Understanding RF Transmission Lines by Measurement and Calculation page 5

4.1) Reflection in a Coaxial Cable

  It is normally necessary to match the load impedance of a coaxial 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 (or a coupler). We supply three special pieces of equipment specifically designed for making this measurement. The RF Bridge which is supplied by Chemandy Electronics has a working band well exceeding the frequencies used in the measurements, it has a fairly flat frequency response from 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 levelling problems within the Signal Generator output circuit. In order to avoid these problems the Signal Generator is buffered with the Broadband Amplifier which is a test aid developed for these measurements. This amplifier increases the signal level internally to approximately + 22 dBm. The signal then passes through an internal 6db attenuator to the output, resulting in greater than +15 dBm at the test aid output. The purpose of the 6 dB attenuator is to protect the amplifier output from damage and to isolate it from the serious mismatches that are applied. The slightly higher output level of +15 dBm, will allow 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 a SHORT and an OPEN of approximately equal electrical length, with the mechanical lengths marked on the box. These equal lengths will become important at a latter stage, when the comparative phases of INCIDENT and reflected signals are measured.

4.2) INCIDENT Direction

  With the DMM connected to the Voltage and Current Detector, voltage output and the Termination Box 50 Ohm LOAD connected to the free end of the 2 metre cable as per Figure 4.1. The Signal Generator is set to 0 dBm at a frequency of 100 MHz. This gives a Broadband Amplifier output of approximately +15 dBm or approximately 1.26 Volt rms. at 0.025 Amps rms into 50 Ohms.

Figure 4.1

[Test set up INCIDENT power]

  The absolute voltage indicated by the DMM is not too important and can vary due to the detector diode characteristics. It is not necessary to linearise this result but the voltage is noted.

  The Termination Box 50 Ohm LOAD is removed and the SHORT is connected. The DMM indication will change very little. This is because the input power to the Voltage/Current detector has not changed and the effect of a completely mismatched SHORT at the end of the cable has no effect on the signal going in the INCIDENT direction.

  The SHORT is now removed and the OPEN is connected. Again there is very little change in the DMM voltage because once more there is an almost total reflection from a complete mismatch. It can be seen that the behaviour of the INCIDENT signal is not effected at the RF Bridge or the Voltage/Current detector by any type of TERMINATION at the end of the 2 m cable. It can be deduced from this that the signal has to reach the TERMINATION (or very close to it) before there can be any effect on the INCIDENT signals behaviour.

  These INCIDENT signal tests demonstrate that the signal at any point in a transmission line are not effected by the conditions further down the line. The charged particles passing along the cable are unable to predict conditions that they have not yet met and therefore are unable to respond to them. The signals can only be modified by conditions as they encounter them. This of course results in the voltage and current of the INCIDENT signal being in phase along the length of transmission line which will be proven in a later paragraph.

4.3) Reflected Direction

The equipment is now reconfigured as per Figure 4.2 with the RF Bridge reversed to measure reflected power.

Figure 4.2

[Test set up REFLECTED power]

  With the 50 Ohm LOAD connected to the end of the 2 m cable the DMM shows virtually zero volts. This is because the LOAD is matched to the characteristic impedance of the cable and there is almost no REFLECTED signal.

  The Termination Box 50 Ohm LOAD is removed and the SHORT is connected. The DMM indication will now be slightly lower to that obtained in paragraph 4.2 due to the attenuation of the cable, but the indication is similar to that obtained in paragraph 4.2 because all of the INCIDENT power to the Termination Box has been reflected by the SHORT, which is a total mismatch.

  The SHORT is now removed and the OPEN is connected. Again the DMM voltage is very similar to that obtained in paragraph 4.2. This is because all of the INCIDENT power to the Termination Box has been reflected by the OPEN which is also a total mismatch.

  The measurements in paragraphs 4.2 and 4.3 demonstrate that the INCIDENT signal power level is unaffected by the termination on the end of the cable. It is clear that there is no effect on a INCIDENT signal from conditions that are further down the transmission line. Also, that an OPEN or SHORT give a total reflection of the signal but a 50 Ohm load gives none. It can be seen that the REFLECTED signal amplitude is totally independent from a constant amplitude INCIDENT signal and is only affected by the match at the Termination Box.



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