This 10 meter dipole
project will enable you to start using your new HF privileges as a
Technician class operator on 10 meter SSB between 28.300mhz and
28.500mhz.
It is a half
wave dipole designed for the center frequency of 28.400mhz but should
cover most of this 200khz spread with low SWR. This project shows the 10
meter dipole in the horizontal configuration but may also be installed in
the inverted V fashion.
Since any
dipole is considered a balanced antenna, it should be used with an air
wound choke to help eliminate rf on the coax. See the "Ugly
Balun Project" for instructions on how to
build one from coax.
Building the 10 meter dipole is very easy and
simple construction is used with very inexpensive materials you may
already have laying around the shack. See drawing
below:
It consists
of 2 equal lengths of #12 or #14 wire insulated in the center and on each
end and supported with rope, nylon cord, etc from both ends and the center
if needed.
The insulators
can be homebrewed from any non-conductive material that will withstand the
rigors of mother nature such as glass, plexiglass, painted wood, pvc,
heavy plastic, commercially made insulators, etc.
The center insulator
actually can be used as a duel purpose device, both for support at the
center and to prevent the two outer radiators from touching. It should be
made heavier and a bit larger to handle the weight of the entire
antenna, coax and support ropes. Remember that it contains all connections
to the antenna and some method must be used to secure them from separating
from each other due to wind load, etc such as plastic wire ties to
eliminate strain on the coax end.
The center support rope at the
insulator may not be needed depending on how much the completed assembly
droops when put up between the end supports. Don't
suspend it between trees unless some method is used on the ends to prevent
tension from breaking the wire if the wind
blows.
It is fed with
standard 50 ohm coaxial cable, from the center of the dipole with the
center conductor attaching to one side of the dipole and the shield of the
coax to the other side. The type of coax is up to you but the larger coax
like RG8 will give a bit less loss depending on feedline length. The coax should come away from the dipole at a 90 degree
angle to the wire if at all possible when tuning and in the final
installation.
It does not matter
which side is connected at the coax just as long as neither side of the
antenna is shorted to the other side, hence, the insulator in the middle.
Seal all connections at the coax end where it connects to the antenna
including the end of the coax. This keep water out.
From the antenna,
the coax should go to an air wound choke AT the antenna as mentioned
above using the same coax that would go to the transmitter.
NOTE: Rather than have connectors at each end of the
air choke, just make the feed line coax about 18 to 21 feet longer than
needed to reach from the transmitter to the antenna and then wind your
coil on the form and attach to the antenna.
Here is the standard
formula used for half wave dipoles that you should already know and it is
used for the overall length of the radiating
elements.
NOTE! It is always better to start LONGER with each half of
the dipole than the formula results below!
No two antennas will perform
the same in all locations with the formula! It is much easier to cut than
add wire to a dipole.
468 / freqmhz = total length in
feet:
Example,
468 / 28.400mhz =
16.478 feet (total length end to end in
feet)
Round off 16.478
feet to about 17 to 18 feet total length, end to
end, for swr tuning.
Don't forget to feed it in the exact
center giving you 2 equal legs on each side of the center
insulator.
Do not attempt to tune the antenna on the ground! It
must be raised to it's final operating height for tuning! If you have an
MFJ 259B analyzer or equal, your tuning will be much quicker and you can
do it off the air!
After you have built the antenna, raise it to
the final operating height, (the higher the better), tune your transmitter
to 28.400mhz, listen for unused frequency and using the AM, CW or tune
mode with very little power, (just enough to get a reading on your SWR
meter), transmit your callsign and say testing; (here again assuming
you have a clear frequency)......immediately check SWR, say you're call
sign again and "Test clear, unkey, trim short equal amounts from each end,
again listen for unused frequency, and repeat on the air "Testing" with
your call sign, keyup, check SWR again, unkey, repeat as needed for lowest
SWR on 28.400mhz so you will be centered in the Tech portion of the
band.
