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. Here’s a photo of my Mk3’s “antenna farm.” My GTI has a more sleek look that I don’t want to clutter. I’m unwilling to cut holes in the roof. Instead, 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 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. In short, removing the sharkfin antenna did not inconvenience me as much as it might others.

Once the antenna is removed, the roof is left with a 40x40mm square hole with unique corners to assure proper antenna orientation. This hold 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 that 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. But there was a flaw that led me to switch to a different solution later. I’ll share both mounts below. Decide for yourself which route is best for you.

My first choice was Breedlove’s three-inch mount, Model #504,  since the square hole is 2.23″ diagonally. I finished it with semi-gloss black paint, which made 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.

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. I don’t remember how I disconnected the factory antenna, 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! But there’s a drawback…

Eventually, I added a GMRS radio to my suite. I wanted to see if any of my existing antennas would tune to GMRS frequencies. For years, my only antenna analyzer was an MFJ-269B, which only measures as high as 170 MHz. I read that “any dual-band antenna that tunes on 2m will tune on 70cm.” I’ve never had a problem operating on 70cm. So, I assumed all was well. Still, I wanted to see if any of my antennas would tune on GMRS frequencies. So, I bought a Comet CAA-500MARKII. That’s when I learned that NONE of my antennas were tuning on UHF frequencies! My troubleshooting lead straight to the Breedlove mount. I think the lugged coaxial connections (shown above) were contributing to this problem. The mount worked fine at 2m and below.

Larsen NMOHFGPS mount shown with WeBoost mag-mount antenna…

Wanting to fix my UHF challenge, I chose Larsen’s NMOHFGPSNOCONN mount. It is a combination mount that features an NMO mount and a GPS antenna in a single package that mounts in a 3/4-inch hole. It’s advertised to work with all 27-6000 MHz antennas. It’s not a good fit for the 40mm hole in my roof since the NMOHFGPS is only two inches wide. My solution was to sandwich a 3×5-inch aluminum panel and a neoprene gasket between the NMOHFGPS and my roof. The new mount works and tunes like a charm! Better yet, I was able to restore a GPS signal to my car’s infotainment system by using an SMA-to-Fakra adapter (photo below). As mentioned above, the GPS signal isn’t critical since I don’t have factory navigation. But it’s nice to have my compass working properly as well as having the clock set automatically. Other than the need for the aluminum plate in my application, this mount’s main drawback is the loss of the HF band below 27 MHz. I mount my Scorpion HF antenna when I want to work those bands.

Between the two solutions I’ve shared above, one only needs to decide between the following: Get the Breedlove mount if you want a strong mount that will work with a small HF antenna like the ATAS-120A and you don’t use UHF; or get the Larsen mount if you want to work all VHF/UHF frequencies and have a use for a GPS signal.

No New Holes,

Scott