AN EXPERIMENTAL PATCH ANTENNA - Part
to the encouraging results in building a single patch for 70 cm, I
decided to design and build an array of patches specifically for the
70 cm uplink for AO40 and AO10 and for use with other satellites
such as FO20, FO29 and UO14 for the downlink.
decided initially to build an array of 4 patches.
first design consideration was to optimise the spacing of the
patches to achieve the maximum gain achievable of 6dB. This figure
is the maximum gain that can be expected by increasing the number of
driven elements in an array by 4. This would result in the array
having a gain of around 15 dBic. To achieve this gain, the apertures
of each element must not overlap and ideally should just touch. Also
all elements need to be fed in phase with equal power division.
aperture or effective area of an antenna can be calculated form
either the gain of the antenna or by measuring the beamwidth of the
antenna. The assumed gain of 9dBi was used and this resulted in the
minimum spacing of 0.82 Wavelengths. Using the measured H-Plane
Beamwidth of 70 degrees resulted in a minimum spacing of 0.84
Wavelengths. Therefore to achieve a 6 dB increase in gain over a
single element, the minimum spacing should be a minimum of about
0.85 Wavelengths. Mutual coupling is not a problem with patch
elements at this spacing as the isolation is in the order of -30.0
next consideration was to design a power division system to provide
equal power and phase to each element. As lead length is quite
critical at 70 cm and the fact that the final array would be mounted
on a single sheet of aluminium, I decided to use micro strip (or
strip line) power division as this could be easily constructed on
the underside of the ground plane. Micro strip is simply a flat
conductor mounted close to a ground plane. It is an unbalanced
transmission line where the width of the strip and the spacing of
the strip above the ground plane determine the characteristic
the elements must be spaced more than 0.5 wavelengths apart, quarter
wave matching was going top be awkward. However if the elements were
spaced 1 Wavelength apart centre to centre, cascaded quarter wave
transformers could be used or better still, 0.5 Wavelength
exponential tapered transformers (this arrangement has considerably
wide bandwidth compared to a single quarter wave transformer).
the final initial design was an array of 4 patches mounted in a
square formation, with element spacing of 1 Wavelength. The screen
size was chosen to be 1200*1200 mm, a little larger than necessary,
however as standard aluminium sheets are 1200*2400mm this was a
convenient size. For rigidity and for mounting, the screen was
reinforced with a frame 1200*1200 mm made of 25 mm square aluminium
tube. A brace across the middle of the screen was decided to ensure
aluminium sheet didn't wobble in the middle.
0.5 Wavelength exponential transformers are used a follow
pair of patches is connected using 2 transformers, transforming 50
Ohms to 100 Ohms. At the junction of the transformers is therefore
50 Ohms (100 ohms in parallel with 100 Ohms). The junctions are then
connected with the transformers providing the same transformation
again except an N type connector is connected at the junction for
connection to the 50 Ohm feed cable.
strip line transformers used were designed using a program called 'TXLINE' which is a free program available and calculates the
characteristic impedance of a number of different types of
transmission line. I did have a fairly accurate formula the
calculated characteristic impedance of a strip line, however I used
this as a check on the final dimensions. The program “TXLINE” is
quite accurate and accounts for the thickness of the strip used and
the 'fringing' effect on the characteristic impedance.
I was fairly optimistic about the success of this project, I decided
to take a bit of time to plan and construct the array.
first thing was to build the screen and frame. The screen was made
of 2 mm thick 1200*1200 aluminium sheet pop riveted (Using stainless
steel pop rivets) to the aluminium frame made of 25 mm square
aluminium tube. Plastic right angle connectors used to construct the
frame. The brace used was also 25 mm square aluminium tube mounted
across the middle of the front of the screen leaving the rear free
to mount the strip line transformers. This was pop riveted from the
rear. A bead of natural curing Silastic was also used as an adhesive
and also a sealer.
patches were made of 2.0 mm aluminium sheet. The mechanical mounting
was identical to the prototype patch using nylon nuts and bolts at
the corners and a stainless bolt to earth the centre. The patch
height was made 12mm. Electrical connection was made using small
countersunk brass bolts, which were inserted, through a countersunk
hole at the patch feed point, and bolted from the top of the patch.
