Joining Pipes & Tubes: 40 Meters

In the midst of the many antenna and other projects going on in parallel I have taken the first steps towards building a 3-element yagi for 40 meters. This is a very large antenna with many challenges. After reviewing the many different techniques I used to join the pipes and tubes I realized it was an almost complete catalogue of the common (and some uncommon) techniques that may be of interest to others.

I built a single dipole-fed element to prototype the mechanical and electrical design of a yagi element. The split feed allows connection of an antenna analyzer. The final antenna will use a continuous conductor on all elements for mechanical robustness and the driven element will use a gamma match. Since a continuous element cannot be precisely tuned with an analyzer this is how I will determine dimensions of the 3 elements.

Precise tuning is critical for a yagi. NEC2 supplemented with SDC (stepped diameter correction) is accurate but is only reliable for unloaded elements. My prototype element is ~90% full size with a modest size capacitance hat.

Although the loading reduces weight a small amount the primary reason is to eliminate the third harmonic resonance on 15 meters. That resonance, if allowed, can have a deleterious impact on my 15 meter stacked yagis. Modelling indicates that a small hat will move the harmonic resonance far enough above 21.5 MHz to maintain the excellent pattern of the 15 meter yagis when pointed towards Europe. The planned 40 meter yagi is in the same direction.

That is all I will say at the moment about the electrical design. It is enough background for readers to understand what I am trying to accomplish. There is much yet to do on that front before I am ready to build the full yagi and lift it to the top of the 150' tower. You don't want to do that job more than once so it pays to get it right!

Taper schedule

For the following discussion you can refer back to the following list of pipes and tubes. All pipes are structural aluminum alloy 6061-T6 or 6061-T6511, or aerospace alloy 6063-T832. Tensile strengths of the alloys are comparable. English units are used since that is how our pipes and tubes are sized.

• 2.375" OD and 2.067" ID schedule 40 pipe
  
• 1.9" OD and 1.61" ID schedule 40 pipe
• 1.315" OD and 1.049" ID schedule 40 pipe
• 1" OD and 0.120" wall tube
  
• ⅝" OD and 0.058" wall tube
• ½" OD and 0.065" wall tube
• ⅜" OD and 0.058" wall tube

The largest pipe's ID leaves a wide gap for the 1.9" pipe. The final element will use schedule 80 pipe which is a close fit to the 1.9" pipe. I am using short scraps for the prototype to permit a centre gap and dipole feed point and those happen to be schedule 40.

The capacitance hats are made from the two smallest tube sizes and attached to the outer end of the 1" OD tube.

Ordinary joints

There are two methods of joining tubes that are common enough that I will not discuss them in detail. The latter of the following two methods was previously described for my 15 and 20 meter yagis.

• Tube slit at one end with the smaller tube slipped inside and held with a gear clamp (commonly called a hose clamp). Length of the smaller tube (portion that protrudes from the larger one) may be adjustable.
  
• Tubes drilled through at the overlap and secured with fasteners. Length adjustment isn't possible.

Both methods require that the two tubes are a close fit but not so tight that it is a press fit. This is where 0.058" aerospace tubing in ⅛" diameter increments comes in handy. They telescope perfectly for making stepped diameter yagi elements. The only tubes of this type in the element are ⅜" and ⅝" OD.

The ⅜" tube can be slit to take a ¼" rod to extend the tip, but I have not done so. It will depend on the results of field testing and the amount of adjustment room I've designed in.

Centre

The dipole feed has the following requirements. 

• Interior non-conducting rigid material to mechanically couple the two halves of the element.
• Backing plate to support the element halves and for attachment to the tower.
• Element insulated from the backing plate.
  

It must be sufficiently robust to survive numerous tram trips up and down the tower for repeated measurements and adjustments. Since it's temporary I didn't want to overdo it or spend any money building it. So I looked through my junk pile and then took a drive to look through someone else's junk pile.

The backing plate is 6" × 27" ¼" steel plate. I cut and drilled it after grinding off the surface rust. It will not be painted and is it already forming a new layer of rust. That's okay since it doesn't need to conduct and it's temporary.

