Hey Friends,
I am working on microstrip antennas and I have stuck at a point where I have to give the feed to the radiating patch. I am working on equilateral patch antennas and I know how to give the coaxial probe but not about of what dimensions should I use. What will be diameter of coaxial probe as I am not aware how to calculate the impedance of an equilateral patch?
So basically I have two doubts.
1. First one is how to calculate the impedance of an equilateral patch antenna and
2. How to calculate the dimensions or diameter of coaxial feed as per impedance of the equilateral antenna?
I am working on HFSS. Kindly provide any technical paper or calculations on them. Thanks in advance.
Regards
Munish Kumar
Dear Munish Kumar
The attached formula is used to calculate the characteristic impedance of coaxial probe, where:
(d) is the inner diameter of coaxial probe
(D) is the outer diameter of coaxial probe
(er) is the dielectric constant
(Zo) is the characteristic impedance
The characteristic impedance (Zo) should be 50 ohm which is also available for fabrication of antennas. Then, u can find the diameters if u know the dielectric constant (er)
The link below makes sense if you want to calculate dimensions of coaxial probe:
http://www1.sphere.ne.jp/i-lab/ilab/tool/cx_line_e.htm
Regards
Thanks Rammah for sharing this. It means whatever the impedance of the patch, the dimensions of the patch should be as per 50 ohms. Isn't it?
Most coaxial cables and connectors in common have a typical value for industrial standard lines of 50 ohm or 75 ohm characteristic impedance. Which is required to be properly matched with the patch impedance of the designed antenna. (Read more about Matching)
Regards
Thanks Rammah. I'll design according to 50 ohms. It means I have to change the feed location for matching with my patch? I am using FR-4, with dielectric constant=4.4 and height=1.6mm. Can you please calculate which dimensions of probe and coax should I take while simulating it? I'll forward my design to you.
Can you also give the accurate feed location formula for equilateral triangle?
Thanks again.
Hello Munish Kumar,
Yes, you need to change the feed location. The feeding point could be placed in any desired location inside the patch in order to be matched with (50 ohm) the desired input impedance. [except at the edges or center of the patch due to E-field =0 at the center which causes zero input impedance while at edge is maximum E-field which causes high input impedance]
You need to be aware of the location of feed point which determines the patch's input impedance at the resonant frequency. If you use the feeding with a 50 ohm standard SMA connector, the input impedance of the patch should be adjusted to approximately 50 ohm for optimum matching.
The link attached below used the same specifications as yours, you may refer to it. It also includes the formula that you needed in order to calculate the position of the coaxial cable.
http://ijettjournal.org/volume-4/issue-4/IJETT-V4I4P340.pdf
Thank you thank you so much Rammah. Actually I have used the formula that you have shared for calculating the dimensions of the 50 ohm coaxial probe. But still I am not able to find the exact feed location for equilateral triangle. I have tried this for the rectangular patch, but this time I am using equilateral triangle for fractals and unfortunately, not able to find the exact feed location. But still I am trying, and if I find any solution for that, I'll definitely share this with you and others.
Thank you so much.
Bahman can you also confirm for FR-4 substrate having permittivity=4.4 and height=1.6mm?
Generally, easily available 50-ohm connectors are having inner conductor diameter of 1.5 mm and Teflon 2.1 epsilon diameter of 5 mm. For finding 50-ohm point of equilateral patch you can use parametric sweep or use its design equations available in Chapter 6, Microstrip Antenna Design Handbook By Ramesh Garg.
Thanks Dibinmary... That's what I actually asked. Thanks for understanding my problem correctly.
Thanks Ayan. Actually, my question is not about how to make coaxial probe in HFSS. It is about what are actual calculations behind it. Moreover, thanks for sharing this guide with us.
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