Complete Guide to Installing NMO Mounts for Mobile Radios

Installing an NMO (New Motorola) mount for your mobile radio is a critical step in building a reliable vehicle communications system. Whether you're outfitting a patrol car, fire apparatus, construction vehicle, or any mobile platform, proper installation ensures optimal antenna performance and years of trouble-free operation.

This comprehensive guide walks you through everything you need to know about installing and connecting NMO mounts to mobile radios, from selecting the right location to making the final connections.

What is an NMO Mount?

An NMO mount is the industry-standard antenna mounting system for mobile radios. Originally developed by Motorola, the NMO interface has become the universal standard for vehicle-mounted antenna installations across public safety, commercial, and amateur radio applications.

The NMO mount consists of two main components:

• The mount base: A threaded housing that mounts to the vehicle surface with an integrated coaxial cable
• The antenna: Screws directly into the mount base with a 3/4"-24 threaded connection

All NMO mount installations require drilling a 0.75 inch (3/4") hole through the mounting surface. The mount includes a super seal gasket that creates a weatherproof barrier between the mount and the vehicle surface, preventing water intrusion when properly installed.

Types of NMO Mounts

Before installation, you'll need to select the appropriate mount type for your application:

1. Standard NMO Mounts

Standard NMO mounts are designed for typical vehicle roof, trunk, or hood installations where the metal thickness is relatively thin (generally up to 1/8" thick). These mounts work with three common cable types:

• RG58 coaxial cable: The most economical option, suitable for shorter runs and lower-power applications
• LMR195 coaxial cable: Low-loss cable offering better performance than RG58, ideal for most mobile installations
• LMR200 coaxial cable: Similar performance to LMR195 with slightly different specifications

2. Thick Surface NMO Mounts

NMO thick mounts are specifically designed for buses, large commercial vehicles, and any application where the mounting surface exceeds standard thickness. These mounts accommodate thicker metal surfaces found on:

• Transit buses and motor coaches
• Heavy-duty commercial trucks
• Utility vehicles with reinforced roofs
• Truck caps and camper shells
• Fiberglass or composite surfaces with backing plates

The extended threading and longer mounting hardware on thick surface mounts ensure a secure, weatherproof installation even on surfaces that would be too thick for standard mounts.

3. High Frequency NMO Mounts

If your radio system operates above 1 GHz (1000 MHz), you must use a high frequency NMO mount. Standard mounts are not designed for frequencies above 1 GHz and will result in poor performance, high SWR, and potential signal loss. High frequency mounts are engineered with:

• Precision impedance matching for frequencies above 1 GHz
• Low-loss cable specifically rated for high-frequency operation
• Enhanced connector quality to minimize signal degradation

Common applications requiring high frequency mounts include cellular boosters, some public safety systems, and specialized communications equipment operating in the 1-6 GHz range.

4. Magnetic Mounts

Magnetic NMO mounts use a strong magnetic base that adheres to metal surfaces without drilling. Perfect for temporary installations, rental vehicles, or situations where you cannot modify the vehicle. While convenient, they may not provide the same performance as permanent mounts due to less optimal grounding.

5. FME Connector Mounts

NMO mounts with FME (For Mobile Equipment) connectors at the cable end offer a significant installation advantage: the connector is already factory-installed on the coaxial cable. This eliminates the need to install connectors to the coax cabling during installation, saving time and ensuring a professional, reliable connection. The FME connector is identified by "FME" at the end of the part number. This is particularly valuable for installers who want to avoid the complexity and potential errors of field-installing coax connectors.

