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Author Topic: Dipole / In-Vee Madness  (Read 370 times)
KC0LTV
Member

Posts: 26




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« on: August 21, 2004, 04:39:31 PM »

I'm very ignorant in this respect, so I'd really appreciate if one of the "old-timers" or elmers here could key me in.

I have an Alpha Delta DXLB-Plus antenna.  Basically, it's a ~50' all-(HF)band dipole.  I'm curious on installing it as 1) an inverted-vee type antenna and 2) a dipole.

I know (or think, at least) with an inverted vee that I would put the center insulator at the apex.  If I do this configuration, I'd like to either mount the apex atop a tree or at one of the higher parts of a 40' freestanding tower I recently acquired.  Do precautions need to be made that the antenna does not come too close (other than touching) to another antenna, such as a yagi mounted on the tower, or that the antenna wire does not touch the tree?  How about with a dipole?

Thank you.
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WB6BYU
Member

Posts: 13163




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« Reply #1 on: August 25, 2004, 04:43:47 PM »

The same rules apply whether you install the antenna
as a dipole or inverted-vee.  (The distinction between,
except for choice of support structures, is insignificant.)

Certainly you don't want bare antenna wires touching
a metal tower or other bare antenna wires.  But beyond
that you really have "guidelines" instead of "rules".

For safety, make sure that you don't have wires where
people can come into contact with them.  The ends of
the wires have the highest voltages, so don't tie them
off too close to the ground.

Try to avoid having insulated wires touching each other
or the metal tower.  If the wires cross at right angles
and are a few inches apart, that is probably OK.  But
a long antenna wire taped to a metal rain gutter isn't
going to work the way you might expect.  You have to
consider the spacing, orientation, and size of the
object in judging its impact on the antenna:  far,
perpendicular, and small are good; close, parallel, and
large are worse.  

Also, running the wire close to bricks, reinforced
concrete, or other lossy materials will tend to absorb
your signal, and should be avoided where possible.
Again, it is relative:  a brick chimney 3' from the
wire probably isn't a problem, but taping the whole
length of the antenna to a brick will isn't good.

In general, if you install the antenna and it seems to
tune up properly, the chances are that it will work,
even if there are other wires, etc. in the area.  If
the resonant frequency shifts significantly or is much
flatter than the specifications show, then there may
be too much interaction.  (The wide bandwidth is a sign
of a lossy environment.)

Probably one of the simplest ways to install an inverted
vee is to use a standoff from the tower itself: a 2x2"
board, perhaps 2 to 4' long with a pulley hanging from
the free end is a good start.  It can be attached to
the tower with U-bolts and a length of synthetic rope
to support the far end.  Run a rope through the pulley
and tie it to the center insulator of your antenna
then hoist it up into the air.  Then tie the ends off
to convenient points (as high or far away as possible
to keep the dipole reasonably flat).  Using the pulley
and halyard makes it much easier to lower the antenna
for maintenance or adjustments.  Use synthetic rope
or similar on the ends:  with the inverted vee there
often isn't a lot of tension on the end ropes, and
nylon "mason's twine" is cheap and works well.

If you are stringing the antenna between two supports,
attach rope to each end insulator and run the ropes
over the trees or other supports and down to the ground
where you tie it off.  Again, this makes it much easier
to lower the antenna when needed, and/or you can hang
a counterweight on the end of one of the ropes to
maintain a constant tension (which is a good idea if
the supports are trees that sway in the wind.)

Polyester ("Dacron") rope is best because it doesn't
decay as quickly in the sunlight.  Nylon may last a
few years - at least long enough that you can try out
the antenna and locate something better.  (And, living
in a farming area, I have been surprised how long baling
twine lasts, even in full sunlight!  But it is very
prone to abrasion, so not suitable for running over a
tree branch.)  Polypropylene (often just "poly") decays
very quickly, and isn't recommended except for very
temporary installations (like testing an antenna, or
Field Day weekend.)

A few feet of synthetic rope between the dipole end
insulator and a tower/mast/post is usually enough to
prevent any significant interaction between them.

My antennas always manage to work in spite of the
occasional tree branch or leaf that touches them,
especially when the antenna uses insulated wire.  And
it won't make much difference if you have to bend the
wire somewhat to make it fit in the available space.
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