problem in coaxial feed in patch antenna

Hello friends...

I am working on micro strip patch antenna but m getting problem in coaxial feed... i don't know what's diameter of cable should be used. Is it different for different patch or there is some standered dia of coaxial? If it is standered then plz tell me what is it...and how to decide particular diameter cable for a particular patch antenna..on what parameter the diameter depends?...Plz tell me as soon as possible...m waiting for ur reply.....thankyou

Which simulator are u using? It sounds like u are using HFSS. HFSS has probe-fed patch antenna example in its fullbook. You can refer to it for the structure and dimension of the coaxial feed. In my opinion, the coaxial feed size is not fixed. It can be modified according to your needs. However, you need to make the characteristic impedance of the coaxial cable equal to 50 ohm. You can rely on the characteristic impedance equation of the coaxial cable to calculate the diameters of the inner/outer conductors.

the HFSS manual shows the usage of probe as specified below:
coax outer: vacuum: 0.16mm radius
coax inner: PEC: 0.07mm radius

usually the rule of thumb is to take the inner probe dimension as the half of the outer probe.

In my opinion, your model should consist of the probe dimension as available with you in your lab. for example, at my workplace we have coax outer with diameter of around 1.25mm and then by following the above mentioned rule, you may easily calculate the probe dimension as the half of 1.25mm.

Moreover, the impedance calculations would only give you the position of the feed and not its dimensions at all!!
you basically have to check for the perfect matching point on the patch marked by the Re(Z11) being close to 50 Ohm.

hope it helps.

Ashish

Mr ashish.mw
sir what should we take the radius of inner and outer coax for 50 ohm resistance which is also available for fabrication of antennas.
and also if we cosider the ratio between the inener and outer diameter constant then we can take any value for them according to our need and is that dia is available at the fabrication time.
moreover , the ratio of the inner and outer diameter depends upon the relative permittivity according to the below formula:Characteristic impedance
Characteristic impedance

then what value of relative permitivity we consider if we are using 2 dielectrics.
thanks
prety