Hi Charlie and All,
First let me say I spent weeks on this years ago, at the request of
a manufacturer who paid for the time. The goal was to evaluate the
article in QST. But what the heck, I could have missed something
so I tried again quickly. If you have any suggestions, I would be
willing to try once more.
I just built your circuit and tested it with a 259B. I checked one coil
resonant on 2.5 MHz with 29 turns 1.75 inch diameter air wound
tuned to ~2.5 MHz with a 220pF mica cap. This is a reasonably
high Q system, Q~230.
Using your coupling loop directly to the MFJ-259, I see a
noticeable SWR meter wiggle at one inch axis spacing.
Using your exact circuit for the detector, I see a detectable meter
wiggle at one inch axis spacing.
Using my Heathkit dip meter with a 1/2 inch diameter coil, I see a
meter wiggle at a five-inch axis spacing.
This does not amaze me, since the detectors INSIDE the 259B are
virtually identical in function to the external detector described here
on TT.
If the coupling loop is exactly the same, and the detectors are also
the same, the results should be the same...and no surprise they
are.
Now the Heath meter has a small coil, so we have to compare it to
a small coil of the same size. To be fair, I reduced the pickup coil
diameter to 1/2 inch and for minimum detectable wiggle I get:
Heathkit 5 inches.
259B ~1/2 inch
External detector ~1/2 inch.
> 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.
How can you disagree when you don't know what I am talking
about, and have never read the article? What I said was the MFJ
coils are more sensitive than the coils in the QST article.
Actually, your pickup loop will work better when testing large
diameter coils because it more closely matches the flux area of the
coil at the axis of the coil when using a large coil. As the coil is
made smaller, the optimum size of the pick-up loop becomes
smaller (unless you place it over or around the coil in the tuned
circuit under test).
In order to evaluate a system, we have to compare apples to
apples. Making all the coils the same diameter shows the true dip
meter is about ten times more sensitive than a passive dip meter.
That is my only point, and I stand behind it. If I wanted to use a 259
as a dip meter, I would have a variety of pickup coils of various
turns count and diameters. That is what is important, since the
metering is identical.
73, Tom W8JI
W8JI@contesting.com
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