Bellow this page
you will find a program that will calculate the vertical
length of your 5/8 antenna
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 feed line 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 a
dipole.
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.
