Thursday, October 31, 2019

Hy-Gain Trap Repair

Traps on Hy-Gain HF yagis are not really designed to be serviced by users. Of course we're hams so we do it anyway. Despite a lot of disparagement of traps including those of Hy-Gain yagis they can be effective performers. With the wide availability of inexpensive Hy-Gain yagis on the used market many hams will find themselves with a good tri-band yagi that has been in the air some time and may need some repair. The traps are usually the only daunting aspect of the job.

I purchased my TH6 and Ham-M rotator from a local ham back in 1985. At that time it had been on his tower for at least 10 years. In those early days the hardware wasn't stainless steel so much of that had to be cut off and replaced. The dreadful BN86 balun that comes with the antenna was soon discarded in favour of a coax choke. A couple of the traps needed service and I puzzled out their disassembly and repair -- I couldn't benefit from an internet search back then.

When done it performed flawlessly until 1992 when I dismantled the tower and antennas and exited amateur radio for over 20 years. I kept the antenna, and indeed I sold very little of my equipment in the intervening years. I restarted in the hobby in 2013 in a small way, gradually increasing the size of the antenna systems. The driven element was put to use as a trap dipole for a short time.

In 2017 the antenna was refurbished and raised to the top of my new 150' tower. A couple of mechanically suspect traps were given cursory attention since it was the onset of winter and time was of the essence. Trouble reared its head the next winter. I had planned to have it up there for no more than 6 or 8 months but changed plans meant leaving it up there another year. Hope is a poor strategy for ensuring antenna reliability.

Again I failed to repair the suspect traps when I had it on the ground this fall. It went intermittent during CQ WW SSB this past weekend. This time I would have to take action. It was a simple matter to isolate the problem. With an antenna analyzer attached I wiggled the elements within reach of the tower. The SWR bounced around when one half of the driven element was shaken. The trap itself wiggled which made the failure point obvious. It was a simple matter to pull the half element out and lower it to the ground.

Trap structure

Hy-Gain yagi traps are not designed to be opened and serviced. Of course countless hams, myself included, do it all the time. The first time a trap is opened it is difficult to separate the parts without causing damage if you are unfamiliar with how they're built. A mistake usually makes it unusable. It is worthwhile to learn how to do it properly if you insist on doing repairs yourself.

The first thing to know is what's inside: the components and how they're put together. All the Hy-Gain traps for the tri-band yagis are the same no matter the model, be it an ancient TH6, a relatively recent TH11, a TH3, an Explorer 14 and so on. Indeed there are only 4 traps for the tri-band yagis:
  • 10 meters: there is a different version for parasitic elements on some yagis
  • 15 meters, for directors and reflectors
  • 15 meters, for the driven element
They are mostly distinguishable without the part numbers printed on the outer body, which is a good thing since the labels fade with age. The 10 meter traps are shorter than the 15 meter traps, the 10 meter and 15 meter parasitic element trap coils are wound with aluminum wire, and the 15 meter driven element trap coils are wound with copper wire.

Compare body lengths and peek in the weep holes and you have all the data you need to identify the trap. That said, I haven't directly compared the two versions of the 10 meter trap. Hy-Gain sells new traps at a reasonable price, and even most of the component parts. That's an alternative to repair. Go to their web site and search for "trap".

I pulled a broken 10 meter trap from my antenna junk box that I scavenged from another ham's antenna nightmare. The coil form and tab connecting the shell (capacitor) to the right tube are broken. I don't know the story of how it broke. Laying out the pieces as I've done gives an idea of what's inside. These traps are really very simple devices and quite robust when not abused in service, disassembly or reassembly.

The three sheet metal screw that secure the coil end and the shell tab are frequently criticized. While not a superb choice the screws are tough and have an integrated lock washer, and when the traps are properly assembled the screws are under very little stress. I judge the design a reasonable compromise between cost and robustness. The trouble comes when the traps are reassembled after service.

The plastic spacers are critical to the strength of the trap. Install them incorrectly and fatigue failures will occur. Their purpose is to keep the inner tube and coil rigid with respect to the outer tube. Since the weakest component is the coil (wire and form) these must not flex under wind and ice loads. When the spacers are properly positioned flex is minimized. It takes two spacers on either side of the coil to prevent the inner tube from flexing.

