RF CONNECTORS, ATTENUATORS AND LOADS, TOGETHER WITH THEIR USES

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There are scores of RF connector types, ranging from the "cheap and cheerful" to the extremely expensive types intended for use in professional and military applications.  Whilst amateur radio operators may be tempted to use the former type, it is often a false economy and the best quality and most appropriate connector for any given application should always be used.  The cost of new top quality professional connectors often precludes their use in amateur projects but there are usually many such devices available second-hand at rallies, at local club junk-sales and on the Internet.

The information on this page has been obtained from a variety of sources including Ilpo J Leppänen OH5IJL, who provided data on the MUSA 4/13 COAX connectors.  Ilpo has a very interesting web site devoted to connectors but unfortunately the text is in Finnish.

Being electro-mechanical devices, connectors are statistically relatively unreliable, along with switches, relays and similar components.  The majority of RF connectors used by the amateur radio fraternity are coaxial types, although single wire antennas, 600ohm open wire line and 300ohm ribbon feeder are sometimes connected to equipment using purpose made connectors, banana plugs/sockets or screw terminals (binding posts in the USA).  The use of crocodile clips should be avoided at all costs, both on the grounds of safety/reliability and RF efficiency.

All the normally available connector types are detailed in this article and the more esoteric types are listed for completeness.  To avoid repetition, the terms normally used when referring to all types of RF connectors, cables and associated devices, are defined below:-

Attenuation              Reduction in signal amplitude resulting from using a connector, cable
                         or other device.  This attenuation is often referred to as loss.
Attenuator               See "Coaxial attenuator" below.
Bayonet fitting          A system in which the connector is retained in its mating part by a
bayonet and slot arrangement.

Body The main part of the connector (same as "shell"), usually connected
to the outer braid of coaxial cable.
Braid The screening layer surrounding the inner conductor of a cable.
Cable clamp The arrangement by which the cable is mechanically retained within
the connector.
Cable mounting The situation where the connector is mounted directly on a cable.
Centre pin The pin connected to the inner of the coaxial cable.  Note that this
"pin" may be either a solid male pin or a socket intended to mate
with a pin.
Characteristic impedance The characteristic impedance of the connector, which should be chosen
to equal that of the associated cable.

Chassis mounting The situation where the connector is mechanically mounted on a metal
chassis or panel. Coaxial attenuator
A device possessing a known signal amplitude reducing factor, which is usually stated in decibels. Coaxial load A device which terminates a coaxial line and dissipates any power being fed down the line. Controlled impedance A term used to describe a connector in which the physical dimensions
are closely controlled such that the characteristic impedance is
maintained within the connector.

Crimp A system whereby the centre conductor and/or outer braid is clamped
by compressing a metal tube around it, rather than employing soldered
joints.
Dielectric The insulating material that separates the centre pin from the shell.
EMP Electro-magnetic pulse, such as those produced by a lightening strike
or a nuclear explosion.
Female A term describing the "socket" element of a connector.  Note that
female shells can be fitted with either male or female pins, or a
mixture of both. Fibre-optic cables and Cables and connectors that employ thin glass fibres to optically Connectors transmit data.

Flexible cable A coaxial cable that may be flexed, as opposed to semi- rigid coax. Hermaphrodite A connector system where the two halves of a mating pair are identical,
i.e. not specifically male or female.

Hermetically sealed A method by which the ingress of gasses, liquids or other contaminants
is prevented by the use of seals and "O" rings.
Insertion loss The RF loss, expressed in dB, introduced by using the connector.
Inter-series adaptor A device for inter-connecting connectors from differing series.  They
may be male to male, female to female, male to female or female to male.
Load See "Coaxial load" above. Low loss Low insertion loss and, by implication, good VSWR.
Male A term describing the "plug" element of a connector.  Note that male
shells can be fitted with either male or female pins, or a mixture of
both.
Mating part The socket/plug associated with a plug/socket.
Non-controlled impedance A term used to describe a connector in which the physical dimensions
are not closely controlled such that the characteristic impedance is not
necessarily maintained within the connector.
Plug The half of the connector pair in which the shell is male.  Note that
male shells can be fitted with either male or female pins, or a mixture
of both.

Push fit The method in which the two halves of connector pairs are retained in
the mated condition by friction. Retained pin A centre pin that is mechanically retained within the connector, rather
than relying on the centre conductor of the cable, to retain it in place. Screw fitting A system in which the connector is retained in its mating part by a
threaded ring.

Semi-rigid cable Coaxial cables employing a solid copper outer in place of the more normal
braid, and are therefore non-flexible.  These cables are normally used
at microwave frequencies.
Shell The main part of the connector (same as "body"), usually connected to the
outer braid of coaxial cable.

