![[Test set up forward power]](../../images/standing-waves-10.gif)
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 good 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 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 + 22 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 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 forward and reflected signals are measured.
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
The absolute voltage indicated by the DMM is not too important and can vary due to the detector diode characteristics. It is not important to linearise this result but it should be 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 FORWARD direction.
The SHORT is now removed and the OPEN is connected. Again there is very little change in the DMM voltage although once more there is almost total reflection from a complete mismatch. It can be seen that the behaviour of the FORWARD signal is not effected at the RF Bridge or the Voltage/Current detector and so it can be deduced that the signal has to reach the TERMINATION or very close to it before there can be any effect on the signals behaviour.
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 reverse power]](../../images/standing-waves-11.gif)
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. The indication is almost identical because all of the FORWARD power to the Termination Box has been reflected by the SHORT which is a total mismatched.
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 FORWARD power to the Termination Box has been reflected by the OPEN which is a total mismatched.
These measurements demonstrate that the FORWARD signal power level is unaffected by the termination on the end of the cable, and that an OPEN or SHORT give a total reflection of the signal but a 50 Ohm load gives none. It is clear that there is no effect on a FORWARD signal from conditions that are further down the transmission line. It can also be seen that the REFLECTED signal is totally independent from the constant FORWARD signal and is only affected by the match at the Termination Box.