Ham antenna base mount types for mobile use

Antenna socket / Antenna receiver / Antenna attachment / Antenna mounting

There are a number of coaxial RF connector (radio frequency connector) types being used in the ham radio world. Each connector is designed to work at radio frequencies in the multi-megahertz range. RF connectors are typically used with coaxial cables and are designed to maintain the shielding that the coaxial design offers. Mechanically, they provide a fastening mechanism while maintaining a low ohmic electric contact allowing very high mating cycles and reducing insertion force.

The SO-239 base mount seems to be the most prevalent in ham radio as it is also the most common base connector on the back of radio equipment. The SO-239 mates with the male PL-259 connector on coaxial RF cable. Although SO-239 is not ideally suited for structural loading present in mobile use (on a vehicle) the ham radio community continue to use it as a means to attach an antenna.

Other antenna attachment types are also common in ham radio, the two most frequently encountered being the NMO mount and the 3/8 x 24" CB radio style mount. The NMO mount is typically used on VHF and UHF application while the other is typically seen more in the HF frequency range. Adapters that go between NMO and SO-239 are common.

What you use for amateur radio on your vehicle should depend entirely on what you are doing with the vehicle, what ham bands you intend to operate with the antenna, and the antennas you wish to use. Most ham antennas for repeater operation on the VHF and UHF range will mount to SO-239. That the the connector type ham antenna manufacturers tend to produce antennas for, for better or for worse.

Various mounting apparatus are used with the different connector types. For example, you will find the 3/8 x 24" antenna receiver on a ball mount, bed clamp, trunk lip mount, mag mount (magnetic), fold over mount, mirror mount, post mount, and other varieties. The mounting apparatus is part of the entire antenna mount including the antenna receiver (the hole that you screw the antenna into or the connector you crew it onto.)

SO-239

Also known as the UHF mount. This type of antenna mount is very common in amateur radio. The SO-239 (so239) accepts connections from PL-259, the male counterpart to the cable to antenna mount connection. The base of the antenna itself will have a PL-239 style connector to fit into the SO-239 base.

3/8 x 24 Stud

Also known as 3/8-24 bolt-in thread mount. This type of antenna mount is very common with Citizen Band radio antennas such as the Firestick. It is a 3/8 inch x 24 thread (0.375" or 10mm). The 24 refers to 24 threads per inch which in the U.S. is considered "fine thread" or UNF.

The antenna uses a 3/8" threaded shaft and typically a 1/2" hole is used to allow room for the insulator bushing.

NMO

NMO is an acronym for New MOtorola. This type of antenna mount is very common in Part 90 and public safety radio systems. Police, fire, ambulance and other public safety vehicles make use of antennas with the NMO mount. This is due to the durability of the design, which is superior to the amateur grade SO-239 mount. The NMO mount was developed by Larsen company for Motorola.

NMO mounts are devised to have a standard threaded connector where you screw connect the antenna of choice.

Type N

It is not common to find the Type N connector being used as an antenna base mount, however, some cases do exit.

Glass Mount

For people that don't want to drill holes or stick magnets on their vehicle, the glass mount capacitive coupling antenna provides an alternative which is also a significant compromise. The antenna uses an inner and outer capacitor place to couple both parts using capacitance rather than a direct electrical connection. The connection may be up to 90% efficient on glass not containing any metal contaminants. Receive sensitivity takes a hit and output power cannot exceed a reasonable limit. Typically the system is used on VHF/UHF antenna systems running less than 50 watts.