Thursday, October 31, 2013

40 Meters Delta Loop Mechanical Design

Finally the weather broke and I had a few hours to construct and raise the delta loop for 40 meters. Cutting to the chase, it works. However it does need some work, both mechanical and electrical. I'll describe the mechanical design in this article and follow up with the electrical design in a subsequent article.

A delta loop for 40 meters is big. It is in fact much taller than the small tower (Site C) that supports it. This requires a mast to support the apex of the loop which needs to be at least 14 meters above grade so that it doesn't grab anyone's neck as they walk under it. The height constraint is tight since it must also be no higher than 15 meters in order to avoid the regulatory process of both Industry Canada and the city.

Since the tower is 8.8 meters high, the support mast must extend 6 meters above the top of the tower. This is not something that is undertaken without due regard to the wind and bending forces on both mast and tower. The entire system must survive year-round weather events from thunderstorm-caused wind squalls to freezing rain.

The adjacent picture shows the tower, mast, modified TH1vn and delta loop. This is not the final configuration since more work on the mast guying was done.

At full height the TH1vn is up just over 11 meters. While this is not clear in the picture the 4-band dipole runs inside the vertical legs of the delta loop. This is deliberate. I needed that orientation and EZNEC modelling judged the arrangement acceptable. That is, very little interaction between the dipole and loop on all bands of interest. This was confirmed by measurement once it was all in the air.

When I was adding 17 meters to the TH1vn I also took down the mast so that I could prepare the fittings for the delta loop mast. There are two parts to this:
  • A second clamp was placed below the TH1vn element/mast clamp for added support and redundancy. Although the dipole doesn't need it (it's very light), the antenna clamp now also supports the weight of the extended mast and the downward force due to the guys pulling on the corners of the loop. It is made of same 1.5" Schedule 40 ABS pipe that couples the TH1vn clamp to the steel (bottom) mast. It is cut on one side and then compressed with a stainless hose clamp.
  • Three of the 4' sections of fibreglass mast that I previously used for the experimental 20 meters delta loop form the extended mast. They nest together and slip onto 4" of exposed steel mast above the TH1vn clamp. A shim made of aluminum flashing was cut and formed to allow a snug coupling of the two masts. This is needed since the steel mast is 1.5" O.D. and the fibreglass mast is about 1.6" I.D. The shim is held in place by the pipe clamp that secures the TH1vn to the steel mast.
The snugness of the fit between the masts is important. It must be tight enough to avoid stress points when the delta loop is tensioned but also be easy to drop into place when strapped at the top of the tower and holding vertical the 12' fibreglass mast plus guys and antenna wire above my head to drop it into place. This is the the most dangerous operation in the entire process. I did a test with a single section of fibreglass mast to confirm that I built it properly. Only then did I assemble the extended mast on the ground and lift it into place. It all went more smoothly than I expected, so that was a relief.

Since the fibreglass mast sections are held together only by gravity I had to temporarily secure them in another way during lifting. The rope stay and the vertical legs of the loop were attached to the guy ring, slipped over the mast then pulled tight and secured with a hose clamp to the bottom of the 3 sections. The lift rope was friction fit to both the top and bottom section, with the top one knotted to allow it to be released with one hand.

As I pulled it up to me I slipped the top knot, continued raising it over my head and (holding it very still) dropped it into place. With that done I removed the lift rope and temporary hose clamp and tied the rope guy to one of the tower guys cables. The loop wire had to be cut at the centre of the bottom (horizontal) leg so that one side could be reeled up and dropped over the other side of the TH1vn. All I then needed to do was loosen the mast clamp, raise it to its full height, swing the TH1vn to its final orientation and clamp the mast.

The reason I've gone to the trouble of described this process in detail is to highlight an important requirement of all tower and antenna work: plan everything in advance (and I mean everything!) and test every crucial step on the ground. Too many hams injure or kill themselves by over-confidence, imagining they can resolve any difficulty they run into, then find themselves in poor position or insufficient main strength at a critical step.

Can you really hold that load above your head without it tipping and crashing to the ground and onto you someone else? Try it on the ground, and do it without moving your feet. If you can't or it's iffy you should resolve the difficulty and not take a chance. Trust me, it isn't worth it. Really.

The following are additional notes on the mechanical design. These can apply to any wire antenna.
  • Shear pin -- The delta loop is a big antenna that covers a lot of ground. It spans 14 meters over the ground, plus the length of ropes to tie down the corners. As the pictures show there are a number of neighbouring old trees which branches that arch over my property, and the delta loop. A branch that breaks in a storm could fall onto the antenna. The south corner tie-down rope includes a length of synthetic twine to act as a shear pin (a mechanical fuse). Its breaking strength is about 40 lbs (18 kg). The idea is that the twine will break when a tree hits, thus preventing more extensive destruction of the mast and tower.
  • Wire selection -- A 40 meters 1λ loop (naturally) requires about 40 meters of wire. In addition to supporting its own weight it must also support the tension needed to reasonably ensure its delta shape. The wire must be up to the mechanical forces. For this antenna I am using 12 AWG insulated soft-drawn stranded copper wire. I would not recommend anything less.
  • Coax deflection -- The weight of the coax distorts the shape of the delta loop, as the adjacent picture demonstrates. This can be reduced with higher tension on the loop's tie-down ropes but at the cost of greater stress on the wire and extended mast. The deflection in my case is large since I tried to avoid excess coax droop which would increase coupling of antenna currents onto the coax, and because I used RG-11/U (¼-wave transformer). RG-59 would cause less deflection, if QRO is avoided.
  • Mast support -- As mentioned earlier, the extended mast must be guyed. Three-point guying is used. Two of those are the vertical legs plus tie-down ropes, and the third is a rope orthogonal to the loop plane that is tied to the tower. Some care must be taken when setting the tensions of the loop and third rope so that the mast is under sufficient tension, does not deflect from vertical by more than a few inches, and the loop itself is secure. I will need to add a horizontal crossbar to the tower to extend outward the tie point for the rope guy. For the present the rope is tied to the top egg insulator of the most suitable tower guy.
In the next article I'll talk about the antenna itself. As I said, it does work but it requires tuning. Putting it through a tuner to get the SWR down in its present state I was able to work 40 meters DX in Europe and Central America even under poor conditions (October 30 evening) and only the 10 watts from my KX3.

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