The Polaplexer Revisited
By Ed Munn, W6OYJ
Introduction
The Polaplexer is a microwave antenna, or antenna feed, which supports two
simultaneous
inputs or outputs that are independent and isolated from each other by use
of
orthogonal (at right angles) linear polarization. The isolation can be as much as
30 -
35 dB. Its main use is to permit full duplex operation with a single antenna,
in which one
port is used to transmit and the other to simultaneously receive.
As long as the transmit
power input to the Polaplexer is limited to 100 milliwatts
or so, the receiver input stages
are unlikely to be damaged by the transmitter's
continuous output.
Polaplexer is a coined word describing a polarization-based diplexer first
developed
in the late 1940s by the late Donavan Thompson, W6IFE. In the 1950s through
70s
it was used it on the 2300, 3300, 5600, and 10,000 MHz amateur bands with reflex
klystron and lighthouse triode oscillators as the transmitter signal sources
and with crystal
diode mixers as the receiver input stages. The Polaplexer, having
only a few dB gain as
an antenna itself, was primarily used as a feed for a parabolic
dish or conical horn
antenna with much higher gain. In more recent times, the
development of ferrite
circulators provided a way to separate receiver and transmitter
signals from a single
antenna using a single linear polarization.
Today, Polaplexers still offer a good approach for microwavers who want to
construct
simple low-power full-duplex transceivers. They are easy to build and low-loss
Polaplexer
materials can be almost free, such as beer cans, coffee cans, etc. More on that
later.
In the simplest type of full duplex system the transmitter's carrier signal is
also
used to provide the receiver's local oscillator (LO) signal. By doing so,
only one
microwave oscillator is required for each transceiver. The distant station
transmits with a
frequency offset equal to the i.f. frequency used by both stations.
When either carrier
frequency is modulated, both receivers will detect the modulation
and the audio will be
heard at both ends, provided the stations are within range
of each other and properly
tuned. The Polaplexers also have to be properly aligned
so that each station's outgoing
transmit polarization matches the receiver polarization
at the distant station. This is
automatically accomplished when each station
uses an outgoing transmit polarization
rotated 45 degrees clockwise from vertical,
as viewed looking toward the distant station.
Polaplexer Theory
The Polaplexer is made from a short section of circular waveguide, closed
(shorted)
at one end, and with input/output ports installed at distances of 1/4 guide
wavelength,
and 3/4 guide wavelengths from the shorted end. The optimum overall
length should
be about 5/4 guide wavelengths. See the attached drawing. At these
distances from the shorted end, a high impedance exists. Each port consists of
a
connector, typically type N or SMA, with a quarter-wave linear probe attached.
The two
probes must be installed so that they are aligned 90 degrees (orthogonal)
from each other
as viewed from the open end. This permits two independent linear
polarized waves to
coexist within the waveguide.
This article describes a Polaplexer design for a joint project with N6IZW and
WB6IGP to build a simple 2.3/2.4 GHz transceiver. The first decision made was
the
operating frequency (FO) for the design. Band edges, frequency stability
of the
oscillators, and choice of i.f. frequency drove this decision. For this
project the bottom
and top band edges are 2300 and 2450 MHz, we will be using
precision frequency
control, and we selected an i.f. frequency of 146.00 MHz.
We can just fit two
frequencies 146 MHz apart inside the band edges at 2302 and
2448 MHz. For the design
of the Polaplexer we can split the difference and use
the mid-frequency. A more precise
method is to take the square root of the product
of the two frequencies. In this case the
mid-frequency is 2375 MHz, and the square
root method gives 2373.9 MHz. I will use
2375 MHz as the design frequency (FO)
in the following example.
Waveguide Propagation Modes and Guide Wavelength
Unlike coaxial cable, waveguides operate over limited frequency ranges
determined
by their dimensions. They have much lower losses than coax at microwave
frequencies.
For propagation to take place in waveguides, the configuration of electric
and
magnetic fields of the wave must satisfy certain boundary conditions. There are
several
possible configurations, called modes. Propagation can occur with very low loss
provided
that the operating wavelength is shorter than a critical value for the mode being
used. This is called the cutoff wavelength, and if the wavelength is longer,
or the
corresponding operating frequency is lower than the cutoff, extremely
high losses will
occur.
For each mode, there is a different cutoff wavelength or frequency, determined,
in
circular waveguide, only by the inside diameter of the guide. The lowest frequency
mode
that will propagate in the waveguide is the Transverse Electric 1,1 (TE11)
mode, and is
called the dominant mode. That is the mode we will use.
Here are the circular waveguide relationships for cutoff wavelength (Wx) and
cutoff
frequency (Fx). Different cutoff coefficients apply if rectangular guides are used.
