About The Nashua Area Radio Society and Our Mission
Cushcraft 40-2CD
|Here, the 40-2CD is mounted at ~4 feet as it nears completion. It is less the strut assembly for the boom (work in progress) |.|
|and the capacitance hats--I have them but will wait until the antenna is ready to go up before attaching them. | |
|The second photo shows the element to boom assembly where I used the SS U-Bolts with saddles from DX Engineering. This really | |
|strengthened the assembly and did not squeeze the boom at all--very strong! | |
|[pic][pic] | |
|Element Strengthening | |
|My station location is within 75 miles of the gulf coast and in the 85 to 90 mph minimum wind range. It is possibly with a | |
|direct hit from a hurricane that we could get winds well in excess of 100 mph. I used the techniques that W6NL recommends for | |
|strengthening this antenna. Check out Physical Design of Yagi Antennas, by David B Lesson, W6QHS (W6NL) book on strengthen this| |
|and other antennas. It is very good! | |
|The 205's heavy duty boom (center sections) was in good shape with only a very slight bend, I did not bother to try to | |
|straighten it, just lined it up with the bend pointing down and will let the strut support pull it up and support it. | |
|I also used the Hy-Gain Boom to Mast mount. | |
|Loading Coils | |
|The following are some pics of the coils of the 40-2CD. Bruce had replaced the self-tapping screw with a stainless steel bolt | |
|and nut. However corrosion had developed with the aluminum tubing, the copper lead to the coil and the stainless steel bolt | |
|met. I cleaned up the connections very good and used some Nolox to control the corrosion between the aluminum, stainless steel | |
|bolt and the copper wire, hopefully this will help control the corrosion. | |
|The coils had two pop rivets on each side to hold these assemblies together. Three of the four assemblies were loose on the | |
|fiberglass forms. I drilled the pop rivets out and used three pop rivets evenly spaced around the tubing on each side. One of | |
|the rivets had to be off center from the other two because they would not all fit inside the aluminum tubing. | |
|Here are some pics of the work that was required on the coils: | |
|[pic][pic] | |
|[pic] | |
|For the new shrink wrap, picked up some 1 1/8" 3M shrink at the local electronic supply house. Used a small torch to shrink the| |
|tubing. Tried a hair dryer but it would not work on the thick material. When heated up, this shrink extruded a clear sealant | |
|that sealed the ends. I am sure that this was some type of water resistance sealant. I don't know what it will be like if I | |
|ever had to take it down--hopefully I won't have to find out! hi | |
|Unfortunately, I did not make any pics of the finished coils. They really didn't look too much different than the original | |
|ones! | |
|Here is a detailed drawing of the LCA | |
|[pic] | |
|Dave, W6NL also posted the following article to the Tower Talk reflector regarding the coils. It is complete and you will do | |
|well to following these procedures. | |
|Improving the Cushcraft 40-2CD Loading Coils (by W6NL) | |
|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 strength. | |
|The LCA consists of a heavy-wall plastic tube, around which is wound 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 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 dummy 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. | |
|Dave, W6NL | |
| | |
|Antenna History | |
|I acquired this antenna from Bruce/WT4I in Florida, after it had fell from about 90' as a result of a hurricane in 2005. The | |
|boom broke at the mount, the aluminum struts were destroyed, the capacitance hats were all broken and the antenna tips were | |
|bent a little. | |
|I was lucky in that, AL/K3VN, from Florida, gave me the remains of a Hy-Gain 205CA that had crashed during a recent hurricane | |
|(2005). There was a good bit of damage to the elements outer pieces, but there was plenty of the larger tubing that was usable | |
|to strengthen the elements of the 40m antenna. I did have to buy a couple of specific sizes to finish the job, but all in all | |
|it was minimal. | |
|Thanks Bruce for the 2L40m and Al for the 205CA! | |
| | |
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