Wednesday, November 6, 2013

Strengthening the 40 Meters Delta Loop

Extending masts far above the top of a tower raises questions about safety. Simplified tower ratings for wind load make strict assumptions on how the tower is supported and antennas are placed on the tower. Depart from those assumptions and you can rip up the manufacturer's headline loading specifications. This may then require the assistance of a structural engineer.

I did not call upon an engineer when I designed and installed the extended mast on my tower to support both the multi-band dipole (TH1vn) and 40 meters delta loop. Which should raise the question: is it safe? I will deal with that question in this article as the major part of the follow-on to the article on the mechanical design of the 40 meters delta loop.

The extended mast rises 6 meters above the tower so that the peak is at 14.8 meters above grade, and therefore just within the Industry Canada's policies regarding the "duty to consult". The tower is 8.8 meters tall, the TH1vn is at approximately 11 meters and the delta loop apex is at 14.7 meters. Let's look at the wind loads.

I'll stick to English units since tubing and most wind load equations and specs are shown in those units. Calculating the projected wind area is not difficult, being mostly a matter of multiplying width and length of tubes and pipes. The approximate projected areas of the major components above the tower are annotated on the attached picture.

The total wind area is 5.3 ft². Golden Nugget tower is rated for an antenna load of 3 ft² when mounted just above the tower top, with the tower bracketed (or guyed) one section down from the top and a maximum of 3 sections below the bracket. My tower is guyed at the top so we have to adjust for that.

First off we need to know the wind area of a 10' tower section. This is tricky since there is shadowing of tower components leeward of components facing the wind. I won't go through the details, and just report that my rough estimate is 3 ft² of cylindrical wind area for one Golden Nugget section.

Extrapolating from this calculation and the manufacturer's headline specification the tower is rated for good for 3 ft² of horizontal antenna load mounted above 3 ft² of vertical load, with the vertical load extending 10' above the tower.

From the picture above you can see that (roughly speaking) there is a 2.5 ft² horizontal load above about 10' of steel mast plus a bit of tower from where the guys are attached. That is well within spec. In an earlier article I described the response of this approximate configuration to a 100 kph wind. That was without the extended mast and the wind area of the TH1vn was just under 2 ft² since it had not yet been enhanced to support 17 meters. It passed that test with flying colours.

However there is now a further 1.8 ft² of fibreglass mast above that. While 5.3 ft² is less than 6 ft² the load is not configured per the manufacturer's spec. For a maximum wind of 140 kph (North America wind zone A) the wind pressure on the fibreglass extended mast is 36 lb (16 kg). At 80 kph, which is the wind it recently survived, the wind pressure is a more modest 7 lb (3 kg).

As I mentioned before that the extended mast bounced around a lot in those high winds, while the lower mast and TH1vn were far more steady. The guyed tower itself showed no stress or movement. The bouncing itself is a concern even if the wind load alone is not since oscillations can multiply the effective wind load. Although I don't have a reference handy I have in the past seen published reports of instantaneous loads due to oscillation of up to 3x and 6x the static wind load. The wind load equations typically only account for 30% additional load due to gusts and turbulence.

It is this concern regarding turbulence plus that of the unknown breaking strength of the fibreglass mast that motivated me to guy the extended mast. This ameliorates most of the residual concern of the load height of the extended mast by reducing the majority of mast motion in the wind and the bending moment on the structure below.

The guying of the extended mast is not perfect since the 3 guys are not close to being separated by 120°. This is because two of the guys are the vertical legs of the delta loop itself. These are angled out from the tower so that the delta loop plane is not vertical, resulting in an interior angle of approximately 160°. There is a back stay now added, with the help of horizontal bar, to act as the third guy. It is secured further down the tower. There is approximately 100° between it and each of the delta loop legs.

You can see this arrangement in the adjacent picture. The perspective is distorted since I took the picture while on the tower, which I did in this way to show more detail.

Balancing the tensions in the loop legs and the third guy took several attempts. The residual curve you can see in the extended mast is quite small and not a concern. The modest amount of bending load on the fibreglass mast in the direction of the rope guy helps to steady it by compensating for the less-than-ideal guying behaviour of the loop legs.

Since completing the construction to strengthen the entire system we have had winds of up to about 50 kph. This is not a proper test but all I can do is deal with what nature provides. In that wind the motion at the top of the extended mast was no more than 2 or 3 cm. So far so good.

In the electrical design article I described the remote placement of the coax choke for the 40 meters antenna. Now that I've finished that part of the antenna I can show what it looks like. The deflection of the loop due to the weight of the coax is less than it was at first since with the completion of the extended mast guying I was able to add substantially more tension to the delta loop. Supporting the coax choke on the steel guy wire substantially reduces the stress and deflection of the loop wire. The red circle on the picture is the feed point, which is ¼λ down from the antenna apex.

The choke consists of 12 turns of RG-213/U with a diameter of 7" (18 cm). This should supply over 1,000Ω of (mostly reactive) impedance at 7 MHz.

As a final step in the construction I moved the bottom corner anchor points on the loop to better account for the splicing in of 3.4 meters of wire in the bottom horizontal leg. With the increases tension on the loop this improved loop symmetry and keeps the bottom leg out of hand's reach. Unfortunately these changes also shifted the resonance of the antenna about 100 kHz higher. Now the SWR at the bottom of the band is 1.4 rather than 1.1. This is still perfectly acceptable so I am leaving it alone at this point.

I will have more to say about antenna performance in future. Although the antenna works there remains the tough challenge of using QRP on 40 meters.

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