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Print PageRF Coaxial Connectors

By Joseph H. Reisert

RF coax(ial) connectors are a vital link in the radio spectrum. Coax connectors are often used to interface two units such as the antenna to a transmission line, a receiver or a transmitter. The proper choice of a coax connector will facilitate this interface.

Coax connectors come in many impedances, sizes, shapes and finishings. There are also female and male versions of each. As a consequence, there are thousands of models and variations, each with its advantages and disadvantages.

Fortunately, coax connectors are broken down into a few dozen "series." There are some series and standard types that are most popular and will work in most installations. This application note will try to give you some guidance as well as take some of the mystery out of selecting the proper coax connector for your application.

Coax Connector Parameters

To properly select a coax connector, you first must know these 10 things:
1. Frequency of operation
2. Characteristic impedance
3. Insertion loss
4. Power handling capability
5. Applications
6. Environmental considerations
7. Mechanical dimensions
8. Durability
9. Finish
Some of these parameters are shown on Table 1.

The impedance of coax connectors is very important to prevent VSWR mismatches. All RF coax connectors are designed to have low VSWR (typically 1.1:1 or better) and very low insertion loss (<0.05 typical and 0.1 dB maximum) over a very wide bandwidth with a minimum of variations. Power handling is a function of the diameter of the dielectric, the dielectric material used internally and the frequency size of the center pin. As with transmission lines, the higher the frequency, the higher the VSWR mismatch and insertion loss and the lower the power handling capability of the connector.

Before selecting a connector, it is important to know the application. Will it be used once or will it be inserted numerous times? Is it going to be outside and need to be weatherproof?

The mechanical dimensions are also important and are a function of the connector series. Also whether it is a female or male connector. The finish and plating are also important. Most connectors use some form of nickel plating, but some of the more expensive types are silver or gold plated.

Finally, connector interfaces may be threaded, bayonet, snap-on or push-on. Furthermore, the optimum connector isn't always possible so tradeoffs are often necessary.

Coax Connector Series
Coax connectors are usually referred to by series designations. Fortunately, although there are thousands of different coax connectors on the market, there are only about a dozen or so groupings or series designations. Each has its own important characteristics, The most popular RF coax connector series not in any particular order are UHF, N, BNC, TNC , SMA, 7-16 DIN and F.

RF Coaxial Connectors
The "UHF" connector is the old industry standby and was really the first connector developed for RP communications above 50 MHz. In the days when it was developed during World War II, 100 MHz was considered UHF.

The UHF connector is primarily an inexpensive all purpose screw on type that is not truly 50 Ohms. Therefore, it's primarily used below 300 MHz. It is also difficult to waterproof. It can handle moderate power (500 Watts through 300 MHz). Although sometimes used up to 500 MHz, it is not recommended above about 300 MHz since it will induce VSWR mismatches and higher insertion loss. The popular UHF male connector is often referred to as a PL-259.

"N" connectors were developed at Bell Labs soon after World War II so it is one of the oldest high performance coax connectors. It has good VSWR and low loss through 11 GHz with wiping contacts and is waterproof. N connectors will handle reasonable power (300 Watts through 1 GHz), have a threaded interface and are the workhorse of the industry.

"BNC" connectors have a bayonet-lock interface and thus has a much lower cutoff frequency and higher loss than the N connector. They are frequently used where numerous quick connect/disconnect insertions are required.

"TNC" connectors are an improved version of the BNC with a threaded interface. Since it is smaller than an N connector, its power handling capacity is reduced.

"SMA" or miniature connectors became available in the mid 1960's. They are primarily designed for semi-rigid small diameter (0.141" OD and less) metal jacketed cable. They are quite small and have extended frequency range.

"7-16 DIN" connectors were only recently developed in Europe. Hence the part number represents the size in metric millimeters and DIN specifications. This connector series was primarily designed for high power applications where low intermodulation is important; such as, locations where multiple high power transmitters and antennas are collocated (like cellular poles). It is much more expensive that most other connector types.

"F" connectors were primarily designed for very low cost high volume 75 Ohm applications much as TV and CATV. They have a very unique construction in that the center wire of the coax becomes the center conductor.

Reverse Polarity Connectors
Recently, the FCC has required that suppliers of RF LANs (local area networks) have an RF interface that cannot be matched by the present available RF connector series. The idea is to prevent connecting higher gain antennas etc. to low power FCC Part 15 spread-spectrum devices. As a result, several so called "reverse polarity connectors" have been designed. The reverse polarity TNC is one of the most popular where the threads are left-hand instead of the conventional right-hand type.

We have tried to review many of the more important parameters involved in the design and specification of RF coax connectors. Many of the most popular connector series have been described along with reference Table 1. Antenna suppliers can often help to recommend the optimum connector for your application.

Table I. Standard coax cable connector series and typical specifications.
Series Frequency GHz Power Watts* Typ. Diameter inches Relative Cost
BNC 0-4 80 0.6 Low
F 0-0.9 N/A 0.44 Low
N 0-11 300 0.8 Moderate
SMA 0-18 100 0.4 Moderate
TNC 0-11 100 0.6 Low
UHF 0-0.3 500** 0.85 Low
7-16 DIN 0-7.5 2,500 1.25 High
* At 1,000 GHz (See text) ** At 300 MHz

Astron Wireless Technologies, Inc. and the author retain the rights to all intellectual property contained herein.
This information should be used as a guideline only to help you in the appropriate selection of an antenna.

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