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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