Tools and Materials Needed

For a professional NMO mount installation, gather these tools and materials:

• 3/4" (0.75 inch) hole saw or step drill bit
• Electric drill
• Deburring tool or file
• Measuring tape and marker
• Masking tape
• Silicone sealant (automotive grade)
• Isopropyl alcohol and clean rags
• Wire strippers and crimpers (if not using FME pre-terminated cables)
• Electrical tape or heat shrink tubing
• Fish tape or wire pulling tool
• Multimeter (for testing)
• Torque wrench (optional but recommended)

Step 1: Selecting the Mount Location

Proper antenna placement is crucial for optimal radio performance. Consider these factors:

Electrical Considerations

• Ground plane: The antenna needs a metal surface (ground plane) to radiate efficiently. The center of a metal roof provides the best 360-degree radiation pattern
• Clearance: Ensure adequate clearance from other antennas (minimum 18-24 inches) to prevent interference
• Avoid obstructions: Keep away from roof racks, light bars, or other metal objects that can distort the radiation pattern

Practical Considerations

• Cable routing: Choose a location that allows clean cable routing to the radio location
• Accessibility: Consider how easy it will be to change antennas in the future
• Vehicle structure: Avoid mounting over structural supports, wiring harnesses, or headliner supports
• Aesthetics: Balance performance with appearance, especially for unmarked vehicles

Step 2: Preparing the Mounting Surface

Once you've selected the location:

1. Mark the center point: Use a marker to indicate exactly where the 0.75 inch hole will be drilled
2. Check underneath: Before drilling, verify there are no obstructions, wiring, or structural components directly below the mounting point
3. Apply masking tape: Place tape around the drilling area to protect the paint and prevent the drill bit from walking
4. Clean the surface: Wipe the area with isopropyl alcohol to remove any dirt, wax, or contaminants

Step 3: Drilling the Mounting Hole

For NMO mount installation:

1. Start with a pilot hole: Use a small drill bit (1/8" or 3/16") to create a pilot hole. This helps guide the larger hole saw and prevents walking
2. Drill the 0.75 inch hole: Using a 3/4" (0.75 inch) hole saw, drill through the surface at a steady, moderate speed. Let the tool do the work—don't force it
3. Deburr the hole: Remove any sharp edges or burrs from both the top and bottom of the hole using a deburring tool or file
4. Clean the hole: Remove all metal shavings and debris, then wipe clean with isopropyl alcohol
5. Apply rust prevention: For steel surfaces, apply a rust-preventive primer or paint to the exposed metal edge

Step 4: Routing the Coaxial Cable

The integrated coaxial cable must be routed from the mount location to your mobile radio. For most vehicle installations, follow this standard routing path:

Standard Vehicle Cable Routing

1. From the roof mount: Feed the cable down from the drilled hole
2. Behind the headliner: Route the cable underneath the vehicle's headliner, keeping it concealed and protected
3. Down the door post (A-pillar): Run the cable down the door post trim on either the driver or passenger side, depending on radio location
4. Into the dash area: Continue routing through the kick panel or under the dash to reach your mobile radio mounting location
5. Connect to the radio: Make the final connection to your mobile radio's antenna port

Cable Routing Best Practices

• Choose the shortest practical path while avoiding sharp bends (minimum 2-inch bend radius for most cables)
• Keep the cable away from high-voltage ignition wires, power cables, and electric motors to minimize interference
• Use existing wire routing paths, grommets, or conduits when possible
• Avoid routing through areas where the cable could be pinched by moving parts
• Use split loom tubing or wire conduit to protect the cable from abrasion
• Secure the cable every 12-18 inches using zip ties or cable clamps
• Leave some slack at both ends to allow for service and prevent strain
• Use rubber grommets when passing through metal holes to prevent chafing

Step 5: Installing the NMO Mount

With the hole drilled and cable routed, install the mount:

1. Apply sealant: Apply a thin, continuous bead of silicone sealant around the underside of the mount flange
2. Feed the cable: Thread the coaxial cable through the 0.75 inch mounting hole from underneath
3. Position the mount: Place the mount over the hole, ensuring the cable exits cleanly without kinking
4. Install the super seal gasket: From underneath, install the included super seal gasket, which provides the primary weatherproof barrier
5. Add the metal washer and retaining nut: Install the metal washer and retaining nut. Hand-tighten first
6. Tighten securely: Using a wrench, tighten the retaining nut firmly. The mount should compress the super seal gasket and create a complete weatherproof seal. Do not overtighten, which can crack the mount body
7. Clean excess sealant: Wipe away any sealant that squeezes out from under the flange
8. Allow to cure: Let the sealant cure according to manufacturer's instructions (typically 24 hours) before installing the antenna