PCB shield boost the unwanted harmonics

[HS]

pcb harmonics

I tuned very nicely a 2.4GHz amplifier to get 19dBm output and I inserted a very nice BPF to kill the harmonics (34dB on 4.8GHz and 40dB on 7.3Ghz).
Everything was nice till the point I added a PCB shield (required by FCC).
Then all the previous killed harmonics went back to life (like my Band Pass Filter was not any more in the circuit.)
Please advise. How can I correct this "unpleasant side effect" of the PCB shield?
Any info or reference to an article will be greatly appreciated.
Thank you in advance !
HS

 

[Azulykit]

2.4ghz pcb shield

This is unfortunately a common problem. You might try a smaller shield or another possibility is a multi cavity shield. The addition of absorber inside the shield could also help. Sometimes even a fence under the shield might be enough. If the shield detuned the filter, moving the shield further away could be beneficial.

My first effort would be to see if I could isolate the problem to a particular component and then the fix would be easier.

 

[HS]

pcb shield fence

The BPF is a ceramic one. Very performant and quite small (2.5mm /1mm/ 1mm). Without shield everything was nice and clean.
I just wonder why FCC is asking a PCB shield since that is not going to make a cleaner signal but worse in spectrum.
I am looking at that tiny board built with 0402 components and I cannot believe that all my work is useless.
Hours and hours of tuning and at the end the tuning is lost because the shield is bringing back the harmonics.
I saw few products on 2.4GHz using shields. How they manage to get rid of those "side-effects" ? How they manage to pass FCC certification ?
HS

 

[spectrabei]

It sounds like you need to beef up the board's ground plane. You can do this by adding a shield on the bottom of the board. You are going to have to re-tune your filters with the added bottom (beefed up) ground plane in place. #1 RULE in HF RF, You Never can have "too much" space for your Ground Plane Footprint.

 

[rfmw]

Probably you're getting coupling from your harmonics-full circuits to the output by shield, if you're using only one shiled for the all circuits. As Azulykit said, multi-chamber/cavity shileding might help you. Enclose the circuit producing the harmonics with its own shield.