| Elements |
My antenna design uses 3/4 inch diameter elements. This is probably
overkill and adds a little weight. The primary reason that I used this diameter element
was that I had many coils of 3/4" CATV hardline in my material surplus pile that I
had accumulated over the years. For a portable version of a rope yagi, a smaller diameter
element is preferred. Going too small (say, less than 1/2" diameter) will start to
add some loss to the antenna efficiency. Any element diameter other than 3/4" will
require a change in the element length and spacing in the antenna design. |
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| Element-to-boom clamps |
The focus is cheap and effective. I went up and down the aisles of my
local hardware store and settled in this clamp design. Each element clamp uses two
L-brackets that are used for furniture reinforcement, a couple of small hose clamps and a
10-24 bolt/nut. I used my vice grips to cinch down the two L-brackets on the rope boom.
Next, I tightened down the two hose clamps to afix the L-brackets to the element. I then
tightened the #10 bolt until the L-brackets deformed a little. When I release the pressure
from the vice grips, the brackets contiunued to grab the rope pretty well. I have pulled
with all of my strength and cannot move an element on the rope boom with this clamp
arrangement. The hose clamps grasp the L-brackets as far as possible to the ends of the
bracket to help maintain the alignment of the element to be at a right angle to the rope
boom. |
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| Rope boom and stabilizing lines |
The boom consists of 1/2 inch rope. The best rope type (stregth, stability
and sun resistance) is polypropelene but was not readily available in my local Home Depot.
So, I went with "heavy duty truck rope" which is a poly rope (not hollow). It
has slightly less strength than polypropelene but is cheap and available. The stabilizing
lines are 1/4" braided nylon rope. This rope will stretch when it gets wet but is
only used to keep the elements in a plane and does not support much of the element load. |
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| Element slides |
I first tried erecting the antenna without this feature. The antenna
turned into a porcupine in a second. We added the element slide feature to keep the
elements loosely attached to the stabilizing lines during erection and antenna rotation
events. I use two nylon tie-wraps on each end of each element. Each stabilizing line
passes through a tie-wrap loop on all elements. The tie-wrap loop is attached to the
element with another tie-wrap as shown in the photo. |
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| Director-end truss/ swivel |
A truss is used at the director-end support to provide an attachment point
for the rope boom and provide tie points for the stabilizing lines. 1 1/2" EMT is
used as the truss boom. The truss boom is u-bolted to a somewhat thick plate. The plate is
attached to the antenna mast using a 3/8' screw pin anchor shackle. Using this shackle
permits the truss to rotate as the antenna is rotated. This simplifies the geometry of the
rope stabilizers and element slides a little. |
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| Reflector-end support |
The end supports are twelve-foot redwood four-by-four's. Redwood is
naturally resistant to termites. In Arizona, its a simple matter to dig a post hole and
drop in a four-by-four. Depending on your locale, your selection of an end support may
vary. Another truss is attached to the rear support. A large pulley is used to take up
tension on the rope boom. Two smaller pulleys are used to apply tension to the stabilizing
ropes. C-clamps are used to attach the pulleys to eye-bolts on the support and truss. When
"rotating" the antenna, simple open up the C-clamps and remove the three pullys
and drag the antenna over to another end support and hook up the three pulleys. After
attaching the three pulleys, take up the tension to get the antenna into its normal shape.
This sounds simple but requires a little practice. |
 
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| Tension system |
A comealong is used to take up tension in the rope boom. Rope saddles and
and rope clips are used extensively to prevent sharp edges from cutting the ropes under
tension. The stabilizing lines are lashed down to tie points on the redwood support in a
manner that requires only one hand. Your other hand is usually busy doing something else.
Sometimes I wish I had yet another hand. |
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| Driven element/match |
Most of the time taken to build the antenna was consumed in
building the driven element. I prefer to use beta matches in my antennas. I have always
had good success at cutting my material close and nudging the adjustments into a good
match. Your mileage will vary. Another advantage of this sloping rope yagi is that the
driven element is near the ground and it is reasonably easy to adjust the match after the
antenna is installed. And, the premp can be placed on the ground nearby allowing some very
low noise temperatures on receive. |
 
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