The inner end of the 10 meter traps (on the right, with the tab) flex the most since the spacers are close together. Over the years the tab can fatigue and break. This is usually only a problem on the driven element since the larger and heavier 15 meter trap on the outer end of the element increases stress on the 10 meter trap.

The spacers are positioned correctly when manufactured. Dimples (small depressions) in the shell hold the spacers in place. There are cavities in the spacers that align with the weep holes so that the spacers don't dam water flow.

To disassemble the trap remove the screw on the shell tab and push the inner tube left with respect to shell as oriented in the picture above. Some force may be needed to push the two left spacers over the dimples. Unfortunately the spacers may suffer some damage but there's no good alternative to getting them out. The spacers on the right will most likely stay where they are during disassembly and reassembly.

To reassemble the trap you push the inner assembly in from the left of the shell. If the rightmost spacers moved reposition them first. The one on the outside comes close to the tab. The other should be pushed from the left to its approximate position. Use the dimples as a guide but don't obsess over getting it perfectly aligned. But do try to have a weep hole aligned with the outer tube weep holes.

Even it isn't in far enough the trap coil screw push it further when it is inserted. Avoid this if possible since is a small risk of damage to the spacer and coil. If the spacer is too far outboard use a blunt metal rod to lightly tap it toward the coil through the outermost spacers weep holes, going around from hole to hole so that it doesn't jam. The most common reason for the spacer to be too far outboard is pushing the inner assembly too far to the right. Move slowly and watch for the tab screw hole to appear. Screw the tab to the inner tube.

Push a spacer in from the left until it is at or near its dimple on the left side of the coil. Hold the outer tube in a non-marring vice while for best control and least risk of damage to the tab. A 1" PVC pipe can be used to slide over the inner tube and apply even pressure to the spacer. Don't push it in too fare (right up against the coil) or you'll have start over again.

Finally insert the leftmost spacer and you're done. Hold each end of the inner assembly check for flex. There should be little to none. Remember to slip on the trap covers. I wrap the covers with Scotch 33+ for extended UV protection. Replacements are cheap if you need them.


For my repair job I disassembled the 10 meter and 15 meter traps. Although only the 10 meter trap was flexing I chose to do both since I had them in hand.

No wire breaks or loose hardware were evident. This can be a problem when the traps are mechanically unstable because the coil wires or forms will flex and eventually break. There was no damage of this kind just some debris and corrosion. An old insect nest was removed.

I undid the screws at either end of the coils and lightly sanded the aluminum surfaces to ensure good electrical contact. The inner end of the 10 meter trap tube was slightly distorted by the flexing so I pulled it from the coil form and gently applied pressure to the tube with a vice to reduce the freedom of motion it had developed. Perhaps a needless repair but I prefer that the form fit snugly.

As expected the spacers were out of position and that is what caused the flexing. The spacers were correctly positioned (as described above) during reassembly. The half element was then put back together and the dimensions checked. The entire job took less than 2 hours despite proceeding carefully. I wish that I'd taken the same care the last time I did this job!

Back in the air

For late October the weather was warm so I rushed to put the driven element back together before the weather turned. The high wind that day didn't dissuade me since the half element is easy to lift. I stuffed it and the insulator back into the boom clamp and attached the clamp and wires going to the balun and hairpin (beta match shorted transmission line stub). Within minutes I was ready to test my repair.

The element no longer wobbled when shaken. I connected the antenna analyzer to the balun and the SWR was what it ought to be. So far so good. To my dismay the SWR again misbehaved when I wiggled the driven element in exactly the same fashion as before the repair. That was not good. Although the poorly assembled trap need fixing the problem was evidently elsewhere.


Further troubleshooting showed there were two problems not one. The first was found by wiggling every wire and connection on the balun, driven element and hairpin stub. The wire from the stub to the clamp on the suspect half element was intermittent.

