Howard Venning, MD of Aspen Electronics explores the nebulous of the coaxial connector and its continual transition through the ages
Coaxial cables first came into existence around 1880, with coaxial connectors following on soon after. So they have been in use for more than 100 years and today they can be found almost everywhere.
From domestic applications like television or satellite installations; on to professional installations in audio and broadcast systems; and also found in military communications to high- energy science projects – and that’s not forgetting many types of test & measurement applications.
The one common thread for all coax connector applications is the requirement to connect cables, typically carrying radio frequency signals, from one piece of equipment to another. Having said that the variables related to this basic application are many, including, cable type and size, frequency, power rating plus the number of connect/disconnect operations. Also the physical design of the mating equipment be it a rack mount transmitter, a PCB inside an item of equipment or an antenna mounted high up a mast to name just a few applications.
Looking back over the last 30 years the number of different types of coaxial connector bought to market has increased dramatically as new applications with the ever increasing rise in operating frequency continue.
Whereas 30 years ago you would have a choice of approximately 10-15 connectors, today the number is more like 50. Whilst this presents the user with an incredible choice, selecting the correct connector has certainly become more complex as the number of connector designs and the expanding number of suppliers increases.
Over the past three decades, the vast majority of general purpose RF applications would have been satisfied by just a handful of connector types. As a generalisation the BNC was used for the majority of test instrument applications, the UHF connector for ‘radio’ applications, the ‘N-type’ for high power and high frequency application, plus the TNC for avionics applications.
Higher frequency applications would rely upon the SMA connector, said to be the ‘most widely used microwave connector on the planet’. Whilst all these connectors are still in use today there are many alternatives.
Finding the right fit solution
In considering newer designs a general rule of thumb applies. As the frequency goes up the centre pin diameter gets smaller, whereas as the power rating goes up the centre pin diameter gets bigger. Therefore, if you have a high power, high frequency application the only answer might be waveguide, a different topic not related here.
One solution would be the 7/16 DIN connector that can handle up to 3000 watts and capable of operating up to 7.5GHz. However, as with all connector specifications these two specifications cannot be used together as the power rating will reduce as the frequency increases.
Looking at higher frequencies, today there are a whole range of connectors capable of handling applications right up to 110GHz. Starting with the ‘standard’ SMA connector, that has a maximum frequency rating of 18GHz, improved versions of this design are available for use up to 27GHz.
Next there is the 3.5mm (sometimes called the APC 3.5 connector) that will operate up to 34GHz. and lastly the 2.92mm connector (often referred to as the Wiltron ‘K’ connector) for 40GHz applications. Note, all these connectors will mate with each other, so being able to recognise these is important particularly if you are making measurements where the results need to be accurate and repeatable.
For even higher frequency applications the 2.4mm (50GHz) 1.85mm (60-65GHz) and lastly the 1.00mm connector (110GHz) can be used.
For connections between printed circuit boards and other system components there are a whole range of connectors where small size and ease of use are the key parameters. The SMB connector was developed to provide a smaller size when compared to the SMA. Its ‘snap on/off’ design made it very easy to use in confined spaces and where production assembly/installation times needed to be reduced.
Where a more permanent connection is required the SMC connector could be used as it is the same size as the SMB, but features a screw thread coupling that improves performance, particularly insertion loss and frequency.
To these older designs we can add a relatively new connector range called the GPO connector. Having nothing to do the post office (a reference older readers might recognise) the GPO connector was designed in the 80’s by Corning Gilbert offering a small sized, push fit, high frequency (40GHz) connector ideal for high density interconnects.
This range has been expanded to feature the GPPO (65GHz) and other variations. As with all good products there are a number of similar designs with a variety of names that offer similar performance.
The design legacy of old & new
Whilst it is good to review the latest designs, we should not forget that there is a lot of legacy equipment in the field that needs supporting, plus applications where an older design of connector is still the best solution. The HN connector, a high power derivative of the N-type is still in use today, as is the SC connector where higher voltages are likely, plus the C-type, which is a larger version of the BNC.
Lastly we should not forget broadcast and other ‘very’ high power applications where the EIA range of connectors is used. These connectors, designed for use with rigid or semi rigid cable (or feeder as it is sometimes called) range in size from 3/8” (inch) to 6-1/8” (inch) designs and are used where kilowatts of RF power needs routing.
This is only a very brief overview of examples of the diverse range of products on the market today, covering probably less than 50 percent of the available connector types!
The answer is to consult a good stockist to discuss a coax connector application and get the best advice of the most suitable product to use.
The SMP Male thread connector