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 1

Background:

Twelve months ago, after about an 18-year absence from amateur radio, I decided to fire up my old IC22A on 2 metres and build some antennas. I was fortunate enough to be introduced to the 2100 net by Ken, VK3HKR and I found that the net was a very interesting forum which discussed anything from painting to heavy technical discussions on how a discone worked.

One of the regular topics of discussion was working satellites. I realised at this point if I was to be able to join in with these activities, I needed to upgrade my equipment. So I lashed out and bought second-hand IC271A and IC471A all mode transceivers.

After building up various antennas and achieving some limited success working UO14, FO20 and FO29 I came to the conclusion that to operate with these birds, especially FO20 and FO29, you really needed some sort of tracking equipment. As my Emotator Rotator was in bits (Having pulled it apart 18 years ago to replace at pot) and my also my QTH being a rented property, I didn't think that the landlord would appreciate the erection of a tower in the backyard!

In one of the 2100 net technical discussions one night, Robert VK3KRB brought up the topic of 'patch' antennas and how patch arrays could be electronically steered. The topic of 'patch' antennas generally drew a blank from every body else, including myself and we all decided that we didn't understand how they worked and these antennas were really used at microwave frequencies and didn’t really have any application for VHF and UHF,

I had heard that there were a couple of high orbiting satellites namely AO10 and

AO40. The big advantage with these satellites is that their footprint at times covers half the world and the prospect that you could work the States and Europe on VHF/UHF was quite exciting. The other advantage was these satellites moved relatively slowly compared to the low orbiting satellites which meant you could point your antennas at them and be able to leave them there for a considerable period of time without the need to track them across the sky.

I then decided to concentrate on AO40 and managed to convert a 2.1 GHz down converter to 2.4 GHz, down converting the downlink on AO40 to 2 Metres. After a lot of persistence, late one Saturday night using this converter with its grid reflector, I managed to hear the middle beacon of AO 40. It was only one S point above the noise. I could also hear some weak sideband signals and some Morse code. In the next few days I managed to transmit into AO 40 on 70 cm and heard myself and couldn't believe the enormous time delay which seemed to be at least 1 second.

My first contact on AO 40 was to Charlie, VR2XMT in Hong Kong and second was to Scott, NX7U in the States. After emailing Scott, and having given him a signal report of 1*1, his report on my signal was 5*6. At this point I realised that my downlink was not working well.

When I was over at Peters QTH, VK3DI, I spotted a large 1.5 m microwave dish lying outside in his back yard, which Peter kindly loaned me. I worked out that if I used this dish for the downlink, the additional gain taking into account circular polarisation, I should achieve an additional 10 db.

After measuring up the dish to ascertain the focal length etc, I calculated that I would need a 1.86 turn helical to optimally illuminate this dish. The problem with this is that a 1.86 turn helical would not perform as calculated as the general formulas for helical antennas were only applicable for a helical of a number of turns (probably 3 to 4 turns minimum). Any way I built a 2 turn helical and this immediately lifted the AO 40 beacon from 1 S point above the noise to 3 to 4 S points above the noise. I could also hear a dramatic increase in the readability of other stations.

Very pleased with this I managed to work a number European stations and even the UK.

I was not convinced that the helical was illuminating the dish as I thought I wasn’t achieving a 10-db increase over the grid antenna.

After looking at the web for some clues, I found a commercially advertised 2.4 GHz dish which had a, you guessed it, a “patch” feed. The radiation pattern was ideal and the axial ratio, which is a measure of the antennas circular polarisation, was near perfect. (A 2 turn helical does not have a good axial ratio).

The “patch” feed had come back to haunt me.

After referring the matter to the 2100 net technical forum, Robert, VK3KRB emailed me some URLs on articles on “patch” feeds. One of the articles was a very detailed description of all the antennas on AO 40 written by the people who built the bird. Low and behold, the antenna used for 70cm was an array of 6 'patch' antennas. Whilst it didn’t give a technical description of how they worked, there was very detailed information on dimensions, feed points, impedances and how to connect to create circular polarisation.

