Help needed for understanding placement of flexible PCB GSM antenna

Question

I am using flexible GSM Antenna for my application.I am sticking the antenna on top of my FR4 based PCB board as shown in the below figure. Note that the PCB, on top side (One towards antenna) has GSM module. So effectively, I am sticking the Antenna on top of the module itself with a simple DST acting as boundary between them. The antenna is connected to PCB via UFL connector through co-axial cable. I have some questions: 1. Does folding the PCB the way I have mentioned (In the middle) affect the performance due to any interference in the radiating arm?

2. Should I use rigid PCB antenna instead of flexible one? Are there any important factors I should consider here in making my design decision?

3. Does keeping the antenna on top of RF module itself with around 0.8 mm separation lead to destructive interference? If yes, how can I tune the physical placement of the antenna for least interference?

P.S - Here is the link to Antenna Datasheet

Answer 1

1. Does folding the PCB the way I have mentioned (In the middle) affect the performance due to any interference in the radiating arm?

The fold will have an impact but it'll be small in comparison to the other issue you have. The antenna is tuned to be stuck to a piece of plastic (2 mm thick ABS in the datasheet). Sticking it to piece of grounded metal (the shield of the GSM module) will completely ruin this tuning. Since the datasheet makes no mention of ground planes you want it to be as far away from any metal as possible.

2. Should I use rigid PCB antenna instead of flexible one? Are there any important factors I should consider here in making my design decision?

The depends upon how you plan to mount it. Look at what the antenna datasheet says about metal, if it doesn't mention anything then they are assuming none near the antenna.

3. Does keeping the antenna on top of RF module itself with around 0.8 mm separation lead to destructive interference? If yes, how can I tune the physical placement of the antenna for least interference?

By putting it as far away as possible.

Answer 2

You go on about interference but that is not an issue.

To have what is called interference in the RF world, you need something else transmitting a signal, the signals then influence each other and disturb reception of these signals.

What you are doing will affect the radiation pattern and impedance of the antenna. For both non-flexible and flexible antennas the optimum radiation pattern (the one you see in the datasheet) is achieved when the antenna is used in free space. This means no conductors nearby. This "nearby" is often assumed a couple of wavelengths distance.

You do have conductors nearby, the traces on the PCB, the shielding can of the module etc. These will affect both radiation pattern and impedance. This will results in degraded sensitivity. Your module will still work but it needs to be closed to the base station.

Many mobile phones use flexible antennas near metal objects (shielding in the phone etc.) so it is common practice. But during design this influence is taken into account using EM modelling software and measurements.