Is it that big a deal if when I'm installing the GPS antenna that I don't use the sticky metal sheet? I've found a good spot for the GPS antenna in the OEM location for my car, fits snugly there, but won't fit the square of metal underneath it.

In the install manual, it says if installing the antenna inside the car to always use the metal sheet otherwise it'll affect the sensitivity of the GPS antenna....and not to cut the sheet into smaller pieces either..,..is it that much of a deal?

Without going into too much detail... yes, it is important in antenna design to define a ground plane. On the other hand.. some GPS antenna have a build in magnet to prevent it from moving around. I don't know if the Pioneer antenna has a magnetic base BTW, I didn't install the unit myself. So suppose this antenna needs a metal plane to have optimal performance. The minimal size does matter, larger metal sheets normally have positive or no influence.

Do you have to use the supplied sheet? Not when the location you plan to go for is a metal construction part of the car, I'am inclined to say. And since this is the location the car manufacturer has 'designed' to be used for a GPS antenna, it might well be a metal base with no (or little metal) in the field of view of the antenna.

The practical approach.... Find a place with relatively little or no structural blocks (buildings) that could interfere with receiption, attach the antenna (with the supplied sheet if no metal base is present) and look at the GPS signal strength page. Then remove the metal sheet, place the antenna in the intended mounting location and see if receiption changes significantly. Remember that the GPS sats are constantly 'on the move' so changes in signal strength will always be present and use as little time as possible when moving the antenna.

well that metal sheet suppied with my unit was too big so i cut it to the size of the antenna and place it on my dash.. No problem from it so far so knock on wood.

You definitely do not need the metal sheet. Just try to make sure that the surface you're attaching the gps to (super powerful magnet) is relatively flat. I don't use the sheet, and I always connect to 14 or so sattellites with a "green" connection in under a minute, so I'd assume it's just fine.

No you dont really need tou use the sheet. If ou mount it to metal you will get slightyly better reception. in my vehicle I didnt want the antenna on the dash or visible so i used a piece of double sided tape and centered in in the top middle of my windshield above my mirror.(inside the vehicle) works just fine with high accuracy all the time.

Mine is installed on my rear deck, with the magnet holding it to the metal deck, and which the fabric cover on top, and I get great reception. I didn't need to use the supplied metal.

Thanks for the replies everyone, that makes me feel a bit better about not using the metal sheet if I can't fit it in. The Pioneer antenna definitely has a magnetic base, just wasn't sure if I HAD to attach it to something metal.

Actually, the OEM spot(R171 SLK) for the antenna is right above the rear-view mirror and there's no metal base for the antenna to attach to, just so happens that the fit is snug enough that I can just jam the Pioneer antenna in there and once I put the rear-view mirror assembly back on, there's no way that sucker's moving anywhere. Once its in there, the only thing between the antenna and outside is the windshield so it should be pretty good reception. If I get a chance, I'll try the practical test and see if adding the metal sheet underneath the antenna improves reception or not.