Shield The screening layer surrounding the inner conductor of a cable. Side cable entry A description of connectors where the cable enters the shell radially,
rather than the more common axial system.
Snap-fit A push-fit connector that employs a definite mechanical indent to retain
it in place. Socket The half of the connector pair in which the shell is female.  Note that
female shells can be fitted with either male or female pins, or a mixture
of both.

Triaxial cable A cable consisting of one centre conductor and two outer concentric
shields with an insulating layer between them.
Twinaxial cable A cable in which two insulated conductors are twisted together and
surrounded by a single shield.
CONNECTORS FOR SINGLE WIRES

The four ways of connecting a single wire to equipment are soldering to a tag mounted on a stand-off or feed-through insulator, using a binding post, using a banana plug and socket arrangement or using electrician's connection block ("choc-bloc").  The original electrician's connection blocks were made of porcelain and these are perfect for this application.   Modern choc-bloc is made from various types of plastic and care should be taken in choosing a type that has good RF performance, such as those employing polythene.  Even with these simple methods, there are potential problems associated with the materials used to provide insulation.  When running powers of around 400W, several thousand volts can exist at the driven end of a long wire antenna when the feed impedance is high (half wave, end fed, antenna).

Stand-off and feed-through insulators should be manufactured from glass, porcelain, PTFE, polystyrene or some similar material.

Binding posts are normally manufactured from bakelite or a similar plastic.  Although these devices are designed to withstand several hundreds of volts at DC or very low frequencies, they are not intended for use at radio frequencies, where they exhibit considerable dielectric loss and parasitic capacity to earth.  When used at RF, heating will occur, which can lead to breakdown.  A solution is to mount the binding post on a sheet of polstyrene, which is mounted over a large hole in the metal panel, thus considerably increasing the effective path-length between the high RF voltage and earth.  Versions of the binding post using ceramic insulation have been produced in the past (e.g. those incorporated into the American WW2 Command Transmitters), which were specifically intended for high voltage RF use.  A similar situation exists with banana sockets and the same methods of breakdown prevention should be employed when using these components.

CONNECTORS FOR OPEN WIRE LINE

As far as connection devices are concerned, all the comments relating to those used with single wires apply equally to those used with open wire lines, except two devices are required.  The spacing between the two connection devices is normally equal to the spacing between the conductors forming the open wire line.  The photograph below shows one of the feeder connections used in the WW2 medium wave broadcast transmitting system, code named Aspidistra, which was located at Crowborough, East Sussex.  Assuming that the line impedance was 600ohms and the transmitter was running 500kW, an RF potential of over 17.3kV would have existed between the conductors.

When using electrician's connection blocks employing plastic insulation, it is advisable to avoid using adjacent "ways" and the metal inserts should be removed from the unused positions.  This is not necessary when using porcelain types.

CONNECTORS FOR 300 OHM RIBBON FEEDER

It is probably advisable to terminate ribbon feeder in a similar manner to that used for open wire line.  Purpose made connectors, designed for terminating ribbon cable, are not generally available, although an examination of the junk-box or a visit to a rally may yield some obscure special purpose device that will fill the bill.   Assuming a 300ohm line impedance and an RF power of 400W, the potential between the conductors will be around 350V, meaning that voltage breakdown should not present too much of a problem.  However, the use of VHF receiver antenna connection strips, which often use Paxolin as the dielectric, should be avoided.  Although they are about the correct pin spacing, the small "figure-of-eight" mains connectors such as those used on some domestic radios, shavers and kettles, should not be used for safety reasons.  Mains connectors must only ever be used to connect mains leads.

 

COAXIAL CONNECTORS

As mentioned earlier, there are a very large number of coaxial connector types.  All the types usually found on new, second-hand, or ex-government equipment are described below.   With all types of coaxial connector, it is vital that the cable is fitted to the connector in the manner defined by the manufacturer.  This is particularly true when considering the coaxial outer braid or shield.  The correct method is seldom the easiest or even the most apparently obvious as many connectors such as N and BNC types use a bewildering array of special washers, ferrules, neoprene gaskets and O rings.  All these items are necessary, so if, when you have finished fitting the connector to the cable, you have something left over, you have made a mistake.  It is quite common to hear radio amateurs complaining that they have a "dodgy cable" or an "intermittent" connector.  It is almost never the cable or the connector that is "dodgy" or "intermittent" but it is usually a case of bad practice, poor assembly, bad soldering or plain carelessness.  A very young G3NPF was once told by his boss that "there were only two people in the Company who could fit connectors to cables, you and me, and you're no good at it".

