Plexiglass Shield

We are installing some card readers for vehicle gates on gooseneck mounts. The spec calls only for the gooseneck, no back box or rain-hood. There IS however an 18" x 18" plexiglass sheet specified to be installed between the gooseneck and card reader. That's dramatically bigger than the card reader.

When I asked the architect why they made this choice, I was told: "[The plexiglass] provides a radio interference barrier with the gooseneck".

Does this make any sense to anyone? I've never heard of the practice, and I have installed plenty of card readers on plenty of metal surfaces without issue.

I can see how spacing it away from the gooseneck with a non-conductive material might have an effect, but plexiglass is radio transparent, right? That oversized sheet isn't actually accomplishing anything is it?

From Plexiglas - General Information and Physical Properties

Interesting trivia: "Plexiglas" is spelled with only one S.

The non-metal spacer does not block RF, but it prevents attenuation of the reader's RF field resulting from the reader being mounted directly to a metal surface. This is typically an issue for longer range readers like the HID 5375. Standard size readers are usually fine mounted directly to metal. For the 5375, spacing it away from the metal surface does impact the read range. But it is all about the spacing, not the material. If you put a thin sheet of Plexiglas behind the reader, it is not likely to make much difference.

We used to use plastic backboxes on pedestals for this reason. I recently came across this one, which looks really nice and is purpose made for this application (but is not 18x18).

This is pretty much what I figured. That what was important was spacing it away from the metal gooseneck, not acting as a "shield".

In that regard, the product you linked is likely superior in that it provides a full inch of spacing, while only 1/4 inch is spec'd. That and the pre-recessed pockets for bolts and screws makes it a better option.