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Design of Directional microstrip patch antenna at 433mhz

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KARTHIK_K_S_92

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HI, I have to design a microstrip directional antenna for Man tracking application at 433MHz Please suggest me how can i and which is the best design so that the size of the antenna also must be around 50mmX50mm at max.
 

A fundamental question : How are you going to feed the patch ? (inset feed, coaxial or proximity ..etc) each has its own pros and cons in terms of performance and bandwidth
Once you know this, you start with the published design equations to define dimensions of the patch and feed location and then build your model accordingly
 

I predict that you will not be able to design useful patch antenna at 433 MHz 50x50 mm.
The suggested size is too small fraction of wavelength to create directional pattern
 

Thanks for the reply .. I would go with coaxial feed .. put simple patch at 433MHz wil be very big in dimension and i want to make it as small as possible!! Can you suggest any other way to go by to design directional antenna?
 

thanks for the reply.. Yea i know that simple patch cant be designed in that size of 50X50mm, I want to know whether is there any method or way of approach to achieve it the ones like Meander (omni) to make it has a directional or any other type of antenna ?
 

Besides all implementation details, some general properties of an electrically small antenna should be considered

- the maximum achievable directivity is that of a small dipole (cosine law)
- the smaller the antenna relative to wavelength, the higher it's resonance Q respectively the smaller it's bandwitdth, Q ~ 1/r³
- the bandwidth of an electrically small antenna can be increased by including loss elements (and reducing the gain respectively)
- instead of an impedance matched antenna, an electrical or magnetic field probe (Hertz dipole or small loop) may be used, by nature it has only low sensitivity.

The microstrip patch concept seems not well suited for the intended purpose, although in principle, a high permittivity ceramic substrate (Er ≈ 50) can be used to make a 50 mm patch resonate at 433 MHz.
 
thanks FvM for the reply.. I would like to know the substrates which i can use ?? Whether it is available ?
 

The substrates are not generally available. I've seen it in an academic project, don't know where they got the material. They are also used by industrial 433 MHz "chip" antennas, e.g. from Yageo. The latter are intentionally low directional.
 

thnx for the reply.. What about using a bowtie antenna?? can it give a directional pattern?
 

Almost all 'wire' antennas are directional. In fact they can be made highly directional but they are not 'patch' antennas. Are you restricted to a patch type? If you want directivity it is a poor choice, especially at relatively low frequency.

Brian.
 

Thnks betwixt. Yes i agree with you regarding wire antenna, but length is huge, I want antenna to be small in dimension! Can you suggest any other way in achieving it?? Why patch antenna means as you know the gain is good in that case so i thought patch antennas are good..
 

For small size and lowest cost (a patch would be expensive) I would suggest a helical antenna. Basically it's a dipole but the length is reduced by coiling the driven element. I use 433MHz antennas in remote solar powered temperature monitors and they are about 5mm diameter and 30mm long.

You will find some ideas in the attached file. I downloaded it to my "data sheets" collection but can't remember it's original URL.

Brian.
 

Attachments

  • 433mhz_antenna.pdf
    227.3 KB · Views: 408
We use 433 MHz antennas significantly smaller than 50x50 mm. But such antennas are not directional. And gain is also low.
 

The only other way to create some directionality is to use other electrically large structures to block the signal in certain directions. E.g. A building, car or body. Effectively doing some of what a large ground plane would do for a regular sized patch antenna.
 
Correct Hagster. This is called reflector.
Curiously enough, some 20 years back there was concept 'super-directive antennas'. The idea i believe was to place several electrically small elements in close proximity should result in directivity ... or something. Few books were written about it but no practical results.
I afraid i digressed ...

- - - Updated - - -

Replying to notions of high permiitivity substrates. There is one readily available material - Distilled water. Er=80. Conductivity of double-distilled water is minimum. Double-distilled water can be purchased in some medical supplies stores.I imagine it can be enclosed in thin wall dielectric container.
I always wanted to try it and never did : ) If anybody had experience using water as dielectric please share the experience.
 
So the entire structure for a superdirective antenna is still electrically large.

I wasnt really refering to a reflector as an absorber would be just as effective in impriving directivity.

The distilled water sounds interesting, but i guess it might be hard to keep is sufficiently clean to maintain its properties.
 

Regarding distilled water as dielectricum, if you look at Er loss factor, it won't work well above some 10 MHz or maximum 100 MHz.

Water permittivity.png
 

Thanks for this data FvM. it is pretty sobering.
Loss tangent at 500 MHz at 25 deg 0.025. For comparison FR4 loss tangent 0.008.

At 200 MHz loss tangent at 25 deg: 0.0097 could be on par with FR4.

Another unfortunate detail Er shifts with temperature. At the same 500 MHz Er:
87.65 at 0C; 78.31 at 25C and 69.87 at 50C.
Probe inserted in distilled water can be used as thermometer : )

- - - Updated - - -

Just for fun i did quick simulation of patch on water substrate as following:
Ground plane 100x100 mm
patch: 42x42 mm
gap between the two 3.2 mm filled with distilled water
First resonancecame up at 440MHz
Bandwidth at -10dB S11: 0.6 MHz
Directivity: 2.3 dBi - the same as dipole. And pattern loos the same as dipole, lightly squished on sides.
It is not directional antenna which proves the point: structure less then 1/4 wavelength will not result in directional antenna.
 

thnks betwixt, I would rather try hellical antenna for highly directional purpose, and one thing can be the loop antenna used as directional at 433MHz range?
 

Thnks FvM it is tough to use Distilled water as a substrate in my case and yea loss will be high and maintaining its properties is tough..
 

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