From: The Crawford Hill
P.O. Box: 65 Colts Neck, N.J 07722
Date : April 1980
Subject : An Off-Set Fed Parabolic Reflector Antenna for 1296 Mhz
The parabolic reflector antenna design presented in Technical Report
#5 was a standard front fed 'dish' type antenna.
The latter is a primary disadvantage, not only because it is physically
difficult to
reach the feed but for EME service both the receiver low noise preamp and
the last
stage(s) of the power amplifier should be located at the feed to minimize
feedline
losses which can be very significant at 1296 MHz.
While all these effects are relatively small (in the order of l db typically )
in terms of the antenna gain, they are definitely detriments to th high
performence required in EME service.
This widely used antenna type provides high gain with good efficiency at
the UHF and
Microcwave frequencies. There are however several
disadvanges to
this standard design:
(a) The prime focus feed is directly in front of and in the center of the
electrical aperture.
(b) The feed must be supported by struts which are also in front of the apature.
(c) With the large reflectors required for EME service the feed is
difficult to
reach for adjustments while in service.
These modules, when mounted at the feed can incease the aperture blockage
and scattering
of energy from the center of the aperture where the energy density is maximum.
The implication of the above is that initial tune-up and subsequent
maintenance
and adjustments of the feed, preamp or PA to optimize the system in service
will place
the person doing the work also in the area of maximum energy. This results in exposure
to high RF fields on transmit and further blockage to the aperture area
for either transmit or receive. In short, "hot" adjustments of the feed
will be hazardous and difficult to perform proporly. The implication of (a) and (b)
are decreased antenna gain by blockage and higher antenna noise by scattering.
It is the express purpose of this report to acquaint the EME enthusiast
with methods and techniques to obtain
Tbe term off-set refers to the section of the paraboloid
off-set from the azis of the paraboloid. The feed horn aperture
(phase center) remains at the focal point of the paraboloid, but the feed is
aimed so as to illuminate the off set section of the paraboloid.
In this report a design directly related to the high efficiency dual-mode
feed horn is chosen as a desirable and compact arrangement. The geometric
considerations presented later, are specifically for this particular
design.
With the off-set geometry the feed is at once out of the antenna
aperture, the feed supports need not be in the antenna aperture at all,
the feed is closer to the ground and most importantly, the feed may be loaded
with modules which may be adjusted without electrical intervention of the
person doing the adjustments. The importance of being able to get to the
feed and the associated modules for initial adjustments of focal position,
impedance matching and optimizing preamp noise performance as well as
maintenance cannot be stressed enough.
A
Elimination of reflections between feed and reflector is especially
important when using circularly polarization. In the front fed design
circularly polarized energy transmitted in one sense of rotation is
reflected back to the feed in the opposite sense.
This means that the transmit-receive port isolation is limited to
where
f is the focallength,
G is the feed absolute gain (typically G = 10 for the dual-mode
feed)
lamda is the free space wavelength.
A 20 ft diameter front end fed reflector with F/D = 0.6 will
have an isolation of about 26 dB at 1296 MHz. For 500 Watts at the feed
transmit port, this will result in 1.25 watts at the receive port!
In the off-set feed design the T-R isolation can be nearly perfect with any polarization.
It is characteristic of the off-set parabolic reflector assymetric geometry to
radiate cross polarized energy with linear polarization("") The cross
polarized radiation increases with shorter focal length and larger
off-set angle. Cross polarized radiation is found not on axis of the
main beam, but in the 45 degree planes close to the main beam.
This radiation is small but it is completely lost energy in terms of
antenna gain.
Another characteristic of the off-set reflector antenna is that with circularly
polarized feed energy, there results no cross polarized radiation but instead a
slight misalignment of the main beam.
(") Radiation Lab. Series, MIT, Vol 12, page 140.
("") T.S. Chu and R.H. Turrin, "Depolarization Properties of Off-set
Reflector Antennas".
IEEE , PGAF Vol AF-21 #3 May 1973, pp. 3339-345.
The main beam misalignment is in the horizontal (i.e transverse ) plane
and is to the left of the center for right circular polarization
for a transmitted wave receding from the antenna. The error in pointing
of the main beam from left to right circular polarization is only a fraction
of the bandwidth which results in a gain difference from left to right C.P.
of only a fraction of a dB. For example, a 20 ft ( 3 meter ) aperture offset reflector
antenna with offset angle of 45 degrees at 1296 MHz ( the design detailed in this report )
the pointing angle will change 0.75 degree from right to left CP while the main beamwidth
is in the order of 4 degrees. The resulting gain difference is about 0.1 dB.
