Conference: SHORTWAVE Date: 4/04/1992 From: DOUGLAS BOZE
Tune-A-Stick: Building a better signal sink.
Recently, Don Kimberlin described a wire-wound broomstick antenna he
built, and the noise cancelling properties it displayed. Intrigued, I
built a similar antenna using about 226 turns of #24 speaker wire
around a 1 1/16" dowel. The length of the wound section was about 27".
These materials were what I had on hand, and the speaker wire allowed
the start and end connections to be at the same end. The other end of
the winding was soldered together. I was surprised by the performance
of this antenna, even though it was indoors rather than out. I reckon
it had over 100' on it, compared to the 65' random wire antenna I was
using outside.
I connected an MFJ antenna tuner between it and my DX-440. It seemed
to reject most of the noise from my PC (a rather poorly shielded 20MHz
286), and even local AM MW bcs. I am, unfortunately, located in the
ground wave of a 50KW AM xmtr, and since the DX-440 is prone to inter-
modulation, this causes some annoyance <G>!
Even so, I found I still needed my 7-element Chebyshev high-pass filter
to attenuate the MW frequencies and the resulting harmonics (particularly
from that station on the hill, at 1360KHz). Nevertheless, it was enough
to make me want to build bigger and better: what I call my Tune-A-Stick!
Bolstered by the results of the test antenna, I set about designing the
outdoor, industrial strength version.
I had some vaguely defined goals to meet:
a) As much wire in one shot as possible, say 1000';
b) Make it compact, even portable (well, within reason);
c) In consideration of item "b", it should be easy to
connect and disconnect.
d) Make it weatherproof.
The results culminated in a 10' length of class 125 PVC pipe (2 3/8"
diameter) and 1000' of black PVC insulated 20 AWG solid tinned copper
wire (Carol C2028-21-01, available from DigiKey as C2028B-1000-ND).
One end has a standard 2" PVC cap, the other end, which was belled,
received a 1-1/2" to 2" adapter, into which was cemented a 1/8" thick
PVC disk with a gold-plated RCA jack (Radio Shack 274-852).
Assembly
Pipe: Nothing special, just used as it came from the local Ace Hard-
ware. Class 125 is all you need, as this is not a heavy device.
I used 2" nominal, which has an O.D. of 2-3/8". One end was
belled, but only becuase it was chopped from a 20' length of
belled-end pipe. You will need two (2) 2" caps, or in the case
of belled-end, one cap and one (1) 1-1/2" to 2" adapter to plug
into the bell.
Caps: One cap will be left as is, the other will get a 1/4" hole
drilled in the center. In the case of my BE pipe, I intended to
cut a PVC disk with a 1/4" hole in the center to fit into the
adapter. Lacking PVC, I used a piece of acrylic. Since acrylic
and PVC will not bond, I used PVC cement in the adapter. This
softened the PVC enough to sort of ooz around the acrylic disk,
such that it will never come out. Put a jack into the hole, and
make sure you can tighten the nut onto it. Now remove it.
Put the cap(s, and adapter) on, but do not cement them! They will
only be pushed on. At a point just beyond the cap, (or bell) on
the pipe, drill a hole just large enough for the wire to pass
through. For the 20 AWG wire I used, that was 1/16". Drill into
the print stripe, to act as a reference for counting the turns.
Remove the cap (or adapter) with the jack, or rather the hole
for the jack, if you've been following the instructions <g>. Now take
the free end of the wire from its spool and pass about 6" through the
hole and out the end of the pipe.
Strip enough insulation from the end to make a connection to the
jack, then slip the nut, and the "chassis" or "shield" washer (the one
with the solder-eye), onto the wire. You can omit the lock-washer if you
need to. Now pass the wire through the hole in the cap or adapter from
the inside (pipe-wise) and solder it to the center lug of the jack. Now
push the jack into the hole, slip the washer over the inside portion and
secure it with the nut. Bend the tab on the washer toward the center to
facilitate the eventual connection of the other end of the wire.
Push the cap onto the pipe (or the adapter into the bell): You
are now all set to wind up a new antenna project! Really! Turn on your
SW receiver, your stereo or TV, becuase you will be spending one to two
hours on a truly boring task.
I just wound each turn next to each other, snugging it up as I
went along. It's easier than trying to spread them out. You can count
the turns if you're nuts. I think I put 1532 +/-5 on my stick. It's all
academic...