It should be noted that some multi-pin connectors, both circular and rectangular, include one or more coaxial "pins" or include only coaxial "pins".  These are usually intended to be used with miniature or sub-miniature coaxial cable and often require special purpose tools to correctly fit the connector to the coaxial cable.  Connectors of this type are very expensive and are normally only found in military or "top-end" professional equipment.

Jack Plugs and Sockets

Strictly speaking, these connectors are not really "coaxial" and should never be used at frequencies above 50kHz.  They are included in this listing for completeness.  There are three normally used sizes, namely 2.5mm, 3.5mm and 6.3mm (1/4 inch) diameter.  All sizes have the same basic format, are available in mono or stereo versions and may be used with screened cable or with separate wires.  Plugs are invariably intended for cable mounting but sockets are available in either chassis mounting or cable mounting versions.  Very occasionally, the smaller diameter types are used as low voltage DC power connectors but this is not good practice as the plug will be "live" whenever the power supply is operating.

Concentric Barrel Connectors

These connectors, although coaxial, are intended for low voltage power applications and should never be used for RF.  There are at least forty different types with various outer and inner diameters.  A representative selection is shown below but all types have the same basic format.  They may be used with screened cable or with separate wires.  The outer sleeve is always the ground connection, with the power being connected to the centre "pin".  The normal convention is that the outer sleeve is the negative side of the supply, with the positive side being connected to the centre "pin", but this arrangement is not always adhered to.

The corresponding sockets are almost invariably chassis mounting types.

Car Radio Antenna Connectors

The RF signal input connectors used on car radios are unique to this application and are non-controlled impedance, unsealed, push-fit male plugs on the cable mating with unsealed chassis mounting sockets.  They are cheaply made and not intended to be repeatedly inserted and removed from the mating socket on the equipment.  The mating socket is usually very cheaply made using paxolin as the dielectric.  The dielectric in the plug is usually polythene or paxolin and the plugs may employ a cable clamp, as shown in the photograph, or they may be crimped onto the cable or be permanently moulded on.  To prevent the plug becoming detached by the fairly hostile vibation experienced in motor vehicles, the centre pin and the earth sleeve are longer than in most other coaxial connectors, providing greater grip.  These connectors are intended to be used in "receive only" applications at frequencies up to 150MHz.

Phono Connectors

Phono connectors were originally developed by RCA in the USA for use on domestic and semi-professional equipment.  They are designed for use at audio and video frequencies, although they have been used on commercially manufactured amateur radio equipment at low power and frequencies up to 30MHz.  Phono connectors are not sealed and their characteristic impedance is not controlled.  The cable mounted half is always the male plug while the female half is always chassis mounting.  Colour coded versions are available.   Several insulating materials are used, depending on the manufacturer and quality of the connector.   The male half of the connector is a push fit into the female socket and is not otherwise retained.  A screw ring retained version is known to exist but this is a very rare "special".  The metal parts are usually tin or nickel plated steel but gold plated versions are available.

Standard TV connectors complying with IEC Spec 60169-2 (also known as Belling-Lee plugs/sockets)

In the UK, the antenna input ports of TV receivers have almost always been fitted with connectors developed in the 1940s by the Belling-Lee company.  With the advent of UHF TV in the early 1960s, tests were carried out to ascertain whether this type of connector could be retained for use at those frequencies.  The tests revealed that the good quality chassis mounted sockets then available were a reasonable match to 75ohm cable but the corresponding cable mounted plug had a lower characteristic impedance of around 50ohms.  Chassis mounted plugs were a reasonable match to 75ohm cable but cable mounted sockets also had a characteristic impedance of around 50ohms.  However, it was thought that this was of more academic interest than practical importance, although modern digital broadcasting protocols may suffer from reflection problems if the aerial system VSWR is too high.  This implies that standard TV connectors are usable up to about 800MHz, exhibiting a reasonable VSWR and fairly low loss, provided that they are correctly fitted to the cable.  The cable braid should be teased out and then trapped between the cable clamping ferrule and the body of the connector.  The centre conductor of the cable must be threaded through and soldered to the centre pin of the connector in the plug versions or soldered to the appropriate solder bucket in the socket versions.

The impedance of these connectors is not controlled but can be regarded as being between 50ohms and 75ohms.  The shells of commercial versions are usually manufactured from aluminium but military versions were made using nickel plated brass.  The centre pin is retained and is usually manufactured from silver plated brass.  The insulating material is usually polythene but other cheaper materials are found in lower quality devices.  Both male and female halves are available in cable and chassis mounting versions and in both versions the male plug is a push fit into the female socket and is not otherwise retained.

