
of both graphs could be the mutual amplitude benefit for both receiving
and transmitting stations. A station receiving such a diverse signal would
be rewarded the benefit of a "polarization match" regardless of
the polarity of the receiving antenna. The typical 10 to 30 dB fades in
a single polarity antenna would be "held up" in amplitude to the
best case fading of the other arriving polarities. The -3dB power split
and resultant 1/2 power per polarity of transmitted power for each of the
separate feeds, more than outweighs the benefit of reduced overall fading
into the noise level at the receiver thousands of miles away. The following
pages describe the practical application of a multi-diversity antenna system
using a dual feed point cubical quad. The driven element only is described
herein as the basis for the antenna. The typical parasitic element design
can be any, and is not considered here. Note: Closed loops are recommended
for maximum Circular diversity.
1.0
OVERALL DESCRIPTION Triple Polarity AB2KC Quad
The
purpose of Triple Polarity AB2KC Quad is as follows:
1.
Provide Horizontal polarization for DX and ground wave contacts while both
stations are mutually horizontally polarized.
2.
Provide Vertical Polarization for ground wave contacts for stations mutually
vertically polarized, and specifically for local mobile contacts that use
vertically mounted mobile antennas.
3.
Provide Circular Polarization for DX contacts to reduce AM / SSB selective
fading due to Faraday shifts in the arrival of the signal. Vertical to horizontal
rotations is a result of the reflections off the ionosphere, resulting in
an undefined received polarity. The affect is also reciprocal for transmission,
in providing a polarization consistent at the receiving station. The power
is split 3dB to either polarity during this mode.
1.1
Two Gamma matching sections in separate weather proofed enclosures.
A
weather proofed containing the wide spaced 100 pF variable capacitor for
tuning, and attachment for the Gamma wire (on cap shaft). An SO-239 female
connector mounted for feedline connection and center of quad loop attachment
permits easy mechanical connections.
Notes:
- Use
two ground lugs to solder the quad loop wire to the SO-239 connector flange.
- Use
a tin plated copper right angle bent strap from the shaft of the cap to
the gamma wire. Seal box against weather. Use Plexiglas spacers to maintain
spacing of the gamma wires to main loop.
- Both
boxes must be identical.
1.2
Two 1:1 Baluns for Isolation
A
balun at each feed point is mandatory, to provide a high impedance to the
outer shield of the feed lines to eliminate loading of the loop. The two
Baluns are made with coax sections. Wound with 4 turns at 3-inch diameter.
USE TEFLON COAX HERE. Each cable used to make the Baluns must be the same
length. Tie up each coil in solenoid form using 6 heavy duty nylon tie wraps
evenly spaced around the coil. The coils must be solenoid wound to reduce
distributed capacitive coupling. The two feed lines to the branch line coupler
must be the same length. The Baluns have PL-259 connectors at both ends
for easy attachment to the gamma boxes that have the SO-239 females ready
for action (Hi).
1.3
Ninety-Degree Phase Shift
To
get Circular polarization… you need a 90 Degree phase shift on one
of the feed lines. This can be done by simply adding a _ wave section of
line to either feed line, and adding a TEE to split the signal. The problem
with that is the TEE connection provides no isolation, and reduces the feed
point to 25 ohms (assuming a pair of 1:1 SWR lines). I took a novel approach
to this problem. I used a “BRANCH-LINE-COUPLER” design, which
is a modified design from common microwave dividing and combining technique.
In essence, it is a 90 degree quadrature –3dB divider, that reciprocates
as the combiner during receive. Normally at microwave frequencies, the network
is printed as stripline on double sided dielectric PC board. At HF frequencies,
I simply used coax! You will need to make a network using two _ wave lengths
of 50 ohm coax, two _ wave lengths of 35 ohm coax (explained below) and
connect it to four SO-239 connectors mounted on a plastic box (no common
ground between connectors).
Here
is how it is connected up: To visualize it, draw a square with 3 inch sides.
On the bottom left corner, mark it “transmitter input”. Mark the
top left corner “Vertical out”. Mark the top right corner “Horizontal
out”. Mark the bottom right corner “termination”. Now, mark
the two vertical sides of the square 35 ohms, and the two horizontal sides
of the square 50 ohms. The two 35 ohm lines are each made with two parallel
75 ohm lines (37.5 ohms close enough!). Be careful to take the velocity
factor of each line into account when determining the _ length needed. Some
75 ohm cable is polyfoam dielectric with a velocity factor of about .8 compared
to Teflon at .66 (MAKE SURE). When complete, each SO-239 connector should
have three coax connections, a pair of 75 ohm lines, and a single 50 ohm
line. The termination connection is for a dummy load connection. When the
entire system is working, there will be no power at this point. If there
is a small imbalance in the system with VSWR or a bird in the antenna, the
difference in amplitude will end up at the termination and NOT AT YOUR TRANSMITTER.
The rig will always “see” 50 ohms!. I use a 200W dummy load, good
for 1KW carrier of AM. I never saw more than 100 watts across the entire
10M band. You can add coax relays and a 3-position RF switch where needed
to feed only the vertical, or only horizontal feed points for “linear
polarization” (not circular, Hi Hi). It’s fun to flip the switch
and recover a severely faded AM signal while receiving. It more fun to give
your fellow ham a circular signal to receive from you! Now all that work
you put into your AM transmitter’s audio will have less selective fading
on the other end! Of course… when the band goes out, none of this helps
and you’re finished!
2.0
SYSTEM PERFORMANCE
2.1
Initial Adjustments/SWR and Isolation
Initial
adjustments of the gamma matching networks proved to be straight forward,
with almost unnoticeable interaction between the two feed points. The variable
capacitor will end up about 1/2 meshed. Make the gamma wire longer to start
with, and use “back-to-back” soldered alligator clips to find
the best length. I did most of the matching with the MFJ SWR antenna analyzer.
After adjusting the dual feed points for minimum VSWR at 28.7 MHz, I fed
one polarity (horizontal) with 10 watts and measured the power coupled into
the vertical feedline. It was less than 0.15 watts, about 18-dB isolation.
Vertical to horizontal isolation is 18-dB.
2.2
Bandwidth
The
bandwidth of each polarity was excellent, with a VSWR of less than 1.4 :
1 from 28 to 29.6 MHz, with only a single parasitic reflector.
2.3
Balance
With
10 watts drive into the transmitter input port, the branch line coupler’s
termination port received only _ watt power worst case, (28 MHz) across
the full bandwidth of 13-dB combined amplitude and phase balance. At 28.7
MHz (design frequency) to 30 MHz, the reflected power to this port was almost
zero. This should produce little or no skewing of the circular rotational
shape in the AM portion of the band. I use a pair of identical VSWR meters
(old Lafayette type) to each of the branch line outputs to monitor each
feedline during circular mode. A cheap power meter in the termination line
gives a good overall indicator of total balance of the system. |