Care & Handling of RF/Microwave Connectors
The Best Way to Connect
The performance of every RF system depends on the quality and application of each component. In industry, much of the focus is on complex, expensive RF equipment. At the same time, it is also prudent to give equal attention to the quality and type of interconnections between components.
RF cables, connectors, adapters, and accessories (for the purposes of this paper, this family of components will be referred to as RF connectors) directly impact system performance. Impedance mismatch, insertion loss, and return loss are just a few of the parameters that impact the reliability, repeatability and accuracy of an RF system. This paper will focus mainly on coaxial connectors — by far the most common type of interconnect. Still, the principles discussed apply to a range of connectors and interconnections.
Some key considerations when working with RF cables and connectors include:
- Type – Select the connector best suited to the requirements at hand. This includes frequency range, power handling, mechanical fit, characteristic impedance, and overall quality.
- Compatibility – The connection should impact the overall measurement performance as little as possible.
- Wear – RF cables and connectors are expendable items. They become worn or damaged and have a finite useful lifetime. They should be discarded once past this lifetime. It is a false economy to retain worn or damaged items which will adversely affect measurement results.
- Characteristic Impedance – The mechanical dimensions of an RF connector determine its characteristic impedance. Overwhelmingly, 50 Ohms is used in modern times, followed by 75 Ohms. Subpar components will cause impedance mismatches, yielding inconsistent and inaccurate results.
- Repeatability – Measurements remain stable and consistent during multiple connects/disconnects over time.
- Measurement Accuracy – The ability to make a measurement within specified error limits. Connector mechanical tolerances, alignment and wear directly impact accuracy.
Connector Types & Examples
Mini-Circuits offers numerous RF connector types to meet the requirements of virtually any measurement. Some common RF connectors are shown in Figure1.
As mentioned, the mechanical dimensions of an RF connector determine its electrical characteristics. This can be illustrated with a simplified model of a coaxial RF connector, such as that shown in Figure 2 for the 2.92 mm RF connector.
Z0 = (60/√εr)ln(D/d)
D = Inner diameter of outer conductor
d = Outer diameter of inner conductor
D = 2.92 mm
d = 1.27 mm
Z0(√εr) = 50Ω
Figure 2: Simplified model of an RF coaxial connector (2.92 mm).
In Figure 2 we see that the characteristic impedance (Z0) of an RF coaxial connector or cable, is determined by the ratio of the inside diameter of the outer conductor (D) and the outsider diameter of the inner conductor (d), and epsilon (ε), the relative dielectric constant of the material separating the inner and outer conductors.
Further, the approximate maximum frequency in GHz for a given RF connector is given by:
fmax (GHz) ≈ 120/D (mm)
This reveals why, in general, RF coaxial connectors are mechanically smaller in size as the frequency range increases.
Connector types can also be categorized into three quality “grades”: Metrology, Instrument and Production. Production encompasses the lowest level of specification requirements, while Metrology the most precise and demanding. This grading allows the user to select a connector grade that best meets their needs, budget and requirements.
Not every connector is available in every grade. For example, an SMA connector is only available in production grade but the mechanically-compatible 3.5 mm connector provides an alternative (generally higher cost) available in all three grades.
Other combinations of connector types are highlighted in the table below, and can be directly mated with each other when required. For example, a 2.92 mm connector rated to 44 GHz can be mated to an SMA connector rated to 18 GHz (good performance would only be expected up to the lower of the two frequencies). Since there are small differences in dimensions between these types, extra care must be taken when mating in order to ensure a clean connection. For applications requiring repeated disconnect/reconnect cycles, best practice is to use a connector adapter instead. Mini-Circuits has a wide range of connector adapters available on our website.
| Connector | Metrology | Instrument | Production | Cut-Off Frequency (GHz) | Sexed |
| Type F (75) | N | N | Y | 1 | Y |
| BNC (50 & 75) | N | N | Y | 2 | Y |
| SMC | N | Y | N | 7 | Y |
| Type N (50 & 75) | Y | Y | Y | 18 | Y |
| APC-7 or 7 mm | Y | Y | Y | 18 | N |
| SMA (4.14 mm) | N | N | Y | 22 | Y |
| 3.5 mm | Y | Y | Y | 34 | Y |
| 2.92 mm or “K” | N | Y | Y | 44 | Y |
| 2.4 mm | Y | Y | Y | 52 | Y |
| 1.85 mm | N | Y | Y | 70 | Y |
| 1.0 mm | N | Y | Y | 110 | Y |
Visual Inspection & Cleaning
Best practice connector care always starts with a thorough visual inspection. Placing a worn, broken or damaged connector on a good connector (or instrument) will likely result in two damaged connectors.
Begin by inspecting the outside of the connector. Observe the condition and quality of the plating. Also check the knurling or connecting nut. Check the alignment of the center pin while noting any damage. Next, look for dirt and debris on the outside or inside of the connector. If an RF connector is damaged or worn excessively, it should be immediately discarded. To assist in the connector evaluation process, Mini-Circuits offers various calibration kits, connector wrenches & gauges.
If cleaning is required, start by gently blowing out any debris with dry, filtered air. Next, apply a very small amount of 100% isopropyl alcohol to a foam swab and use this to clean the connector. Foam swabs are preferred over cotton swabs because the cotton tends to snag on the connector center pin. Additionally, anything less than 100% alcohol contains water, which is less than ideal for electrical components. Be sure to clean all surfaces completely, and repeat cleaning if needed. Foam swabs and alcohol cost significantly less than RF connectors. Finally, dry the connector with another round of dry, filtered air.
