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
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
6. Environmental considerations
7. Mechanical dimensions
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.
||Typ. Diameter inches
| * At 1,000 GHz (See
text) ** At 300 MHz
Technologies, Inc. and the author retain the rights to all intellectual
This information should be used as a guideline
only to help you in the appropriate selection of an antenna.
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