# Understanding RF Transmission Lines by Measurement and Calculation page 3

The attenuation of a cable is a function of the size and resistance of the conductors, the dielectric loss, the length of the cable and the frequency of operation. It is possible to establish the attenuation at different frequencies and with different cable lengths using the Voltage and Current Detector.

The measurements in this paragraph establish the attenuation of two different length Coaxial Cables. These cable attenuation measurements are more easily performed on a higher loss coaxial cable e.g. RG58, but they can be performed on RG223 if care is taken. The measurement frequency is increased to 100 MHz in order to be able to compare results obtained with the cable manufacturers data.

INPUT POWER FOR 2 METRE CABLE

The test equipment is connected as per Figure 2.2. The Signal Generator is set to give an output of +13 dBm ( or 1 Volt rms ) at a frequency of 100 MHz. The input power to the 2 Metre Coaxial Cable is obtained by first recording the indicated input Voltage and Current . The change of frequency will cause a small error because of the frequency response of the Voltage / Current Detector but the voltage and current results do not need to be linearised because the measurements are made close to the original calibration level. The results can now be used to calculate the input power to the Coaxial Cable using Formula 3.1 and this becomes P1.

P = V x I Watts  Formula 3.1

The input power should be approximately 20 mW at +13 dBm.

OUTPUT POWER FOR 2 METRE CABLE

The equipment is now reconfigured as per Figure 3.1.

Figure 3.1 With the Signal Generator still set to give an output of  +13  dBm at a frequency of 100 MHz. The output power from the Two Metre Cable is obtained by first recording the output Voltage and Current. These results can now be used to calculate the output power from the Coaxial Cable using Formula 3.1 and this becomes P2. We can now calculate the attenuation in dB.

Power Ratio dB = 10 Log P1/P2  Formula 3.2

This is the insertion loss at 100 MHz for 2 metres of cable.

ATTENUATION OF 1 METRE CABLE

The Two Metre Cable is now replaced with the One Metre Cable, the input power and output power measurements are repeated for the 1 metre of cable and the insertion loss calculated.

As can be seen, the insertion loss of the Two Metre Cable is approximately twice that of the One Metre Cable, thus proving that attenuation is proportional to cable length. The attenuation of coaxial cable is normally quoted in dB/100ft or dB/10m and a comparison can be made between the results obtained and the cable manufacturers data.

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