[TowerTalk] Open Wire

[Rob Atkinson]

I think the sensitivity of balanced antenna systems to their
environments is exaggerated.  I have a 130 foot center fed dipole up
50 feet that I run the W7FG ladder line to.  The ends dangle down
around 20 feet.  One side is near my aluminum sided house.  That end
dangles down near a pine tree.  The other side is over a tree and ends
near a 50 foot aluminum mast and its end hangs down in the air near
the mast.  If these antennas are all that touchy mine should be near a
fiasco but it works quite well.  Is it perfectly balanced?  No, one
side has a few percent more current than the other.  Is that
important?  Not as far as I can tell.  If it were more like one side
had 75% of the current the other had then sure, there would probably
be a problem.  Which is why I don't recommend unbalanced antennas such
as asymmetric dipoles and end fed wires with ladder line.  However,
anyone who wants to feed a dipole with coax, knock yourself out, but I
would not want to hoist the weight of a balun and RG8 size coax very
high in the air.   There are physical advantages to parallel wire
line--light weight if you use hollow plastic spacers, easy attachment
to feedpoint, splices are a piece of cake as your tools outside are
clippers, wire stripper and a torch and solder, and your connectors
consist of banana plugs and jacks.
Ladder line please; not window line.

Having written all that, I just might take down one of my dipoles and
feed it with coax and see how it is.   I freely admit that I might
have to eat crow afterwards.

Thanks to Paul W9AC for posting the URL to the W5 website.  That was
extremely interesting and useful especially the data on feedline
lengths.  I think I managed to get near the sweet spot lengh (one of
them anyway) on both of mine.

73

Rob K5UJ


<<<<<<<<Because it cannot be choked, any imbalance in the antenna system makes
ANY feedline radiate (and receive). This happens whether it's coax or
a so-called balanced line, BECAUSE of antenna imbalance. The virtue of
coax is that you can choke it to kill the radiation. The shortcoming
of balanced lines is that you CANNOT choke them effectively.

It doesn't take much to make an antenna unbalanced -- ground slope, or
the antenna slopes, or one end is close to a building or tower or tree
and the other is not, or the two sides of the antenna are of unequal
length.

This is a big deal if you live in an urban neighborhood, with noise
sources all around you. We have few weapons in this fight, and two of
the most important are 1) antenna directivity and 2) moving the
antenna further from the noise source. A feedline that radiates (and
receives) defeats both of those weapons. >>>>>>>