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Author Topic: Future 20M Rock-Mite needs a Tuna  (Read 702 times)
K3ESE
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Posts: 57


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« on: September 10, 2002, 05:48:37 PM »

so...let's say I DO manage to construct my 20M
Rock-Mite. Then what? Anyone have ideas on an inexpensive,
easy-to-construct tuna kit for a complete beginner? Or
have one I might acquire? Or ideas for getting it on
the air without a tuna, or any other kind of fish?
Thanks!
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WB6BYU
Member

Posts: 13284




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« Reply #1 on: September 12, 2002, 03:51:48 PM »

First, make a simple half-wave dipole, and you won't
need any sort of tuner.

There may be some situations where an end-fed wire is
preferable:  you don't have the weight of the coax to
support, and it is cheap, as well as being smalller and
lighter to carry.  (And sometimes a single wire is
easier to install than a dipole, though I've installed
portable dipoles in many situations without problems.)
In this case, I'd opt for a full-wave wire - about 65'
long, using stranded, insulated wire.  (Perhaps around
#22, or whatever you can find.)  Because this will have
a high input impedance, you will need a tuner to match
it.  Simple!  An "L" network will do the job:  put a
coil in series between the wire and the coax to the
rig, and connect a variable capacitor between the wire
and ground.  You will need to experiment with the right
values, but a 150pf variable capacitor is probably a
good place to start.  Adjust coil and capacitor to get
minimum SWR.

Now, as long as you are using the same wire on the
same band, the tuner settings won't change much.  You
should be able to wind an equivalent coil on a T-37-6
toroid core, and replace the variable capacitor with
a fixed capacitor in parallel with a 30pf or so trim
cap.  Then just throw the wire over a tree branch and
adjust the trim cap for minimum SWR (or best received
signals).

By establishing the antenna you will be using, you can
optimize the tuner for the specific impedance it has
to match.  The trim cap should handle variations in
how the wire is installed.

Such a long wire may cause RF on the rig if not used
with a suitable "ground".  Don't worry about pounding
an 8' ground rod for portable operation - just lay a
quarter wavelength (16') radial wire on the dirt if
you don't have a large chunk of metal (such as a pack
frame or bicycle) to use for the "ground" connection.
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