Understanding RF Transmission Lines by Measurement and Calculation page 4

3.3) Coaxial Cable Attenuation with Frequency

The Two Metre Cable is now measured at a lower frequency and the result obtained compared with the earlier result in paragraph 3.2 to demonstrate the decrease in insertion loss. In this measurement, RG58 will again give easier results than RG223, but it requires the same cable as used in paragraph 3.2 in order to compare the results.

INPUT POWER

The test equipment is connected as per Figure 3.2. The Signal Generator is now set to give an output of +13 dBm ( or 1 Volt rms ) at a frequency of 1 MHz. The input power to the Two Metre Cable is obtained by first recording the input Voltage and Current. The power is then calculated with Formula 3.1.

P = V x I Watts Formula 3.1

Figure 3.2

[Test set up for cable impedance measurement]

OUTPUT POWER

The equipment is reconfigured as per Figure 3.3 With the Signal Generator still set to give an output of +13dBm at a frequency of 1 MHz. The output power from the Coaxial Cable is obtained by first recording the output Voltage and Current. These results are used to calculate the output power from the Coaxial Cable with Formula 3.1

Figure 3.3

 

[Test set up for cable impedance measurement]

 

ATTENUATION

The attenuation of the coaxial cable at 1 MHz is now calculated using Formula 3.2

Power Ratio (dB) = 10 Log P1/P2Formula 3.2

 

This is the insertion loss at 1 MHz for 2 metres of cable. When this result is compared with the insertion loss at 100 MHz for the same 2 metre cable measured in paragraph 3.2, it can be clearly seen that there is a decrease in the attenuation of the Two Metre Cable with a decrease in frequency. Thus proving that the attenuation of a coaxial cable is dependent on frequency. It should be noted that there was no correction for the Voltage / Current Detector frequency response and a more accurate result can be obtained by normalising the frequency response. This achieved by measuring the input power to a termination using the configuration of figure 3.4 The signal generator being set for 100 MHz and then 1 MHz with the voltage and current being noted at both frequencies.

Figure 3.3

[Test set up for cable impedance measurement]

The power obtained at 100 MHz is then divided by the power obtained at 1 MHz. This gives a factor to be used to multiply the measurement results obtained at 1 MHz.

Normalisation Factor (f1)= P100 / P1Formula 3.3

 

Normalised Power (P1N)= P1 x f1Formula 3.4

 

Where P100 is the power into a termination at 100  MHz

P1 is the power into a termination at 1 MHz

This technique can be used at any number of frequency points and will remove variations in signal generator output level, Voltage / Current Detector frequency response and any interconnecting cable attenuation.

 

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