Belling-Lee connectors were never intended to be used with high RF powers but they have been used successfully at 50MHz with powers of 100W.

Versions of these connectors have been produced, in which the cable is connected to the connector elements by means of small screws.  These types of connector should be avoided as the VSWR is not likely to be within acceptable limits and the losses are likely to be much higher than those associated with the "standard" types.

Miniature Belling-Lee Connectors

Miniature versions of the standard TV antenna connectors have been made and were used on some military and professional equipments.  In the photograph below, the standard size plug is shown for comparison.

Pye Connectors

Pye connectors were developed by the Company of the same name in the 1940s and were used on many British military equipments.  By modern standards, they are truly dreadful connectors, being unsealed with a totally uncontrolled characteristic impedance.  Although, during WW2, they were used at frequencies up to 100MHz and perhaps even higher, it was because there was nothing else available in the UK, rather than because they were suitable for purpose.

The two halves of a mated pair are retained using a wire clip.  The female half was always chassis mounting and was made of nickel plated brass or steel and the dielectric was usually paxolin.  The female shell was always fitted with a male centre pin.  The male half was always cable mounted and had a female centre pin.  The dielectric was usually polythene and paxolin.  As the male and female halves of the connector were made of dissimilar metals, there was always the risk of serious corrosion in damp environments, especially as they were not sealed in any way.

It is not recommended that this range of connectors be used by home constructors unless they are to be used with existing equipment that cannot be modified for historical interest reasons.

Pattern 12 or "Miniature Pye" Connectors

Pattern 12 or "Miniature Pye" connectors are not, as their name would imply, small versions of the classic Pye connectors described above.  They are totally different, hermetically sealed, fairly small, coaxial connectors with retained centre pins, much favoured by the British Military in the 1950s and 60s.  They are available with either male or female shells and either may be chassis or cable mounted.  All types of shell may be fitted with male or female centre pins.  The two halves of a mated pair are retained using a threaded ring.  The characteristic impedance of this range of connectors is not controlled but is thought to be around 50ohms and they may be used up to 100MHz and 100W.  The dielectric appears to be polythene.

It is surprising that a search of the Internet does not provide much information about this very useful and fairly rugged range of connectors.  It is not thought that these connectors have ever been used on commercially made amateur radio equipment but there is no reason why the home constructer should not use them, assuming he has acquired adequate numbers of spares to account for future requirements.

F Series connectors

F connectors are 75ohm devices that are universally used in TV satellite systems for connecting the LNB to the set-top box.  These connectors use the solid inner conductor of the coaxial cable as the center pin.  Fitting is accomplished by folding the braiding of the cable back over the outer PVC sheath and "threading" the connector shell onto the cable, allowing the cable inner to protrude through.  There are several sizes of connector shell to cater for different types of cable and it is essential to ensure the connector shell is the correct size.  It is also essential to ensure that any connectors used in the open air are adequately waterproofed using self amalgamating tape.  The connectors are usable up to 1GHz.

General Radio Hermaphrodite Connectors

The General Radio Company in the USA developed the GenRad 874 connectors which are hermaphrodite and have a nominal characteristic impedance of 50ohms and good performance up to 8.5GHz.  There are locking and non-locking versions.

GR also developed the GR900BT series of hermaphrodite coaxial connectors for use in critical laboratory applications.  These locking connectors have a characteristic impedance of 50ohms and good performance up to 8.5GHz.  The principle of operation is similar to that of the Dezifix connector.

Dezifix Connectors

Dezifix connectors are very large, quick release, hermaphrodite coaxial connectors, retained by a ring operated cam locking device.  These connectors are precision devices, normally only used on test equipment and are generally available in 50ohm, 60ohm and 75ohm versions but could easily be made for other characteristic impedances if required.  The dielectric is usually polystyrene and the metal parts are silver plated.  The external diameter of the shell is the same for all impedances but the internal diameter and the diameter of the centre pin are chosed depending on the impedance required.  It should be noted that all impedance types are mechanically inter-mateable with each other, meaning that it is essential to ensure only similar types are inter-connected, otherwise serious electrical mis-match and high VSWR will result.  This type of connector was developed by the Rohde and Schwarz Company and used primarily by them.  The principle of operation is similar to that of the GR900BT connector.

Burndept Connectors

The Burndept Company developed this series of fairly large, non-controlled impedance coaxial connectors in the 1950s that were used in a variety of professional and military applications.  They are not sealed and mated connectors are retained by a threaded ring.  Axial or radial cable entry versions were made.  These connectors are no longer manufactured and their use in amateur radio applications should be limited to interfacing with existing equipment where modification to more modern connectors is not an option.

