Post by Bill TurnerPost by Geoffrey S. MendelsonI have an MFJ modle 249 (the original with a meter) antenna analyzer.
MFJ sells a kit of two coils to make it into a dip meter.
I would like to use it to measure the resonant fequency of a traps I
am building, but am unable to purchase the coils.
Is there a way to make then on your own?
Don't bother. I have one and it's a very poor dip meter. Get a Heathkit
or Eico or Millen instead. You'll be much happier.
The MFJ 269 with the dip-meter coils, is probably the least-sensitive
and least-usable dipmeter I've tried to date. It really isn't good
for very much at all.
I don't think the coil construction is terribly critical, though,
since the coil inductance is not being used as part of the oscillator
tuned circuit. The MFJ uses only two coils to cover a wide range of
frequencies, rather than the half-dozen or so coils used by real
tank-circuit dippers.
You can try simply winding a few turns of hookup wire on a convenient
tubular form, hooked up to an RCA or BNC or PL-259 glued to the end of
the form, and see if you can get an adequate dipping for your
purposes.
The higher-frequency MFJ coil seems to be about 4 or 5 turns of wire -
length is about 1/2" and diameter is somewhere around 3/8". The
lower-frequency coil is somewhere around 12-14 turns of wire,
close-wound on a 1/2"-diameter plastic form.
As dippers go, the Heathkit HD-1250 solid-state model (it's a
dual-gate-MOSFET-based gate-dipper rather than a grid-dipper) is
better than the MFJ. It's not great, but it's probably adequate for
most purposes.
Haven't used a Millen but I've heard that it's better yet. Haven't
used an Eico, or one of the B&W model 600 acorn-tube-based grid
dippers.
The Measurements/Boonton Megacycle Meter is the ne plus ultra of
dippers, as far as I know. It's big and perhaps a trifle clumsy, but
it has a strong oscillator which *really* dips nicely with even very
loose coupling to the circuit being measured. If you ever run across
one, grab it.
For comparison: when testing an air-core inductor in parallel with an
air-variable cap, I could barely get a dip reading at all with my MFJ
and either of its coils. The coil had to be poked right up into the
inductor and into contact with its windings to create enough coupling
for a dip... and of course this tends to de-tune the coil and makes
the measurement less accurate.
A Heathkit HD-1250 would get a usable dip with its coil as far as an
inch away from the inductor.
A Boonton got a dip three or four inches away.
--
Dave Platt <***@radagast.org> AE6EO
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