I was economical with the aluminum tubes and pipes I had on hand, saving the longer pieces for other projects. The stinger was 21' long, with 19' (6 m) projecting above the tower top. That was topped by over 3' (1 m) of 1" PVC pipe to get more height for the parasitic wire element support ropes. The total amount of aluminum and PVC above the tower was 7 meters long. That's a lot.
Although the stinger is guyed by the catenaries for the wire elements the tension produces a downward force (compression). It was obvious during setup that a butt joint between 1-½" x 0.095" aluminum tubes was not up to the stress. My hope was that it would last long enough that I could focus on other projects until I would have to replace it. All seemed well for a year as it survived one wind storm after another. Then the joint suffered a fatigue failure and the top half of the stinger fell down.
Stinger version 2.0
The stinger needs to be robust but not necessarily lightweight. The stinger must resist modest horizontal tension of the parasitic element catenaries. It must also be up to the compression force due to those same catenaries, especially with regard to high stress points where the yield point could be exceeded in a high wind and icing, or from tension imbalance among the 4 catenaries. It is laterally stabilized by those same catenaries which act as guys.
Being lightweight is beneficial when installing the stinger since it is long and must be lifted overhead to be dropped into the tower top. It can be assembled in pieces and raised from below at the price of more time and effort. As you will see I did a bit of both.
Rather than a butt joint between two lengths of 1-½" aluminum tubes the main improvement is a butt joint between two lengths of 1-½" aluminum pipe (1.9" OD). This allows me to use the existing 2" saddle clamps that secure the stinger to the tower plates. I also have a supply of these surplus pipes on hand and I know where I can get a few more at a good price.
Pipe fitting
My introduction to fitting pipes together for antenna construction was simply out of expediency: I had sources of cheap surplus aluminum pipe and tubes are expensive. I later came across the same idea in W6NL's book Physical Antenna Design (now out of print), which increased my confidence.
Although aluminum pipe is almost always 6061-T6 -- excellent tensile strength -- they have a seam, hard as it can be to find one on these aluminum pipes. They are theoretically weaker than seamless tubes although I have yet to see a seam failure. Indeed, some of my stock comes from commercial antennas that have survived harsh Canadian winters at great heights.
Aluminum pipe follows the same size schedules as steel pipe in the US and Canada. That's very convenient. Plastic pipe that is now in common use for water pipe and conduit -- ABS and PVC -- are similarly sized. I'll provide several examples of how pipes and tubes can be mated with respect to my 80 meter stinger, and additional ideas covered in earlier articles. There are other combinations of pipes and tubes that can work well together.
On the left are two schedule 40 pipes: a 1-½" pipe inside a 2" pipe. The inner pipe OD is 1.9" and the outer pipe ID is 2.067". The gap is 0.167", or 0.083" all around. Depending on the application the pipe can be simply bolted together. For improved rigidity a shim made of aluminum flashing can be used. An alternative is to increase the amount of overlap to reduce wobble, at the expense of greater weight.
The middle example is almost the same except that the outer pipe is schedule 80 with an ID of 1.939". That is a much better fit. Both are options to butt join two lengths of 1-½" pipe. I considered using the schedule 80 pipe until I found what is, to me, a more favourable solution. I prefer to save the schedule 80 pipes for my various yagi projects.
On the right is the 1" schedule 40 PVC pipe that is fit to a 1-½" aluminum tube at the top of the stinger as a non-conductive extension for the catenary attachments. The OD is 1.315" and the ID is 1.049". The 1" pine dowel purchased at a hardware store provides structural strength and fits well enough for the intended use.
This size PVC pipe will also fit well over 1" tubes. I may use PVC pipe as an insulator on the driven element 1" centre segments if I decide to use a beta match. Insulation is not needed for a gamma or T match.
There are other applications of pipes that I will discuss in future articles about those projects. Consult charts of pipe sizes and trawl through surplus yards for cost effective solutions in your antenna farm.
Stinger butt splice
In my junk box are Hy-Gain yagi parts that have been collected over the years. Hy-Gain booms are mostly 2" OD and spliced at the centre for the longer yagis. I have two of these surplus brackets. I tested a bracket on 1.9" OD pipes and was successful in achieving a secure fit despite the smaller size.
I butt spliced 10' and 7' pipes in the bracket. Holes were drilled through the pipes to make use of the bracket holes for that purpose in lieu of using the inner perimeter holes intended for a mast clamp.
I briefly experimented with plastic pipe as insulators to electrically isolate the pipes. I would need do this to attach a switchable coil to add 160 meters to the 80 meter array. I slit a scrap length of 2" white PVC pipe to slide over the pipe. It had to be slit since the ID is slightly less than 1.9".
At the end of the dressed pipe is a round insulator made of pressure treated lumber, cut with a hole drill, to provide mechanically robust isolation between the upper and lower pipes. The lower pipe does not need an insulating sleeve except perhaps to achieve a consistent diameter within the bracket.
