On Sep 9, 10:52 pm, Clayton wrote:
Thanks for the replies everyone

Scott,
Could you please explain how I can use a grid dip meter to measure
the
resonant frequency and adjust the number of turns and/or wire spacing
to
get it where it needs to be?
I just picked up a grid dip meter but have never used one.

I'm not completely understanding the math required in figuring
inductance out...
I am able to figure out that with my coil form and number of turns the
coil I have is around 6.5uH but I cant figure out how to work wire
gauge into the equation? I have been trying to find someplace that
explains it but am unable to.
Could somebody here maybe explain how the math works when you throw
wire gauge into the equation?

Tam,
I am replacing the wire in the trap because when I was cleaning the
corrosion off,the last inch or so of the wire broke off and since the
wire is aluminum I figure I would be better off replacing it then
trying to repair.

Thanks for the help everyone
Its most Appreciated
Regards

The link Owen put in his posting is to a pretty complete on-line
inductance calculator. The one I use is similar, in that it uses a
helical waveguide approach, though it probably does not include all
the "tweaks" mentioned on the link page. Wire gauge plays a part in
several ways. If the basic formula uses coil inside diameter, then
note that larger wire puts the current further out, on average. If
the wire spacing is much greater than the wire diameter, the current
in the wire will be nearly radially symmetric: if you look at a cross-
section of the wire, the current density will depend on the distance
out from the center of the wire because of skin effect, but will not
vary much at any angle for a given radius. But if the turns are
closely spaced, proximity effect will re-distribute the current on the
wire so it's no longer radially symmetric. That will affect the
inductance.

A grid dip (or these days, just dip) meter is used to find
resonances. You want the trap, when it's in the environment it's used
in on the beam, to be parallel resonant so it presents a high
impedance to signals at some frequency. So in a 20M-15M beam, an
element with a trap resonant at 21MHz will look like a high impedance
to 15M signals, and decouple the ends of the elements from the inner
sections, which in turn are designed to operate on 15M. On 14MHz, the
traps look inductive (the inductive reactance is lower and the
capacitive reactance higher than at 21MHz, so the _parallel_
combination looks like a net inductance), and that means they look
like loading coils. As a result, the elements on 20M including the
outer sections and the traps-that-look-like-loading-coils are shorter
than if they didn't include the traps. You can extend that reasoning
to a tri-band beam.

You'd probably do well to practice with your dip meter with some
resonant circuits "on the bench." You want to couple the meter to the
coil of the resonant circuit to get a dip. To start, you can couple
it closely, but the dip will be hard to read well, since the resonant
circuit you're measuring and the one in the dip meter will interact
strongly, "pulling" the dip frequency. So when you get a strong dip
near a reasonable frequency, pull the dip meter further away, till you
get just a gentle dip. Then the frequency readout (assuming the
frequency scale on the meter is accurate) should be accurate. As with
most instruments, it's good to get some experience with a dip meter so
you have an idea when it's giving good readings and when it's in
error.

Cheers,
Tom


Cheers,
Tom