Faraday Cage - Block UHF but not GPS

[Teknube]

Hi,

Simply:
I’m trying to block the EMF from a 450mHz UHF radio antenna above my office room without reflecting GPS signals that another antenna up there needs to recieve. I’ve tried alum foil, it seemed to reflect both, disrupting the GPS system.

Elaborating:
I’m operating a GNSS base station at my office. Above my room is a GNSS (GPS) dome antenna, and a UHF radio antenna.

The GPS antenna relies on a clear view of the sky without relfecting signals. It does not work well near tall buildings, trees, etc.

The UHF antenna broadcasts from my office with fairly high wattage. I’ve tested an found there is around 5-15 uT of EMF at my desk.

I’m interested in blocking out the UHF from my room without upsetting the GPS system. I’ve covered the attic floor in aluminum foil, sucessfully reducing EMF to 0-1 uT in my room, but the GPS could not read well, I assume due to signals reflecting off of the foil.

Does anyone have a suggestion on how to block/absorb UHF so that the GPS signals will still work? Would grounding a shield/“cage” allow absorption rather than reflection? Would a properly sized mesh allow GPS through but reflect/absorb the 450mHz UHF?

Thanks!

 

[polarmystery]

Maybe try adding a high-pass / notch filter to the GPS receiver. GNSS (Had to look it up) appears to operation in the GHz range, and your UHF of concern is half that. Should be somewhat straight forward without having to use any kind of grounding / shielding / cage / etc.

 

[Teknube]

Thank you for the recommendation. It turns out that sort of filter is active on the GNSS system, and the dome style antenna is designed and set to only recieve signals from above, being that any signals reflecting off of the foil were not a problem. I just had an unrelated setting wrong on the system that caused it to not operate. The foil solution does not disrupt the system, and measures to be completely blocking out EMF. There is a slight reduction in the UHF range with the foil, but tolerable.

So the foil barrier is sucessful.

For others interested, the foil barrier is spread without gaps across the attic floor, and all sheets are connected. There is a grounding wire from the foil system to an unused coax plug in the wall that is tested to be a good ground.