The 5/8 wave antenna consists of a vertical radiator
which is fed at the base of the antenna. A matching device of some sort
must be added between the antenna and the feedline if you wish to feed
it with coax. Adding a coil in series with the antenna at the base is one
such method of matching.
So why would anyone use a 5/8 wave antenna if they have
to go through all that extra work? After all, a ground plane antenna provides
a nicer match. There are a couple of answers. The first is GAIN.
The computer shows that the antenna (mounted 1 foot above ground) has a
gain of about 1.5 dBd higher than a dipole's gain (also mounted
1 foot above ground.)
The second reason you may want to use the 5/8 wave vertical
is to obtain a lower angle of radiation. A half wave antenna's radiation
peak angle is 20 degrees. You'll find that the 5/8 wave antenna's angle
of radiation is just 16 degrees making it an even better dx antenna.
You may have noticed a pattern developing here. A quarter
wave ground plane antenna has a radiation pattern that produces maximum
gain at about 25 degrees and a half wave antenna drops that angle to 20
degrees, and the 5/8 wave antenna further drops that angle to 16 degrees.
So why not just keep extending the antenna out to one full wave? Well it
would be nice if it worked but unfortunately the wave pattern begins to
create very high angles of radiation beyond 5/8 wave. So we've reached
the maximum gain at this point and extending the antenna any further just
reduces the gain where we want it (low angles). Of course if you are interested
in very short skip, extending the antenna will produce nice gains over
All antenna lengths depend on various factors. Some of
these factors are:the height above ground, the diameter of the wire, nearby
structures, the effects of other antennas in the area and even the conductivity
of the soil.
This page allows you to calculate the length for a 5/8
wave antenna. It uses the standard formula, 585/f (178.308/f for metric)
MHz to calculate the element lengths. If you've experimented with 5/8 wave
antennas before and know of a better formula for your QTH, feel free to
change the formula to suit. This formula is for a wire antenna. Of course
if you construct your antenna out of tubing, the total length of the antenna
will be shorter, for example I've found that 21.5 feet seems to give the
maximum gain for a frequency of 28.5 MHz when using 1" tubing. And 22.5
feet seems to be the best length for wire at the same frequency. Since
the formula calculates the antenna to be about 2 foot shorter, be
sure to experiment and perhaps add a little to your final length.
Enter the formula for the antenna calculation
Your 5/8 wave antenna's vertical length is