UHF Series

UHF connectors are often, erroneously, called "PL259" (plug) and the "SO239" (socket).  These are the Amphenol catalogue numbers, not the generic series designations.  The UHF series was developed in the USA in the 1940s and were the first purpose designed coaxial connectors.  They are usable up to 300MHz, which was regarded as UHF at the time.  These connectors are non-controlled impedance, non-sealed types with screwed sleeve retention and are very rugged.  They are used almost exclusively on amateur and PMR equipment up to 150MHz and on many test instruments and items of ex-government equipment.

There are many varients to cater for a variety of cable sizes and cost considerations.  The better quality types employ PTFE as the dielectric and the metal parts are silver plated.  Cheaper versions employ several different dielectric materials, some of which melt at soldering temperatures and the metal parts are often nickel plated.  These cheaper types should be avoided if possible, particularly when fitting the connector to large diameter coaxial cable such as RG8U or RG214.  Considerable amounts of heat are required to adequately solder the braid to the shell and this can melt the dielectric in cheaper types of UHF connectors.  The center pin is always mechanically retained in the dielectric.  All types are available with normal axial cable entry or as right angle or elbow versions.

The male connector shown below can be used with RG214 cable but if it is to be fitted onto RG58U cable, a reducer (also shown) should be used.  Some versions intended for use with RG58U and similar cables employ a crimp connection for the braid.  The use of these types should be avoided unless the correct crimping tools are available, as it is virtually impossible to achieve satisfactory crimps without them.

BNC Series

BNC connectors are controlled impedance types employing a bayonet retention system to permit quick mating and unmating.  They are available in 50ohm versions for use up to 4GHz and 75ohm versions for use up to 1GHz.  The mating parts of the 50ohm and 75ohm versions are identical, the impedance change being effected by reducing the dielectric in the higher impedance version.  Consequently, the two versions are intermateable but the VSWR could be slightly degraded if the impedances are mixed.  Better quality versions employ retained centre pins and the dielectic is almost always PTFE.  Different cable sizes are catered for by the use of appropriate clamp rings and ferrules.

The BNC series of connectors is widely used in amateur, professional and military equipment of all types.  Some versions employ a crimp connection for the braid.  The use of these types should be avoided unless the correct crimping tools are available, as it is virtually impossible to achieve satisfactory crimps without them.

TNC Series

The TNC series is essentially the same as the BNC series except that retention is achieved by using a threaded ring in place of the bayonet arrangement.  This results in a more stable connection, with less likelyhood of electrical noise and ingress of contaminants.  50ohm versions are useable up to 11GHz with precision designs able to work up to 18GHz.

The TNC series of connectors is widely used in professional and military equipment of all types.

C Series

The C series are controlled impedance types employing a bayonet retention system to permit quick mating and unmating.  They are larger than the BNC types and are consequently more rugged, being similar in size to the UHF and N series.  C connectors are only available in 50ohm versions and are useable up to 11GHz.  They can be used reliably at high voltages.

The C series of connectors is used in professional and military equipment of all types.

N Series

The N series are threaded coaxial connectors developed in the 1940s and were one of the first connectors capable of carrying microwave-frequency signals.  Originally, the connector was designed to carry signals of up to 1 GHz in military applications, but the common Type N of today handles up to 11 GHz comfortably.   More recent precision enhancements to the design have increased this to 18 GHz.  The N connector is a controlled impedance type and comes in 50ohm and 75ohm versions.  The male plug is retained by a threaded ring.  The 50 ohm version is widely used in land mobile, wireless data, paging and cellular systems.  The 75 ohm version is primarily used cable television systems.  Connecting these two different types of connector to each other can lead to damage or intermittant connection due to the difference in diameter of the center pin.  Unfortunately, many type N connectors are not labeled, and it can be difficult to prevent this situation in a mixed impedance environment.  The 50 ohm type N connector is favored for home constructed wireless LAN antenna systems, which run at 2.4 GHz or 5 GHz.  50ohm N connectors are also commonly used on amateur radio equipment operating at frequencies above 150MHz.  N type connectors are capable of operation at power levels up to 1kW.

Both male and female connectors can be either cable or chassis mounting and the metal parts are usually silver plated.  All types are available with normal axial cable entry or as right angle or elbow versions.  Precision types are often made from silver or gold plated stainless steel.  The dielectric is usually PTFE, even in the cheaper varieties.  Most types provide neoprene seals to render a mated pair substantially waterproof, provided the cable has been correctly installed using all the correct O-rings amd special washers.  The braid is normally clamped to the shell, rather than relying on a soldered joint.  Some versions intended for use with RG58U and similar cables employ a crimp connection for the braid.  The use of these types should be avoided unless the correct crimping tools are available, as it is virtually impossible to achieve satisfactory crimps without them.

