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[SOLVED] does a 5mm RF Track require Control impedance?

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Lambda

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I have a new project that will use one of these surface mount GSM-GPRS Modem Module.
The RF antenna connector is not on the module itself and need to be solderer of the Main PCB
connector:**broken link removed**
the distance between the module edge and the connector will be max 5mm(0.2").
This would be the only controlled impedance track on the entire pcb.
I really don't want to have to specify a specific material, stack-up... just for 5mm of RF tack.
The PCB specifications must be as loose as possible to reduce costs.
Does 5mm of RF track at gsm frequency ( around 900MHz), absolute need controlled impedance? or would a loose approximation of the track impedance sufficient for 5mm?
thanks
 

Lambda said:
Does 5mm of RF track at gsm frequency ( around 900MHz), absolute need controlled impedance?
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As a rule of thumb, lines shorter than 1/20 wavelength have little effect, regardless of the line impedance.

f=900MHz -> wavelength in air = 333mm
If we assume FR4 and an effective permittivity of 4.5, the guided wavelength is 333mm/sqrt(4.5) = 157mm
If we now apply the rule of thumb, lines shorter than 157mm/20 = ~8mm have little effect and do not need impedance control.
 

Below λ/10 or even λ/20, the trace effect can be calculated as lumped element. But I guess, there's nothing against dimensioning it systematically around 50 ohms?

P.S.: In contrast, if you make the trace very thin, you'll probably see a slight mismatch effect of the created series inductance.
 
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    Lambda

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Many thanks to you both.
 

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