( W in cm, F in MHz, Diameter D in cm. ) Mode Cutoff wavelength Cutoff Frequency --------------- ------------------ ----------------- TE11 (Dominant) W1 = 1.71*D F1=30000/(1.71*D) TM01 W2 = 1.31*D F2=30000/(1.31*D) TE21 W3 = 1.03*D F3=30000/(1.03*D)
It is best to choose a waveguide diameter so that the desired operating frequency
falls between the TE11 and TM01 mode cutoff frequencies. This will absolutely
prevent
the possibility of multiple moding within the guide, which could produce
SWR problems.
For the polaplexer port design described in this article, the chance
of initiating the TM01
mode is small, and I decided to push the design above
the TM01 cutoff frequency in
order to use an easily available waveguide size.
However, the operating frequencies must
be always kept below the cutoff frequency
of the TE21 mode because the type of
monopole probes used will certainly excite
the higher E-field modes.
For a particular frequency and propagation mode in waveguides, the wavelength
will differ from that in free space, often by a considerable amount. In measuring
where
to install the ports or terminations along the length of a waveguide, you
must first
calculate and use this "Guide Wavelength". Here is the formula
for Guide Wavelength:
WO=30000/FO (FO in MHz)
WO WO=design wavelength in cm
WG= ---------------------- WG=guide wavelength in cm
SQR(1-((WO/W1)^2)) W1=TE11 cutoff w.l. in cm
At this point I suggest you do some calculations to determine the approximate
range of diameters you can use for your chosen operating frequency. It will be
bounded
at the outside limits by the TE11 and TE21 cutoff frequency limits. One
way to get
started is to calculate the diameter of guide that produces a TE11
cutoff wavelength about
fifteen percent longer than that of the lowest operating
frequency you will use. This
leeway is my empirical way to avoid unreasonably
long guide wavelengths. With this
approach you calculate the smallest diameter
you might use.
Example: FL =0.85*2302 = 1957 MHz
DL=30000/(1.71*FL) =30000/3346 =8.96 cm or 3.53 in.
Now calculate the diameter that will give a TE21 cutoff wavelength about five
percent higher than your highest operating frequency. This will give you the
largest
diameter you should use.
Example: FH =1.05*2448 = 2570 MHz
DH=30000/(1.03*2570) =30000/2647 =11.33 cm or 4.46 in.
Probe Length vs. Diameter
You probably know that the driven element of simple monopole (ground plane
vertical)
antennas is usually a little shorter than a calculated free-space quarter
wavelength.
In fact, the thickness or circumference of the element has a bearing on both
the
resonant frequency and the bandwidth of such an antenna. The fatter the probe, the
shorter its length and the broader its bandwidth. The two probes inside a Polaplexer
essentially perform the same function as a monopole antenna and their length
must be
shortened accordingly. George Tillitson, K6MBL, described an empirically
based
formula for probe length versus thickness in a March 1977 Polaplexer article
in "Ham
Radio Magazine". Here it is:
For FO=operating frequency in MHz, WO=Operating wavelength in cm
and PM=Probe diameter in mm
P=0.31416*PM/WO (ratio of probe circum. to wavelength)
L=2950.7*(1+P-SQR(P))/FO Probe length in inches. Or L=74948*(1+P-SQR(P))/FO Probe length in mm.
The LO insertion adjustment screw
In the simple full duplex transceiver, we need to be able to upset the isolation
between the transmit and receive polarization modes, so that some of the transmitter
signal will be coupled to the receiver port to provide adequate local oscillator
injection
for the mixer. This can be accomplished by inserting an adjustable
length conductive
post (a screw) on a line midway between the two polarization
planes. A good location for
this is to position it one-third guide wavelength
from the shorted end of the Polaplexer.
With this article I have included a pair of software design programs for
Polaplexers
written in BASIC. One is for inch dimensions, and the other is for metric.
From
the web page you can save these programs as "plain text" files with your
browser.
They should run when loaded with a ".bas" extension in QBasic
or in the earlier DOS-
based GWBasic or IBM Basica. With this software, try some
waveguide diameters
between the limits you calculated above, using your chosen
design frequency and see
what the Polaplexer length dimensions and cutoff frequencies
will be. This will prepare
you for your search for circular waveguide material.
Going to the Supermarket ??
Tin cans are the favorite material to use for Polaplexers (actually they are steel)
because it is easy to directly solder the probe connectors to the can, or solder
on an extra
bottomless can when you need to reach an optimum length. Unfortunately
they are
harder to find these days, aluminum being much more common. When exploring
the
aisles with your tape measure you should also check the rims to see if they
are shiny
(steel) or dull (aluminum). I prefer unpainted cans with paper labels
that can be removed
to allow easier soldering. Another thing to observe is the
relative number and depth of
the strengthening ridges used crosswise to the length
of the can. If they are too deep it's
harder to flatten them out in the place
where you want to install a connector. If the can
has a paper label, you can
run your fingernail along the paper to judge these ridges. This
search process
may require several trips to the grocery store, and your spouse or friends
will
probably not want to be seen with you during these events.
