CABLES & CONNECTORS SUPPLEMENT

PIN HEIGHTThe importance and role of RF coaxial connector pinheight and its impact on electrical and mechanicalperformance.

T he job of an RF coaxial cable is to trans-mit a signal from one point to anotherwith as little signal or power loss over its

length and to keep its signal isolated from orto the outside world. A coaxial cable’s struc-ture consists of three major components: acenter conductor, a dielectric and an outerconductor. These three components are puttogether in a way to maximize the cable’s per-formance over its intended frequency rangefor the desired application. A jacketing or cov-ering is added to the cable to protect and sealthe RF structure from outside elements andgive the cable additional environmental ormechanical advantages.

A coaxial connector is used to attach the endof a coaxial cable to enable it to be connected toanother cable or RF device. It also has a centerconductor, a dielectric and an outer conductor.The center conductor of the connector is typical-ly referred to as the contact. The portion of theconnector that is intended to mate with anotherconnector is called the mating interface. Theseconnector interfaces typically come polarized in

male and female configurations(see Figure 1). The RF coaxialconnector’s center conductor orcontact is normally arranged intopin and socket configurations tofacilitate positive electrical con-tact between the male and thefemale connector. The matingportion of the male contact thatengages the mating female spring

fingered socket is referred to in the industry asthe mating pin. It is the height or length of thispin and the position of the contacts within theconnector’s interface that this article addresses.

Pin heights are often taken for granted. Weseldom think of its importance in traditional pinand socket configurations because they seldomfail in their application. RF connectors, particu-larly those used in microwave applications, re-quire tightly toleranced interfaces to achievethe intended electrical and mechanical perfor-mance of the design. Most of a connector’s in-terface dimensions are achieved through thetolerancing of the machined components thatmake up the interface. Setting and maintainingthe pin height or the location of the pin re-quires that special attention be paid to correctlypositioning it during the assembly of the con-nector components. Some connector designsrequire that the position of the center pin beset during attachment to the cable.

What is pin height? As mentioned earlier,most RF connector center contacts are de-signed to have a solid pin that inserts into aspring finger female contact or socket. In or-der to maintain the characteristic impedanceand ensure electrical performance of the coax-ial transmission line, most connector contactsincorporate a smaller diameter-mating pin

DAVID CRITELLITimes Microwave SystemsWallingford, CT

Fig. 1 Polarized male andfemale interfaces. ▼

Reference Plane

Female Male

Reprinted with permission of MICROWAVE JOURNAL® from the March 2007 issue.©2007 Horizon House Publications, Inc.

with a shoulder on one of the matinginterfaces. This contact shoulder islocated within the connector at apoint where the mating connectorseats against it and is optimized in away to minimize electrical reflectionscreated by discontinuities betweenthe two connector interfaces. Like-wise, the face of the mating female orsocket contact (which is also referredto as the shoulder) must also be locat-ed within the connector at a positionthat optimizes performance. Thispoint where the two connectors cometogether is called the connector refer-ence plane (see Figure 2).

The connector performs bestwhen the shoulders of the male con-tacts are closest to the referenceplane without going above it. Theidea is to create the smallest gap pos-sible between the two mating contactshoulders to minimize its impact on

electrical performance. The size ofthe gap that can be tolerated is de-pendent on the intended operatingfrequency of the connector. Loweroperating frequencies can toleratelarger gaps between the contactswithout degrading electrical perfor-mance. However, coaxial connectorsare designed to operate across broadfrequency spectrums so the contactgap must be designed to performwith minimal electrical degradation atits highest operating frequency.

The pin height is measured fromthe connector’s reference plane to theshoulder of the contact. High and lowlimits are characterized to ensureproper engagement length of themale pin with the mating connector’ssocket contact. The tip of the malepin also has a characterized taper tominimize mating forces on the femalecontact and to guarantee high matingcycles and long life.

Manufacturers control most of thepin’s positioning and dimensioningthrough close tolerancing of the ma-chined components associated withthe connector’s interface. However,close or tight tolerancing of the con-nector’s interface components addscost and seldom guarantees that the

contact will be positioned to achieveoptimal electrical and mechanicalperformance of the connector. There-fore, special attention needs to begiven by the manufacturer to fine-tune the pin height and gap at the in-terface by resetting the contact’s posi-tion during assembly to optimize per-formance. The responsibility ofsetting the contact into its optimalposition may also be passed on to theend user if the connector is suppliedwith a loose contact to facilitate easeof cable termination.

