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Common Coaxial Connectors

Below, I present a digest of important information about coaxial connectors. Most of these are for RF and microwave frequencies, but a few of the more common ones used for lower frequencies are included for completeness.

Why different connectors?

Many coaxial connector types are available in the audio, video, digital, RF and microwave industries, each designed for a specific purpose and application. Much of the development of the smaller connectors that perform well into the GHz and millimeter wave range has been conducted by test equipment measurement companies. One of their considerations is the number of connect-disconnect cycles that a connector pair can withstand while still performing as expected.

Why different sizes and frequencies?

The frequency range of any connector is limited by the excitation of the first circular waveguide propagation mode in the coaxial structure. Decreasing the diameter of the outer conductor increases the highest usable frequency. Filling the airspace with dielectric lowers the highest frequency and increases losses. The mating process typically changes the geometry of the mating surfaces and resistance loss at those interfaces as well as geometric changes result in variation of impedance and loss.

Some RF connectors are sexless (such as the HP/Amphenol APC-7 and the General Radio GR874 and GR900BT). Most connectors have female structures with slotted fingers that introduce a small inductance. The fingers accommodate tolerance variations, but reduce repeatability and may ultimately break after 1000 connections. There are slotless versions of connectors available, but they are, for the most part, relegated to instrument interfaces. Slotless female connectors are very difficult to clean and require very careful connection and disconnection.

Sex and Connectors

Those unaccustomed to the use of the terms “male” and “female” to describe connectors will have to get used to this time-honored engineering nomenclature. Those of us who work with them regularly use the terminology without a second thought. One day a number of years back, my daughter (who was about 8 years old at the time, if memory serves) was in the lab with me while I was working at the network analyzer with one of the grad students. She overheard our conversation, peppered as it was with the terms “male connector” and “female connector”. After we were done, she asked me why the connectors were named that way. Well, this was a conversation I had expected to have in somewhat different circumstances, but I gave her a quick summary of the “how-babies-get-made” story, followed by the analogy that is implied by the connector terminology. She thought about it for a few moments when I was done, and then said, “Daddy, that’s just weird.” It’s hard to argue with that.

Connecting and Disconnecting

RF and microwave connectors are precision-made parts, and can be easily damaged by mistreatment. You should start with all connector surfaces as clean as possible, using a solvent such as alcohol or a special-purpose cleaner to do the job. Use as little as you can, and in no event contact dielectric spacers or resistive materials (as used in loads) with the solvent, since these can be irreparably damaged by the solvent. As a general rule, if the connectors have threaded sleeves, you should rotate these to tighten, leaving the rest of the connector (and cable) stationary. The idea is not to grind the mating RF surfaces against each other: these are not garden hoses! If other parts of the connector are twisted while tightening or loosening, damage can easily occur.

Connecting 7 mm connectors is somewhat different, and perhaps counterintuitive. These are sexless connectors, and the mating surfaces mount flush and are held together by a single rotating sleeve. The mating sequence is:

1. Each connector has an outside rotating sleeve. On one connector, rotate the outer sleeve so that the threaded connector sleeve extends completely out from the outer sleeve. Do this on any fixed-mounted connectors, such as those on the test ports of a network analyzer. On the other connector, rotate the outer sleeve so that the threaded connector sleeve recedes completely into the outer sleeve.

2. Mate the surfaces flush and rotate the forward sleeve to engage the threads of the other connector.

3. Complete connection is made when the forward rotating sleeve is tight and the other sleeve is loose.

Caution: one sleeve must be loose. Tightening down both sleeves can cause connector damage.


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