13. Wire, Wire, Everywhere

 

Tech-Talk
Part 12

Welcome to Part 12 of our discussion.

Last time we started to look at simple dipole antennas.  Again, here's the picture we've all seen in the Antenna Book and other publications:

Especially as we get down to 40 and 80 meters (not to mention 160M!), space becomes an issue -- an 80M dipole is about 132 feet long.  Fortunately, it's not strictly necessary to have such nice straight wires.  For example, you can construct your dipole as an inverted V.  One advantage to this is that you only need one tall support -- although you do want to keep the ends up high enough to avoid contact with humans and wildlife.  With the wires at a 120° angle to the ground, you only need 115 feet of space for that 80M antenna.  At 90°, it's just 93 feet.



Perhaps surprisingly, there's not an awful lot of difference in performance when you slope the ends down.  Gain is reduced a bit, and the pattern becomes more omni-directional, but you'll still work plenty of stations.


Similarly, you can bend the ends horizontally without much sacrifice.  Or bend them both vertically and horizontally if you need to.  There may be some changes in the pattern, and it may tune a bit differently, but you'll be on the air with a decent antenna.

The reason this works is simple -- in a dipole type antenna, the center third of the antenna does most all of work.  Relatively little signal is radiated by the ends.  Now, you may be thinking "Hey!  I could put a loading coil at the end of the antenna and really shorten it!"  And you'd be almost, but not quite, correct.  The farther away from the feedpoint you go, the more inductance (that is, the bigger the coil) you need.  Moving the loading coil in toward the middle of the element will allow a manageable size.

Designing, building, and installing an antenna like that is beyond the scope of this series of articles, but some research will give you a load of information to work with.  In particular, there's a great article in the October, 2003 issue of QST magazine describing just how to figure it out.  There's some math involved, but if you're handy at all with a spreadsheet or scientific calculator, you can easily make the proper coils.  ARRL members can now access back issues of the magazine as PDF files on the League's web site.

That's it for this month -- just a short one so I can continue preparation for Hamvention.  We'll continue with antennas next time.  Until then,

73 for now
John Bee, N1GNV
Quicksilver Radio Products