Once you have tuned
it for lowest SWR, make certain you have all supports firmly tied down and
your ready for some fun on 10 meter SSB!
Added notes of
interest:
If the dipole is
installed in a horizontal (flat top) fashion, it will tend to be
bi-directional, meaning that it will transmit and receive equally well at
90 degrees (broadside) to the antenna and very little off each
end.
A more popular
method of installing it is in the inverted V fashion which will yield you
a more omni (all direction) pattern. You will have to compensate with the
swr tuning using this method as it may be different from horizontal
mounting.
For an inverted V,
simply have the center higher than the ends, like an upside down V. Don't
bring both ends together. Use about a 45 degree angle from the center
insulator.
Performance:
The dipole has no
gain compared to a yagi (beam) antenna. The dipole is used as a reference
antenna for comparison of other types of antennas and is usually
referenced as 0db gain or in techincal terms 0dbd. The "d" at the right
side of 0dbd represents the reference (dipole).
It is a standard, basic antenna and the most widely used of all
antenna types. A reference to a dipole is much more
realistic when comparing antennas.
Until the 11 year
sunspot cycle starts to climb, activity on 10 meters (DX) will be very
difficult and limited.
The band will be
most active during the daylight hours (when it is "open"), but very
good "local" contacts will be made using ground wave coverage anytime, so
don't expect to set the world on fire until the 11 year cycle really
starts to climb! Then the world will open up to you with stations from
around the world.
When the sun spot
cycle is at it's peak, it is very common to work around the world with 5
watts SSB using a 10 meter dipole!
More about dipoles!
A dipole antenna is a straight
electrical conductor measuring 1/2 wavelength from end to end and
connected at the center to a radio-frequency (RF) feed line that is
connected to the transmitter/receiver.
This antenna, also called a
doublet, is one of the simplest and most basic types of antennas,
and makes up the main RF radiating and receiving element in various
sophisticated types of antennas.
The dipole is inherently a balanced
antenna, because it is bilaterally symmetrical or contains
equal conductors on each side of the feed point.
Ideally, a dipole antenna is fed
with a balanced, parallel-wire transmission line. However, this type of
line is not extremly common and it's impedance does not match the output
of most ham transceivers. It is extremly low loss however, and due to this
fact, it is often used by hams and can be matched to most 50 ohm output
radios. This is usually done with a matching transformer called a balun,
which is a contraction of the words "balanced" and "unbalanced".
An
unbalanced feed line, such as coaxial cable, can be used, but to ensure
optimum RF current distribution on the antenna element and in the feed
line, a balun should be inserted in the system at the point
where the feed line joins the antenna.
How high should it be in the air?
For best
performance, a dipole antenna should be at least or more than 1/2
wavelength above the ground, the surface of a body of water, or other
horizontal, conducting medium such as sheet metal roofing. The antenna
should also be at least several wavelengths away from electrically
conducting obstructions such as supporting towers, utility wires, guy
wires, and other antennas. This is very difficult to do with most ham
installations.
Dipole antennas
can be oriented horizontally, vertically, or at a slant. The polarization
of the electromagnetic field (EM) radiated by a dipole transmitting
antenna corresponds to the orientation of the element. This means that if
the antenna is installed with it's wire horizontal to the ground, it
radiates a horizontally polarized field. If it is installed in a vertical
position, then it would be said to have a vertical polarized field in
reference to the ground. When the antenna is used to receive RF signals,
it is most sensitive to EM fields whose polarization is parallel to the
orientation of the element.
The RF current in a dipole is maximum at
the center (the point where the feed line joins the element), and is
minimum at the ends of the element. The RF voltage is maximum at the ends
and is minimum at the center.
The RF current portion of the antenna is where the
maximum rf field is radiated.
To learn even more
about antennas in general, it is highly recommended that you get a copy of
the ARRL Antenna Book seen below!
Enjoy and welcome to the world
of
HF....73
