Grid Dip Meter

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Rick Thurstenson:
What is a grid dip meter. How is it used?

Greg Miller:
It's a device used to measure the frequency or resonance of a tuned circuit. It does other things but this is my understanding of it's main purpose. I think the name is derived from the fact that as one turns the Grid Dip Meter's dial the meter "dips" when it reaches the resonant freq.


I just found my copy of "How to use Grid Dip Oscillators" by Rufus P Turner and here's a coupla blurbs from the preface:

"It was first intended as a simple device for measuring the resonant frequency of a tuned r-f circuit."

"... this instrument is employed not only for resonance measurements but also as a signal source, monitor, and field-strength meter, and for checking such characteristics as capacitance, inductance, Q, bandwidth, and the properties of antennas, transmission lines, crystals, and filters."

73,
Greg, W4IK

Dale Hunt:
The grid dip meter has an oscillator which is tunable over a wide
frequency range (generally using plug-in coils.)  When the coil
is held near a tuned circuit and the oscillator is tuned through the
resonant frequency of the circuit, the interaction causes a change
in the operating conditions of the oscillator.  The early devices used
tubes, and had a meter to measure the grid current (or the effect
that the grid had on the plate current), hence the name "grid dip
meter" since the meter indication "dips" a the resonance point.
Since most modern versions are solid state, they would be "gate
dip meters" or "base dip meters", but there really is no reason to
discriminate on the basis of the active device type, so I try to use
the generic term, "dip meter" to include all versions.

Generally, the dip meter is a hand-held box with a plug-in coil
sticking out one end and a calibrated frequency dial.  To measure
the resonant frequency of a tuned circuit, hold the dip meter so
the circuit coil couples to the dip meter coil (either side-by-side,
or end-to-end).  Slowly tune the dip meter through the frequency
range until you see the meter dip.  Reduce the coupling between
the coils (further apart) until the dip is just noticable - this will be
the most accurate measurement (otherwise the tuned circuit can
pull the meter off the indicated frequency - but in my experience
the frequency calibration is only approximate anyway.)

On of my first applications for a dip meter was to determine the
tuning range of a military surplus receiver.  I measured the resonant
frequency of each tuned circuit with the variable capacitor at
minimum and at maximum.

There are lots of things you can do with a dip meter.  I use one to
measure the resonant frequency of wire antennas (the reflector
elements for quads, for example) by wrapping a couple turns of the
antenna wire around the dip meter coil.  They are particularly
useful to check the resonant frequency of short mobile antennas
and tuned loop antennas.  You can determine the
approximate inductance of a coil by putting known capacitor
across it and finding the resonant frequency of the combination.
I've used the oscillator as a signal source for antenna testing or
receiver allignment.   With the oscillator off, it becomes a tunable
RF detector:  not particularly sensitive, but I've used it to sniff the
RF along my feedline to find where I was losing power.

:
I am currently doing a project on landmine detection.  In this project, the aim is to find a way of detecting the tripwire that comes from a landmine.  
I have been using a grid dip meter to detect the presence of this tripwire.  The tripwires are going to be obscured by vegetation that will be quite thick.  
So far, i have been able to detect the tripwire up to a distance of 0.8m.  To do this, the coils for the grid dip meter has been changed to a single circular loop of wire approximately 27cm in diameter.
Ideally, the detection range should be about 5 metres.  

What would be the maximum range in which these grid dip meters would work at?
How could the range be improved? Or how could the coils be changed to make it more directional?
Furthermore, does anyone know the effect of vegetation (or any other obstacles i.e. trees) on the coupling between the grid dip meter and tripwire?

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