With that disclaimer out of way let's dive in. You will learn how I went about the physical construction of 5-element stacked yagis for 15 and 20 meters. The yagis are complete but not tuned. That's my next task. Then I have to raise them and put them to work. I am nowhere near done.
Perils of aluminum shopping
I envy Americans, at least when it comes to aluminum tubing. Despite Canada being among the biggest global producers of aluminum finding what a ham needs is challenging. On the positive side, aluminum tubes, pipes and other shapes are readily available and economical.
Nesting tubes for telescoping tapered elements requires close tolerance of inner and outer diameters. Milling and finishes affect the usability of tubes since the measured and published dimensions may not match or be consistent. The English dimensions used in the United States for tubes and pipes are most common in Canada due to the close trade relationship and despite this being a metric country. When it comes to yagis this fact is helpful. I have heard that telescoping available metric tube sizes can be difficult.
The 0.058" and 0.120" wall diameter high tensile alloy tubes in stepped ⅛" sizes work best for yagis. The former is unavailable here and the latter is uncommon. These tubes are classed as aerospace tubing and though widely available in the US must usually be imported from there. That can be costly since shipping can double the price. Shipping of longer tubes is especially expensive
To keep cost as low as possible I chose a taper schedule after discussions with local suppliers and after considering the tooling requirements to adapt tubes that were close to being suitable for telescoping. All my tubes are 6061-T6 alloy except for the 6063-T832 aerospace tubes I imported from the US. I did my own importing since in every case it was far cheaper than getting it via a local metal supplier.
I received an unwelcome lesson in the difference between mill and other aluminum finishes. More on that later. First I'll describe my taper schedule and construction techniques.
Element taper schedule
The longest half elements are the 20 meter reflector at 219.5" and 15 meter reflector at 143.4". Here are the half-element taper schedules. The centre 1" tube is twice the half-element length.
- 1" OD, 0.120" wall: 60" (20m); 30" (15m)
- ¾" OD, 0.125" wall: 60" (20m); 40" (15m)
- ⅝" OD, 0.058" wall: 36" (20m); 24" (15m)
- ½" OD, 0.065" wall: variable length element tips
The approximately ⅛" wall for the inner segments increases the wind and ice survival. For those in the US, it may be more convenient to nest 0.058" tubes in adjacent ⅛" diameters. Ultimate survival was not calculated but interpolated from published designs. These yagis should survive this 135 kph wind zone with capacity to spare, even with modest icing.
The ¾" tube fits snugly in the 0.120" wall 1" tube with no friction at all. I consider this a lucky break since I expected 0.125" to be available yet the 0.120" was available and inexpensive. Since I bought so much they gave me a substantial discount.
The ¾" tubes were machined to fit a ⅝" tube at the outer end so it didn't really matter whether the wall was 0.120" or 0.125". More on that machining below. Fitting the ½" tips into the ⅝" tube with its 0.509" ID should have been easy but wasn't, as I discovered to my dismay.
In tandem with this major project I have also developed a taper schedule for 40 meter yagi elements that utilizes the same taper schedule for the outer halves of the elements. The inner halves will be substantially heftier. I'll leave this construction project for a future article until I've built one of these monsters.
Stepped diameter correction (SDC)
My EZNEC models for the yagis were updated with the final taper schedule for the elements. Using the built in SDC add-on for the NEC2 engine I adjusted the tip lengths to return the yagis to their original designed frequency ranges. This process retains the performance of the designs. For the lucky few with NEC4 although the SDC algorithm is superfluous it is still necessary to adjust the element lengths for the taper schedule.
You must scale the yagi elements since without the SDC for the final taper schedule the performance changes can be substantial. This was most evident when I scaled the 20 meter yagi. Surprisingly the 15 meter yagi, despite having the same taper schedule, required almost no change to the element lengths.
I found it helpful to scale the element in steps, starting with the inner tube section and working outward, checking yagi performance after each step. The changes at each step are not always in the same direction so that a later section change cancels the previous deviation. This is what happened when I scaled the 15 meter elements.
Drilling out thick wall tubes and pipes
Reaming out the ¾" tubes to ⅝" must be done precisely. Ideally it is done on a metal lathe in a machine shop. They will do it but it can be costly since I have 40 half-elements to be machined in this manner. So I chose to do it myself.
