Latitude D20 series weak wireless

We are an IT company and have been selling Latitudes to our business customers for 2 years.
Recently we have noticed a problem with the D520 D420 D820 Latitudes.
In our workshop and on site we carry D410 and D610 Latitudes.
There appears to be an issue with the D20 series namely weak wireless signal strength, we have tested several machines in various environments and locations with the same result:
The D20 series has a significantly lower wireless signal strength than the D10 series in every situation (tested with intel and Dell wlan cards with same result)
Has anyone else noticed this issue? We are hearing rumours in the market about this but Dell Tech support are denying any knowledge at the moment.
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