NMO antenna mounts

What is an NMO mount?

People who mount antennas to their vehicles, but need a universal connector because they’re probably going to change the antenna a few times over.

Why an NMO mount?

The NMO mount has two advantages. First of all if you install it right it has two waterproof seals. There’s a waterproof gasket which goes up against the metal in the vehicle. This keeps the car from rusting out. There’s another waterproof gasket that sits inside the antenna and keeps the connection point waterproof. This means the antenna doesn’t corrode from the inside. Both of these are good thing.

By the way, if you don’t want to drill holes, there are magnetically mounted NMO mounts too. Obviously they’re not quite as secure but then again you don’t have a hole in the car so there’s that.

The other advantage is that when you connect an antenna, it’s not permanent. It’s secure enough, it won’t fall off, but you can disconnect one antenna easily and add another one. It turns out this is a pretty good thing.

Types of NMO mount

Through-hole NMO mounts

Through-hole or “thru-hole” NMO mounts are one of the most common types of NMO mounts. They are the best choice for an antenna installation that has to withstand rugged use.

Through-hole NMO mounts are for long-term or permanent installation on a vehicle. Holes are drilled through the vehicle roof, bonnet, or trunk, through which the NMO mount is passed. The antenna can then be securely screwed onto the exposed contact on the surface of the vehicle.

Through-hole NMO mount sizes

These NMO mounts come in two sizes, which are based on the size of the hole that must be drilled through the vehicle roof:

• ⅜ inch through-hole NMO mount
• ¾ inch through-hole NMO mount

Benefits of through-hole NMO mounts

Through-hold mounting offers the following benefits:

• Higher resistance against mechanical stress, vibrations, and shock.
• More secure, more resilient antenna installation.
• High mechanical strength because the mount runs through the roof.
• Fewer drilling holes are required for installation
• The mount is sealed with O-rings on both sides for waterproofing that prevents rust from spreading through the metal mounting surface of the vehicle.

Through-hole NMO mount specifications

Through-hole NMO mounts commonly have an all-brass (78 Rockwell Hardness) or chrome body with a gold-plated contact. This ensures corrosion resistance and efficient power transfer so that the antenna performs well. The ground lug is silver-soldered, eliminating static and radio frequency noise. The threading of the outside ring of the NMO connector has a threaded size and pitch of 1 1/8” x 18.

Magnetic NMO mounts

Magnetic NMO mounts are the next major class of NMO mounts. They use a powerful magnet to strongly attach the NMO mount to the vehicle roof or trunk. They can be attached confidently to any vehicle with a metal roof and work with all types of antennas that have NMO mounting requirements. The mag mount comes with a length of suitable coaxial cable attached, which may be terminated with a connector.

Mag mounts offer secure but temporary mounting on any vehicle with a suitable metal surface. Unlike through-hole antennas, the mobile antenna mounting remains easily removable.

The magnets used in mag mounts.

Magnetic NMO mounts use two types of ultra-strong magnets to achieve their hold:

• Neodymium magnets are a type of rare-earth magnet, made from alloyed neodymium, iron, and boron. It was created by the auto industry and is the strongest type of permanent magnet available.
• Ferrite magnets are made from a ceramic material that combines iron(III) oxide, strontium, barium, manganese, nickel, and zinc. The material is not usually electrically conductive but is strongly magnetic. It clamps directly onto any ferromagnetic surface, like steel such as mild steel.

Mag mount performance

The performance of mag mount NMOs is affected by the ability of a mag mount to form a ground plane surface at a proper resonant distance with a DC ground. With magnetic NMO mounts the base of the antenna is above the ground plane surface so the resonant length of the antenna may be off. The antenna also lacks a DC ground connected to the metal of the vehicle body because paint and finishes are in the way.

So amateur radio enthusiasts get around this problem by cutting the antenna to a resonant length (compensating for additional height) for the lowest possible SWR then placing the antenna on a mag mount where the SWR will be higher. Magnetic mounts usually perform best at higher frequencies (UHF versus VHF).

Though the placement of a mag mount is temporary, the mount can damage vehicle paint.

