RF Coax Cable Connectors
- an overview of the different types of RF connectors that can be used with
coax cable or feeder including the UHF (SO239 / PL259), BNC, TNC, N-type, SMA,
SMB, SMC, MCX, etc..
Coax cable connectors, often called RF connectors are in
widespread use. Wherever radio frequency or RF connections need to be made there
is the possibility of using coaxial connectors. Where signals reach frequencies
above a few million Hertz, these coaxial connectors need to be used. The need
for their use arises because it is necessary to transfer radio frequency, RF,
energy from one place to another using a transmission line. The most convenient,
and hence the most commonly used form of transmission line is coaxial cable
which consists of two concentric conductors, an inner conductor and an outer
conductor, often called the screen. Between these two conductors there is an
insulating dielectric.
Coaxial cable has a number of properties, one of which is the
characteristic impedance. In order that the maximum power transfer takes place
from the source to the load, the characteristic impedances of both should match.
Thus the characteristic impedance of a feeder is of great importance. Any
mismatch will result in power being reflected back towards the source.
It is also important that RF coaxial cable connectors have a
characteristic impedance that matches that of the cable. If not, a discontinuity
is introduced and losses may result.
There is a variety of connectors that are used for RF
applications. Impedance, frequency range, power handling, physical size and a
number of other parameters including cost will determine the best type for a
given applications.
UHF connector
The UHF connector, also sometimes known as the Amphenol
coaxial connector was designed in the 1930s by a designer in the Amphenol
company for use in the radio industry. The plug may be referred to as a PL259
coaxial connector, and the socket as an SO239 connector. These are their
original military part numbers
These coaxial connectors have a threaded coupling, and this
prevents them from being removed accidentally. It also enables them to be
tightened sufficiently to enable a good low resistance connection to be made
between the two halves.
The drawback of the UHF or Amphenol connector is that it has
a non-constant impedance. This limits their use to frequencies of up to 300 MHz,
but despite this these UHF connectors provide a low cost connector suitable for
many applications, provided that the frequencies do not rise. Also very low cost
versions are available for applications such as CB operation, and these are not
suitable for operation much above 30 MHz. In view of their non-constant
impedance, these connectors are now rarely used for many professional
applications, being generally limited to CB, amateur radio and some video and
public address systems.
N-type connector
The N-type connector is a high performance RF coaxial
connector used in many RF applications. This coax connector was designed by Paul
Neill of Bell Laboratories, and it gained its name from the first letter of his
surname.
This RF connector has a threaded coupling interface to ensure
that it mates correctly. It is available in either 50 ohm or 75 ohm versions.
These two versions have subtle mechanical differences that do not allow the two
types to mate. The connector is able to withstand relatively high powers when
compared to the BNC or TNC connectors. The standard versions are specified for
operation up to 11 GHz, although precision versions are available for operation
to 18GHz.
The N-type coaxial connector is used for many radio frequency
applications including broadcast and communications equipment where its power
handling capability enables it to be used for medium power transmitters, however
it is also used for many receivers and general RF applications.
BNC connector
The BNC coax connector is widely used in professional circles
being used on most oscilloscopes and many other laboratory instruments. The BNC
connector is also widely used when RF connections need to be made. The BNC
connector has a bayonet fixing to prevent accidental disconnection while being
easy to disconnect when necessary. This RF connector was developed in the late
1940s and it gains its name from a combination of the fact that it has a bayonet
fixing and from the names of the designers, the letters BNC standing for Bayonet
Neill Concelman. In fact the BNC connector is essentially a
miniature version of the C connector which was a bayonet version of the N-type
connector.
Electrically the BNC coax cable connector is designed to
present a constant impedance and it is most common in its 50 ohm version,
although 75 ohm ones can be obtained. It is recommended for operation at
frequencies up to 4 GHz and it can be used up to 10 GHz provided the special top
quality versions specified to that frequency are used.
TNC connector
The TNC connector is very similar to the BNC connector. The
main difference is that it has a screw fitting instead of the bayonet one. The
TNC connector was developed originally to overcome problems during vibration. As
the bayonet fixing moved slightly there were small changes to the resistance of
the connections and this introduced noise. To solve the problem a screw fixing
was used and the TNC coax cable connector gains its name from the words Threaded
Neill Concelman.
