RF Cable Basics
Radio frequency is in use since long to carry E-M waves from source to the destination. The medium between the source and destination can be either air or cable. The antenna is used to guide E-M waves in the air. RF cable is used otherwise to interface E-M waves between the two systems or between DUT and test equipments. For longer distances rf cable is not suitable due to loss, instead antenna is used.
As for difference frequencies of operation different antennas are used similarly for different radio requencies different cables are used. As mentioned for testing purpose such as for connecting DUT with RF signal generator or analyzer or with spectrum analyzer or power meter RF cables are in wide use.
Another application is to connect one module with the other inside the system such as RF Transceiver or SSPA etc. For example, connecting Reference oscillator output to the input of local oscillator/synthesizer and to connect LO output to the input of up-conversion/down-conversion stage.
RF cables are used mainly based on three generic applications.
• Flexibility of use in the system viz. semi-flexible,flexible,rigid or semi-rigid type or coaxial cable.
• based on connector type where they need to be connected viz. SMA,N-type,TNC etc.
• Frequency of operation
Other than the above three generic parameters the most important specifications are insertion loss and return loss. Insertion loss is the loss of the RF signal through the cable.For example, if insertion loss mentioned is say 0.5dB at one frequency and input power is say 5dBm then output power will be about 4.5dBm. Similarly return loss should be as high as possible. It is the measure of how much power will be reflected and how much will be passed through the cable. Usually RF cable manufacturers provide tabular and curve of insertion loss and return loss with frequency range so that users can decide the best cable for their application. The summary of the specifications of cable rf are outlined below.
Specifications of cable
• Connector type
• Cable type
• Frequency of operation
• Insertion loss or attenuation per meter
• Return loss
• Impedance of operation(50 Ohm/75 Ohm/95 Ohm)
• Dielectric material used
• Signal delay
• Operating Voltage
• Military or non-military use
Figure depicts connectorized rf cable and open ended one used for debugging in the assembly of rf components and rf testing. The rf cable mentioned is of flexible type. connectorized one has one side SMA and the other side TNC connectors.
To buy the Cable other than above specifications one need to specify class under which they would like to be used such as RG or M(military) application. Following are few of the types, here 'R' stands for Radio frequency, 'G' stands for Government, 8/11 is the number assigned to the government for approval and /U stands for universal specification.RG 8A/U is designated for 50 ohm impedance cable,RG 11A/U is designated for 75 ohm impedance, and RG 22B/U is designated for 95 ohm impedance.
Popular RF cable manufacturers
There are many manufactures of cables used in RF, the popular among them are listed below.
W. L. Gore & Associates, Inc. http://www.gore.com/
HIRAKAWA HEWTECH Corporation,Japan. www.hewtech.co.jp/e/
Carlisle Interconnect Technologies, www.tensolite.com
Radiall is a global leader in design, development and manufacturing of RF coaxial connectors, RF cable assemblies and more. website:www.radiall.com
Lighthorse Technologies, Inc., manufacturer of cable assemblies, website: www.rfconnector.com