Ordinary muffler clamps attach the element halves to the plate. Each clamp is wrapped with a thin, flexible material that I thought was rubber but turned out to be some kind of plastic. I inserted strips of galvanized sheet on bottom and top to prevent the tightened clamps from splitting the stuff.

An old piece of 2×2 seasoned maple joins the element halves. It can support the full element weight but it is not a perfect fit so there is droop. The droop is partially remedied by the backing plate, as we'll see later. I will tape the gap so that rainwater doesn't reduce its insulating properties.

The pipes are tapped for #8 stainless screws to connect the element to the analyzer or coax. The nuts under the heads are needed to prevent the screws from striking the wood since I didn't have shorter screws at hand.

Screw pressure clamp

The 1.9" OD pipe fits very loosely in the larger pipe with its 2.067" ID. Since this is a temporary joint to be replaced with a pipe with thicker walls I opted for a pressure clamp using screw. It's simple and robust enough for the prototype.

There are four ¼" stainless hex head bolts in tapped holes on the 2.375" pipe. Screwing them down presses the smaller pipe against the opposite wall of the larger pipe. The bolts double as set screws to prevent slippage. Nuts can be added under the bolt heads and tightened onto the pipe to avoid accidental loosening.

This method is not recommended for a permanent installation yet it seems to be reliable. The pipe overlap is 12", the maple insert is 24" long and each 2.375" pipe is 24" long. Thus the smaller pipe and maple insert just about touch. This was done to maximize the overlap and strength of the joints.

Slit tube reducer

Element taper does not require using every available step size. For an optimum balance between weight, cost and strength 40 meter elements have a strong centre and step down to much smaller diameter tubing. A reducer is needed for the larger steps. Often these can be made from short sections of intermediate tube diameters. This may not be possible when pipes are used because NPS pipes often don't telescope well with other pipes and tubes.

The first large step is between 1.9" and 1.315" OD pipes. The radial gap is (1.61 - 1.315) ÷ 2 = 0.1475". A tube or pipe of this non-standard wall thickness must be fabricated. Rather than machining a reducer I fabricated a two-step reducer using material on hand. The reducer is 6" long to match the pipe overlap.

A 1.5" OD tube with a wall of ~0.095" was cut to size and slit with a hacksaw. When the slit is slightly spread it fits tightly over the smaller pipe. Another 1.5" pipe of 0.065" wall thickness was cut into 3 lengthwise pieces and one is used to fill the remaining gap. The fit is tight. All of this was determined by calculation and experiment.

When assembled two holes at right angles are drilled through the pipes and reducer for ¼" bolts and nylocs. One of those bolts is centred on the double thickness of the reducers. Both pipes must be aligned so that the element is straight. I moved the drill press to the floor and levelled the pipes with the drill press's work surface.

The finished joint is strong and straight. I don't anticipate any problems. As with all joints the mating surfaces are sanded to remove oxide and then coated with conductive grease. I have always had good success with Noalox but there are alternative products available from electrical suppliers.

Flashing reducer

Sometimes the fit is close but not close enough. There is a tight fit is between the 1" OD tube and the 1.315" OD pipe with its 1.049" ID. There's a radial gap of 0.0245". I keep a roll of aluminum flashing handy for these situations.

One layer of flashing made a reasonably tight fit. I have long lost the packaging and I don't recall the flashing thickness. Since a second layer made it too thick the flashing may be ~0.015". I used #10 screws (3/16") to secure the joint.

Because of the large stress on the joint at this mid-point location on the element I opened up just one of the holes for the screw head to press against the flashing and 1" tube. This ensures a solid electrical connection.

Both sides of the flashing are coated with conductive grease. The overlap is 4".

Closed tube reducer

For ⅛" step sizes and 0.058" walls it is easy to make a reducer. All you need is a short length of the missing step size(s). This is what I needed to make the step from 1" to ⅝". The 0.120" wall thickness of the 1" tube is 2 steps on it own so a ¾" tube fits nicely. Since I didn't have that size with a 0.058" wall on hand I made one from 0.125" wall thickness tube.