More important than positioningthe contact pin for optimal perfor-mance is positioning it to mate withany connector within its series (forexample: SMA series, TNC series, Nseries, etc.), regardless of who manu-factures the connector. For this rea-son, interface standards have beendeveloped. There are a variety of in-dustry, commercial and governmentstandards in place that regulate inter-faces dimensionally to guarantee in-termateabilty between manufactur-ers’ connectors. Government and/orindustry standards govern most of thewell-established connector interfaceson the market. However, newer prod-ucts with uniquely designed inter-faces may not yet be adopted by astandards group and have no govern-ing standard. In these cases, the man-ufacturer that developed the uniqueinterface is responsible for specifyingand controlling the connector inter-face tolerances. Sometimes connectormanufacturers will work together todevelop a unique interface and to-gether develop the interface standardfor it. The QMA series interface wasdeveloped in this way.

Every connector series interface isunique with respect to other series in-terfaces. An SMA interface is uniquelydifferent than a TNC interface, SMAand TNC interfaces are uniquely dif-ferent to the N series interface, and soon. They all have different interface di-mensions with respect to their size andthe location of the contact’s pin andshoulder position with respect to theconnector’s reference plane. There-fore, the gauging requirements foreach interface are different. Most con-nector reference plans are located atsome depth below the face of the con-nector (reference Figures 1 and 2).Special gauges are available for mostwell-established interfaces to make it

CABLES & CONNECTORS SUPPLEMENTReferencePlane

Male ContactShoulder

Female ContactShoulder

▲ Fig. 2 Mated connectors makingelectrical contact at the connector referenceplanes.

Female Contact Finger

▲ Fig. 4a Misaligned pin caught betweeninsular and female contact fingers.

Contacts Making Contact With Each Other

Stressed Contact Fingers

▲ Fig. 4b Contact between connectorcontacts could break female contact fingers.

▲ Fig. 3 Gauges used to measure connectorinterfaces.

CABLES & CONNECTORS SUPPLEMENTly, it leads to increased contact gapsbetween the mating contacts andcontributes to higher than intendeddiscontinuities in the connector inter-face and can degrade electrical per-formance from consequent high re-flections. Power levels could begreatly reduced if the pin is posi-tioned so far back that the matingcontact fingers ride on the taperedsection of the male pin (see Figure5). Significantly reduced power han-dling will result if the contact fingersride on the taper section of the malepin.

End users attaching connectors tocable also need to be aware of the ef-fects of incorrect pin heights. Con-tacts of connectors that are designedto attach to cable can be suppliedloose by the manufacturer or cap-tured within the connector.

Loose contacts are generally sup-plied to simplify the connector designand facilitate ease of assembly to thecable’s center conductor while keep-ing the pricing in line with the user’sexpectations. This means some de-gree of accuracy is left up to the as-sembler to ensure proper location ofthe contact with respect to the con-nector’s reference plane. Loose con-tacts can be either soldered orcrimped onto the cable’s center con-ductor. They can be set into positionby either sitting the contact directlyon the cable core, using a dielectricspacer or shim, or positioning thecontact with a shim that is later re-moved after attachment of the contactis achieved. The idea is to provide theproper electrical spacing to optimizethe cable-to-connector terminationelectrically and provide the properpositioning for the contact at the con-nector’s interface (see Figure 6).

Spring fingered center contact de-signs, most commonly made withheat treated beryllium copper, havegreatly simplified the termination ofconnector contacts to cable centerconductors. This design allows the ca-ble’s conductor to simply be pushedinto the contact without the need tocrimp or solder to maintain the elec-trical connection between the cableand the connector. The simplicity incable termination comes from thefact that the contact no longer needsto be supplied loose and can now besupplied captured within the contac-tor assembly. Therefore, the assem-

bler does not have to deal with sepa-rately terminating the contact to thecable and does not need to worryabout setting the contact’s properplacement location with respect tothe connector’s interface. The key tothis style of contact in providing a re-liable connection lies mainly with themanufacturer properly machining,forming and heat treating the con-tacts to provide the needed tight fitonto the cable’s center conductor. Be-cause the center contact is designedto fit tightly, not only must the centerconductor be trimmed to the properlength but also the end needs to befree of all burrs and slightly pointed

easy to measure the location of thecontact from the reference plane.Nonetheless, it is possible to measurethe contact location with a standarddial indicator depth gauge (see Figure3). In the photograph the three smallergauges are special interface gauges.The gauge mounted on the post is astandard dial indicator depth gauge.The importance of controlling pinheight cannot be downplayed. Theheight of the pin cannot and shouldnot be ignored. Problems result bothmechanically and electrically if the pinis positioned too high (long) or too low(short) with respect to the interface asit is specified.

