Designing an Inverted F-Antenna

I want to use an Inverted F-Antenna for my next project, but I'm struggling to find some good design information.

I have read that the length of the arm "L" is roughly a quarter wavelength of the desired resonant frequency.

I also read that the antenna impedance is actually determined by the shorted stub left of the feed and the open stub right of the arm. So by moving the feed point "L1" the impedance can be tuned to 50ohm. This have I seen done in practice by trimming the length "L" of the antenna, but this would change the resonant frequency and the impedance simultaneously, or maybe I'm missing something.

So how do I determine the best location (L1 distance) for the feed point in my initial PCB design ?

And how do I trim the resonant to say 868MHz and also the impedance to 50ohm ?

• Found a TI design note that gives a design for an inverted F that will work at 868 MHz. Actually, I found a couple of such design notes, and neither of them actually says how to design it. They just say "make it look like this" and give dimensions and which one to change for certain frequencies. Nothing, though, that says how to determine the dimensions. – JRE Aug 11 '16 at 11:39
• I have found dozens of articles describing the inverted F antenna, but no one of them gives any information on how they determined the dimensions. Not even a description on how they got to the result. – JakobJ Aug 12 '16 at 6:00
• If your problem is to design an antenna for a prototype, I would advise you to use an existing design in application notes. For most product antenna design, engineers use application notes and if they really need to design something specific (e.g; a loop antenna), simulation is used (with CST for example). Pure equation solving is quite complicated and almost impossible for complex designs. I would definitely answer your question with "simulation" but this may not be a satisfying answer. – Julien Sep 10 '16 at 17:55
• I already figured it out, I did trial and error approach. My question was more regarding getting it as close as possible in first iteration. I thought there was some rules of thumb, regarding where to try and place the feed line in the beginning. The antenna was placed inside a partially conduction casing, so I already knew, that it was gonna be hard to determine the dimensions analytically. – JakobJ Sep 12 '16 at 13:21
• I have the same question and when I googled, I found this research paper that talks about relation between different parameters of Inverted F Antenna. Analysis of Inverted-F and Loaded Inverted-F Antennas for 2.4 GHz ISM Band Applications. (2009) – Joana Rigbi Oct 4 '16 at 16:18

1 Answer

This have I seen done in practice by trimming the length "L" of the antenna, but this would change the resonant frequency and the impedance simultaneously, or maybe I'm missing something.

The impedance is determined by the length from the feed pin to the short post. Trimming the "L" dimension of your PIFA will accomplish this but in a detrimental way that will change your resonance frequency. Moving the feed closer to the short post will decrease impedance and moving it further will increase impedance.

...the input impedance of the antenna is dependent on the distance of the feed point from the grounded end. The portion of the antenna between the feedpoint and the ground plane is essentially behaving as a short-circuit stub. Thus, the designer can match the antenna to the system impedance by setting the position of the feed point... Wikipedia.com - Inverted F-Circuit

Impedance is done by a calculation of measurements, as can be seen in the diagram below:

I recommend this whole .PDF file as it explores the subject of PIFA circuits in clear language.

As you can see, the resonance of the antenna is based off of the length and width of the plate (L1 and L2 here respectfully).

I have yet to find a reputable source on determining the location of the feed but will update this post when I do.

Update: the feed location (aka transmission line) is highly characteristic to your substrate and dielectrics. I am still trying to find a way to soundly calculate it. I have found some software which may help you calculate the transmission line characteristics under the microstrip section. I don't have a Windows computer so I can't say for sure.