It is desirable to employ circular polarization for the EME path to eliminate Faraday
rotation fading through the ionosphere. The offset reflector antenna is suited
to circular polarized applications. As mentioned previously the off-set feed virtually
eliminates the coupling between transmit and receive ports via relections
from the reflector surface back into the feed. It is now highly desirable to conciderthe circular
polarized feed methode.
This scheme takes advantage of the isolation between opposite sense circularly
polarized ports to eliminate the nedd for a T-R relay or switch, With this
methode if all EME stations transmit one sense CP and receive in the oppsite sense
then all will be compatible with each other and with their own echoes. The
obvious advantages of this scheme are elimination of Faraday fading, no polarization
tracking or searching, no T-R switch required in the antenna feed, complete
polarization compatibility and eliminating of cross polarized radiation which
result in maximizing gain.
Another advantage of the off-set geometry is for multi band operation where the feed package may be easily changed by virtue of its accessibility and non critical space requirement. That is the feed and modules may be packaged in almost any size and shape since there is no blockage problem. It is conceivable to mount two complete feed packages on swinging booms to facilitate band changing.
For strictly EME service with good foreground clearace in the direction of the moon orbit, the lower edge of the reflector may be mounted neat the ground and may also be the location of the elevation axis. In this arrangement the feed will be easily accesible even for moderate elevation angles. For a given aperture size and with an off-set angle of 45 degrees, the feed will be 16% closer to the reflector in the off-set design compared with the front fed design.
If there is any disadvange to the offset reflector antenna design it is the different and somewhat awkward geometry. The reminder of this paper will present geometric details and suggestions for the consruction of a specific parabolic reflector antenna design.
It is also a basic property of a parabolic reflector that the total
length of each reflected ray from the focal point to the aperture
plane is a constant length. This latter property means that energy
at a fixed frequency originating at the focal point will be distributed
over the aperture plane with uniform (constant) phase.
These properties cause the beam forming characteristics of a parabolic
reflector antenna much the same as a flashlight produces a beam of light
when the bulb filament is located at the focal point of the small
mirrored parabolic reflector in the flashlight.
In the design described in this report, we choose a region just of the
paraboloidal axis so that the feed antenna is outside of the region
(aperture area) of the reflected rays. The term off-set therefore
designates that the reflector surface region used is off-set from the
paraboloidal axis but the feed horn antenna is still located at the
focal point. The feed horn axis is however tilted to illuminate the
choosen ares of the reflector surface and the center of the feed
horn aperture (phase center) is located at the focal point of the
paraboloid.
Since all rays emanating from the focal point behave in the same
way, we may choose to direct energy from the focal point to any
region of the paraboloid which will then form a beam.
A cross s ectional drawing of this geometry is shown below for the
particular case of the dual mode feed. The dual mode feed is choosen
not only because of its high illumination efficiency as pointed out in
report #5, but because it will also result in the most compact
form of the off-set antenna design.
The constraint of feed illumination is the same as with the front
fed case, that is, that for maximum gain the -10 db contour of the
feed radiation pattern should fall at the edge of the reflector surface.
The spillover will be the same in both cases and the gain between the two
designs in terms of aperture efficiency will be the same.
The difference in effective gain between the two cases is due to blockage
by feed and struts which will be 0,5 db or more.
Since the feed radiation characteristic of the dual mode horn are circularly
symmetric, the -10 db radiation surface contour may be represented
by a cone whose apex is located at the focal point of the paraboloid and
whose half cone angle is 45 degrees. The geometry of the off-set fed
parabolic antenna may therefore be simply described as the intersection
of a cone and a paraboloid,
Figure l, shows various useful views of this geometry with simple
formulae to calculate all necessary dimensions and angles. The views shown
by the drawing are specifically for the dual mode feed with off-set angle of
45 degrees.
Several useful properties of this geometry which may be
readily proven are :
(I) The projected area of illumination of the reflector in the direction
of the main beam (parallel with axis of paraboloid) is a circular disc
(the aperture area).
(2) The edge or the rim of the illuminated area of the reflector is
elliptic and lies in a tilted plane. These characteristics make it easier
to visualize the physical consruction. Be assured therefore that even though
the off-set feed geometry is awkward its physical properties are described by
simple geometric figures.
In the aperture view, to the right in Figure l, the dotted radial lines
represent lines of the same parabolic shape. If a trussed rib type
construction is employed these trussed ribs can all be made on the same
template but of different lengths. All the ribs will converge at the vertex
V at the bottom of the aperture where a support hub may be used as the
main structural mount.
Figure 2 is a
sketch of a complete off-set fed reflector antenna and
mount. The mount consists of a well anchored vertical post which serves
as the azimuth pivot. It is located at the center of a circular track near
ground level upon which the mount will turn on wheels.
