Universal NMO/PL-259/HF Stud Antenna Mounting System
|This universal mounting system was developed to
allow light-weight NMO- and PL-259- based VHF/UHF antennas to be used on a
permanent mount in town, while still allowing larger heavier HF antennas
(that use the customary 3/8-inch-24 threaded stud) to be used on the same
mount on cross-country trips.
Click this picture for large (approximately 1 MB) animated GIF image showing various HF/VHF/UHF antennas using the same mount.
Larsen NMO-HF-THK Mount
This setup is based on the very-high-strength Larsen
NMO-HF-THK mount installed through a heavily-reinforced 5/8-inch hole in the
center of the trunk lid. (Click thumbnail at left
for closeup view view of the Larsen NMO-HF-TK.)
[The "HF" in the model number refers to really high
frequencies; i.e. microwave. The contact pad on the top of the mount can be
pried off, exposing a miniature constant-impedance microwave coax connector.
Larsen offers several GPS, 1.9 GHz (PCS) and 2.4 GHz (WiFi) antennas that
can screw onto the mount and use this matched loss-loss connector. ]
Reinforcement added to the Larsen mount under the rear deck.
The large plate is an 6" by 10" piece of 1/8" aluminum
to spread the stress of the heavy mount over a larger area. The 8'-wide
aluminum flashing, running to the left, is used as a ground strap from the
trunk lid to the car body. Finally, a 3" square steel washer with a 5/8"
hole in it (that I just happened to find at a local hardware store)
completes the sandwich.
Homebrew NMO-to3/8-24 Stud Adapter
Small VHF/UHF antennas (and the Yaesu ATAS-100/120
"mini-screwdriver" HF antenna) can use the NMO mount directly, or with an
NMO-to-SO239 adapter. However, most HF antennas use mounts based on the
3/8'-24 threaded stud instead. The homebrew adapter below was developed to
allow these antennas to mate with the NMO mount.
|The adapter started with a pair of
commercially-available adapters. One converts from NMO to SO-239. (One of
these used alone allows the ATAS-100/120, with it's recessed PL-259
connector, to mount to the NMO). The second one converts from PL-259 to
|Screwing the second one into the first yields an
|This adapter, by itself, is nowhere near strong
enough to support large HF antennas. The base does not spread the twisting
force, caused by air drag on a large antenna when the car is in
motion, over enough sheet metal. About the largest antenna it could support,
unaided, was a 6-meter quarter-wave whip (i.e. about 54 inches or 1.4M).
This adapter needed to be reinforced and built out to a larger diameter.
|After pondering how to do this for several months,
I spotted this plastic pipe fitting in the plumbing department of a large
Incredibly, it's height was within a couple of millimeters of the combined adapter stack. The diameter at the top was 2 inches -- exactly the same diameter as the base of my W6HIQ "Hi-Q" HF screwdriver antenna. The bottom flared out to cover an area of sheet metal 2 1/2 inches in diameter.
I filed and sanded the top of the fitting until it was exactly the height of my adapter stack, inserted the adapter stack inside the pipe fitting, and poured liquid epoxy casting compound into the space between the two.
|After allowing the epoxy to cure for a week, I
topped the assembly with a 2-inch steel fender washer, glued a rubber
washer/gasket cut from an old inner tube to the bottom, and spray-painted
the adapter matte-black to match the antenna that would be mounted above it.
|The height of the all-metal adapter stack is
intentionally about 1mm shorter than the pipe and epoxy surrounding it. The
result is that the plastic parts are in compression, not strain, when the
adapter is mounted on the car with the antenna on top of.
As of this writing, the antenna and adapter have survived over 5,000 mobile miles (8,000 Km), many at over 85 MPH (130 Km/H), including some rather severe wind buffeting and storms on the high plains of Nebraska and Colorado with no problems.