The only technical description referred to a patch antenna being equivalent to a couple of slot antennas. I thought it was time to re-familarise myself with the principles of a slot antenna, so I dug out and old textbook, which dealt with slot antennas and after skimming over a lot of maths the final analysis, is relatively simple.

A slot antenna is simply a slot cut in a large sheet of metal. However if you were to make a dipole out of the piece of metal, which was cut out to create the slot, there is an interesting relationship between that dipole and the slot created.

The two antennas are complimentary and almost exact duels, in other words everything is opposite i.e.

1.      The centre feed point impedance of a dipole is minimum at the centre corresponding to a current maximum and impedance maximum at the ends corresponding to a voltage maximum.

2.      The centre feed point impedance of a slot is a maximum across the centre of the slot corresponding to a voltage maximum, and impedance minimum at the ends corresponding to a current maximum.

3.      The impedance of a centre fed resonant dipole in free space varies from about 60 to 70 ohms (depending how fat it is) and the impedance of a centre fed resonant slot varies from about 590 to 510 ohms (depending on how wide the slot is). The fatter the dipole the lower the impedance, the wider the slot the higher the impedance.

4.      A horizontal dipole creates horizontal polarisation whereas a horizontal slot creates vertical polarisation.

But how does this relate to the 'patch' antenna?

Whilst having to do further reading on the subject and not knowing whether this is strictly correct, the analogy I see is that a “patch” antenna is the unbalanced form of slot antenna as the quarter wave ground plane antenna is an unbalanced form of a dipole.

The “patch” antenna has 2 vertical slots and two horizontal slots created by the space between the edges of the patch and the ground plane. The horizontal slots radiate vertical polarisation and the vertical slots radiate horizontal polarisation. The patch is grounded in the centre so the feed point impedance varies from 0 Ohms at the centre to somewhere near 300 Ohms at the centre of the patch edges (assuming my analogy is correct). So the “patch” antenna is like 2 separate antenna sets in one (although I am not sure whether you could feed each antenna set with a different transmitter)

Having both polarisations available lends this antenna to creating either right or left hand circular polarisation by connecting the horizontal and vertical slots together with a quarter wavelength of coaxial cable to provide the 90 degrees phase shift. This is similar to how crossed yagis are connected to create circular polarisation.

Construction:

Initially I was ready to built one of these for 2.4 GHz using the dimensions quoted however after further discussions with the 2100 net, Dave, VK3AAD suggested I build one for 70 cm to make sure it works before building the 2.4GHz version

It is strongly that prior to building on of these antennas you:

1.      Read the article on the AO40 70 cm 'patch' antennas http://www.amsat.org/amsat/sats/phase3d/antennas.html

2.      Adhere closely to the dimensions given.  

The ground plane I used in the prototype was a piece of aluminium sheet 600 * 600 mm. The size of this is not critical and this can be reduced somewhat. I believe that the ground plane should be at least 100 mm larger than the patch (providing 50 mm extension of ground plane on each side of the patch)

The patch is made of 1 mm brass sheet and measures 315*315 mm. I decided to use brass sheet as this would make it easy to solder a connection to (There is no reason why you couldn’t use aluminium sheet). This was found to resonate at 438 MHz. This is slightly smaller than quoted in the Amsat article as I found that the original size of 0.47 wavelengths (325*325mm) resonated somewhere below 430 MHz.

The centre of the patch is grounded and spaced from the ground plane a distance of 12mm, by using a 3/16 stainless steel bolt with two stainless steel locknuts used as spacers. The patch is supported at the corners at a height of 12 mm using ¼ inch nylon bolts with two nuts used as spacers. The support points in my prototype are 40 mm from each corner along the diagonals of the patch.

Feeding the “patch” antenna is achieved by running the coax from behind the antenna, through a hole in the ground plane, connecting the outer to the ground plane and the inner to the feed point on the patch. As is the case generally when working with UHF, all lead lengths should be kept to a minimum. The outer braid of the coax should be continued through the ground plane and cut just short of the connection to the patch.