If tin cans are not classy enough for your application, you can of course use
other
cylindrical materials. These can range from sections of copper or brass
pipe or tubing, to
rolled sheet metal for the larger diameters. The plumbing
or hardware store may have
what you want.
Constructing the Polaplexer
I decided to use a "one pound" coffee can as the basis for the 2.3/2.4
GHz
transceiver project, based on knowledge that others had used this type of
can for this
same band. They measure 3.875 inches inside diameter. I ran the
software program
"POLPLXIN.bas" for that diameter, and a design frequency
of 2375 MHz. Here are the
results:
TE11 Mode cutoff frequency = 1782 MHz TM01 Mode cutoff frequency = 2327 MHz TE21 Mode cutoff frequency = 2959 MHz Guide Wavelength (TE11) = 7.53 inches 1st probe spacing = 1.88 inches 2nd probe spacing = 5.64 inches LO adjust screw spacing = 2.51 inches Overall Length = 9.4 inches Probe length (for # 18 wire) = 1.07 inches
At the grocery story I found a standard coffee can and a shorter gourmet coffee
can, both "tin" with shiny steel rims. The combined length was about
9 inches. Both had
paper labels. I saved the coffee in plastic containers, soaked
the labels off, and cut out
the bottom of the short can. With a heavy duty Weller
soldering gun, I carefully aligned
the cans and tacked them together in four
places, rim-to-rim, before soldering all around
the rims. To check the joint,
I doused the room lights and moved a flashlight beam
around the seam from the
outside as I peered into the open end; missed spots were
obvious, marked, and
corrected.
I then scribed two perpendicular lines across the bottom center of the assembly,
and extended these orthogonal lines along the sides of the cans to indicate where
the
probes would be placed. Because the closed (shorted) end of the can is inset
slightly from
the rim itself, I used a steel rule to measure the inside depth
of the can from the open end.
I re-calculated the depth for the probe locations
to correct for this inset and marked them
on the orthogonal lines.
I prepared two SMA connectors by cutting and soldering probes to them, made of
# 18 wire. Then I drilled the mounting holes in the can and flowed some solder
around
the edges before soldering the connector/probe assemblies to the Polaplexer.
Last, I
drilled a hole slightly smaller than the size of the LO adjustment screw,
and flowed some
solder around the edges of that hole. I ran a brass nut part
way up a 2-inch brass screw
and force-started the screw into the hole. Then I
tightened the nut down to press against
the can and flowed solder around its
edges to permanently affix it to the can. This
concluded the construction of
the Polaplexer. It can be easily mounted to a supporting
bracket or strut by
using a large metal hose clamp.
See photos of the completed
polaplexer.
Test Results
With a 25 milliwatt source and a power meter, I measured the isolation between
ports at just over 30 dB at both 2302 and 2448 MHz. When two of these Polaplexers
were used as antennas with 25 milliwatt nbfm transceivers, I measured over 30
dB signal
margin on an 18 mile clear path before full quieting began to fall
off.
Acknowledgments
In 1955 I built my first beer can Polaplexer for 3335 MHz, under the watchful
eye
of Donavan Thompson, W6IFE. He and Bill Baird, W6VIX are both silent keys
now, but
the San Bernardino Microwave Society which they founded that year has
encouraged
many of us in a lifelong enchantment with amateur microwave communications
and
experimentation. In San Diego, Kerry Banke, N6IZW and Chuck Houghton, WB6IGP
have played a similar role in recent years. With their concept of a simple 2.3/2.4
GHz
transceiver, they stirred me into once again into the world of Polaplexers.
It's been fun!
References
1. "A Radio Club for Microwave Enthusiasts", W. H. Baird, W6VIX, QST,
Dec 1957.
2. "Let's Go Microwave", A. D. Bredon, W6BGK, QST, Jun 1958.
3.
"Standards for Amateur Microwave Communications", Richard Kolbly, K6HIJ,
HAM RADIO, Sep 1969.
4. "A Low Cost Amateur Microwave Antenna",
Richard Kolbly, K6HIJ, HAM RADIO,
Nov 1969.
5. "Microwave DX — California
Style", Richard Kolbly, K6HIJ and Ed Munn,
W6OYJ, QST, Sep 1970.
6. "The
Polarization Diplexer — a Polaplexer", George F. Tillitson, K6MBL, HAM
RADIO,
Mar 1977.
Submitted by Ed Munn, W6OYJ, 6255 Radcliffe Drive, San Diego CA 92122. Phone
(858) 453-4563
or email to : edmunn@compuserve.com
Last updated Oct 11,
1999