WA8LMF Home Page | Main Mobile Mobile Page | Main Ham Page |Updated 21 December 2009

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.)

This mount is machined out of stainless steel, and is far stronger than the customary NMO mounts made of brass. It designed for installation through surfaces up to almost an inch (2.5 cm) thick instead of the thin sheet metal most NMO mounts are made for, allowing for additional layers of material to be sandwiched under the sheet metal. 

This mount inserts through a 5/8" hole (same punch used for SO-239 coax sockets and 8-pin mic jacks) rather than the more common 7/8" hole used by most NMO mounts. The mount has to be pushed up from below the surface, rather than dropped in from above like the usual NMO. The fitting at the right is then screwed on from above. The "wings" on the underside are then tightened with an open-end wrench to pull the top fitting tight against the upper surface. The mount grips and sandwiches far more sheet metal than the two claws of the typical NMO mount.

[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 3/8-inch female.
 

NMO-to-SO239

PL-259 to 3/8ths (Bottom)

PL-259 to 3/8ths (Top)

   

  Screwing the second one into the first yields an NMO-to-3/8ths adapter.


 
  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 hardware store.

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.


Conveniently, the casting epoxy was available in a small size at a local arts & crafts store. (I had feared I would have to go to an industrial plastics supplier and buy in gallon-sized containers.)
 
  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.
 

Top View Of Finished Adapter As Used With Screwdriver Ants.

Adapter with Added Nipple for Hamsticks, CB Whips, Outbacker, etc

Bottom View Showing NMO Fitting and Rubber Gasket.
  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.