It seems that after 45 or 50 years the stranded wire had few intact strands remaining. Nothing lasts forever. I now realize that this was the same intermittent problem that developed earlier when the antenna was on top of the 150' tower.

Hy-Gain yagis of that vintage did not use stainless hardware. The screws and nuts are pretty well welded onto the aluminum tubes of the stubs. Since I was not prepared to tram the antenna to the ground to destructively remove the hardware I used a hose clamp to attach the new wire to the tube.

The wire quality is not the best but it should hold through the winter. I put a few layers of tape on the wire insulation where it might touch the boom clamp for added protection from high voltage and stray capacitance.

Finally the antenna survived the wiggle test. The SWR tests perfectly on all three bands.

The next problem was further down the tower. Where the short length of RG213 from the antenna to connects to the long length of Heliax the UHF to N adaptor was visibly bent underneath the layer of weatherproofing. Although the SWR was fine in the shack this could not be left as is due to risk of imminent failure.

The Heliax terminated with a male N and the RG213 had a male UHF. Adaptors with an N female on one end and a UHF female on the other end are rare. The 10 meter length of RG213 I grabbed from my junk box had a heavily taped N female on it that looked perfect for what I needed to hook up the newly side mounted TH6.

When I cut away the layers of plastic and rubber I discovered the problem. This old bit of coax was older than I realized. Inside was an adapter I home brewed back in the 80s for LDF4 runs, all with N male terminations.

Oh, to be young and foolish again! Unwrapping this contraption brought back memories. The shells of the N female and PL259 reducer were merely pressed together. It had no mechanical strength at all. I'm surprised it lasted as long as it did. Only the tape holding the halves together allowed for electrical continuity.

The abomination was replaced with an N female barrel connector and a more common N male to UHF female adapter. I have both in abundance since I routinely connect Heliax main runs to RG213 and LMR400 terminations. I taped up the joints and this time I really did had the problem resolved.

Rush jobs coming up

The weather is finally turning colder, windier and wetter. Time to finish the many antenna jobs I have ongoing is running out. Just this week I found time to (finally!) complete my 80 meter vertical yagi. An article on that should be out next week. The 20 meter and 15 meter stacks are slowly progressing, with two yagis tested and working. I am rebuilding the boom of the upper 20 meter yagi so that it is stronger and lighter. Permanent gamma matches to replace the testing/tuning gamma matches are under construction. Other hardware to side mount and mast mount the yagis has recently arrived or will be fabricated shortly.

The time taken to repair the TH6 delays every other job making this problem very unwelcome. The antenna was really proving its worth during CQ WW SSB before it quit working. As the number of towers and antennas grows the probability of a problem arising increases. Problems come in all types. For example, this week I discovered a small pit dug into the bundle of Heliax and other cables running from the 150' tower. Some animal, probably a skunk, dug up a wasp nest that was down there. I inspected the cables -- thankfully there was no damage -- and refilled the hole.

Maintenance is never ending in a growing antenna farm. I am spending some time on the air, entering contests and working DXpeditions. I count myself fortunate to have the time to do all of this and for the friends willing to help even when the weather is unpleasant. They seem to enjoy being a part of this journey.

However it is better to do things right the first time and avoid unwanted repair jobs.

Friday, October 25, 2019

Tower Lift Failures - Are You Prepared?

First the good news. My new 140' tower is complete. At the time the adjacent photo was taken the only thing left to do was to tighten the top set of guys and realign the tower. There are no antennas on the tower at this point. A few of the yagis for the 15 and 20 meter stacks can be seen on the ground biding their time.

The reason I delayed completing the tower was to build the mast and rotation system which required keeping the top two sections on the ground. With that job completed the tower sections could be raised. More on that in a subsequent article.

Unfortunately the final lift was not without mishap. No matter how carefully you plan and execute an operation of this scale there is an ever-present possibility that something will go wrong. It is never routine.

Several of the hams who have generously given their time to this project have commented that I make these big jobs look easy. Of course they are not easy. It helps to have experience but that is no insurance policy. What experience does provide is a bag of tricks to avoid or recover from many of the things that can and do go wrong.