Like the BNC connector, the TNC connector has a constant
impedance, and in view of the threaded connection, its frequency limit can be
extended. Most TNC connectors are specified to 11 GHz, and some may be able to
operate to 18 GHz.
SMA connector
This sub-miniature RF coaxial cable connector takes its name
from the words Sub-Miniature A connector. It finds many
applications for providing connectivity for RF assemblies within equipments. It
is often used for providing RF connectivity between boards, and many microwave
components including filters, attenuators, mixers and oscillators, use SMA
connectors.
The connectors have a threaded outer coupling interface that
has a hexagonal shape, allowing it to be tightened with a spanner. Special
torque spanners are available to enable them to be tightened to the correct
tightness, allowing a good connection to be made without over-tightening them.
The SMA connector was originally designed in the 1960s for
use with 141 semi-rigid coax cable. Here the centre of the coax forms the centre
pin for the connection, removing the necessity for a transition between the coax
centre conductor and a special connector centre pin. However its use extended to
other flexible cables, and connectors with centre pins were introduced.
SMA connectors are regularly used for frequencies well into
the microwave region, and some versions may be used at frequencies up to 26.5
GHz. For flexible cables, the frequency limit is normally determined by the
cable and not the connector.
SMB connector
The SMB connector derives its name as it is termed a Sub-Miniature
B connector. It was developed as a result of the need for a connector
that was able to connect and disconnect swiftly. It does not require nuts to be
tightened when two connectors are mated. Instead the connectors are brought
together and they snap fit together. Additionally the connector utilizes an
inner contact and overlapping dielectric insulator structures to ensure good
connectivity and a constant impedance.
SMB coaxial connectors perform well under moderate vibration
only and the 50 ohm versions are often specified to 4 GHz. 75 ohm versions of
the SMB coaxial connector are also available, but there are often not specified
up to the same frequencies, often only about 2GHz.
SMB coaxial connectors are not as widely used as their SMA
counterparts. They are used for inter board or assembly connections within
equipment, although they are not widely used for purchased microwave assemblies
in view of their inferior performance.
SMC connector
A third SM type connector is not surprisingly the Sub
Miniature C or SMC coaxial cable connector. It is similar to the
SMB connector, but it uses a threaded coupling interface rather than the snap-on
connection. This provides a far superior interface for the connection and as a
result, SMC coaxial cable connectors are normally specified to operate at
frequencies up to 10 GHz.
SMC coaxial cable connectors provide a good combination of
small size and performance. They may also be used in environments where
vibration is anticipated. In view of their performance they find applications in
microwave equipment, although they are not normally used for military
applications where SMA connectors tend to be preferred.
MCX connector
A number of mico-miniatiure RF connectors have been developed
by a variety of manufacturers to meet the growing demand for cost effective,
high quality smaller connectors. These are finding high levels of use, for
example in the cellular phone industry, where size, cost and performance are all
important. In fact the MCX is about 30% smaller in both size and weight than an
SMB connector to which it has many similarities.
One connector that falls into this category is the MCX (MicroCoaX)
coax connector. This was developed in the 1980s by Huber and Suhner of which MCX
is a trade name. The MCX connector has many similarities with the construction
of the SMB connector using a quick snap-on interface, and utilising an inner
contact and an overlapping dielectric insulator structure.
The MCX connector is normally specified for operation up to 6
GHz, and it finds applications in a variety of arenas including equipment for
cellular telecommunications, data telemetry, Global positioning (GPS) and other
applications where size and weight are important and frequencies are generally
below 5 GHz.
MMCX connector
Another connector which is being widely used is the MMCX
connector. Being some 45% smaller than an SMB connector, the MMCX is ideal where
a low profile outline is a key element. It is therefore ideal for applications
where board height is limited, including applications where boards may be
stacked. As such it is being widely used in many cellular telecommunications
applications.
The connector provides a snap fitting and also utilises a
slot-less design to minimise leakage.
Overview
There is a great variety of RF coaxial cable connectors in
use today. The list above describes some of the more popular types of RF
connector, but there are nevertheless more varieties available. When choosing a
coaxial cable connector, the requirements should be carefully matched to the
available options to see which RF connector will provide the best choice. In
this way the best compromise between size, weight, performance and cost can be
achieved.
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