Replace an old co-ax fed 40M dipole with a new one, fed with 300 Ohm ladder line and in doing so provide a "workshop" for the necessary techniques, and document the process for "posterity."
Here's the center
connection of the old dipole. It shows a little
wear and tear, but really not too bad after three years in the
air.
It was wrapped in electrical tape, which did a pretty good job of
keeping
out the elements.
The main reason for rebuilding (or replacing) it was that the old wire was full of splices after having been adjusted for various positions over the house, and repairs where it broke once, and neede further adjustment after its last move about 6 feet east. Since it had to be taken down, it seemed sensible to rebuild it. Marshall also decided to replace the original 50 Ohm coax with 300 Ohm ladder line for the feed. 300 Ohm is a little harder to find than 450 Ohm, but has lower wind resistance and is generally easier to handle. It presents all of the advantages of balanced transmission line (the main ones we were looking for were very low loss and the ability to tune the antenna for other bands), and at the design frequency is a closer match to the impedance of the dipole. Still another advantage of ladder line as compared with coax is that on "awkward" bands you have some options. You can connect both legs of the ladder line to your antenna tuner and feet it as if it were a random wire antenna, or connect either of the two legs and let the other one float.
Marshall decided to use the existing ropes to attach the antenna to the trees, but he described how to put the ropes into place in a "new" system. The perfect installation, he said, would be to attach the ends of the antenna to two structures which won't move. If you can't do that, then one fixed and one moveable (e.g. a chimney and a tree) is still better than two trees, because trees move a lot. One end of this dipole is a youngish pine tree, which really isn't as tall as would be desired, but it does have the advantage that is is fairly "whippy" and the other tree can "pull" the dipole (and the attached pine tree) without breaking the wire. If the tree had been bigger, it would have been advisable to put the rope through a pulley and counterbalance the antenna with a bucket full of rocks.
Marshall
(N1FN) uses one of his XYL's prized crystal wineglasses to demonstrate
the basic principles of resonance. When a wet fingertip is
rubbed
around the rim of the glass, the glass vibrates at its characteristic
(or
resonant) frequency, with a very noticeable ring. At
resonance, a
very large percentage of the applied power is converted to sound, and
little
power is required to sustain the oscillation. Alter any of
the physical
parameters even slightly, as Marshall showed by touching the body of
the
glass with another finger, and the glass stops ringing. The same
principles
apply to radio frequency oscillations in a wire. Thus a
dipole operated
at its resonant frequency is very efficient in that most of the power
fed
into it is radiated as electromagnetic waves. Off the
resonant frequency
efficiency drops noticeably. It will still be useful as an
antenna,
but it will not be as effective. A resonant antenna offers
3dBi gain
(compared to a theoretical isotropic radiator). An important point to
remember
is that adjusting the impedance match at the transmitter end of an
antenna
system has absolutely no effect on the efficiency or resonance of the
antenna
itself. The term resonance is often used inappropriately, for
example
"adjust your antenna tuner for resonance..." which is one reason the
usefulness
of a resonant dipole is often overlooked. Often operators
will use
a tuner to adjust the impedance of a wrongly cut antenna, with no idea
of how much better it would work at that narrow area of resonance.
The
new dipole was made from all new materials. Mighty Samson,
the QRP
dog offered the use of a "dogbone" insulator for the center connection,
but we went with a standard plastic one, with small ceramic insulators
for the ends, as you'll see in the illustrations below. For
the wire
we chose CQ 14awg Solid ccs (copper-clad-steel) wire from the Wireman,
also the source for the 300 Ohm 18awg 19-strand copper transmission
line.
Choice of wire depends on several variables, but the main ones that
were
taken into account here were ease of working, dimensional stability
(stranded
wire tends to stretch), and low noise induction (stranded wire offers
more
wind resistance; movement of the strands, and corrosion build up all
contribute
to noise).
Hams
often think the first step in building a dipole is to do the
calculations,
but as discussed above there is some pre-requisite homework!
At any
rate, the math is pretty simple, using the standard formula of 468/f
where
f is the frequency in MHz, and the answer is in feet. An easy
way
to remember the formula, as demonstrated by Marshall, is the cheer
"Two,
four, six, eight! Which antenna will we make?
Dipole! Dipole!
Yay!" Corny, but effective. This dipole was
intended for 7.040MHz,
so we divide 468 by 7.040 and get 66.48 feet total length.
