[TowerTalk] Re: Dipping with 259B and such

[N0TT@ juno.com]

>W8JI writes:
>The 259B has a detector right across the output port that 
>measures voltage there, as well as a detector that measures 
>current in series with the output port (which is what the described 
>detector measures), and the display will clearly reflect changes as 
>small as one bit out of 256 bits (0.4%) change in current and/or 
>voltage.

I *tightly* coupled the MFJ's "Dip Meter Adapter" with a tuned
circuit and "yes" there IS a dip on the 259B's meter, BUT,
the dip was shallow and "inprecise"...something on the order of 2mm
movement at the end of the meter.  I also used an older model
MFJ 259 with virtually the same results. Read on.... 

Using a Millen "GDO", model 90652, there was a very deep dip
(as expected) with the same tuned circuit.  Of course looser
coupling and a calibrated receiver or freq counter would be
used with the GDO for a more precise measurement. 

Using the homebrew external "grid" dip circuit that was described
earlier, there was also a very deep dip displayed on the
external meter with comparatively LOOSE coupling with the tuned
circuit.  In fact, with a single turn loop, I could get a very
reliable dip with the 1-turn loop about 1 coil diameter away from
the end of the tuned circuit.

>With an external detector, all we can do is measure the very same 
>parameters. It is no different than what is already inside the unit.

The external detector in this case is quite sensitive.  May I suggest
that it be TRIED by yourself and others to see what the results are?
One can even use clip leads to hook up the circuit...5 minutes, tops.

>The problem is the lack of mutual coupling, nothing else. That 
>problem can not be cured unless the coupling coil is part of a 
>system that is resonant with high levels of regeneration.
>
>Understand I am not saying it doesn't work, just that the system 
>will always lack the high mutual coupling that exists when the dip 
>coil is part of an oscillating tank circuit. 

OK, that's a "given" and you are correct.  Of course the 259B (etc)
is not simply not designed to serve the same function as a GDO.  

>While there are better form factors and inductance values for one 
>application or another (which may, in a given test, make one 
>system behave better than another), overall the system has many 
>times less sensitivity than a regular dip meter. 

>>N0TT wrote:
>> 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.

>Ironically, assuming the are built and used correctly, the MFJ coils  
>are more sensitive than the adapters described in QST for equal 
>diameter coils!! I'm positive of that.

I don't know what adapters you are referring to in QST.
As far as the usefulness of the "Dip Meter Adapter" coils that
are available for the 259B (etc), I disagree...
I have really TRIED to use those coils with many circuits with
virtually no success.

When something doesn't perform like we want it to, we, as
experimenters like to improvise, improve and expand the usefulness
of existing equipment, antennas, etc.  'Nuf sed  :)

73,
Charlie, N0TT

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