Microwave connectors

Updated December 30, 2012

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Chess-set photo courtesy of TVB of leapsecond.com!

Attention everyone: we recommend that you use our section on connector care as a mandatory reading assignment before any of your new hires are even allowed in your laboratory! Check out our connector care poster in our download area!

Here is a clickable index to help you navigate through our material on connectors:

A proposed color-code for connectors! (separate page)

Microwave connector overview

Connector sex (separate page)

Connector species and their frequency limitations

Type N connectors

SMA and SSMA connectors (separate page)

SMC connectors (separate page)

Precision connectors (separate page)

7 mm connectors

3.5 and 2.9 mm connectors

2.4 and 1.85 mm connectors

1 mm connectors

How (not) to trash a cal kit (separate page)

Push-on connectors (separate page)

Blind mate connectors

SMB connectors (separate page)

SMP connectors (separate page)

SMPM connectors (separate page)

Adapters

Waveguide-to-coax transitions (new for January 2013!)

Connector care - mandatory reading! (separate page)

Storing connectors

Connector torque (separate page)

How to deal with stuck connectors

Gauging connectors

Cleaning connectors

Possible future topics

Microwave connector overview

Microwave connectors are now a two billion-dollar-per-year industry. There are a wide variety of microwave connector families and more choices are added each year. Microwave connectors are used to make low VSWR coaxial connections, and can be used as the terminations ("ends") of coaxial cables, installed into housings, and increasingly onto circuit cards.

Most microwave systems use 50 ohm characteristic impedance but some use 75 ohms (BEWARE! type some connectors such as type N come in both 50 ohms and 75 ohms. Don't mix these up!) All connectors described on this page are 50 ohms unless noted. For the equation for the characteristic impedance of coax, see coaxial transmission lines, or go straight to the coax calculator.

Elbow connectors

The term "elbow" is another bit of connector slang, it means a right-angle bend connector or adapter. Right-angle connectors come in two types: 90 degree, and swept (see photo below). In general, the swept connectors are better at high frequencies, and the outer jackets are molded rather than machined. If you took apart a 90 degree elbow you'd see why. It's because the dielectric typically is not continuous at the 90 degree junction, so there is some parasitic inductance that screws up the VSWR.

Connector species and their frequency limitations

One property of any coax transmission line is that it has a cutoff frequency above which the desired TEM mode will no longer propagate (see transmission lines for a discussion of cutoff frequency, or go straight to the coax calculator). The maximum frequency of operation of coax connector families cannot exceed this limitation. The industry-accepted frequency limits of connectors families are generally 95% of their theoretical cutoff frequency.

The following table lists all of the major connector families. Generally, cost increases proportionately with frequency limit. And "air" costs a lot more than "PTFE", which may seem counterintuitive, until you see how the center conductor has to be supported in an air-dielectric connector. Some connector history is included in the right-hand column.

Here's another way to look at connector frequency limits:

Type N connectors

These are cheap and rugged, for these reasons you will find them all over your laboratory.

Adapters - within a species

Within any connector family there are three adapters you can use. A male-to-male adapter refers to an adapter with two male ends. This is often referred to as a "barrel" adapter. A female-to-female adapter has two female ends; it is often referred to as a "bullet" adapter. An adapter with one male end and one female end is often referred to as a "connector saver". This is because this type of adapter is often screwed onto an expensive piece of test equipment or component that requires a lot of connect/disconnect cycles. If an incident occurs where one of the connectors is damaged, it is far cheaper to throw away and replace the connector saver than to repair expensive equipment that it is protecting. Below is a picture of an SMA connector saver, barrel and bullet adapter.

Adapters - between species

Between species adapters are a huge part of the RF connector industry. For sake of argument, if we accept that there are 10 different connector species (and there are a whole lot more than 10), and each has two sexes, using the "choose" function we can arrive at the total number of possible products:

N=20!/(2!*18!)=190

This is why connector catalogs are so thick. Of course, there are some extreme permutations that would have a limited market, so they are deleted from the lineup. For example, no one is going to buy a 1 mm to type BNC adapter.

So what is the frequency range of a bastardized connection, where one species meets another? You should not exceed the lower frequency limit, i.e. if you are connecting 2.92 to 2.4 coax, don't try to take data above 40 GHz.

What other ways can you get in trouble? How about mating a cheap SMA connector to an expensive 3.5 or 2.92mm connector. The tolerances are quite different.. here's a rule that was provided by The Professor!

...even though combinations like SMA and K connectors are thread-compatible, plugging one into the other can damage the sleeve in the receptacle (jack) side due to differences in the diameter tolerance.  The rule I use to avoid that is, OK to plug a K plug into an SMA jack, not OK to put an SMA plug into a K jack.  The jacks seem to be the more fragile side of the connection.  

Possible future topics

Bulkhead or flange-mount connectors

Feedthrough connectors

Hermetic connectors

Crimped versus soldered connectors for cables

Captured versus floating center contact