NMO Connectors / Mounting


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Probably one of the most popular RF connectors used on mobile mount antennas is the NMO type connection. NMO stands for “New Motorola Mount and was a standard configuration developed by Motorola for their land mobile radio antennas quite a few years ago. This standard is still used today by many different manufacturers on their antennas. NMO is a style of connection which the antenna will screw on to and can also be referred to as a NMO mount. Both terms call out the same type of connection. The picture to the right shows what the NMO connector looks like with the antenna removed from the mount. The mount can either have a coaxial cable terminated to the back side or a coaxial connector such as a type N or UHF. The “button contact” in the center is tied to the center conductor of the coax cable, and the outer brass threaded area is connected to the braid of the coax. Typically the mount is made out of brass and the center contact is gold plated. The outside ring is threaded to a size and pitch of 1 1/8” x 18.

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NMO mounts come in various styles and the only difference is how the NMO connector is terminated on the opposite side. As we move from left to right, you will see a NMO mount to be used in a ¾ inch diameter hole with a length of coaxial cable attached. This would be used to mount an antenna on a roof of a vehicle. In the lower center is a mount which would terminate in a N-Type female connector on the bottom. This would be used to mount an antenna on an item such as a control box. Above that is a magnetic mount base and to the right is another NMO mount with a cable. What is unique about this version, it is designed to mount in a 3/8 inch diameter hole in panels which are up to ½ inch thick. We’ll look closer at the installation process of the 3/4 and 3/8 inch cables later. These are just a few examples of the styles of NMO mounts available. When the antenna is mounted properly on these mounts, you will have a water tight seal where the NMO mount penetrates the metal base. This is because on the underside of the brass nut, there is a rubber O-ring seal. We also have a seal when the antenna is screwed on to the NMO mount. The result is a connection would be waterproof and can be exposed to the outdoor elements.

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Depending on the manufacturer, the appearance of a NMO type mount on an antenna can vary. However, even though they appear different, since they are NMO type mounts, they will work with any NMO mounts we discussed earlier. As you can see in the picture, the center contacts vary on these antenna bases. The one on the left uses a simple “leaf spring” contact for the antenna connection to the center pin on the NMO mount. The center and right antenna use a spring loaded contact for the center connection. The advantage of the spring loaded contacts is that they are less likely to have connection issues over time. I have seen some of the “leaf spring” types deform and loose contact after several years of use.

Now let’s have a look at how the cables are installed. As I mentioned, when the cables are installed properly, this will give a weather tight seal from the NMO mount on the “outside” to the cable or termination on the inside of the metal panel. The most practical application for the NMO type cables is to mount an antenna on a vehicle. And there are two types typically used and we’ll start with probably the most common style, the ¾ inch NMO mount. The name of this mount implies the size of the hole that has to be drilled in order to secure this mount. A ¾ inch hole is drilled in the metal surface typically with a special hole-saw such as this.
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Notice how on the just above the teeth on this hole saw, there is a “stop”. This saw can only be used in thinner gauge metals. This is intentional because when you are drilling a ¾ inch hole in the roof a vehicle, you do not want to accidently come into contact with the head liner on the interior. Here is a side view of how it stops a pre-determined distance when drilling. There are several manufacturers who make this type of hole saw for installing the NMO type cable mounts. A Ripley brand is shown, but Laird also makes one. Typically, on this style of hole saw, the blades are replaceable so you don’t have to replace the complete unit.

The ¾ inch hole mount cable is shown below in the left picture. In the right picture is a 3/8-inch hole mount which is designed to be used with thicker materials. The 3/8-inch hole mount will go up to about ½ inch thicknesses, which allows it to be used on thicker materials such as plastic or fiberglass. Besides the brass nut on the top of the 3/8-inch hole mount, there is an adjustable nut on the bottom that is adjusted upward against the bottom of the roof or panel.

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Due to the design on the ¾ inch NMO mount it is installed from the outside of the vehicle to the inside thru the hole you drilled. Otherwise you start off by feeding the bare end of the coax cable first thru the hole until you are about ready to slide the mount into the hole you made. The advantage of this is it allows you to “fish” the cable thru a vehicle without having to pull the interior head liner. Here you notice how the top side nut is screwed up to allow it to be slipped into the hole. The mount is held at a slight angle like this so you can maneuver it into the hole.
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Another feature to point out on the ¾ inch hole mount, is that the outer threaded area is slightly larger diameter than ¾ of an inch. So even with the outside nut off the mount (as pictured here), it is impossible for it to fall thru the hole you drilled.
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The bottom side of this mount has a slightly “toothed” area which will actually bite into the metal so that when the top nut is tightened there is usually no need to use a spanner wrench to hold the center area to keep it from spinning.

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On the 3/8-inch hole mount, it must be installed up thru the bottom. The 3/8-inch hole is not large enough to fish the mount thru as it is done on the ¾ inch hole mount. As you can see in this side shot, the bottom nut has been adjusted up to accommodate a thinner mounting material. You can see the potential on how much thicker the mounting surface can be for this mount. Also on this mount, there is no need to hold the bottom nut while tightening the top brass nut. It is designed to “bite” into the bottom side so it does not spin while tightening the top.

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Depending on the antenna manufacturer or model, some do require an additional gasket to be placed between the mounting surface and the bottom of the antenna. Typically these gaskets will be supplied with the antenna itself. They fit snugly around the outside nut on the NMO mount.
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And this is the finished result of the gasket around the NMO mount.
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And last the antenna is screwed on to the mount. It is suggested to tighten the antenna slightly with a wrench. For the style of antenna show, I have simply used pliers on the knurled chrome piece and snug it slightly at that point. That will ensure the antenna will not loosen if in a high vibration application such as construction or farming equipment.

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Some antennas do not require the separate gasket because there is an O-ring built into the bottom side of the antenna. Here is an example of that style of antenna being installed on the NMO mount. With either style of antenna used, there is essentially a double seal made at the hole once the antenna is installed. The NMO mount (3/4 or 3/8 size mount) has an O-ring gasket on the bottom of the top nut and the antennas themselves also have a rubber seal. The result is the sealing properties are very good. Imagine an emergency vehicle driving at high speeds down the high way in heavy rains. There will be no leaking at that point when the installation is done properly.
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The NMO mount is an excellent choice to use when an antenna needs to be installed for an outdoor application. Whether it’s on a roof of a vehicle or top of an outdoor control box, this system will easily provide years of reliable operation.
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Applicable Part numbers

Connector Part Numbers
931-1303-ND 931-1343-ND 931-1302-ND 931-1309-ND 931-1305-ND 931-1342-ND 931-1306-ND 931-1307-ND 931-1304-ND 931-1308-ND 931-1311-ND 931-1312-ND 931-1310-ND 931-1313-ND
CAB.V09 CAB.W05 CAB.V08 CAB.W09 CAB.V11 CAB.V05 CAB.V12 CAB.V13 CAB.V10 CAB.W08 CAB.W11 CAB.W12 CAB.W10 CAB.W13