I can understand your confusion. You are right about the term Grid Dip
Meter. It is a holdover from when everything was powered by glass bottles
with grids in them. The modern ones work the same way but use transistors,
However, the term remains with us out of tradition. Grid Dip Meters (and
their modern equivalent) don't use a direct connection to the item they are
measuring. They are like a tunable oscillator with a deliberately exposed
coil. Things that are in proximity to the exposed coil absorb a bit of the
oscillator's energy. If the object is resonant, it absorbs a lot more and
you see a pronounced dip in the GDM meter as you sweep the GDM through the
resonant frequency of the object. So, in the case of unbroken elements like
directors and reflectors of yagis, you don't have to break the element in
the middle to make the measurement. The same goes for resonant circuits.
You just bring the GDM near the circuit and turn the dial looking for a dip
in the meter reading.
In some cases, the thing you are testing doesn't couple well with a GDM, so
you wrap a few turns of wire around the GDM coil and then around the thing
you are testing. It might be necessary to do that with an antenna element.
Also, it might be necessary to greatly detune the other elements in the yagi
so they don't couple to the element you are testing. You would do that by
either removing them or by hanging pieces of wire off each end of other
elements to make them electrically longer. The ideal thing would be to test
each element by itself at its normal operating height. Easy to say, hard to
As for the MFJ antenna analyzer, that is a very different beast. As you
say, it needs a direct connection at the feedpoint as if it were a
transmitter. I am not sure you can indirectly couple an antenna analyzer to
an element like you are suggesting. Someone else here might have a better
answer on that subject.
Dudley - WA1X
From: "Mark Beckwith" <email@example.com>
Subject: Re: [TowerTalk] Grid dip meter and Yagi elements
Content-Type: text/plain; charset="iso-8859-1"
> Why does it need a gap in it?. It is an
> inductor and it has a resonant
> frequency. Use the coil with the frequency
> range of the antenna and tune to
> find the dip.
I can't believe I'm going to show such ignorance, but here goes: We had
"grid dip meters" when I was a kid. I think they were called this because
they had a vacuum tube in them. I think vacuum tubes had something called a
"grid" inside them.
I am assuming you guys talking about "grid dip meters" are not really
talking about these old devices. What is the modern equivalent? I have an
MFJ 259B, for instance. It is easy to get the resonant frequency of a yagi
element split and insulated in the middle. Can I use it to inductively
couple to a continuous yagi element or otherwise determine that element's
resonant frequency in any way?
Forgive my untechnical perspective, but the only reason I care about towers
and antennas is to make as many QSOs in radio contests as possible. This
has incidentally helped me conquer my fear of heights.
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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