The centre pin is normally mechanically retained in the dielectric but some cheaper connectors have a centre pin retained only by the inner conductor of the coaxial cable.  This latter type should be avoided if possible.

SMA Series

The SMA series are miniature, precision, 50ohm controlled impedance, threaded connectors originally developed for terminatng sem-rigid coaxial cables in military applications where they are useable up to 18GHz.  Versions exist for fitting to flexible cables where the upper frequency limit is reduced to 12.4GHz.  Special versions are available for use up to 26.5GHz.  The SMA series of connectors are manufactured from beryllium copper, brass or stainless steel and are normally gold plated.  The dielectric is PTFE and in the semi-rigid versions there is no separate centre pin, the inner of the cable forming the center connection.  To achieve optimum reliability and performance, particularly under adverse environmental conditions, it is essential to tighten the retaining ring to the correct torque, for which purpose custom made torque spanners are available.

SMA series connectors are being used in the latest amateur radio equipment and are frequently used in professional communication and test equipment.  Some versions intended for use with flexible cables employ a crimp connection for the braid.  The use of these types should be avoided unless the correct crimping tools are available, as it is virtually impossible to achieve satisfactory crimps without them.

SMB Series

The SMB series can be regarded as "snap-on" versions of the SMA series and are useable up to 4GHz.  SMB series connectors are being used in the latest amateur radio equipment and are frequently used in professional communication and test equipment.  This series of connectors is normally only used with flexible cables.

SMC Series

The SMC series is a threaded version of the SMB series and is useable up to 10GHz.  SMC series connectors are being used in the latest amateur radio equipment and are frequently used in professional communication and test equipment.  This series of connectors is normally only used with flexible cables and is not inter-mateable with the SMA series.

MUSA Series

"MUSA" is an acronym for "Multi-User Steerable Array" or "Maximum Use (of) Space Available".  The first acronym relates to a connector apparently used for manually switching signals in very early experimental radar systems.  Both acronyms may refer to the same series of connectors, the latter referring to a series of connectors used by the BBC and other broadcasting authorities in 75 ohm video distribution systems, typically used with a double screened, application specific, cable.  There are at least two different connector ranges in the MUSA series, that shown in the first picture below may have been designed and developed by the BBC but it was also often used in British telephone exchanges, so may have been designed by the GPO when they were responsible for the UK telephone system.  The second picture shows a similar connector, designated 4/13 COAX, which may have been designed by Siemens and is used mainly on professional broadcasting equipment.  The MUSA series of connectors are not impedance controlled but are designed to be nominally 75 ohm.

             

F & E Connectors

This series of connectors was originally designed and manufactured by a company called "Films and Equipment Ltd." and comprised a cable mounting male plug that mated with a chassis mounting female socket.  They are intended for use with co-axial cable and are fairly robust with the mated plug retained in the socket by means of a knurled threaded ring.  The connectors are not impedance controlled and probably have an upper usable frequency around 30MHz.  These connectors were mainly used in the film and television industries but were occasionally used on military equipment.

Series 100/200

A range of non-controlled impedance, miniature screw coaxial connectors, also known as PET connectors (after the Precision Electronic Terminations Company who first produced them).  The impedance of these connectors is not controlled although it approximates to 50 ohms.  They are primarily intended as high voltage connectors, being rated at 3kV DC (Series 100) and 30kV DC (Series 200).  Both series are available with either free (cable mounting) or chassis mounting plugs and sockets.  Elbow versions, "T" junctions, inter-series adaptors and dust caps are also available.  Mated plugs and sockets are retained by means of a knurled threaded ring.

OTHER COAXIAL CONNECTORS

The connectors listed below are not found in equipment intended for amateur radio applications but are included for completeness as they may be found in surplus commercial or military equipment.  If you know of any other rare or obsolete types, please let Tony know via .

AFI         Coaxial connectors intended for board-to-board applications which are available as
AFI-75 for up to 3GHz operation in 75ohm systems and as AFI-50 for up to 6GHz
operation in 50ohm systems.

AMC
Micro-coaxial snap on connectors available in 50ohm versions only and usable up to
6GHz.
2 Pin UHF Two pin connectors, externally identical to the UHF series, and usable up to 300MHz.
They are non-controlled impedance types and are intended for use with Twinax cables.