If the pin is too high or forwardthe pin could misalign during mating.The male pin could get caught be-tween the insulator and the fingers ofthe female contact (see Figure 4a).This typically results in a broken fe-male contact. Also, if the contact’s pinposition is too high, the contact’sshoulder could end up above the con-nector’s reference plane (based onthe fixed length of the pin-to-shoul-der dimension subjected by mostconnector specifications). If the con-tact shoulder ends up above the con-nector reference plane there is thepotential that it will make contactwith the mating contact and breakthe female contact fingers (see Fig-ure 4b). These two conditions, themale pin getting wedged between theinsulator and female contact, and thecontact shoulders making contactwith each other, have the potential toput enough force on the female con-tact finger to crack it. These cracksare internal to the connector and notvisible to the end user. Cracks of thisnature will ultimately lead to an inter-mittent signal, and an eventual cata-strophic contact failure and kill signalintegrity.

If the contact is positioned too lowor short the contact could lose itscontact with the mating female con-tact. This sometimes happens whenblind mate connectors are positionedbeyond their specified mounting tol-erances. It can also happen in con-nectors that do not have captivatedcontacts. Non-captive contacts havethe potential of being pulled backinto the connector away from themating connector by forces applied toit by the attached cable, thus shorten-ing the mating pin height. Additional-

Contact Fingers Riding on Taper of Male Pin

Contact Gap Too Large

▲ Fig. 5 Increased contact gaps lead topoor electrical performance.

Connector Contact

Cable CenterConductor

ContactGap at Cable Termination

ContactGap atConnectorInterface

Cable Dielectric

ReferencePlane

▲ Fig. 6 Connector contact requires aspecific gap at the mating interface and cabletermination to optimize electricalperformance.

Cable Conductor Trim Too Long

Cable Conductor Bottomsin Connector Contact

Contact Pushed AboveReference Plane

▲ Fig. 7 Connector contact can be pushedabove the reference plane by incorrectlytrimmed cable center conductor.

at the end for proper fit. This part ofthe assembly to the cable is still leftup to the end user and requires someattention to cable prep detail.

Regardless of the type of contacttermination to cable, the correctlength of cable center conductormust be exposed to accept the centercontact. Exposure of too little cablecenter conductor could result in aweakened and compromised contactattachment for connectors supplied

with loose contacts. Exposure of toomuch cable center conductor willcause the cable conductor to bottominside the contact and not allow it tobe positioned on the cable as intend-ed. This results in pins that are posi-tioned higher in the connector inter-face with respect to the referenceplane. The connector contact couldbe pushed so far forward that it endsup above the reference plane of theconnector and potentially cause dam-age to the mating female socket asdescribed earlier (see Figure 7).

There are a few connector designsthat do not use the traditionalpin/shoulder configuration in themating interface. Although most con-tacts are attached to a cable’s centerconductor using methods such as sol-dered, crimped or a spring fingeredsocket, there are a couple of cases insome SMA, F and other connectorseries where the cable center conduc-tor is actually used as the center con-tact mating pin of the connector.There are still other designs that usea separate contact but have no shoul-der at the connector interface. Inthese designs, the mating pin diame-ter is carried straight into the trans-mission line section within the con-nector; thus, no shoulder exists at theconnector reference plane (see Fig-

ure 8). In these cases, one still mustbe conscious of where the end of themale pin is located in relation to thereference plane even though there isno shoulder to worry about. This isbecause there is a limit to the amountof male pin that the mating femalecontact can accommodate.

Even after the contact is fixed tothe cable center conductor and theconnector secured, the position of thepin can still be moved from its set po-sition. There are influences on theconnector pin position other thanthose subjected by the connectormanufacturer and end user during as-sembly to the cable. The connectorpin or contact can be subjected toforces after assembly that influencesthe contact’s position within the con-nector with respect to the connector’sinterface. High stresses can be sub-jected by cable flexure, pulling on theconnector, and expansion and contrac-tion of the connector and cable di-electrics over temperature that causepin movement. These stresses build incompression or tension depending onthe force applied, and push the con-tact forward or pull it backwards intothe connector away from its optimallyset position. When that happens, thecontact could be moved beyond thedesign or specification limits of theconnector interface resulting in one ofthe performance issues described ear-lier.

