Adapting the Factory Antenna Hole to NMO

When it comes to automobiles and ham radio, there are those who will do whatever it takes to make their radio station operate at its best and those who will compromise the radio’s performance for cosmetic reasons. 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.” I didn’t want to cut a hole in my roof; but the trunk lid was fair game since it’s a replaceable part. My GTI does not have a horizontal trunk lid. That left the roof as my only option with a ground plane. I’m unwilling to cut holes in the roof. Instead, I chose 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 “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 hole is far too large to accept an NMO mount since most require a 19mm hole. I used a Breedlove #504 Mount for a few years. It’s very strong and reinforces the roof’s sheet metal. At the time, my antenna analyzer was an MFJ-269B, which only measures as high as 170 MHz. I read a saying 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. Eventually, I bought a Comet CAA-500MARKII antenna analyzer that measures up to 500 MHz. 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 its lugged coaxial connections were the cause of that problem. The mount worked fine at VHF and below.

Wanting to fix my UHF challenge, I chose an EM-MG11006-SP, by Electro-Magwave. It is a combination mount that features an NMO mount and a GPS antenna in a single module that mounts in a 3/4-inch hole. It’s advertised to work with all 30-1000 MHz antennas. It covers the 40mm hole on my roof and has a silicone rubber gasket. The new mount works and tunes very well! 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) to connect the GPS antenna to the factory GPS feedline. 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 car’s clock set automatically. Despite the 30-1000 MHz rating, my Yaesu ATAS-120A, shown shown in the photo album below, tunes on all bands down to 7 MHz. That indicates good bonding to the car. I wonder if the 0-29 MHz omission was out of concern for mounting a heavy HF antenna? I mount my Scorpion HF antenna when I really want performance, especially when driving.

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 lowered the aft 12 inches of the headliner. The working space is 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. The mount seals nicely!

I want to also share a mount that I tried and DO NOT recommend: Larsen’s combination mount, the NMOHFGPSNOCONN. It’s similar to the EM-MG11006-SP, except Larsen’s mount features a plastic base, which prohibits it from achieving a proper bond to the body of the car (click this photo for a larger view). Only a 1/2-wave “no ground plane” antennas will tune. The EM-MG11006-SP’s base is made of aluminum, which means any antenna will work! Buyer beware!

I bought my mount from The Antenna Farm. They’ll install most common connectors for a modest fee. I chose an SMA male on the NMO feedline and an SMA female on the GPS feedline. I also had them cut back the cables to 24 inches so that I can easily remove the mount for cleaning or maintenance.

No New Holes,