The 50-Ohm feed point is located at 0.078 wavelengths from the centre of the patch along the line joining the centre of the patch and the middle of one of the patch edges. Feeding the patch at this point will excite the slot closest to the feed point and the other parallel slot.

Connection to the other pair of slots is done in exactly the same manner.

Circular Polarisation:

To create circular polarisation, with a linear antenna array we require the following:

1.      Two identical antennas polarised at 90 degrees to each other.

2.      A phase shift in the feed to the antennas of 90 degrees so that one of the antennas is either leading or lagging the other    by 90 degrees depending on whether you want Left Hand (LHCP) or Right Hand Circular Polarisation (RHCP).  

The concept of creating circular polarisation with 2 linear antennas can be quite confusing especially when it comes to determining the which connection creates RHCP as opposed to LHPC.

There are a number of important points regarding circular polarisation:

1.      The direction of rotation of circular polarisation is the direction of rotation of the electric field vector in the direction of propagation.

2.      The polarisation rotation is as viewed from behind the antenna.

3.      RHCP is the most the common.

4.      In the case of an axial mode helical, the direction of rotation is the same as the winding direction.

5.      When using linear antennas to create circular polarisation, impedance matching is vital. A quarter wave piece of coax is a 'dangerous' length to work with because any mismatch here will produce impedance transformation leading unequal power being delivered each linear antenna.

6.      For crossed yagis (one at 90 degrees to the other) feed the 90 degree one first, with a quarter wave connection to the 0 degree one for RHCP. Vice versa for LHCP.

7.      For 'patch' antennas feed the horizontal slots first, with a quarter wave connection to 90-degree connection to the vertical slots for RHCP. Vice versa for LHCP.

8.      When the antenna is driving a dish the polarisation is reversed i.e. for RHCP drive the dish with LHCP.

9.      Another simple way to change from RHCP to LHCP is to add a half wavelength of coax to the existing quarter wavelength of coax.

10. All lengths are electrical lengths and the velocity factor of the coax must be taken into account together with the electrical lengths of connectors etc.  

Because the patch antenna can be tapped anywhere between the centre and the outside edge, the 50 Ohm points on the vertical and horizontal slots can be joined by quarter wave length of 50 ohm coax. This provides the 90-degree phase shift for circular polarisation.

By paralleling the 50-ohm points, the impedance at those points now becomes 25 ohms. By now tapping the 100-ohm point, the impedance at this point now becomes 50 ohms after connecting the 50-ohm points with the quarter wavelength of 50-ohm coax. The 100-ohm point is found at 0.115 wavelengths from the centre.

In my prototype, I actually used N type connectors to tap the 50 and 100-ohm points so that I could do various measurements. As a result, when taking into account the electrical length of the connectors and connection to the feed point, I had to add either a half or full wavelength of coaxial cable to the quarter wavelength because the resultant piece of coax to make up the electrical difference was too short and would not reach the connectors!!

Testing:  

Initially I tested the antenna connected for vertical polarisation. The VSWR at 438 MHz was less than 1.1 : 1 as driving the antenna with 25 watts from my IC 471A, there was virtually no reflected power as read on my Bird Wattmeter on the 25 watt range.

With the antenna at a height of 6 metres at my QTH in Bentleigh, all the local repeaters (RMU,RHF,RMM,RSE,RGL) were full scale on receive. Repeaters RAD at Mitcham was about strength 5 and RPU at Arthur’s Seat was strength 2.

I did some further tests with Rod, VK3DQF in Beaumaris and determined there was good cross polarisation rejection and using Rods 70 cm helical there was about a 3db increase going from linear to circular polarisation.

The real test was a couple of nights later when I decided to see if I could get a signal into AO40. A 5*3 signal report from Rene, DJ1KM in Hanover, Germany completely exceeded my expectations. Not bad when AO40 was more than 60,000 kms away during this QSO.

I am so impressed with the performance of this very compact antenna; I am now designing an array of 4 patches to be used for further satellite work. I have also build a patch feed for 2.4 GHz to feed the 1.5 metre dish, but haven't fully tested this.

Also see patch antenna article Part#2 Patch Antenna Pt2

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Page last revised 05 May, 2025