Mirror NMO mounts

Mirror mounts are NMO mounts that can be attached to the side mirror of a semi truck, tractor-trailer, or even the rails and roof racks of a car. It is extremely versatile and a popular alternative to roof or trunk mounting and NMO antenna.

The mirror bracket mount is usually made from chrome or stainless steel with the 1-1/8-18 pitch thread that is common to NMO mounts. The brackets vary in complexity, from basic L-shaped brackets with a hole that a thru-hole NMO mount can be inserted into, to full NMO mounts equipped with coax cables and connectors

Thick NMO mounts

A standard NMO mount is designed to go through surfaces that have a maximum thickness of 2.5 mm, in line with the original Motorola specification. Thick NMO mounts are specifically designed to provide through-hole mounting through much thicker materials. They offer the secure and rugged mounting of a thru-hole NMO mount on vehicles with thick metal surfaces like construction vehicles.

Thick NMO mounts can be used on ¾ or ⅜ inch holes. They are full-adjustable and suitable for surfaces with a thickness of ⅛ or ½ inch.

Typical specifications and features of a thick NMO mount include:

• A precision 50-ohm interface for the antenna and connector
• Low-loss cable and connector pre-installed
• Brass and stainless steel construction; strong and corrosion resistant
• Self-adjusting locking nut to speed up installation
• Heavy-duty, non-rotational centering washer
• Non-slip clamping pressure

[E] Trunk Lip Mount NMO mounts

Though mobile antennas perform best mounted on the roof of a vehicle or trunk deck as described above, a trunk lip mount or lip mount provides a convenient and non-permanent alternative for locating an antenna. Trunk lip mounts are designed only for trunk lip use and are often used with VHF and UHF antennas. They come with coax and can be adjusted to compensate for the angle of the trunk, up to a 17 degree angle. Trunk lip mounts are designed to fit over an edge of 1/4” thick.

The mount is attached to the trunk lid by being slipped over the edge then tightening the set screws that ground the antenna. The screws need to penetrate the metal and so can damage paintwork.

There are also universal lip mounts can be used on almost any lip on a vehicle. They can be positioned in any position from horizontal to vertical. These mounts are adjustable in several planes so that the antenna can be upright in virtually any angle. Universal lip mounts are available with or without an attached coaxial cable.

Benefits of using a trunk lip mount NMO mount

• This mount does not need holes to be drilled
• Installation is quick
• The mount is fully and conveniently removable
• Many vehicle owners prefer the appearance as the difference in signal quality and coverage is negligible compared to a roof mount.

retevis MR200 U/V Dual Band NGP Antenna and NMO Mount

Excellent single-section magnetic antenna designed foremost for cars and vans. Characterized by very good technical parameters - a very large extent, minimal amount of interference and a high quality, strong magnetic base with a diameter of 9 cm.

NMO antenna mounts FAQs

Where should mobile antennas be mounted on vehicles?

NMO antennas require a ground plane. This is the key determinant of antenna location on the vehicle and its performance. The ground plane dimensions required vary by the type and frequency of the mobile antenna. For example, a 5/8 wave antenna that operates at 150 MHz needs a 42-inch diameter ground plane. The ground plane diameter reduces as the frequency of the antenna increases; at 800 MHz an 8-inch ground plane is adequate.

Key locations for mounting a vehicle antenna:

1. The roof - this is the optimum location for location a mobile antenna.
2. Trunk - this is good location for placing antenna, ideally the center of the trunk lid.
3. Front fender
4. Rear fender
5. Mirrors
6. Rear window

The best positions have a good amount of ground plane and clearance from obstruction. Of, course a soft-top roof or plastic shell will not be suitable as the mounting material is not conducive.

What is a ground plane?

A ground plane is a metal (electrically conducive surface) that sits beneath an antenna. Ground plane-dependent mobile antennas mounted on a vehicle use the metal of the vehicle as a ground plane, to reflect and radiate radio frequency signals.

NMO antennas use the ground plane to perform optimally. Ideally the surface should be as large and as flat as possible. An inadequate ground plane limits the reflection of the sign and the transmission range of the antenna. It can also damage the radio by altering the signal wave ratio causing feedback into the radio attached to the antenna. Antennas that are ground-plane independent do not need a metal surface.