antennamicrostrippcb-antenna
I have a query regarding PCB microstrip antennas.
Most of my antennas have been straightforward with the U.fl being placed close to the RF feed point. Maybe a PI filter is added.
But I have seen many PCBs with unique antenna shapes. Point in case, image attached. How are the parameters for these ascertained?
First, I would look up the datasheet of the RF chip to make sure that the output impedance is 50 Ohms (it doesn't have to be).
Then, peel the coax of your antenna in such a way that it looks like a 3 toothed fork with the inner conductor being in the middle. This is really important for best performance (otherwise your antenna may not work well due to poor balancing).
Using a sharp knife, razor blade (etc) remove the mask on the a small section of the trace leading up to the antenna so that bare copper is visible.
Cut that trance after to pad to isolate your old antenna.
Expose copper on both sides of the center trace (your ground).
Solder the center conductor of your coax to the former antenna trace pad and solder the coax outer shield grounds to the pads.
Keep these unwound wires as short as possible.
Keep in mind, that you will still lose a couple of dB in the transition and cabling.
An antenna with a gain of 9 dBi means it has a radiated power per square metre in its "best" (or optimum) direction that is 9 dB greater than an isotropic antenna in any direction. An isotropic antenna emits the same power density in all directions and although it sounds really useful is an impossible dream BUT useful as a concept.
This means that the 9 dBi antenna MUST not be as good as an isotropic antenna in some directions and this could be a big problem to some applications.
On the other hand, a 2 dBi antenna is like a half wave dipole and it will produce a doughnut shaped power density that has 2 dB gain in X and Y directions: -
The down side is that it produces very little power (theoretically zero) in the Z direction. So, if you add all the power densities up and calculated an average you would find that the average power density produced is the same as an isotropic antenna.
What you gain in one respect you lose in another because a high gain antenna becomes very directional.
Best Answer
Designing (PCB) antennas is a science by itself !
The 3R rule you mention probably applies to a certain type/configuration of antennas only. The fact that there are many uniquely shaped antennas tells you that there are many different solutions.
Usually these antennas are designed by a specialist using 3D Elector-Magnetic simulators like CST or HFSS. This software + some imagination + experience + a little bit of magic can sometimes result in a usable antenna ;-)