Improving the Cushcraft 40-2CD Loading Coils
The Cushcraft 40-2CD uses 4 loading coils, which they call LCA (Loading
Coil Assembly). I had one that had burned out, and when I fixed it, I
found a much better way to construct the assembly for mechanical
stength.
The LCA consists of a heavy-wall plastic tube, around which is would the
tinned or plated copper wire of the loading coil. At each end, the
plastic form telescopes into the aluminum element tubing. The coil wire
is looped under self-tapping screws and the plastic form is fastened to
the aluminum tube with 2 large rivets. The assembly is covered with a
heat-shrink tube.
My antenna, which was the subject of the QST article, has been through
an extraordinarily stressful life and doesn't owe me anything. We've
had winds up to 134 mi/h, and ultimately the 120' tower, which was
overloaded by my commercial renters, broke a guy anchor rod and the
whole thing came down. When I was salvaging the pieces to put the
40-2CD back up, I found that one of the traps had failed to an open
circuit where the wire was terminated. The wire had a copper ball from
melting in an arc, and my conclusion is that the failure had occurred
before the crash. It appears that the wire had fatigued and broken
because the rivets had become loose from flexing in the wind, allowing
motion of the plastic form in the aluminum element tubing. The cracked
wire then arced until the gap was too great to bridge.
When I had originally built the antenna, I was aware that the
self-tapping screws had a reputation of rusting out from moisture
getting into the trap from the inside (although the outside is covered
with heat-shrink, the inside is open to the weather through the element
joints). I had replaced the self-tapping screw with a 10-32 x 1"
pan-head stainless machine screw with internal-tooth lock washer and
nut. I've heard that Cushcraft now uses an approach like this in new
antennas. Because I've had some failures of stainless hose clamps have
seen some rivets eat an oversized hole in element tubing, I now assemble
all highly-stressed element joints with two stainless bolts with nylon
insert locking nuts (whether I use a hose clamp or not; as an aside, I
buy exact bolt lengths, such as 7/8" and 1 1/8", so I don't have
anything sticking out of the nut to snag on guys or whatever).
When I was reassembling the traps, I drilled out one of the rivets that
was roughly at a right angle to the coil-terminating screw and put
another 10-32 x 1" screw through the element and plastic form (at both
inner and outer ends). This double shear and compression results in a
very much stronger mechanical assembly that I believe will not become
loose over time. If I were building the loading coil from scratch, on
assembly I would also coat the tips of the form with something
waterproof and rubbery such as polyurethane caulk (much better than RTV,
butyl or silicone, but for some reason not easy to find). I would also
consider using the heat-shrink tubing that comes with a meltable sealant
on the inside surface.
The only mechanical failures I had experienced with the original
strengthened antenna were U-bolts eating their way through relatively
thin element-to-boom plates and failure of inner ends of the outer boom
sections, which were too thin to withstand the torsional load of the
heavy reinforced elements. This is no knock on Cushcraft, as the
antenna was exposed to winds far above even the 118 mi/h upgrade level I
had worked to.
For the reborn antenna, I used a single 24' length of extruded 2"
diameter 0.12" wall 6061-T6 tubing for the boom, and made new element
and boom mounting plates from 3/8" thick 6061-T6 plate. Both the
elements and boom are mounted with 2" plated muffler clamps (with the
heavy material, you can really tighten up, so there's no tendency for
the elements to rotate on the boom). The boom is guyed from above with
two cables on each side, after the fashion of the HyGain LP1009 (the
guys come from a single point on the mast down to each end of an 18"
aluminum angle attached to the boom as if it were a short dummyh
element). This seems to prevent failure from rotary motion of the
elements and boom, and is working out well on other antennas I have up.
After I straightened the elements and put the antenna back up, the
40-2CD works great again, even in comparison to my reference antenna, a
near-full-size capacitively loaded rotary dipole in a somewhat more
favorable location. We've had our first big winds of the season without
any antenna failures, so the mechanical setup seems sound. As a final
comment, I got immediate and very helpful response from Cushcraft when I
needed replacements for the parts that had been scraped off in the
crash.
Dave, W6NL
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