The University of Western Australia

PHYS 2003

Aim
The aim of the investigation was to verify that electric field and magnetic field are alternating
when in a standing electromagnetic wave of a Lecher line by determining key features of the
waves such as maxima or minima.
When both ends of the wire are shorted, electric field and magnetic field at the shorted end
will be at a minima and maxima respectively. As a result, voltage across this point can be
observed to be approximately zero while the current will be a maxima.
Method
Transmitter with a frequency of 477.4125 MHz and 5W was connected to the wires using
crocodile clip cables and the metal plates were used as shorting termination at each end of the
wires. The position of these plates was adjusted along the wires to achieve the lowest
reflected power observed on the SWR meter in order to create standing waves.
With the wires shorted together at the end, the waves are reflected back up towards the
transmitter with the same frequency. As a result, the reflected waves interfere with the
outputting waves, creating a sinusoidal standing wave of voltage and current.
Using a voltage detector device with a lamp indicator, the nodes were able to be determined
by using pegs to mark the positions where the lamp was off. For the first and last nodes
observed, the extrema were noted to find the difference between them in order to calculate
the uncertainty. Similarly, a current detector with a lamp indicator was used to determine the
magnetic field minima, and these positions were marked with pegs too. These positions were
then measured using a measuring tape to determine the wavelength and subsequently the
speed of light.
Another part of the lab was to experiment with an open end by removing the metal plate
furthest from the transmitter.