How
to make a TuneAStick resonate
Don
Kimberlin discussing with Douglas Boze (DB)
in rec.radio.shortwave, April 13, 1992
Douglas
Boze: With all that inductance, theoretically it should
take a gnat's whisker's touch to adjust a piddling small capacitance
to tune it, particularly at higher freqs.
Don
Kimberlin: It would probably be useful to start off by estimating
just how much inductance you have. The formula for that is pretty
simple:
L = F x n(^2) x d microhenries
Where
L = inductance,
F = form factor, the ratio of diameter to length (d/l),
n = number of turns,
d = diamter of coil (inches)
You'll wind
up with a lot of zeroes in the arithmetic, but this is the simplest
way to state the formula in a fast text file.
Douglas
Boze: I do know, however, that each pair of turns
creates a parasitic capacitor (so that'll be about 766 of 'em),
and that this makes the TuneAStick (TAS) selfresonant. I don't
know how to determine what the capacitance might be.
Don Kimberlin:
Every inductor has some selfresonant frequency, but at the
sort of dimensions I recall you mentioned, the added capacitance
per turn is probably not particularly significant in the HF range.
More significant to practical use would be the seriesresonant
frequency range you can tune the stick to (meaning) connecting
the variable capacitor in series with the TuneAStick. (One position
of the MFJ tuner effectively removes its inductance from the circuit,
for an easy way to have a capacitor only in series.)
You can estimate
that frequency range by computing the series resonant frequency
at the minimum (10 uuFd) and maximum (324 uuFd) values of the
capacitor in the MFJ box. The formula for estimating those frequencies
is:
1
f = 
________
6.28 \/ L x C
Where:
f = frequency in kilohertz,
L = inductance (from preceding) in henries,
C = capacitance (min and max) in farads.
Again, using
the arithmetic stated this simply will give you a lot of zeroes
to work with.
But,
once you know that, you'll have some notion of where in the band
the TuneAStick is resonating, and approximately what capacitor
position should be optimum.
But what happens if you discover the TuneAStick has
too much inductance?
You have the inductor in the MFJ box and its switch to use to
reduce the inductance. Just like resistors with DC, inductors
in parallel will reduce the total inductance. So, you can hook
up the tuner's switchable inductor in parallel with the whole
length of the TuneAStick to try obtaining an inductance that
can be resonated in the 330 mHz region. Now, if that turns out
to be a very small amount, it can start to be a short circuit
at HF, so you may need to put a ridiculously large capacitance
in series with the tapped inductor, something that looks like
zero ohms to the RF current, but blocks the DC path through the
switchable inductor, to prevent that. Several hundred uuFd should
do the trick  and that's nothing more than another variable capacitor
with its plates closed, if you have one lying around.
Douglas
Boze: I have read somewhere that above and below the resonant
freq (fo) the circuit takes on different properties, becoming
more like a capacitor one way, an inductor the other. I can't
remember which.
Don Kimberlin:
That will become important only after you are sure you have gotten
the TuneAStick to be resonant.
Douglas
Boze: The upshot is that it does not consistently display
any "peaking".
Don Kimberlin:
Probably needing some understanding of it as indicated above.
That sort of thing isn't a problem. If something is inductive,
you then tune it with series capacitance, while if it is capacitive,
you tune it with series inductance. (Of course. most inductors
aren't readily adjustable, so that gets changed in lumps, as in
the tapped inductor of the MFJ box.)
Douglas
Boze: The big difference is in the amount of rejected
signals, such as the computer or local AM bcs (like the one on
the hill above me). No filter needed.
Don Kimberlin:
That's the big advantage you've already obtained, and being able
to resonate the TuneAStick will help that advantage by maximizing
the RF current in it.
Douglas
Boze: I am going to work on the tuner, or "waffle box"
(since it waffles about the signals ), this weekend. I had removed
the tapped inductor from the MFJ, but I think I erred in that
it should be connected in parallel with the TAS, thus bringing
the total L down to something the 240pF varicap can tune more
surely.
Don Kimberlin:
Sounds like you're on that path already. Do let me know what sort
of results you get. The preceding is in the hope it will help
you get the TuneAStick into resonance, or keep it in a meaningful
frequency range to resonate it.
On a longer
view, if you get it resonant, then I suspect it will exhibit some
directionality off its end, and perhaps be useful to "point at"
the desired signal, to offer some (perhaps rather small) amount
of rejection of unwanted RF as well as local noise, like a helix
does at VHF or UHF.
Do keep reporting what results you get. Sounds like you're on
a good path there.
Origin:
Don Kimberlin, AET BBS  (704) 5457076, 87,000+ Files (6300 megs)(1:379/16)
