Jetta HF antenna improvement project

In 2009 a 2006 Jetta TDI was my newest vehicle.  But it proved to be a handful when it came to installing my FT-8900 and FT-857 within.  While the VHF/UHF antennas were easy enough with trunk lid mounts, no choice of magmount placement would yield a good result for HF and my Yaesu ATAS-120A antenna.  Click on any photo below to enlarge it.

Step one one was to mount the rigs on an aluminium plate to make easy install and removal to swap between vehicles.  This plate sat upright behind the rear seat in the trunk.  A 12V AGM battery was added to the trunk and powered from the cigarette lighter outlet via a simple charging circuit that also prevented backflow into the car.  A mount was added for each of the two VHF/UHF antennas on opposite sides of the trunk lid.  The challenge to overcome was poor HF performance and some cleanup of power wiring.

Radios and power – before
Antenna mounts – note paint cleaned off inner trunk lip

Anti-seize conductive paste was used anywhere the paint was removed.  This not only inhibits rust but allows good conductivity.

Inside of trunk lid just before completion of step one

Using the shield from RG8 as grounding straps in the corners was useless.  There isn’t enough surface area and the RG8 shielding is fragile with the lid opening and closing.  Magmounts for temporary HF antenna placement yielded very poor results.  I needed a better solution.

 

Step two was to improve the capacitive coupling and bonding of the trunk lid to the rest of the car, and to add a proper mount for the ATAS.  I also had to improve the power connection from the secondary battery to the car.

Half the copper bonding strips in place
Paint removed, hole drilled and strips screwed into place sandwiching copper paste
All four strips in place and anchored

With the bonding strips in place, now it is time to focus on the antenna mount.  If you’ve never seen the ATAS-120A, it is a stout antenna and a windcatcher.  With the very thin metal on the trunk lid, seemingly about as thick as a soft drink can, I needed a backer plate for support to protect the trunk lid from damage.

The parts, waiting to be put in place

Enter some .200″ thick aluminum plate cut to size.  Also shown is the Larsen NMOHFTHK mount.  We then removed the paint from the inside of the lid to match the size of the plate.

Test fitting

From this point we made a sandwich consisting of the mount, the plate, copper paste, the trunk lid, and the top of the NMOHFTHK.  I topped it off with a NMO to SO239 adapter to match the antenna.  Cables were dressed and held in place with zip ties and routed neatly.

Assembly fully in place with dressed cables

The only remaining step was to clean up the power to the secondary battery.  As long as I was in the trunk we pulled the inside panel and tapped into the rear of that 12V convenience outlet directly.  Naturally both sides of the battery are fused and have quick disconnects for service to either the radios or the vehicle.

After power wiring cleanup

 

The result was totally worth it – within 6 months I’d worked six of seven continents on 15, 20 and 40m.  There’s a total of 30 hours of labour into this installation, and the good news is that next time it won’t take me nearly as long to figure it all out. Many thanks to my buddy VE3HJ Andy Jancik for his expertise and shop space for this project would not have been completed without him.

Terrible photo of the final outcome

I’m still hunting for Antarctica 10 years later though, perhaps one day 🙂