We all know the horizontal
dipole is one of the easiest antennas to build and is very common with new
and old hams alike. The vertical dipole, also easy to build, is not real
popular with hams. For me, space (I operate portable mostly) and how long
it takes to install the antenna (which is most always in the dead of
winter for me!) are big concerns.
Also, how many supports you want to
deal with (again, time vs. frostbite!) will set the design criteria.
For horizontal use, you need two supports minimum for the ends, and if
the center insulator droops too much, you'll need another there. With a
vertical dipole only one support is required and you don't need all that
much space either.
I'm also intrigued with this antenna for other
reasons, one being I like verticals anyway and the other is you don't need
a ground radial system (this also got me thinking about bending the
antenna and changing the orientation for a V Beam.....but that's another
project for later on!). I often operate portable on the tundra and always
had problems getting a good ground and tuning some bands was a nightmare
because of it.
Let's build a half wave or full wave vertical dipole with a
novel matching method!
These plans will help you build a vertical
dipole using a coil for better matching and use tubing or thin
walled pipe for the upper leg and wire for the bottom leg just to lighten
things up, (or tubing also for the bottom leg if you
prefer). The project is based
on ideas from an article by Lew McCoy W1ICP (SK), "Let's Talk Antennas-
Part III".
Do you
want a half wave or full-wave?
DIPOLE BASICS
The dipole antenna, developed by
Heinrich Rudolph Hertz around 1886, is an antenna with a center-fed driven
element for transmitting or receiving radio frequency energy. These
antennas are the simplest practical antennas from a theoretical point of
view (1) See References below. The vertical dipole or its electromagnetic
equivalent, the monopole, could be considered one of the best antennas for
land mobile radio applications. It is omni directional (in azimuth)
and, if it is a half-wavelength long, has a gain of 1.64 (or G=2.15
dBi) in the horizontal plane (2). (enter the horizontal V beam which we
will discuss later!) A vertical dipole antenna has constant gain in any
horizontal direction, but zero gain in vertical direction
(3).
Using the standard formula of
468 divided by frequency (MHz) we get the following lengths for each leg
of our antenna: (going lower than 20M the lengths get
unmanageable)
Frequency |
1/2 WL |
1/4 WL |
28.500 |
16.4' |
8.2' |
24.950 |
18.8' |
9.4' |
21.300 |
22' |
11' |
18.150 |
25.8' |
12.9' |
14.250 |
32.8' |
16.4' |
Adding a coil to get a
better match and shorten the antenna!
Let's use Lew's idea from his
article (4), and add a coil in line with one half of the antenna
to bring the impedance closer to 50 ohms which actually shortens the
antenna about 10%!!!
Coil Construction and placement in a half wave
dipole
5 Turns
X 1" diameter X 4 inches long
Turns have 1/4 inch separation
Use #8
solid wire or 1/8 inch tubing
(same coil design is used for 20 thru 10 meters for
the half wave antenna. Some
experimentation may have to be done with the coil length for a full wave
design).
The coil is
installed in series with the center conductor of the feed line and the
uppermost vertical radiator. Attach one side of the coil to the upper
radiator and the other side of the coil to the center conductor of the
feed coax. The shield side of the coax connects to the lower radiator. The
upper and lower radiators are insulated from each other. Adding a 1:1 air
choke at the feed point to reduce feedline radiation is your
choice.
See drawing below:

Drawing 1. Feed point and
coil detail. Suggested Method of mounting
There are many methods of
mounting the vertical dipole and you will have to use one that fits your
needs, your ingenuity, materials on hand etc. Shown above in the drawing
is just one of the methods. The bottom "radiator" can be tubing or wire,
your choice. The important thing is that the vertical dipole is not
mounted on or near a metal mast support! The feed line should come off the
antenna perpendicular (90 degrees) to it if possible but this
will depend on many variables with your installation.
I learned this coil
addition trick from an article in CQ Magazine by Lew McCoy W1ICP
(4) (SK). We can also use stiff wire for the last 2 feet or so of the
antenna (as described in article) to cut down on weight. This approach
just may be what the doctor ordered for 1/2 WL construction!
Several years a go there were a dozen or so of these rigid
horizontal dipoles hanging off towers in the Anchorage area (yes, I miss
Alaska!), and good reports were common. The rigid dipoles were economical
and easy to build and they worked DX! NL7VJ (now KL5T) and NL7TB (now
KL7JR) made hundreds of DX contacts with this antenna design from Alaska
and while operating portable from the Yukon Territory from 1991-1995.
Quite to our amazement we found we could get multi-band use to boot (tuner
required)! You can use wire for the bottom leg of the vertical dipole to
make it even lighter!
With that said, I'll design a
FULL WAVE vertical dipole seen in the drawing below for 21.300mhz,
which should provide 10-20 meter coverage using a tuner.
From
the chart below, 21 mhz is the center band. The 10% length reduction is
for use with a coil.
As stated earlier, the coil dimensions may
have to be adjusted accordingly for full wave operation and lowest swr and
matching. The use of a good antenna analyzer will be a big bonus if you
have one!
Of course you can pick your favorite band and see how far you
can take it. The bottom leg should be at least 6 feet from ground for
safety reasons if left hanging in free air. Multiband operation should be
possible with the use of a tuner.
Frequency |
1/2 WL Length total
per leg |
(minus 10%)
|
28.500 |
16.4' |
14.76' (use
14'-8") |
24.950 |
18.8' |
16.92' (use
16'-11") |
21.300 |
22' |
19.8' (use
19'-9.5") |
18.150 |
25.8' |
23.22' (use
23'-2.5") |
14.250 |
32.8' |
29.52' (use
29'-6") |
(Remember, above dimensions in green are for a coil in line.)
If you just
want to build a standard regular length dipole, then leave out the coil
and use the lengths in black above.

Drawing 2. Full
wave vertical dipole on 21.300mhz
Although I haven't built the vertical
version yet, there should be no reason why one couldn't expect good
results as mentioned above for the half wave version. You can use wire for
the entire antenna, support from PVC or hang vertically from
overhead.....
One other option...use it as a
"horizontal-vertical"!
There's always next summer! Experiment and have
fun!
73, John KL7JR
kl7jr@yahoo.com
References:
1. Dipole antenna-
Wikipedia
2. Antenna System Guide, NIJ Guide 202-00
3. JPL's Wireless Comm. Ref. Web- Antenna
Gain
4. CQ Mag. Nov. 1991, p 44, "Let's Talk Antennas- Part III"
by Lew McCoy W1ICP (SK)