Dip Meter Antenna Measurements: Resonant Frequency
A dip meter or grid dip oscillator, GDO can be used for many antenna measurements including measuring an antenna resonant frequency.
Dip Meter Tutorial Includes:
Dip meter / GDO basics How to use a dip meter / GDO Using GDO to measure inductance Using GDO to measure capacitance Measuring antenna resonant frequency Measuring feeder electrical length
This is probably the most obvious use for a dip meter in connection with an aerial. However it is not necessarily one of the easiest measurement to make as there a number of pitfalls.
In common with other measurements for measuring the resonant frequency of a tuned circuit the basic idea is to couple the coil of the meter to the circuit under test. When the meter is tuned to the resonant frequency of the antenna then the meter current will dip. The centre of the dip indicates the resonant frequency of the antenna.
When performing this measurement it is best to perform it at the antennal itself and not via a feeder. Whilst performing it via a feeder may seem perfectly in order it is found that the feeder will introduce a number of spurious dips and it may be difficult to identify the correct response.
When checking the antenna some way of coupling the meter to the antenna must be found. For an antenna in the HF section of the spectrum it is possible to take a loop of two or three turns from the feed point of the antenna and loop this over the coil of the dip meter. It may even be possible to use this method having a single turn loop at the low end of the VHF portion of the spectrum, but as the frequency rises then it may introduce some inaccuracies. The best way if sufficient coupling can be obtained is to short out the feed point and place the coil as close as possible to the antenna.
It will be found that the best dip using the coupling loop method of is at a current maximum i.e. at the feed point of most aerials. If the other method is used then the best dip will be obtained at a point of voltage maximum. This can always be found at the end of an antenna.
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