My first attempt (left) did not go well. I simply put a ⅝" bit in my hand drill, lined it up by eye and had at it. The bit suffered a lot of chatter and even with frequent progress inspections the bit went off centre over the 3" depth of the cut. For my second attempt I first used a 9/16" bit then used the ⅝" bit. The tube wall is thicker, close to the ideal 1/16" since there was less chatter. Doing the cut in two stages help keep the hole almost perfectly centred.
I decided to keep going and do all 40 of them. It took some time. Each tube required at least 10 minutes of work. There were several that drifted off centre and had to be cut off and redone. The 2" to 3" shortening of these tubes has a negligible impact on the SDC. I learned a lot about drill rotation speeds for cutting aluminum and the dos and don'ts of lubricating aluminum. After an initial trial I discarded the lubrication and did the drilling dry, which went faster and with fewer mishaps.
Chuck driven reamers might have done a cleaner job but for the amount of material to be removed. It would have had to be done in more steps and at greater expense since I did not (yet) have any reamers.
I'd hate to do this job again but it did work out pretty well. That is, except for one difficulty which I did not entirely appreciate beforehand.
In the picture you can see the drilling jig and the two drill bits. The technique of screwing together two wood blocks to secure round tubes without crushing or marking them is one I learned a long time ago when I built a bicycle frame from very thin wall chrome-molybdenum steel tubes.
With a standard ¾" bit (not a wood bit) you make a hole as shown. Use a drill press to make it perfectly vertical. The halves are then unscrewed and a sander applied to one or both interior faces to reduce the diameter a tiny amount. Put the halves back together and lightly hold in a vise. Insert the tube and tighten the vise. The tube won't turn except under very high torque.
Pounding a round peg into a round hole
The problem in a nutshell: you can't fit a ⅝" tube into a ⅝" hole. Well you can, but just once since it won't come out after you've pounded it in. This is an example of a press fit and it is totally unsuitable for building yagis.Even 0.001" makes a difference. My local machinist took one of my ¾" tubes and hand reamed the ⅝" opening to 0.626". The tube now fit though with some friction. After hearing his quote to do all 40 on the lathe I decided to order a reamer online and do it myself. I chose 0.627" to ensure the tubes could be pulled apart in the future.
I use the same ¾" jig to hold the tube. Since I don't have a handheld drill with a suitably large chuck I reamed the tubes manually. I used vice grips and protected the tool with a metal wrap. Unlike a fluted drill bit the reamer is very unlikely to wander off centre. Shaving 0.001" all around is doable by hand without only a little effort.
When the reaming was done the ⅝" tube slipped in easily and hand no discernible slop. Then I did the remaining 39. All this work did save on importing aerospace size tubing, however I might have chose otherwise if I'd thought through the troubles of reaming tubes.
The woes of mill finish
My reaming woes were not over! An unexpected and larger challenge lay ahead. The 200' (60 meters) of ½" tubes I purchases for element tips came with a mill finish just like all the other tubes locally sources. They did not fit into the ⅝" tubes. I was surprised since the 0.058" wall leaves an ID of 0.509", which should leave plenty of room.
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| Mill versus mirror finish |
He explained that this oversize diameter is quite common on small aluminum stock. Worse is that for a press fit of similar aluminum alloys a thin layer of material is pushed along the surfaces and can lock the tubes together. That is, once you press it in it won't come out again. I had already discovered this the hard way.
The aerospace tubing, the machinist explained, goes through a grinder that produces surfaces with precise tolerances and a mirror finish that the market demands. The consistent 0.009" gap allows easy nesting to any depth. But I had 200' of tubes I didn't want to waste so I took the reaming challenge.
This was a bigger problem that reaming the ¾" tubes since tips must be adjustable and that requires greater depth of insertion. I first ordered a 0.511" reamer reasoning that an additional 0.002" should be plenty. It wasn't. It was better than a press fit but not enough to allow the tubes to slip together without any binding.
My next step up was a 33/64" drill bit with a shank that would fit the chuck of my handheld drill. Reaming 0.509" to 0.5156" is difficult to do by hand so I chose to do it with power. I did experience binding of the bit inside the ⅝" tube when I rushed the job so I took it in easy steps, being sure to regularly allow the flutes to clear. It wasn't fun but finally it was done and the ½" tubes slipped in to 5" depth. In a pinch the tube can penetrate 7" since I went deeper with the 0.511" reamer.