Most of these stresses can be over-come by incorporating some form ofcaptivation into the connector design.The roll of captivation is to hold thecontact into position within the con-nector under a predetermined loadcondition to prevent it from movingforward or backward with respect tothe connector reference plan. Mostconnectors use Teflon as a dielectricmedium because of its good electricalcharacteristics and relative cost.However, Teflon is a soft material andmay not be suitable to use as a reli-able captivating material in high per-formance connectors or in environ-ments where mechanical stressescould be high. For this reason, othertypes of materials and captivatingschemes have been developed to holdthe contact in its place.

There are many different typesand ingenious captivation methodsused to capture a contact within aconnector. Epoxy pins and dielectric

CABLES & CONNECTORS SUPPLEMENT

No Contact Shoulderat Reference Plane

Cable Center ConductorUsed as ConnectorMating Pin

Cable

ReferencePlane

End ofMale Pin

▲ Fig. 8 Examples of connector designswithout a contact shoulder.

CaptivatingEpoxy Pin

(a)

(b)

CaptivatingDielectric Bead

▲ Fig. 9 Epoxy pin (a) and dielectric bead(b) used to captivate connector contact.

(a)

(b)

▲ Fig. 10 Multi-port Interconnect Systems(a) and MILTECH Cable Assemblies (b)developed by Times Microwave Systems tomeet today’s tough application requirements.

etary materials and captivation meth-ods. This need for super performingconnectors and cable assemblies hasresulted in new generations of connec-tor products. Figure 10 shows some ofthe newest products developed byTimes Microwave Systems to meet to-day’s toughest application require-ments.

The position of the pin is taken forgranted because most users have nev-er seen an application failure result-ing from an improperly positioned orshifted contact pin. Even so, someengineers are haunted by system-re-lated performance issues caused by asingle RF connector within a systemwith a misaligned pin, broken contactor open circuit caused by an improp-erly positioned or shifted pin. Forthose, the importance of maintainingproper connector pin height will re-main a critical performance issue.

There are many factors used tocontrol the correct pin position with-in a connector interface. Manufactur-ers work to tighten tolerances onconnector interfaces by tightly toler-ancing components during manufac-turing to achieve the desired electri-cal and mechanical performance ofthe mating pin. Special care is takenin setting the pin to the optimalheight during connector component

assembly. Specifications have beencreated by standards and governmentcommittees to govern pin height onthe most common connectors toguarantee intermateability withinconnector series. Additionally, therehas been a myriad of captivations em-ployed to limit or even prevent pinmovement in the harshest environ-ments. All these efforts demonstratethat pin height is being given the at-tention necessary to make it worryfree for the end user. ■Teflon is a registered trademark of DuPont.PEEK is a trademark of Victrex.Ultem is a registered trademark of General Electric.Torlon is a registered trademark of Solvoy Advanced

Polymers.Vespel is a registered trademark of DuPont.

David Critelli is aregional sales managerfor Times MicrowaveSystems, responsiblefor all OEM sales insouthern New York,northern New Jerseyand most of NewEngland. Beforejoining Times, heworked for AEP andRadiall Co., where he

was an engineering manager. He holds both aBS degree in computer science and an MBAdegree and has over 25 years experience in themicrowave cable and connector industry.

beads are the most common methodsused to hold contacts in place underhigher load applications.

An epoxy pin is a dielectric epoxythat is injected through a hole in theconnector dielectric and around thecontact and in some designs through ahole in the connector body. The epoxycures into a hard plastic material thatmechanically captures the componentstogether (see Figure 9).

Dielectric beads use a short lengthof dielectric material having relativelyhigh mechanical strength. These beadsare typically machined or molded. Theconnector contact and body are config-ured in such a way that they capturethe bead to prevent movement (seeFigure 9). Some popular high strengthbead materials that have relatively gooddielectric properties are PEEK,Ultem, Torlon and Vespel.

The material and method used forcaptivating a connector center contactis typically selected based on a mechan-ical force requirement and a price-to-performance trade off.

Other manufacturers of higher per-formance connectors and cable assem-blies that design to meet the highestapplication requirements with respectto temperature, flexing and vibrationwhile maintaining the highest electricalperformance develop their own propri-

CABLES & CONNECTORS SUPPLEMENT