The mount frame should be triangular with running wheels near the
corners. This provides a simple three point support which will not teeter.
The circular track may be of poured concrete with footing suitable to the
local climate and just wide enough to support the total antenna load
(estimate 1000 pounds for a 20 ft reflector).
The wheels can be solid rubber type found in hardware stores as
replacements for garden equipment and need not be very large in
diameter. Several wheels may be stacked at each corner of the frame
for extra load bearing surface. Pneumatic type wheels might be satisfactory
if inflated to high pressure to prevent tilting of the mount frame with
unbalanced loads.
The central pivot should also provide for a trust bearing to hold the
frame down securely onto the track.
A track diameter of no less than 15 feet is recommended for this size antenna
(23 ft aperture).
The central pivot should be set plumb in concrete and the circular track
leveled carefully. Since the antenna structure will turn slowly around
the pivot, the actual bearing may be very crude.
Azimuth drive may he through friction drive directly to a support wheel.
If the drive motor is coupled through a worm gearbox, braking will be
automatic. Azimuth angular readout may also be taken conveniently from
an idler wheel which runs on the track.
By sizing the idler wheel diameter it may be used to drive a synchro
directly for fine readout and through a suitable gearbox for coarse
readout. A bench mark calibration of true North (or ary other reference)
will permit rapid check and realignment of the readout should it slip
or accumulate error. A readout accuracy of 0.5 to l.0 degree should
be achieved.
Once the aaimuth pivot, track and frame have been completed, construction
of the reflector frame with elevation pivots can be started. The elevation
pivots can also be relatively crude since rotation will always be slow.
The reflector frame should be constructed in a "stow" position, that is
with the the radio beam pointed straight up. This frame as well as the
aximuth triangular frame may be fabricated of relatively heavy material
to provide rigidly and strength for the actual reflector and feed support.
Details odf the reflector frame are shown approximately by Figure 2 with
additional stiffeners and supports added where necessary. The depth of the
frame from elevation pivot to reflector vertex need not to be more than
3 feet for this size antenna.
Since this design does NOT use counter weights to balance the elevation
forces, the structure is inherrently unbalanced except at one elevation
angle. For this reason the elevation drive suggested is a system of dual
lead screws or hydraulic jacks one adjacent to each pivot bearing as
shown by the sketch. Also, because the antenna is near the ground, it will
be difficult to track the Moon to rise and set times due to obstructions
in the radio beam. The range of elevation movement may therefore be limited
for your site thus minimizing the requirements of the drive system.
Auxiliary drives may be required to "stow" the antenna.
Construction of the reflector should begin with fabrication
of an accurate reverse template and pivot assembly which will be the
guide for placement of the trussed ribs and as final surface alignment
tool. The template pivot should be secured to the reflector frame
at exactly the vertex of the paraboloid. Actual construction of the
trussed ribs and supports is left to the individual's imagination.
Most of the support will be near the vertex and towards the center
of the reflector. The rim need not provide much support in this
design. Reflector construction using the materials and techniques
described in an article by VK3ATN in HAM Radio Magazine, p 12,
May 74 is recommended.
The feed support tower is added last and may be partially secured to
the reflector by a few thin steel wires for added rigidity.
Surface material for the reflector should be chosen according to the
highest frequency of operation. As a reminder, the surface material
hole size should not exceed 0.l wavelength at the highest
frequeney. The material need not be bonded together electrically
at overlapping joints, provided no joints are closer together than
a wavelength at the lowest operating frequency and the overlaps are
at least a quarter wavelength at the lowest operating frequency.
Deviation of the surface from a true paraboloid should not exceed
0.05 wavelength at the highest frequency especially towards the
center area of the reflector where the energy density is highest.
If the rib construction is not too accurate, trimming up the surface
is highly recommended.
Refer to Figure 1 for more details of the geometry and
placement of the feed. The circular rings around the vertex shown
in Figure 1 are added on top of the ribs and are the actual surface
material support.These circular arcs may be made of lighter material
an placed about a foot apart.
In the stow position, the reflector frame should come to rest
and should be secured to the azimuth frame. This position provides the
least wind loading.
A project of this magnitude should be given careful consideration
especially with regard to expense, construction, time site location and
family-neighborhood complications. In fact if your site is not acceptable
it would probably be advisable to join forces with a nearby EME enthusiast
who has an acceptable site. Remote control operation might be a solution for
joint operation.
Since a reflector antenna may be used over a wide frequency range by
changing feeds, it would be highly desirable to make the reflector
surface as accurate as possible in anticipation of use above 432 MHz,
possibly 1296, 2300 and 4000 MHz the satellite TV band.