To add insult to injury, after all the foregoing woe I ordered more of the ½" tubes for other yagi projects. It looked different: the finish was shinier and there was source and material lettering on the tubes, unusual with mill finish. After putting calipers on a sample and comparing with the previous order I grabbed a ⅝" tube and lined up the two. The new ½" tube slipped right through the ⅝" tube just as it should. I'm tempted to run back and pick up more in case they switch suppliers again. No reaming needed for this batch.
Joining tube sectionsApart from the element tips all tube joints are screwed together. This forms a reliable bond and because these joints are fixed there is no need to make them adjustable. Many commercial and home brew yagis use slotted tubes and hose clamps since they are easier to construct.
Each joint -- 1" to ¾" and ¾" to ⅝" -- uses two #8 stainless screws with a flat washer and nyloc. Two holes are drilled through at right angles, one near the edge and near the back of the 3" overlap.
I borrowed a trick from other yagi builders that achieves a superior mechanical and electrical connection. One side is drilled wide so that the screw head rests on the inner tube. When tightened the opposite sides of the tube are firmly pressed together. This is better than relying on screw torque to distort the outer tube so that it presses against the inner tube. With small screws and high tensile strength tubes it may be impossible to adequately distort the tube.
I made drilling guides out of PVC pipe with the hope of achieving enough consistency that tubes could be interchanged and the holes would be aligned. That didn't work out since that close a tolerance was too much trouble and not really necessary. I soon dispensed with them. However the tubes were clearly marked and stored in groups to avoid mismatches.Because of the tight tolerances of tube diameters (see above) all holes were carefully deburred, inside and outside. All joints, including the screwed ones, were sanded to remove oxide and coated with an aluminum joint compound (I use Noalox, and there are many others on the market) for a good electrical connection and ease of sliding tubes together and, years in the future, sliding them apart.
As mentioned the tips are adjustable by putting two slots in the ⅝" tube and compressing with a stainless hose clamp. I took some care to cut the two slots straight and opposite to each other with two cuts with a hacksaw. The slots were cleaned with a triangular file and a short bevel placed at the bottom of the slot to reduce stress when compressed The twin slots are wide enough that when the hose clamp is tightened the ½" tips are firmly held.Element to boom clamps
For 15 and 20 meter elements with 1" × 0.120" centre section the clamps don't have to be excessively large. Following the advice in W6NL's Physical Design of Yagis the plates are ¼ × 4" × 6" 6061-T6511. I cut the plates by hacksaw from a long length of the extruded alloy, thus saving cutting fees and getting a sore arm. High tensile strength aluminum isn't easy to cut.
Galvanized muffler clamps secure the plate to the boom. Each size of boom section, ranging from 2" to 3", has its own clamp size and bolt pattern. The galvanized u-bolts for the elements are the same. I could have used stainless hardware at more expense and order lead time, but it is not necessary. Electrical continuity is via aluminum to aluminum contact not through the hardware. Although there is no texture on the muffler clamps the torque from the element is modest even in a turbulent and strong wind so it shouldn't rotate on the boom.Using the formulas in W6NL's book I calculated the effective diameter of the plate clamps -- 1.673" -- and inserted that into the EZNEC models. The SDC algorithm takes care of the rest. The boom under the clamp electrically shortens the element a small amount -- estimated ~6% of boom diameter by both W6NL and W2PV -- which is pretty well negligible when considered in combination with the effect of tower, guys, other antennas and hardware "bumps".
Gamma match
Originally I planned to use a beta match and acquired the fibreglass tubes needed to mechanically join split driven elements. Instead I am using gamma matches to reduce the mechanical complexity and to gain the gamma's modest ability to attenuate common mode on the coax shield. Beta and gamma matches are about the same amount of work to tune the impedance match and both require similar shortening of the driven element (capacitive reactance).
The prototype 20 meter gamma match is shown mounted on one of the 20 meter yagis. The gamma rod is ½" tube, making it half the diameter of the centre section of the driven element which is a typical (recommended) ratio.
The fixed spacer is PVC pipe mitered to the top 1" tube and secured with a tie wrap. The tie wrap will be replaced with a more durable clamp when the antenna is tuned. The gamma rod fits snugly through a hole in the pipe. The slider for impedance matching is a strip of 1/16" thick mild aluminum alloy.