Now, you will have to leave enough wire to return down the
length of the pipe to connect at the washer on the jack. When the wire
comes off the spool, tape the last turn, and gently stretch the wire
towards the jack. If it doesn't reach, unwind enough to pass the end
with about 6" to spare.
You should now start at the beginning of the winding and snug it
up by twisting the turns in the applied direction while pushing toward
the starting point, while working toward the free end. Locate the point
where the last turn crosses the print-stripe. Back off a quarter turn.
Place your drill bit next to the preceding winding on the stipe and
drill a hole.
Now comes a tricky part. Tape the end of the winding to keep it
from exploding (and your patience with it). Poke the end of the wire
through the hole _and out the near-end of the pipe_. Pull all 8-9' out.
Tie the end around a heavy object, like a large screwdriver. Remove the
jack-cap, if you haven't already done so, and make sure it is clear of
the bore of the pipe. Pick up the end of the pipe, make sure the wire
isn't in knots or tied around your cat, and _throw_ the screwdriver down
the length of the pipe.
Remove the screwdriver once the wire pops out the jack end of
the pipe (it will not be needed for reception <g>), strip some
insulation from the wire end and solder it to the washer on the back of
the jack. Gently push the cap onto (or the adapter into) the pipe, and
cap the other end.
This completes the Tune-A-Stick! To support it, I made a pair of
stands out of 1/4" acrylic, each about a foot high, with a hole at the
top bigger than the O.D. of the antenna. This allows them to be tilted,
creating a pair of splayed feet (like a sawhorse). I have a flat roof,
you see.
It could be suspended by rope, but I would recommend securing
the ends about a fourth of the pipe's length in from each end, as PVC is
not structural, and will sag if not properly supported.
Since the wire insulation is black PVC, I sprayed the exposed
portions of the pipe with a coating of black Plasti-Dip (sp?), a liquid
vinyl product. Orient the print stripe down, to place the wire holes on
the underside.
And that wraps it up! I modified my MFJ random wire antenna
tuner, but that project can wait. I'm still fiddling with it anyway. The
results will follow as soon as my fingers uncramp! You may all, of
course, distribute these instructions as you like (or don't like).
To paraphrase (well, SWL wasn't what he was talking about) Dr.
Timothy Leary, "Tune in, turn on, and try out!"
- -- --->>-Doug->
Origin: The Boardwalk! - (206) 941-3124 - Federal Way, WA (1:343/47)
------------------------------------------------------------------------
Conference: SHORTWAVE To: DOUGLAS BOZE From: DON KIMBERLIN Date: 4/13/1992
DB> ...With all that inductance, theoretically it should take a
DB> gnat's whisker's touch to adjust a piddling small capacitance to
DB> tune it, particularly at higher freqs.
...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.
DB> I do know, however, that
DB> each pair of turns creates a parasitic capacitor (so that'll be
DB> about 766 of 'em), and that this makes the Tune-A-Stick (TAS)
DB> self-resonant. I don't know how to determine what the
DB> capacitance might be.
...Every inductor has some self-resonant 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 series-resonant
frequency range you can tune the stick to (meaning) connecting
the variable capacitor in series with the Tune-A-Stick. (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 Tune-A-Stick is resonating, and approximately what
capacitor position should be optimum.
...But what happens if you discover the Tune-A-Stick 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 Tune-A-Stick to try obtaining an inductance
that can be resonated in the 3-30 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.
DB> I have read somewhere that above and below the resonant freq
DB> (fo) the circuit takes on different properties, becoming more
DB> like a capacitor one way, an inductor the other. I can't
DB> remember which.
...That will become important only after you are sure you have
gotten the Tune-A-Stick to be resonant.
DB> The upshot is that it does not consistently display any
DB> "peaking".
...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.)
DB> The big difference is in the amount of rejected signals, such as
DB> the computer or local AM bcs (like the one on the hill above
DB> me). No filter needed.
...That's the big advantage you've already obtained, and being
able to resonate the Tune-A-Stick will help that advantage by
maximizing the RF current in it.
DB> I am going to work on the tuner, or "waffle box" (since it
DB> waffles about the signals <g>), this weekend. I had removed the
DB> tapped inductor from the MFJ, but I think I erred in that it
DB> should be connected in parallel with the TAS, thus bringing the
DB> total L down to something the 240pF varicap can tune more
DB> surely.
...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 Tune-A-Stick 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: AET BBS - (704) 545-7076, 87,000+ Files (6300 megs)(1:379/16)