I should like to offer comments on and some alternative
recommendations for the described construction of the 66-foot long FCP.
It has already been pointed out that the commercially
manufactured “450-Ohm open-wire line” is not suitable for the counterpoise.
That is indeed true. A yardstick will quickly show that with less
space for “windows” than “panes” that black stuff measures 58 percent
closed. Hardly “open-wire”! The surge impedance is around 390 Ohms dry
and less when wet. ‘Nuff said.
I homebrew my open-wire feedlines, using #14-gauge solid
copper wire, either enameled, or bare wire that is roughened and then painted
gray. Spaced 2”, the surge impedance is
very close to 500 Ohms. I use solid
copper wire, not copper-clad steel.
First because my feedlines are not under mechanical stress and therefore
steel’s greater strength is unnecessary.
Second, and importantly, because one antenna when used on multiple bands will
almost
certainly have standing waves on its feedline, and it has been my experience
that, with standing waves present,
the feedline suffers permeability losses if itsr conductors contain iron or
steel. In fact, I have also experienced permeability losses when using steel
or steel-cored conductors in matching stubs. And I would expect similar losses
in any impedance transforming device, balun, transformer, etc.
Please spare me any lectures about skin effect and how the rf just whizzes
along without attenuation. Let me assure you that when an rf conductor,
albeit with its unquestioned skin effect, is in intimate contact with another
substance
it WILL BE affected. For example, it
is perhaps not widely known that the intimate contact between pvc insulation
and house wire will
change the velocity factor of a single conductor (although to a much lesser
extent than does the insulation of a coaxial cable). And just as with a
conductor’s
insulation,"intimate contact”
certainly prevails between a conductor's surface and its core.
Therefore, in your FCP use solid copper wire, bare or
enamel-insulated, not stranded (which has higher losses because over time an
outdoor environment
will change the Ohmic relationship of the individual strands to one another,
and thereby may also intensify the inductive effect of the spiraled strands).
By all means avoid copper-clad steel.
14-gauge wire is perfectly adequate; the extra expense of 12
gauge is not warranted, nor is the extra weight desirable.
One does not need spreaders made out of small diameter pvc
tubing with end caps and weep holes.
The weight, bulk and expense of such spreaders is not justified.
I have read, but not verified, that bamboo has a more
favorable strength-to-weight ratio than steel.
Regardless, it is wonderful stuff for making open-wire spreaders. At an Asian
food market I bought 200 bamboo
chopsticks for five dollars. Snooping
around I’ve found bamboo skewers in various lengths and diameters for very
little money. For 14-gauge wire I use
bamboo skewers that are about 3/16” diameter.
I look over the chopsticks or skewers and discard the misfits, about
10-15 percent of the lot.
Here’s how I make my spreaders for the 500-ohm feedline
mentioned above:
I measure 2.5” lengths.
At the cut marks I use a compact tubing cutter to scribe a groove all
the way around the skewer. I cut with a
fine tooth hand saw. Using a divider
set to 0.25” I make a prick mark at that distance from each end. I use a naked
hacksaw blade to cut a slot to
a depth of 0.25” in one end of the spreader.
Placing a piece of cardboard in that slot, and with the spreader in a
bench vise, I visually align my hacksaw blade at the spreader’s other end with
the cardboard, and again cut to a depth of 0.25”. In this way I achieve a
spreader with two parallel slits. At the base of each slit, I use a #50 drill
bit secured in a pin vise to make a circular hole through the sawn slit. I
then widen only the slit using a couple of
naked hacksaw blades taped together and/or a small file. The slit should be
just wide enough to
tightly pass the 14-gauge wire and the hole should grip it tightly when the
slit ends of the spreader are pinched together.
Spreaders are baked in a <180 degree oven for 24 hours,
and while still warm dipped in diluted gray exterior latex paint.
I stretch the two parallel 14-gauge wire conductors between
two trees. For bare wires, I rough up
their surface by running a piece of coarse sandpaper along them. The Eastwood
Company also sells an acid etch
that would probably do the same thing.
I paint the bare wires with the same gray paint used on the
spreaders. Then I snap the spreaders on
to the wires and adjust their spacing, which can be anything between 9 and 15
inches. I secure the spreaders using
50-lb. test Dacron kite line with a constrictor knot at each spreader’s
tip. Another technique is hot glue
applied with a gun and then pinching the spreader tips with spring clamps while
the glue sets. After the spreaders have
been fastened, I paint them thoroughly with gray latex paint.
If you wish to have the described 4” spacing for the
160m folded counterpoise, you should start with 4.5” long bamboo skewers.
73,
Charles, W2SH
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