I drilled 3" lengths of the ¾" tube as described in the (previously linked) article about building the 15 and 20 meter yagis. I then reamed the reducers to 0.627". The resulting reducers are an excellent fit. With the sunk screw heads the electrical and mechanical bond is strong.

Element tips

The ½" tube is joined to the ⅝" tube with a slit and gear clamp. This joint is the primary one for adjusting element length for tuning so the ½" tube is longer than required for a solid mechanical joint.

The 0.065" wall of the ½" tube must be reamed to accept a ⅜" tube. The few thousandths reduction of wall thickness is easily accomplished with a hand drill and ⅜" and fluted bit. It does not bind or wander off centre, difficulties that can arise when more material must be removed. A reamer is a better choice than a fluted bit for this job, but most ham workshops may not have these reamers.

The job was done in a minute. Unfortunately there is little adjustment room in this joint, perhaps 2". Should more length be needed I will use longer ½" tubes or insert a ¼" rod into the ⅜" tube with a slit and gear clamp.

Capacitance hat clamps

The capacitance hats have a ½" centre and ⅜" tips. They are joined as described above. There is little room for adjustment and that is acceptable. For now I am using fixed length hats and tuning the element by adjusting the element tips.

The 4 arms of each hats are made of two of these assemblies. Each arm is 43" long (1.1 m). The ½" tube is drilled with two ¼" holes (reamed slightly larger) to fit a 1" u-bolt.

While strong enough for the prototype this is a poor joining method. Those ¼" holes weaken the ½" tube, and greater contact area is needed between the tubes for reliable electrical contact and to prevent bending and slippage.

A cursory search did not find turn up suitable commercial clamps with the attributes I need so I have a few design ideas that I will experiment with in my workshop in the coming weeks.

This is the furthest outboard I am willing to mount the capacitance hat. Loading increases (element shortens) the closer the capacitance hats are to the tips. However the tubes further out have narrower walls and the weight would increase droop and reduce survival from ice loads. Since my primary objective is to tame the third harmonic and not to greatly reduce element length the chosen position is an acceptable compromise.

Fully assembled element

The assembled element weighs in at 42 lb (19 kg). The steel backing plate and clamps are a further 10 lb (4.5 kg). I expect the final weight with an aluminum element-to-boom clamp to weigh ~48 lb (22 kg). That is a typical weight for 40 meter yagi elements. The capacitance hats added back most of the weight saved by the 10% length reduction.

Droop and flexing is not as bad as it appears in the photo! It's due to a combination of perspective and flexing at the improvised centre joint. The tips are not touching the ground. Using the 2.375" pipe as a guide the droop to the element tips is only 2'. With a continuous pipe at the centre the droop is expected to increase a small amount, perhaps to as much as 3' at the tips.

That is quite good for an element that is 62' (19 m) long. Ice loading is a greater danger than wind at this QTH and I still need to do the calculations to confirm that it will survive our weather. Similarly constructed 40 meter yagis have successfully survived severe weather, albeit with substantial bending.

Above is a close up of the element centre so that you can better see the causes of the additional flexing. The ¼" steel backing plate is bending more than I expected. However it can withstand the abuse. Compression of the wood fibre of the maple causes misalignment of the pair of 2.375" pipes. Despite the bending and misalignment I have little doubt that it will survive repeated trips on the tram.

Not shown are holes near the top edge of the plate that are for  bolting the antenna to the tower. The orientation of the backing plate will be vertical.

Coming up: lifting and tuning

I am planning to test and tune of the 40 meter element in mid-July, once I get a few other projects out of the way. I expect it to be an interesting exercise, figuratively and literally.

When I have collected all the data that I need to design and build a 3-element yagi based on this element design I will build an element with a continuous centre pipe. It will be fed by a gamma match, in keeping the the "plumber's delight" construction.

I will either side mount it at 100' on the 150' tower or (if I'm brave) raise it to the top of mast. The latter position will produce better data on its robustness and give me a temporary high 40 meter antenna to work distant DX. This could be valuable is the (likely) case that the yagi can't be completed in 2020.