Adapting the Factory Antenna Hole to NMO

When it comes to ham radio and cars, there are those who will do whatever is necessary to make their radio station operate at its best and those who will compromise the radio’s performance to maintain the car’s factory looks. With my Mk3, my line leaned a little more toward radio performance since the car was older and had more of a utilitarian look. I wouldn’t cut a hole in the roof, but I did drill one into the trunk lid. Here’s a photo of my Mk3’s “antenna farm.” My GTI, and Mk7s in general, has a more sleek look that I didn’t want to flaw to the same degree. I certainly was not willing to cut any holes for one or more antennas. I wanted to find a way to use the existing hole in the roof.

The factory “sharkfin” houses antennas for the Sirius satellite receiver, the CarNet module (similar to GM’s “OnStar”), and the GPS. I have no use for those systems, even the GPS since my GTI did not come with a navigation system (I use my smartphone when I need navigation assistance). The AM/FM radio, uses a “diversity antenna” that’s built into the rear window. Regardless, I’m more of an SD card listener. In short, removing the sharkfin antenna did not inconvenience me as much as it might others. In fact, I like the idea of “unplugging” my car from whatever or whomever may be watching. Besides, everyone knows that our smartphones are already ratting us out, right?  😉

Once the antenna is removed, the roof is left with a 40x40mm square hole with unique corners to assure proper antenna orientation. The NMO mount is the most common amateur radio mount used in mobile applications. However, the factory hole is far too large to accept an NMO mount since most NMO mounts require a 19mm hole. Breedlove Mounts, Inc has a solution in the form of a wide selection of very strong NMO mounts. Most of his mounts are designed for heavy-duty applications. Coincidentally, the extra material used to reinforce a car’s sheet metal just happens to be great for putting an NMO mount over top a large, oddly shaped hole.

I chose Breedlove’s three-inch mount, Model #504,  since the square hole is 2.1″ diagonally. The mount is bare aluminum, which I’m sure looks good in most applications. I wanted it to look as “factory” as I could make it. So I finished it with semi-gloss black paint, which makes it look like part of my antenna instead of a plate sitting on my roof. Some have said that it looks like a magnetic mount, except for the absence of coaxial cable. This mount is certainly strong enough to hold a large HF antenna, such as the Yaesu ATAS-120A shown on my Mk3 at the top of this page. I’ve decided to use only VHF/UHF in this car. So my 6/8-wave antenna is probably the largest antenna this car will ever see.

The underside of the mount is relatively easy to reach. I did not need to remove the headliner. Instead, I simply removed the top rear trim panel and then separated the tops of the side trim panels to allow me to lower the aft 12 inches of the headliner. The working space is really tight, barely enough space to capture this photo, but everything is accessible. Disconnecting the factory antenna was a bit of a challenge when working in the cramped space. I don’t remember how I did it, but I recall viewing some YouTube videos before trying it myself. The first minute of this video gives good visuals. I’ve included a few extra photographs below. I left the headliner loose for a few days so I could inspect for leaks during rain and car washes. Breedlove’s mount seals nicely!

Big Brother Is (Not) Watching,