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Build a 10 Meter Technician Class
Dipole 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:
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 = total length in feet
Divide by half
for each side of the dipole
468 / 28.4 =
16.478 feet
Round off
16.478 feet to about 17 to 18 feet total for swr tuning.
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". 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.
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