BMA
Slide on connectors available in 50ohm versions only and usable up to 22GHz.  These
are "blind mate" connectors that can accommodate a certain amount of axial and
radial misalignment.
BNO Two pin connector, similar to the BNC type but intended for use with shielded
balanced twin cables (TWINAX).  These connectors are usable up to 200MHz but are not
controlled impedance types and they are not intermateable with BNC types.

BNT
Bayonet connectors intended for use with triaxial cables and inter-mateable with BNC
types.
DIN 1.0/2.3Miniature coaxial connectors with either threaded or push fit coupling which
are available in 50ohm and 75ohm versions and are capable of operation up to 10GHz.

DIN 1.6/5.6
Coaxial connectors with either threaded or push fit coupling which are available in
75ohm versions only and capable of operation up to 1GHz.
DIN 7/16 50ohm coaxial screw coupling connectors.  Suitable for high power applications up to
7.5GHz.

FME
Connectors used for mobile antenna applications which can be adapted for either UHF,
Mini-UHF, TNC, BNC or N connector interfaces using adapters.  Available in various
versions, some of which are usable up to 2GHz. The characteristic impedance of all versions is 50ohms.
G Slide-on alternative to Type F connectors with 15A continuous current rating.
Available in 75ohm versions only.

HN
Medium sized, weatherproof connectors available in 50ohm versions only, and capable
of operation up to 4GHz.
Mini BNC Miniature versions of BNC series connectors with a controlled impedance of 75ohms
and a usable frequency range up to 4GHz.  This series uses crimp fitting only.

Mini UHF
Miniature versions of UHF series connectors but with a controlled impedance of
50ohms and a usable frequency range up to 2.5GHz.
MCX Also known as MICROAX.  These micro coaxial connectors with snap-on coupling are
available in 50ohm and 75ohm versions and are usable up to 6GHz.

MHV
Miniature high voltage coaxial bayonet connectors, intended for RF applications up
to 300MHz, with a working voltage up to 2.2kV DC.
MMCX Miniature MICROAX connectors with snap-on coupling, available in 50ohm and 75ohm
versions and usable up to 6GHz. PC 3.5 Connectors inter-matable with the SMA series but usable up to 33GHz.
QLA 50ohm subminiature connectors with quick-latch coupling mechanism.
QMA Quick release versions of the SMA connector, usable up to 6GHz. QWS Specialised connectors incorporating quarter wave coaxial stubs and consequently
manufactured for specific frequencies.  Intended for applications requiring EMP
protection and available in 50ohm versions only with either 7/16 or N-type
interface.

SC
Medium sized, screw coupling, coaxial connectors with constant 50ohm impedance,
usable up to 11GHz. Series 480 Large coaxial RF connectors, intended for power levels up to 100kW
. Series 500 Large, high voltage, coaxial connectors, with a working voltage of up to 50kV.
SHV
High voltage coaxial bayonet connectors, intended for RF applications up to
300MHz, with a working voltage up to 5kV DC.
SMS Subminiature coaxial connectors with slide-on coupling mechanism, usable up to
4GHz.

SMP
Subminiature connectors, similar to the MMCX series but available in both
push-on and snap-on versions with a frequency range up to 40GHz.
SSMB Micro-miniature versions of SMB connectors, usable up to 4GHz.
SMZ
Similar connectors to the SMB series, usable up to 3GHz. TRE Non-controlled impedance screw connector developed in the 1940s for use in
British military equipment.

ZMA
Coaxial bayonet connectors, intended for use up to 18GHz in very high vibration
environments.

Tony has several other types in his junk box but their designations are unknown.  A selection is shown below and if anyone can shed any light on their designation, please contact Tony via .

CONNECTORS FOR FIBRE-OPTIC CABLES

Strictly speaking, fibre-optic connectors are neither coaxial nor RF devices but are included here for completeness.  They often, but not always, externally resemble high quality coaxial cable connectors but internally they are very different.  This type of connector does not contain any electrical pins, the interface between the fibres in the plug and socket relying on the ends of the fibres being ground optically flat with great precision.  There are also a great number of other types and styles of fibre-optic connectors, which are too numerous to detail here.  It should be noted that multi-pin connectors, both circular and rectangular are available, which include only fibre-optic "pins".  All types require special purpose tools to correctly fit the connector to the fibre-optic cable.  Connectors of this type are very expensive and are normally only found in military or "top-end" professional equipment.


CABLE COUPLERS ("BARRELS")

There is often a requirement to join two lengths of cable together.  If the connectors on the cables are from the same series, female-to-female or male-to-male couplers can be used.  These devices are often called "barrels".  Couplers for most connector series are manufactured but it is not difficult to make one's own.  It is well worth the effort of producing a well engineered unit, as "ad hoc bodges" are seldom very satisfactory, particularly at the higher frequencies.  If the connectors on the cables to be joined are from different series, a between-series adapter should be used.

