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Re: [TowerTalk] Re: Grid dip meter and Yagi elements

To: "Jim Idelson" <>,"TowerTalk Post" <>
Subject: Re: [TowerTalk] Re: Grid dip meter and Yagi elements
From: "Tom Rauch" <>
Reply-to: Tom Rauch <>
Date: Tue, 3 Aug 2004 21:46:53 -0400
List-post: <>
> The difference between an antenna analyzer and a grid dip
meter is primarily
> the input impedance. The low input impedance of the
antenna analyzer makes it
> difficult to use in a loosely coupled manner. The high
input impedance of the
> dipper allows easier coupling with a tuned a tuned

The actual mechanism is Q of the coil in an active
oscillator tank circuit. The coil in a dipper is part of an
oscillator circuit, and feedback in the oscillator cancels
resistive losses in the coil. Since coupling coefficient
between two loosely coupled resonant systems is increased as
Q increases, the GDO couples well even to low-Q circuits.

Another way to view it is with flux density. The stage is
oscillating, so the magnetic flux surrounding the coil is
very strong even at very low power. The strong field couples
very well to anything around the coil.

The coil HAS to be part of the oscillator tank or the above
comments won't be true!

Now look at a MFJ-259. You have a non-resonant coil that has
very low Q, down in single digit numbers or less. The
magnetic flux levels are very low, and the coupling
coefficient is very low. This is why an "add-on" kit for an
antenna analyzer isn't worth a hoot for checking anything
except very high Q circuits.

I recommended not selling the thing, but since the article
in QST about using a coil on an analyzer to make it work as
a GDO won some sort of award for being a good article I was
out voted.

I measured a difference of over 20:1 in sensitivity between
a GDO and the very best adaptor I could build for an antenna
analyzer. It is still my opinion that anyone needing a GDO
buy one, and not waste money on an adaptor for an antenna

73 Tom


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