An Experimental Patch Antenna for 70 cm - By Greg Chenco VK3BLG 

Preparatory material for an article published Amateur Radio (Wireless Institute of Australia's national magazine).

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AN EXPERIMENTAL PATCH ANTENNA - Part 2

Further 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.

I decided initially to build an array of 4 patches.

The 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.

The 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 db.

The 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 impedance.

As 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).

So 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.

Six 0.5 Wavelength exponential transformers are used a follow

Each 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.

The 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.

CONSTRUCTION 

As I was fairly optimistic about the success of this project, I decided to take a bit of time to plan and construct the array.

The 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.

The 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.

The 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.

The 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 50 Ohms.

This 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.

The 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.

Initially the first patch was tuned to be resonant at 436 MHz. Once the dimensions were established, the other 3 patches were cut to size.

A week later (after a couple of very late nights constructing) the array was complete.

TESTING

After 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.

A 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).

With 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!

Since then, I have had quite a few QSO's on AO40 with reports of an exceptionally strong uplink signal.

With 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.)

Since 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.

I 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 replicated.

SUMMARY

After 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.

There 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.

I 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.

Also see patch antenna article Part#1 Patch Antenna Pt1  

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Page last revised 05 May, 2025