A dip meter or grid dip oscillator, GDO can be used for many measurements including measuring the electrical length of a feeder.
A dip meter provides an easy method of measuring the electrical length of a piece of feeder. A knowledge of this length can be of value in a number of applications especially if the aerial installation is used for transmitting.
In order to make the measurement the feeder must be disconnected from the aerial and left open circuit. The other end should then be coupled to the dip meter.
Then with the meter in its oscillator mode it should be tuned from its lowest frequency upwards until a dip is noted. This frequency should be noted as it is the primary resonant frequency. However it is wise to check this by tuning further up in frequency to the next few dips. These are harmonics of the fundamental resonance and they should be at multiples of the frequency of the first dip. If all is correct then the frequency of the first dip corresponds to a quarter wavelength.