There is as yet not coax connector or pigtail to terminate the transmission line. For tuning a variable capacitor will be mounted. After tuning it will be replaced with a hardier capacitor of the same value, either a high-Q, high voltage fixed capacitor or (more likely) a length of RG213 (with covering and braid removed) slid into the gamma rod. The latter method is popular since the gamma rod can be slid to adjust the capacitance. However it is not so convenient for initial coarse tuning.
The ends of the slider were manually wrapped around tubes to create the required shape. The wrap is stopped at approximately 330°, the end bent and the bolt holes drilled. When tightened the grip is very good. A conductive grease is recommended on the interior clamping surfaces.
A difficulty was encountered while forming the gamma rod end. I could put the bend in the right place at the beginning of the process but found it difficult to prevent the long straight section from creeping along the form during the last half of the forming. As a result a couple of the shorting straps are ¼" short. After recalculating the gamma match with the reduced values my concern was assuaged. The gamma capacitance value barely changed and the short position goes outboard no further than another inch.
Booms
The four booms were constructed from a mix of pipe and tube. I tried to find a balance between weight, wind load and cost. Most of my pipe is surplus and inexpensive. When I can't get what I want I either modify the boom design to use what I have or I buy new material as needed. Large tubes and pipes are readily available new and are not too expensive.
The side mount yagis use the largest diameter tubes. Their relatively thin wall makes them unsuitable for rotation at great height. These were built last year and stored until I was ready for them.
The rotatable yagi booms are heavier, narrower and have a modest wind area. They are a mix of surplus pipe and new tubes. The 20 meter rotatable boom is the heaviest with a centre section that is 2-½" schedule 40 pipe (2.875" OD). I made liberal use of my relative strength spreadsheet to contrast and compare alternatives.
Some choices were made based on good fit between various sized tubes and pipes. In one case I had couplers machined but those didn't work out and have been put aside for a future project. I am not bothering to describe details of the booms since my choices are unique to my circumstances.Waste
Machining, drilling and cutting the elements, booms and clamps produces a lot of aluminum waste. Constant cleaning of the tools and work surfaces was necessary to avoid mishaps.
No matter how well I cleaned up each day aluminum shards appeared everywhere throughout the workshop, on my clothes and hair and in the house. The latter occurred despite cleaning of clothes and boots. The stuff is insidious.
Regrettably there isn't enough metal weight to make recycling worthwhile. However it does occupy a large volume since the shards, especially the strings and spirals spun off drill bit, pack loosely. It'll all be thrown out.
Of greater concern is steel waste since it is darker and therefore more difficult to see. That matters since it is far more likely to cause cuts and slivers than the softer aluminum. Fortunately there is less steel waste, the bulk of it from fabricating the rotation and support system for the mast and yagis.
Putting it all together
Here we have the first of the assembled yagis: the side mount yagis for 15 (left) and 20 meters. The gamma matches are not yet ready. That and initial tuning is coming up. It isn't a trivial task since the feed points are far out on the boom. Access to the feed point is required for every tweak to the gamma match. More on this later.
Element positions were previously marked on the boom. I attached the element clamps then the elements, tightened everything and sighted along the boom to align the elements. To do this properly the boom must be lifted above the ground. That will be enough height to clear the fragile gamma match. I use old cable reels for supports, in this case from 500' spools of EHS guy strand.
Was it worth it?
That's a very good question. For myself the answer is yes despite all the difficulties. It was a superb learning experience and, aside from the time invested, economical in comparison to commercial products. There is also a sense of accomplishment overcoming the challenges of design and fabrication.
Of course the project is not complete. After the tuning the yagis must be raised and fed for stacking. The switching system for choosing lower, upper and both for the 15 meter and 20 meter stacks will be a commercial product. Little money will be saved building my own and it will look and work better. There is no shame in buying some products even for a devout home brewer.
As I finish this article friends are scheduled to come over to assist me with tuning the yagis. Three are built and the fourth (20 meters) is awaiting completion of the boom. I'll have more to say once the next stage is completed. For the present I am relieved to have all this aluminum out of my garage workshop so that I can move around and access things that have been out of reach for weeks.
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