[TowerTalk] Re: Dipping with 259B and such

[N0TT@ juno.com]

Hi Tom,

>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. 

OK, good...that's reasonable.
>
>Using your coupling loop directly to the MFJ-259, I see a 
>noticeable SWR meter wiggle at one inch axis spacing.

That's one thing I haven't tried recently...connecting a loop
directly to the 259B.  It seems to me that I may have tried
that before ICW a QST article...perhaps
the same article you were talking about and I didn't have
much success at the time.  So, I again tried connecting
the one-turn loop directly to the 259B and I *DO* get what I would
call a "reliable" dip at one-inch axis spacing....that's with
a 7mhz self-resonant trap coil about 2" diam with a QU of about
200.  The 259B meter deflects about 1/8".  More about that later.
But before I go on, and in reference only to the directly connected
pick up loop, the bit of boiled crow I have before me is tough
as shoe leather but it goes well with humble pie...hi hi :)  I
should have remembered to try the direct connection method.
>
>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.

Yes, the GDO WILL be more sensitive, but of course one still needs
to know the "exact" resonant frequency of the circuit we're trying
to measure unless we just want to "ball park" it.  Most DGO scales
are not calibrated "good" enough to say, prune a length of coax
for a specific frequency.  In that light the 259B methods are, in
my case, easier to use.
>
>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.
------------------------------------------
Repeated from the previous post:

>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.
-------------------------------------------

I was refering to the "Dip Meter Adapter" coils..# MFJ-66.  NOT
the QST article.  I was comparing the circuit in my previous
post with the MFJ-66 coil.

>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).

Yes, I agree...no problem there....reasonably match the pickup coil
with the coil diameter being measured. 

The point I was trying to make is that with the MFJ-66 coil I was using
with the 259 and 259B, I could NOT get a "reliable" dip on the
259/259B SWR meter....only a very small dip was displayed, thus making
it difficult to "accurately" determine the resonant frequency of the
coil being tested.  I found that to be the same with other circuits I
have measured.

>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.

No argument there about a good GDO "winning" the sensitivity issue
over the other systems we have been chatting about.  
>
>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.

I'll take your word for it that the metering is the same...no
schematic in my MFJ manual.  I agree on the pick up coil
configuration.

I can only say that in my experience that the circuit (I'll call
it the "HB meter" here) I had published works *much better* than
the MFJ "Dip Meter Adapter" (#MFJ-66 coils) when I properly used
it with the 259/259B. And per the above, even the single loop
directly connected to the 259B works better.  I even tried to
measure some of the coils (with a parallel capacitor of course)
from my Millen GDO using the MFJ coil...they are about the same
physical size as the larger MFJ coil.  Same poor results.  A
good dip was obtained with the HB meter and the same 1-turn loop
connected directly to the 259B. 

In all fairness here, I tested the MFJ "Dip Meter Adapter"
coil (the larger of the two) to make sure it was OK.  The
inductance measured about 3.17 uH with a Qu of 100 on my HP Q
meter.  Would you happen to know if that is reasonably close
to MFJ specs?  No specs in my manual.

I found the HB meter much easier to read.  I mentioned
earlier in my original post of 9/16 that I use a large
size 50 uA meter..the case measures 4-1/2 inches wide.  That
greatly adds to the readability/resolution as compared to the
physically small (but adequate for their intended use)
meters on the 259/259B.

73,
Charlie, N0TT

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