Divide
by 2 to get the length of each side, or the length of the two pieces of
wire we need to cut. In this case, each side of the dipole is
33.24
feet long, or 33 feet 2-7/8 inches. The calculations were
done twice
to confirm the result.
At the center connection, the measurement is technically to the point at which the transmission wires are no longer balanced, in the case of ladder line, or open out in the case of coax. Generally it makes so little difference at HF frequencies that Marshall just measures to the inside of the loop, as with the ends.
Here (L-R) Wayne
(N0POH), Dennis (W0GD), and Michael (KB0ZTN) measured
the first wire, using a 25' measuring tape. 25' is measured
off from
the 0" end of the measuring tape with the wire stock behind the tape
and
the bend in the end of the wire going to the 25' position. At
that
point the near end of the wire is picked up and moved to the 8' 2-7/8"
mark, and the wire is bent back at the 0" end of the tape, giving a
piece
of wire with a bend ten inches from one end and another bend 33' 2-7/8"
away. Then the wire can be cut a further ten inches or so
beyond
the measured point. Wait! Measure it again before you cut
it!
Or, as carpenters say, "measure twice, cut once."
The second wire is not measured with the measuring tape, but
by holding
it out alongside the first piece. That way, even if you are
slightly
out in your original measurement, the two pieces will be the same
length.
It is better to have two pieces that are identical in length but
slightly
long or short, than one leg which is the correct length and one which
is
not.
Nate, KD0UE, casts a calibrated eye over the two wires as
Wayne holds
them.
The free end of the wire is twisted around the running wire, and will
usually follow its own path. That is, you don't have to worry
about
whether the twists are too close together or too far apart, although
you
do want the loose end to wrap pretty tightly around the running
wire..
Since this is the center of the antenna, additional wire length is not
necessary and five or six turns around the running wire would be
adequate.
If the antenna needs to be lengthened later, it will be done at the
tree
ends, not the center, but we just went with the whole ten
inches.
It makes no electrical or radio difference.
The two horizontal legs of the dipole are soldered at the center insulator. We used a 230W soldering iron, and ordinary rosin core solder. This was brand new wire, so it took the solder well. Older wire should be cleaned before soldering.
This
is a closeup of the center insulator with both legs attached, but
before
the feed line is connected.
The stripped ends of the feed line are wrapped tightly around the
antenna
wire on each side of the insulator, following the groove made by the
twisted
wire. The connection is then soldered, making sure that
solder has
flowed evenly over the entire surface of feedline and twisted antenna
wire.
The completed center connector is shown at right. If the transmission
line
is particularly heavy you might want to take a loop of it over the top
of the insulator and tape over the entire insulator with electrical
tape.
With co-ax this approach also helps to keep water out of the line.
As an optional final touch, the holes are filled with flexible silicone
sealant. With most good quality ladder line it is not
necessary to
seal the end to keep water out of the wire (which IS necessary with
co-ax).
This was done mostly to keep the little spiders from nesting
there.
And before somebody starts yelling at us, the use of an acetic acid
cured
sealant doesn't matter here either-- this is heavy-duty wire and the
small
amount of vinegar in the glue is not going to cause any problems.
We had to carefully "pre-position" the antenna with parts of
it laid
over two of the guy lines for a GAP vertical, so we did it BEFORE
attaching
the ends of the dipole to the end insulators and ropes.
As shown above, the rope and the wire OVERLAP each other in the body
of the insulator. Picture it as if the insulator weren't even
there--
a loop of rope goes THROUGH a loop of wire. We used nylon
sash cord
for the ropes-- it has proven to be pretty reliable and has a degree
of
"give" which helps keep the antenna from breaking when the wind
blows.
The knot shown is not the same as the double-half-hitch used on the
other
end. We don't think this knot has a name.
At last it's time to hoist away. We used five men and a dog
for
what is essentially a three man job-- one in the center to guide the
wires
and feed line over the guy lines and other low-level obstacles, and one
on each end to pull on the ropes. The trick was to hold one end secure
while raising the other end into position, then raise the held end,
then
stand back at a distance to judge how much more tension (or less)
either
end might need so that the antenna is almost but not quite
flat.
A little bit of droop in the middle is not only acceptable, but indeed
gives a much greater capacity to withstand wind stress.
Mighty
Samson, the QRP Dog, says "Nice job, guys!"