I've been following this thread with interest, largely because I'm
ignorant about the utility of these meters. Jim, your description
fits with what I had concluded about them, that being if you had
pretty much anything else to use, it would be better. I understand
how they work but using a GDO always seemed pointless.
I have seen these things on eBay for years, but I still cannot think
of a single instance where it would be of more utility than more
modern antenna tools, so I never bought one.
My question is, am I wrong? Is there some situation I have not yet
encountered where only a GDO will do? Jim, you said that for some
systems it's convenient. I can't picture one for the life of me.
I usually use a HP network analyzer for general things, and the little
Autek boxes for spot checks.
Should I go buy one? Every so often one will turn up with a complete
set of coils...does anybody use one for general purposes? If so,
could you talk about why you prefer it? I want to know if I'm missing
something.
Thanks in advance,
Jeff/KD4RBG
---- Original message ----
>Date: Thu, 20 Mar 2008 15:43:40 -0700
>From: Jim Lux <jimlux@earthlink.net>
>Subject: Re: [TowerTalk] Using a dip meter
>To: Scott McClements <kc2pih@gmail.com>
>Cc: TowerTalk <towertalk@contesting.com>
>
>Scott McClements wrote:
>> Hi,
>>
>> Being a new ham, I have never used a grid dip meter or actually seen
>> someone use one. I've read the historical movement from people using
>> grid dip meters to "SWR analyzers". I've also read that using a a real
>> grid (tube) dip meter is the way to go.
>
>Depends on what you're doing.
>
>For instance, a slotted line is a fine way to illustrate SWR in an
>educational setting, but I'd hate to have to use one instead of a VNA.
>
>
>The grid dip basically has an oscillator that is loosely coupled to the
>system under test, and when the resonant frequency of the system under
>test matches that of the oscillator, it changes the grid current
>(because there's power transferred into the thing you're measuring).
>
>Certainly, for some systems, the coupled oscillator (Grid Dip) is
>convenient. However, I'd venture to say that for general purposes, a
>modern analyzer is better. The Grid Dip will tend to be much less
>sensitive to interfering signals (because the detection mechanism is
>narrow band). Some modern analyzers are narrow band, others aren't (i.e.
>the MFJ 259/269 is an oscillator driving a bridge with a wideband
>detector, so an interfering signal gets detected in the bridge)
>
>
>
>
>>
>> That being said, I need to determine the resonant frequency on some
>> parasitic elements for a cubical quad I am building. Not having time
>> to scour the used market for recommended makes/models, I bought the
>> MFJ-201 Dip Meter new. Ok, time to test this thing out - I read the
>> vague owner's manual and headed outside to try to dip my 75 meter
>> loop. I made a small loop in the wire in a corner close to the ground
>> and moved the coil on the MFJ dip meter around the loop (in the loop)
>> after setting the sensitivity as directed by the manual. I spun the
>> thumb wheel all the way from 2.0Mhz up to 8Mhz.
>>
>> No dips - no nothing. Same experience I had when I tried using the dip
>> adapter coil for my MFJ-259B (which I have read is also worthless).
>>
>> Ok - what am I doing wrong? The owners manual talks about "loosely
>> coupling" to the circuit under test. What does that mean exactly?
>
>There is no "exactly" about it.. it's an art or craft, not science or
>engineering. You want it coupled tightly enough that power gets sucked
>out of the oscillator in your hand, but loosely enough that it doesn't
>change the frequency of the oscillator or kill the oscillation. That
>depends on the drive level in the oscillator, the Q of the oscillator's
>tuned circuit vs the load, the Z of the oscillator vs the Z of the load,
>etc.
>
>(This is why vector voltmeters and bridges with tuned detectors were
>invented, after all, eventually leading to the modern network analyzer)
>
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