The inner of the phasing cable was soldered on to the head of the
bolt. The phasing cable used was RG58U as this was the best
available at the time. As the phasing cables are quite short, loss
is not a problem. However it would be preferable to use RG223 which
is a much better quality cable as the characteristic impedance is
much more accurate, which is desirable for a phasing cable.
outer of the phasing cable was soldered to an 'F' type
female-to-female connector, which has the short end cut off, the
inside removed, forming a sleeve, which the coax can be fed through.
The modified 'F' type connector can then be used as a feed
through for the phasing cable, earthing the outer to the ground
plane. A 3/8-inch hole is drilled through the ground plane to mount
the modified 'F' type connector. This arrangement allows a very
short connection to the underside of the patch.
feed connection to each patch also uses a modified 'F' type
connector as a sleeve. Using a piece of the centre conductor of
Belden 9914 cable fed through the centre of the sleeve and air
spaced, forms a short coaxial line with an impedance very close to
is soldered to one of the brass bolts for connection to the patch.
The other end is soldered to the end of the strip line transformer.
strip line transformers are mounted a distance of 5.0 mm from the
ground plane. Small 5.0 mm spacers made from polystyrene foam and
are glued to the strip line and ground plane using natural curing
Silastic. The dielectric constant of polystyrene foam is almost 1
and it has very low loss.
the first patch was tuned to be resonant at 436 MHz. Once the
dimensions were established, the other 3 patches were cut to size.
week later (after a couple of very late nights constructing) the
array was complete.
spending so much time on this project, I must admit I was a bit
apprehensive about connecting the array to the IC471A via the Bird
Wattmeter, as this would be the moment of truth. No problem, to my
surprise the VSWR at 436 MHz was 1.2: 1 and no reflected power was
measured at 440 MHz. The real test would be the AO40 uplink.
couple of days later, with a favourable pass of AO40 over Europe, I
tried the array out at an elevation angle of around 20 degrees (with
nowhere to mount the array, I could only rest the array propped up
on a seat on the front veranda).
about 25 watts output (18 watts at the antenna), transmitting CW
resulted in a return carrier from AO40 which was about half an S
point below the level of the beacon, a sensational result. Signal
reports on sideband varied from 5*6 to 5*9. In a QSO with Dom, I8CVS
in Italy, he complained that my signal was stronger that his and he
was using quad 22 element yagis on the 1,296 MHz uplink!
then, I have had quite a few QSO's on AO40 with reports of an
exceptionally strong uplink signal.
the problem that AO40 had just before December 2001, which resulted
in the satellite pointing in the wrong direction near and at apogee,
I decided to built one single 2 metre patch antenna and mount this
with the 70 cm array on the same boom, to try out FO20, FO29, U014
and AO10. Excellent results with all these although there is very
little activity on FO20 and FO29, and, AO10 is very unpredictable
because it has no spin stabilisation and it is usually pot luck if
its antennas are pointing in the right direction (not to mention it
must be in sunlight to work as the batteries went open circuit some
years ago so all power is derived directly from the solar cells.)
the construction of the array I done further extensive reading on
patch antennas, and gained a much better appreciation of how they
work and a reasonable understanding of the design process.
am currently working on single feed, circular polarised patch design
which should simplify the construction and make the construction of
higher gain arrays more achievable. Initially I am designing a
smaller, simpler 4-patch array, which should have the same
performance as the prototype array, which can be more easily
having done a reasonable amount of initial work with patch antennas,
I would conclude that patch antenna arrays are one of the most
versatile and exciting antenna systems I have studied. Although the
technology was developed more than 20 years ago, application has
been mainly for commercial antenna systems in the microwave
spectrum. I have not seen any development of these antennas for use
in the UHF area (Except for the antennas on AO40) where I believe
there is huge potential. Although I have not seen any reference to
patch antennas in any of the well-known amateur radio handbooks, I
believe this is one of the few technologies left that has not been
fully explored and developed for use at UHF.
are a significant number of features of these antenna systems which
allow the realisation of an antenna system with a level of
performance that is difficult to achieve using the more traditional
technology of yagi and reflector arrays.
plan to write another follow up article with a bit more technical
detail than is included in this article. This will deal with further
developments, a list of features and applications of these antennas
that distinguish them from most other antenna systems and an update
on the design of single feed, circular polarised patches.
antenna article Part#1 Patch