![[Test set up for forward power]](../../images/standing-waves-10.gif)
The attenuation of a cable is a function of the resistance of the conductors, the dielectric loss, the ratio between diameters of the inner and outer conductors, the length of the cable and the frequency of operation. 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 previously in order to compare the results.
INPUT POWER
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 1 MHz.The input power to the Two Metre Cable is obtained by first recording the input Voltage and Current and then linearising the results. The power is then calculated with Formula 3.1.
Formula 3.1 P = V x I Watts
OUTPUT POWER
The equipment is now reconfigured as per Figure 3.1. 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, and then linearising the results. These results are now be used to calculate the output power from the Coaxial Cable using Formula 3.1
ATTENUATION
The attenuation of the coaxial cable at 1 MHz is now calculated using Formula 3.2
Power Ratio dB = 10 Log P1/P2
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 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 is normally necessary to match the load impedance of a cable in order to ensure the maximum power transfer and minimum reflection. When a signal in a coaxial cable is passed to an unmatched termination there is a reflected signal returned from the termination end of the cable towards the signal source. The power level of this signal can be measured with the aid of a directional device known as an RF bridge. The RF Bridge supplied by Chemandy Electronics has a working band well exceeding the frequencies used in the measurements, it has a frequency response from 0.3 MHz to 300 MHz and is a 50 Ohm instrument.
The measurements now become a little more difficult because of the high level of reflected signal. This reflected signal can cause leveling problems within the Signal Generator output circuit. In order to avoid these problems the Signal Generator is buffered with the Broadband Amplifier. This increases the signal level internally to approximately + 23 dBm. The signal then passes through an internal 6db attenuator to the output, resulting in greater than +15 dBm at the attenuator output. This slightly higher test level will also cause the detector circuits in the V/C Detector to work over a more linear range.
In order to be able to reflect the signal, we use the Termination Box. This has a 50 Ohm termination and also short and open circuits of approximately equal electrical length. These equal lengths will become important at a latter stage, when the comparative phases of forward and reflected signals are measured.
FORWARD POWER
The test equipment is connected as per Figure 4.1.
Figure 4.1
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