Male-to-female right angled, or elbow, couplers are available for some series.  These are used to arrange for a right angle change of direction in the cable without needing a sharp bend in the cable itself and without altering the sex of the cable end connection.  These are used to connect to items of equipment where access is very limited.

"T" JUNCTIONS

It is often necessary to connect an item of test equipment or a resonant quarter-wave or half-wave stub part way along a cable.  The best way of achieving this is to insert a "T piece" into the cable and these are available from connector manufacturers, or they can be home-made.  It is well worth the effort of producing a well engineered unit, as "ad hoc bodges" are seldom very satisfactory, particularly at the higher frequencies.  Commercial "T-pieces" are available for most of the more common connector series.

BETWEEN-SERIES ADAPTORS

Commercially manufactured adapters exist which will connect virtually any one of the above connectors to any other.  All combinations of connector series and male and female gender are catered for.  These adapters can be quite expensive but it is not difficult to make one's own.  It is well worth the effort of producing a well engineered unit, as "ad hoc bodges" are seldom very satisfactory, particularly at the higher frequencies.  Those shown below are representative of what is available commercially but there are countless other variations.

It is often necessary to terminate coaxial cables in a manner allowing connection to test equipment equipped with terminals.  Adaptors are available for this purpose and a selection is shown below.

RESISTIVE TERMINATIONS

50ohm and 75ohm resistive terminations, or loads, fitted with integral coaxial connectors, are available for many of the connector series detailed above.  These loads can be quite expensive but it is not difficult to make one's own.  The load shown second from left in the photograph below is a home made 75ohm load using nine 680ohm carbon resistors in parallel.  It is well worth the effort of producing a well engineered unit, as "ad hoc bodges" are seldom very satisfactory, particularly at the higher frequencies, where it is important to reduce parasitic inductance and capacitance to a minimum.  The item shown on the right hand side on the top row in the photograph below is actually a zero ohm load or "standard short circuit", used in some transmission line measurements.

Coaxial loads come in all power ratings, ranging from milliwatts to kilowatts.  The left hand picture below shows a home constructed, oil-filled load, fitted with a UHF type connector, rated at 500W continuous or 1kW for very short periods.  This is a 50ohm load and is usable up to 30MHz.  The part of the device mounted above the actual load housing contains a metal bellows to allow for expansion of the oil, which occurs after extended use at high power levels.  The right hand picture shows a commercial load rated at 100W, which is usable up to 500MHz.

     

COAXIAL ATTENUATORS

Coaxial attenuators are devices that enable a known attenuation, or loss, to be inserted in a coaxial line system.  They are used to reduce the amplitude of a signal that would otherwise overload a receiving or measuring system, to provide a measure of isolation between items of equipment, albeit with a reduction of signal amplitude and for a wide range of other measurement aplications.  These attenuators come in a huge range of power ratings, frequency ranges, connector types and characteristic impedances, the latter being normally 50ohms, although 75ohm versions do exist.

The photograph below shows, starting at the top, a high power (100W), 10dB attenuator usable up to at least 100MHz, a 6dB microwave attenuator usable at frequencies between 1GHz and 12.5GHz, a 10dB attenuator fitted with hermaphrodite connectors (left) and a 60dB attenuator for use up to 12.5GHz (right).  The bottom row shows a 10dB attenuator fitted with BNC connectors and usable up to about 100MHz, two microwave attenuators fitted with SMA connectors and usable up to 12.5 GHz and a home-made 40dB attenuator usable up to about 30MHz.

Attenuators of this type can be manufactured with any value of attenuation, ranging from 1dB to 100dB but the most useful are those with attenuations of 1dB, 3dB, 6dB, 10dB, 20dB, 30dB, 40dB, 50dB and 60dB.  Attenuators with other values are manufactured for special purposes.  Power ratings can range from a few milliwatts to hundreds of watts.

Note that attenuators intended for use in RF applications are normally quoted as having a dB value, whereas those intended for use with oscilloscopes are often rated by voltage division factor, such as "x10" (20dB) and these often have very high characteristic impedances, high voltage and low power ratings and a limited frequency response.

There is no technical reason why RF attenuators cannot be used with open-wire and ribbon transmission lines but these would need to be balanced types and, as far as I know, are not manufactured commercially.  Balanced attenuators for use at audio frequencies are manufactured and usually have a characteristic impedance of 600ohms.

 

This article is intended to give an overview of the subject and none of the items has been dealt with in depth.  Further information is available from connector manufacturer's literature or from the Internet.


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