Capacitance Hat Clamp

While building the 40 meter yagi element (recently described in a couple of articles) I ran into difficulty mounting the capacitance hats on the element. Rather than fuss with it I improvised a solution suitable for the experiment but not for the final antenna. For a permanent antenna I needed a better clamp.

Look back at that earlier article and you will see that the temporary joint ran the arms of a u-bolt through the ½" capacitance hat centre tubes and seated on the outer edge of the 1" element tube. A round tube crossing a round tube touches at a single point. This is electrically and mechanically unsound.

It is sure to malfunction, and that is unacceptable for an element of an antenna as difficult to access and repair as a 40 meter yagi. There are four of these on an element so the total number of these joints is 12 for a 3-element yagi. That's trouble with a capital "T".

This style of joint is common on yagis. You will find round tubes crossing in capacitance hats (see Cushcraft XM240 method at right), element-to-boom clamps (see Hy-Gain clamp below), among others. Small elements make do with curved clamps formed from aluminum sheet. Large elements use plates and u-bolts to clamp both tubes to either side of the plate or, in Hy-Gain's case, large formed sheets. My home brew HF yagis use plates and u-bolts for the element-to-boom and boom-to-mast clamps.

I browsed web sites and antenna manuals to find commercial clamps of the type I needed. That turned nothing suitable. Clamps were too big, too small or were not strong enough. Formed clamps only fit the specific tube diameters for which they are designed.

Making my own fitted clamps from aluminum alloy sheet was not an option since it is difficult to form with the tools found in a home shop. High tensile strength alloys require careful attention to the bend radius to avoid weakening or breakage. Soft alloys that are easier to form are too weak for 40 meter capacitance hats.

Plates and u-bolt clamps are easy to make but are too heavy for the outer reaches of a 40 meter element. Those used in the capacitance hats of the W6NL 40 meter Moxon and similar antennas require element trusses to survive severe weather.

My work shop has no shortage of tools or aluminum scraps of various shapes and sizes. So I pondered the problem while staring at boxes of metal scraps. Before long an idea leapt forth. I pursued it further and developed a mental picture of a design. On a rainy day I put it to the test in my work shop.

It's a simple device that anyone can make and I am documenting it since others might find it useful in their antenna projects. The 4 pieces I needed for one element took 1 hour to complete. I spent more time on the first of them as I fine tuned it to my satisfaction. I doubt that the design is original since, in retrospect, it's pretty obvious.

The custom tube saddle is made from a 1-⅜" length of ¾" OD × ⅛" wall 6061-T6 tube cut lengthwise. The only tools required are a hacksaw, vise and a set of flat, round and half-round files. You likely have all of these in your toolkit. The saddles are combined with the same 1" u-bolts I used in the experimental antenna to make the capacitance hat clamps.

The critical items are the half-round file and tube. Both must have dimensions that match the diameters of the crossing tubes. In my case the ID of the ¾" tube matches the ½" of the capacitance hat tube diameter. My set of half-round files -- coarse, medium and fine -- have a curvature equal to 1" diameter that is a perfect match for the 1" element tube. For files that don't match the tube curvature some finesse is required to shape the saddle.

The lengthwise cut of the tube should divided it into two equal halves or the larger of the pieces will not fit over the ½" tube or it will bite into it and be difficult to move or remove. A file can correct a small overbite but not a large error.

The chosen length allows indentations for the arms of the u-bolt. Thes hold the saddle in place without requiring precision drilling of holes for the u-bolt. This also shaves the weight by a few grams. Oversize indentations compensate for less than perfect machining to achieve a flush fit onto the 1" tube.

The central saddle (depression) was made with the aforementioned half-round files. It must be centred for the arms of the u-bolt not to interfere with a good fit to the 1" tube. It is acceptable to file the indentations deeper to compensate for being a little off centre. By using ⅛" wall tube for the saddle the file can cut deep and contact area maximized.

For thinner wall tubing you should stop filing when the centre of the saddle first cuts through the tube. This won't significantly weaken the saddle but it you file any deeper the saddle won't seat on the 1" tube. That would undo all the effort to have a large contact surface.

Because the filing action is mostly linear there are visible striations. This is not a problem since the texture can reduce slippage. I smoothed them a little with a fine half-round file. Conductive grease will protect against oxidation in the tiny gaps. Use the grease on the inner surface of the saddle as well. The u-bolt I show in the picture is plated steel. Although stainless is a better choice galvanic corrosion won't disturb the clamp's electrical bond provided by the aluminum saddle.

The result is a solid mechanical and electrical joint that resists slippage even with low torque on the nuts. I didn't fit washers or a top plate for the picture but you should do so to prevent abrasion and distortion of the smaller tube. Don't over tighten! Nylocs are recommended.

Don't be intimated from a little workshop improvisation. You can save money and enjoy the pride of making antenna parts yourself.