SHF Communication Technologies AG
Wilhelm-von-Siemens-Str. 23D • 12277 Berlin • Germany
Phone ++49 30 / 772 05 10 • Fax ++49 30 / 753 10 78
E-Mail: sales@shf.de • Web: http://www.shf.de

Tutorial Note #2
Microwave Connectors

SHF reserves the right to change specifications and design without notice – SHF Connectors - V003 – 06.07.2009

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Introduction
In many RF and microwave applications, it is important to have the right choice of connectors, adapters,
as well as coaxial cables. Proper selection, application and maintenance can result in prolonged usage
cycle and optimum system performance. The purpose of this tutorial note is to provide some backgroundtechnical information on coaxial cables and connectors in general, to enable customers to make an
informed decision in the selection process.
For connecting high frequency devices, SHF offers a wide range of connectors, adapters and cable
assemblies (semi rigid & flexible). These products are manufactured by KMCO & Totoku of Japan.
Through their use in SHF’s test & measurement series these products have been extensively tested andproved to be the best for high frequency, and broad band applications.

Cut Off Frequency
At low frequencies, electrical signals propagate in coaxial lines and connectors in the TEM (TransversalElectro Magnetic) mode. Above the cut off frequency fc (or first mode frequency) higher order modes suchas the H11 mode can also propagate. To ensure that only the TEM mode propagates, thus keeping thesignal clean, the frequency needs to stay below fc.

fc ≈A2 ⋅ c0π ⋅ ε r ⋅ (A + B )B

There is a wide variety of microwave connectors available for general RF and microwave applications.
For our instruments and components the following connectors are important: SMA, 3.5 mm, 2.92 mm,©
©
©
K , 1.85 mm, V , 1 mm and W1 .
Calculating fc by using the coaxial diameters of above mentioned connector standards shows fc is higher
than the frequency specification, in other words: It is possible to use connectors at higher frequencies if
performance degradation is accepted.
Nevertheless, the most significant bandwidth restricting factor is the frequency of the first mode. At thisfrequency the transfer function exhibits a null. If the application is for broadband communication signals(i.e. PRBS or similar type of data) the impact of this first frequency null might be negligible. However, thefirst mode is potentially damaging the signal if the system is supposed to transmit narrowband RF signals
at or around the moding regime.
The table on the following page shows these key specifications of the most commonly used highfrequency coaxial connector interfaces.

SHF reserves the right to change specifications and design without notice – SHF Connectors - V003 – 06.07.2009

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Upper Frequency
Specification (GHz)Cut Off Frequency
fc (GHz)

1Torque Wrench(N cm / in lb)

SMA

12.4 – 2756 / 5

3.5 mm26.53490 / 8404590 / 8505590 / 865

7190 / 8110

13345 / 4Standard
MIL-std-348A
MIL-C-39012
IEC Std 169-15
IEEE-std-287
IEC Std 169-23
IEC Std 457-5

IEEE-std-287

2.92 mm
K©
2.4 mmIEEE-std-287
IEEE-std-287

1.85 mmV©
1 mm
2IEEE-std-287

W1©

Attenuation
From the above relation between cut off frequency and size it seems that the smaller a connector thebetter. However, there are two important reasons that prevent smaller connector and cable geometries:
Cost: very small connectors are difficult to manufacture and therefore expensive
Loss: the smaller the geometries are the more loss will be caused by the skin effectAttenuation of the TCF flexible cable assemblies series-2
The number behind the TCF is the outer diameter ( ·10 mm)

1Some SMA connectors are only useful to 12.4 GHz, most SMA connectors go up to 18 GHz. IEC Std 169-15 specifies up to 24 GHz
and some manufacturers specify up to 27 GHz.
2IEEE 287 specification may still be 34 N-cm, however Agilent found out, that 45 N-cm gives adequate repeatability.

SHF reserves the right to change specifications and design without notice – SHF Connectors - V003 – 06.07.2009

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Connector CompatibilityCA

BCA

B

This is the schematic of a microwave connector (male and female connector is shown). The importantdimensions are:
A: Inner diameter of outer conductor (together with the diameter of the center conductor and thedielectric constant of the insulator determines the maximum frequency up to which the connector will
have mode free operation. From this diameter the name of many connectors derives (e.g. 2.92mm or2.4mm).
B: Diameter of the center conductor. At a given outer conductor diameter this determines the Impedanceof the connector (Z = 120·π Ω· ln (A/B) / (2·π·√εr) ).
C: Center pin recess: It is very important to make sure, that this recess is within the specified tolerances.
If you connect a connector with excessive center pin protrusion not only the other connector might bedamaged but also the mechanics inside the component or system!

Due to these mechanical restrictions, the following connectors are compatible:SMA332.4 mm

1.85 mm

V©

1 mm

W1©

SMA3

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3.5 mm

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2.92 mm

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K©

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3.5 mm

2.92 mm

K©

2.4 mm

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1.85 mm

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V©

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1 mm

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W1©

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Use caution when mating SMA connectors to precision connectors; especially female precision connectors can be damaged easily.

SHF reserves the right to change specifications and design without notice – SHF Connectors - V003 – 06.07.2009

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Bending / Cleaning Of Coaxial Cables
Important to know about coaxial cables is that bending the cable might move the center conductor. Inother words: after bending the cable a center pin protrusion on one end and excessive center pin recesson the other end could be observed.

A professional tool for bending semi rigid cables:
KMCO Hand Bender 2200

Center pin recess degrades the return loss. Center pin protrusion is more harmful as it might damage thecomponent that is connected to this end.

Before bending:After bending:Center PinProtrusion!

The position of the center pin should be measured by using a gauge. If no gauge is available the centerpin recess should at least be verified by microscope (20 times magnified).
It should always be kept in mind: An excessive recess will only degrade the return loss, whereas centerpin protrusion might cause damage!

It is also highly recommended that regularly visually inspection of the cleanliness of the connector isperformed.

Connector Cleaning Procedures:•

Before using a Microwave connector it should be visually inspected. Recommended magnification:
20x.

•If it is necessary to clean it, all devices and persons have to be grounded!

•Avoid using connectors that are mechanically damaged (scratched mating surface, eccentric orbended center conductor)!

•For cleaning, a swap with clean isopropyl alcohol that is free of water should be used. Applying too
much force on the center conductor or flowing alcohol over the insulator bead causes damage.

•Any alcohol and particles must be blown off with clean compressed air. The spray can has to be in anupright position otherwise the propellant might leave a residue on the connector.

SHF reserves the right to change specifications and design without notice – SHF Connectors - V003 – 06.07.2009

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SHF Communication Technologies AG
Wilhelm-von-Siemens-Str. 23D
12277 Berlin
Germany

Phone: ++49 30 / 772 05 10
Fax:++49 30 / 753 10 78

E-Mail:sales@shf.deWeb:

http://www.shf.de

SHF reserves the right to change specifications and design without notice – SHF Connectors - V003 – 06.07.2009

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