I put the idea aside as not quite ready for implementation. It can be retrofit later. I first need to ensure high voltages between the pipes when the coil is active (not shorted) cannot jump the gap through the slit while maintaining high mechanical strength. It may be as simple as a wrap of thick polyethylene sheet and a seal to keep water out.
Upper stinger
Spliced to the upper pipe is a 7' (2 m) length of 1-½" x 0.095" aluminum tube. This is the only piece of the original stinger used in version 2. The 0.11" gap (the pipe ID is 1.61") is filled with a wrap of aluminum flashing coated in conductive grease. Stainless screws with nylocs hold it together. The 1" PVC pipe (1.315" OD) with its inner wood dowel are attached to the 1-½" tube (1.41" ID) in a similar fashion.
The original holes for attaching the rope catenaries are reused, and holes drilled through the wood dowel inside. The raw pine is protected by a cap of pressure treated wood, the top of which is sealed with caulk. The hose clamp adds tensile strength to the PVC and wood to better withstand the tension on the catenaries. The tension isn't high but I want to ensure years of trouble-free service.The final stinger is ~2' (60 cm) longer than the original. This is intentional. I found that with my parasitic T-element design there is some slack on the vertical wire. The longer stinger removes the slack, and is easier than rebuilding the wire elements.
Installation
The new stinger was raised in two steps. The lower 10' pipe with Hy-Gain bracket went up first and dropped into the tower clamps until the bracket rested on the top clamp. The nice thing about aluminum pipe is high strength-to-weight ratio. The 10' of pipe with bracket attached is only ~9 lb (4 kg). It is easy to hold it vertical over my head as I fuss with inserting it through the tower clamps.
The catenary ropes are tied to the top of the stinger and detached from the wire elements in preparation for the next step. I briefly considered leaving the elements attached until I realized that the lateral tugs of those small weights would prevent safe lifting of the 17' long upper stinger.
With everything in place I lifted the upper stinger and dropped it into the Hy-Gain bracket. Once that was secured the elements were reattached. The complete stinger was then pushed up through the tower top and clamped in place.Back on the ground I tensioned all the catenaries. The test for tension equalization was to have the stinger straight and in line with the tower. That was after the adjacent picture was taken.
Although a simple procedure it is tedious. I took an overnight break at one stage when I felt that I was too tired to do the lift with complete safety. A brief delay is preferable to an unnecessary risk.
Matching network changes
The new stinger's mechanical length is 2' longer and its electrical length is 3' longer. The reason is explained below. But having done so the new electrical properties of the driven element must be dealt with. For a λ/4 monopole on 80 meters the approximate rate-of-change (dF/dL) is10 kHz/6 cm; that's ~150 kHz lower due to the 3' extension.
There is no reason to make the driven element resonant at any particular frequency. The low impedance still requires a matching network, both as an omni-directional vertical and in yagi mode. I am using a switchable L-network.
My next task is to remeasure the antenna's impedance across the band, in both omni-directional and yagi modes. I will then use TLW to determine what changes are required. They should be small. I'll describe the details in my final article about this antenna, which will be written once it is complete and fully operational.
Catenary rope length changes
Changing the stinger height presents an interesting geometry problem: to lengthen the catenary ropes in a manner that keeps the parasitic elements vertical and preserves yagi performance. My first inclination was to ignore the problem since the change is quite minor. However it isn't difficult to check this on paper so I took that precaution.
Since the two sides of the right angle triangle are approximately equal each increment in height lengthens the full catenary (including the T-top of the wire element) by 0.7 increments; that is, 1.4' for a 2' height increase. However we only need to lengthen A to B, the distance from the top to the vertical wire, since we can freely add rope at the bottom. With A and B only 40% of the total length -- 10.5/25.5 -- we need only 40% of 1.4' or 6" (15 cm).
By not lengthening the top rope the wire element will lean towards the driven element ~4" (10 cm). As confirmed by modelling (and as you'd likely guess) this is negligible. Since I had no slack to lengthen the upper rope section I built and installed long insulators between the rope and upper end of the T shaped wire element. These are made from PVC pipe. The height of the vertical component of each wire element was increased by 0.8' (25 cm) -- 40% of 2' -- which took up all of the slack. Mission accomplished.
Returning to work
With the stinger rebuilt, better and stronger than before, work can resume on the yagi. All the parasitic switch boxes are installed and working. Tuning of the elements is partially done. Then comes the final step: the main switching system at the base of the driven element (tower).
Progress on the antenna has slowed due to more urgent projects, especially the 20 and 15 meter stacked yagis. Not to mention 6 meter DXing and otherwise simply enjoying the warm weather. With my 80 meter interest being DXing and contesting the 80 meter yagi is not urgently needed. It can wait until late summer, but may be completed earlier depending on circumstances.
In consideration of the weather and my busy schedule don't be surprised by a slowed rate of articles through the summer.
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