To maintain consistent performance and reach their expected lifespans, connectors should be cleaned often. After cleaning, if the connector appears damaged or worn, discard it and replace it with a new connector. These components are intended to be expendable.
Connection Guidelines & Torquing
The following steps should be employed when making an RF connection:
- Always use RF connector care best practices
- Take electrostatic discharge precautions
- Inspect and clean the connectors
- Replace any damaged or worn connectors
- Align the connectors axially
- Make physical contact
- Engage the connector nut
- Apply even force until finger-tight
- Properly torque per the connector specifications with a calibrated torque wrench
When a properly-calibrated torque wrench is utilized, the connector should be held so that its body does not rotate when torque is applied, only the connector nut does. To prevent overtightening, a torque wrench that ‘breaks over’ is preferred to one that ‘clicks’ when the proper torque is reached. Once the handle of the torque wrench initially ‘breaks-over’ by just 10⁰ or 20⁰, immediately stop applying torque. The proper amount of torque has already been reached.
Some connections may require you to support one or both of the devices being connected, to relieve stress on the main connector body. Supporting the devices being connected may also prevent the rotation or movement of anything other than the connector nut(s) being tightened. At times a user will ‘spin’ a bullet or barrel adapter body to save time, or worse yet, the user will even ‘spin’ the body of a calibration standard. The center pin and receptacle are both plated, metal surfaces that engage with the receptacle being slightly spring-loaded. The ‘spinning’ of connector bodies can induce significantly greater wear than proper insertion and extraction, and can dramatically shorten the lifetime of a connector.
Mechanical Inspection & Gaging
Even after proper torquing and cleaning, connectors sometimes require a more thorough mechanical inspection. A set of precision connector gages, such as the Mini-Circuits N-type connector gage kit (https://www.minicircuits.com/pdfs/ACUDIAL-N.pdf) shown in Figure 3 is ideal for such an evaluation. For instance, the test ports on RF instruments and devices under test (DUTs) should be gaged regularly. As a recommended best practice, RF connectors should be gaged prior to their first use, any time a connector fails visual inspection, and approximately every 100 connections of use.
Gaging properly measures the center pin depth of a connector against the gage standard in the gage kit. Improper pin depth can result in connector impact damage or poor connectivity. This is illustrated below.
Figure 4: Connector center conductor insertion depth. Too little depth (left), excessive depth (center), and correct depth (right)
In order to properly gage a connector, follow these steps:
- Calibrate using the included gage standard
- Properly clean and visually inspect connectors prior to gaging
- Visually inspect connectors’ gages and clean gage standards if needed
- Choose the proper sex of gage standard (male or female)
- Axially align the connector with the gage standard
- Rotate and finger-tighten the gage nut without rotating the body of the gage or standard
- Properly torque the gage standard
- Lightly tap the side of the gage to ensure it is fully seated
- Zero the connector gage
- Carefully remove the gage from the gage standard
- Measure the test connector with the calibrated gage
- Clean and visually inspect test connector
- Axially align the test connector with the gage
- Rotate and finger-tighten the gage nut without rotating the body of the gage
- Properly torque the test connector
- Lightly tap the side of the test connector to insure it is fully seated
- Record the pin depth reading from the gage
- Take multiple measurements to reduce uncertainty
- Verify the pin depth measured is within the tolerance of the test connector specification
- If the test connector is not within specification, have it properly repaired or discard it
- Carefully remove the test connector from the gage
- Repeat the procedure for other RF connectors to be gaged
Keep in mind that any damaged or worn connector identified during this process should be discarded and replaced with a good connector. As mentioned before any damaged or worn connector allowed to remain in the test system will damage other connectors and wreak havoc on measurements, potentially costing thousands of dollars to repair and replace.
Connector Savers
As opposed to high-value test instrumentation, which can incur very high servicing/repair costs even for very minor damage, connector adapters are relatively low cost, disposable items.
To avoid the risk of damage every time a new connection is made to a test instrument, it is recommended that ‘connector savers’ be used between the test instrument ports and the RF cables or adapters connected to the instruments. Connector savers are sacrificial adapters, often with the same connector type on each end, that remain fitted to test instruments to prevent damage and wear to the instrument’s own precision connectors. They are especially valuable in use cases that require a high number of connections and disconnections. Connector savers should be inspected and replaced regularly. Mini-Circuits offers a complete set of connector savers and adapters to ensure repeatable and reliable test measurements and to afford robust protection of instrumentation ports.
Handling & Storage
When RF cables and connectors are not in use they should be stored in a clean, dry location. They should be regularly inspected, gaged and cleaned as required, and proper torque should always be utilized. Specifications for connectors should be reviewed and care taken not to touch their mating surfaces to prevent the transfer of oil and dirt from one’s hands. Further, when not in use, mating surfaces should be protected with plastic endcaps. Similar practices should be applied to RF connectors on test instrumentation.
Staying Connected
RF connector care best practices directly impact the accuracy, repeatability and overall quality of measurements. RF cables, connectors and adapters should be inspected, cleaned, and gaged regularly to ensure optimal performance. When not in use they should be properly stored and protected. Worn or damaged connectors should be disposed of and replaced. Consistent record keeping can ensure RF connector maintenance is carried out regularly. RF connector care warrants the same level of attention that other parts of the overall test process do. Conforming to best practices for RF connector care results in higher-quality measurement results while reducing overall costs.
Courtesy of Mini-Circuits
