Tom,
Actually, the 259B is *NOT* used as a grid dip meter in this
application...the external meter is the "dip" indicator and
it really is quite sensitive. Perhaps I didn't make that clear.
BTW, MFJ sells a set of 2 plug-in coils for the 259. It's
called a "Dip Meter Adapter" Model MFJ-66. I found that
item to be absolutely worthless for "grid dip" applications.
73,
Charlie, N0TT
On Mon, 17 Sep 2001 08:08:57 -0400 "Tom Rauch" <w8ji@akorn.net> writes:
>> This is what I did to make a *USEFUL* "dip" circuit for
>> my MFJ 259-B. Of course it should work with other
>> RF generators too, but the MFJ (or equivalent) is
>> hard to beat for convenience and reasonable accuracy.
>
>In my opinion, there is NO "better way" to build a dip meter with an
>antenna analyzer. The sensitivity will never be good.
>
>The reason sensitivity is poor is because the coupling inductor has
>very little current and very little external flux.
>
>In a "real" dip meter, the inductor is part of an oscillator tank
>circuit. The Q of that inductor is extremely high because the
>inductor is part of a system with more feedback than loss (it is
>oscillating). This means with even modest amounts of power the
>magnetic field is very strong. Mutual coupling to any other circuit is
>
>very high, because the field is intense.
>
>(If you look at any formula for coupling, you will see Q is a strong
>factor)
>
>You can never do that with an inductor driven by an amplifier, and
>the more resistors you add to the circuit the worse the problem
>gets. I ran tests here, and the article in QST describes a system
>that actually has less coupling and is less useful than simply
>plugging an inductor right into the output jack of the antenna
>analyzer!!!
>
>Using an analyzer and a coil to make a dip meter is a very poor
>system, it is usable only when the external system has reasonably
>high Q and the coils are laid right against each other. That perturbs
>the system you are measuring, and gives inaccurate results.
>
>If you want a dip meter, buy a real dip meter. It will work a lot
>better.
>73, Tom W8JI
>W8JI@contesting.com
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