New Capacitance Hats for the 40 Meter Yagi

The design of the capacitance hats for the 3-element 40 meter yagi has proven to be inadequate. I misjudged the various mechanical stresses and the effects of wind and ice. The antenna is very big, with a huge wind load, and it is greatly exposed to the weather at its height of 43 meters above ground.

Two capacitance hat arms broke off since it was raised. That's out of 24 arms: 3 elements × 2 hats per element × 4 arms per hat. Modelling shows that the performance impact is not all that bad, and on the air it continues to perform very well. But I can't afford further breakage. 

The time has come to deal with the flaw. I don't hide my mistakes on this blog. This is about learning, both for me and for you. There is no shame in admitting mistakes and letting others see how I deal with them. Hence this article where I describe the flaw and my revised design.

I was aware of the flaw when I designed and built the capacitance hats. I judged (but did not calculate) that the joint to the element would be sufficiently robust. When I raised the experimental dipole and left it on the tower for close to a year, one objective was to see whether the clamps would survive. They did so I kept the design. There are so many challenges with an antenna this size that every work item that I could eliminate left more time to deal with others.

Let's look at those stress points. The centre tube of the capacitance hat arms are ½" × 0.065". The u-bolt is ¼". The pair of bolt holes per u-bolt arm are half the tube diameter, or about ⅓ the circumference. That's a lot to ask from 6061-T6 alloy. 

I had intended that the thick mating clamp made from a ¾" × ⅛" tube mitigate the stress where it crosses the 1" element tube, while also preventing slippage. It seemed an elegant solution at the time.

After surviving for one year at 46 meters height in the experimental dipole I decided it would do. I was simply lucky in hindsight. There is stress from the nut which squeezes the tube. There is also stress from wind and ice load on the arms, which range in length from 42" to 48". The fatigue was worse than I anticipated.  Both broken arms failed at the outer edge of the bolt holes.

Originally I planned for the ¾" tube to remain whole and enclose the ½" tube. A 3" length would have been enough. I cut the tube longitudinally to save weight when I decided that was the greater concern.  The "dimple" where it crosses the element could have been kept with the whole tube.

I considered several alternatives to replace the capacitance hats. I kept the same arm tubes but with a different design at the centre where stress is highest. The same u-bolts secure a 3" length of ¾" × ⅛" 6061-T6 angle stock. The ½" tube passes inside a length of ⅝" × 0.058" tube. The tubes are secured to the angle stock with #8 stainless screws. The ⅝" tube is slit at both ends and compressed with a #8 stainless screw. The slit and screw are needed for a good electrical bond at the joint, and not so much for mechanical strength.

The length of the ⅝" tube is 8". These first arms of the new design are for the driven element. They may be shorter or longer for the director and reflector elements when I build them. Although the capacitance hat arms on the driven element are okay, they are easily accessed from the tower by rotating the element on the boom. I can thus test the new design without undue effort.

Installing the new hats wasn't very difficult, apart from the climbing. I turned the yagi so that the driven element could be rotated in both directions to near vertical without hitting a guy or other obstruction. It is high enough above the lower 10 meter yagi of the stack to not hit it, and the upper side of the element passes between the elements of the upper 10 meter yagi.

One of the 4 capacitance hat arms being removed from the driven element shows clear evidence of fatigue. Within days or weeks this arm would have broken off. The other three were fine, so far. Eventually they would likely fail.

The plated u-bolts are beginning to rust after 18 months. It isn't a surprise. Galvanized or stainless bolts are used everywhere else on the antenna but were not easily available in the 1" size when I was building the antenna. The rust does not affect antenna performance since there is aluminum to aluminum contact via the clamp. I'll look for better bolts before all the other capacitance hat arms are repaired.

To avoid accidents, I first replaced the inside arms. The outside arms, which are at the bottom when the element is rotated on the boom, prevent it sliding off. Despite my care I managed to drop one of the new capacitance hat arms. 

It is tricky to bundle them for climbing and safely extract them one at a time. Due to interference of the element with the boom truss and the capacitance hats with the tower, I had to reach outward about 2' to do the work while holding all the pieces of the assembly. The dropped arm survived the fall from 130' (20' below the top of the tower) despite bouncing off a guy. The arms are long but light so they (happily) have a slow terminal velocity!

The new arms look good on the driven element. It isn't critical to get the two arms snug to each other since on 40 meters an inch makes very little difference. They are wider than the original arms due to the angle stock alongside the tube and I had to keep them clear of the screws and nuts securing the 1" and ⅝" element sections.

Now I wait to see learn how they perform over the coming months. If they do well I'll proceed to replace the capacitance hats on the other elements. Apart from the ⅜" tubes, the rest of the original arms will be discarded.

So near and yet so far

Accessing the capacitance hats on the driven element is easy. The same is not true for those on the director and reflector.

This is one of those nasty challenges you will inevitably run into with big towers and antennas. Replacing the capacitance hats on the director and reflector will not be easy. I have done some planning with a friend on a method of doing the replacement that does not involve taking the antenna down or a large crane. It won't happen sooner than late summer so we have time to get it right.

It is better to build it right the first time so that you never run into this predicament. I do pretty well but I'm far from perfect. To err is human.