No one was injured and the toll on tools and equipment was minor. I enforce safety practices on site. By describing what happened and what may have precipitated the mishap I hope to instill a sense of respect in readers for towers big and small. Mistakes can be very costly.

Photo credit: all but two of the pictures in this article are courtesy of Alan VE3KAE who was assisting me that day.

Lift procedure

Like most hams I don't do tower lifts the way professionals do it. The cost of tools, equipment and manpower is too high and unjustified for a ham. For those without the ability to do the work it is worthwhile to hire those professionals. I won't say more about how they do these jobs.

Hauling 150 lb loads 150' vertically upward is not a job for a few muscular hams. A manual winch with a 4:1 or greater advantage isn't as easy as you might guess, and it takes a long time. Power is required. A vehicle or tractor comes in handy and I've used both over the years.

A pulley at the tower based turns the downward rope from the gin pole horizontal. Find a suitable attachment point (tow hook, trailer hitch, etc.), hook up securely and without risk to the vehicle and away you go. The biggest problem is that you have more power than needed. An automatic transmission with a hydraulic clutch helps modulate the power. A rope rather than steel cable provides elasticity that allows a safe and graceful recovery when the load inevitable snags a guy or other tower protrusion.


When the tower section was up about 90' the rope seized. The load would not move up or down. A brief inspection discovered that wheel on the bottom pulley had split in two. The rope was caught in between the broken halves and was partially severed.

No disaster at that point but not an easy problem to resolve. The axle kept the rope from popping out and dropping the load. Had it broken through the load would have dropped at least 40' and possibly hit a guy at the 65' level. Damage to the car was likely. Had the rope been severed the load would have gone into free fall, an even worse outcome.

Since the situation was stable I had a few minutes to consider the problem. We took care to stand to one side of the tower while doing this. Had the rope or axle broken we would have had time to jump away from ground zero. Hard hats and steel toe boots are no match for a free falling load.

Unfortunately there is no picture of the seized and broken pulley since we were fully occupied dealing with it. The picture shows the split wheel and axle after being extracted from the pulley during the recovery procedure.


As the saying goes: necessity is the mother of invention. The come-along (hand winch) and a bunch of old guy grips were at hand in preparation to pull in the top set of guys. While a poor fit the grip was wrapped on the top side of the rope, a thimble inserted and the the come-along hooked to the tower bottom.

Once the tension was transferred the load was safely suspended. The pulley was then broken apart and the rope freed.

A new and larger pulley was attached next to the old one on the same cable wrapped around the tower base. The rope was wrapped around the pulley wheel and the pulley was reassembled. Only then did we pause to have a close look at the rope cut.

No more than 25% of the rope fibres were severed. It still had ample capacity to support the load. The car was backed up to take up the tension and test this assessment. Had it broken through the come-along would have held the rope.

The rope was again inspected under load. We decided to continue the lift with the damaged rope and new pulley.


The lift was completed without incident. Before climbing up the come-along was reattached to the rope. This eliminated further risk from the damaged rope. My helper used the come-along to lower the tower section into place when I was in position.

The two pictures were taken at almost the same moment. Alan took the one on the left -- one of the rare pictures of me in this blog. You can see me retrieving or replacing my phone for the picture on the right.

Notice that the gin pole has taken some abuse during this problem plagued lift. It looks bad but all that happened is that the pole rotated and pivoted on the top pin so that it is no longer perfectly vertical. This has happened before and is not a risk. All that said it is safer for the rigger to be at the top before the lift, staying above the load rather than climbing below it. On that day the ideal wasn't attainable.

After clearing the tower top the load typically dangles on the far side of the gin pole, only occasionally drifting in the breeze to where you see it in the pictures. The tower has only one climbing face so I cannot climb another face as the load drifts. We monitor it in case something untoward occurs.

A few minutes after reaching the top the section was lowered and bolted in. The guys were then lifted and attached to the segments on the top section. Success at last. After a break we pulled the guys to the anchors and called it a day.


Equipment doesn't come with a best before date. Everything should be inspected before an operation. Had I looked closely at the pulley it's possible I'd have seen something. Often the damage isn't visible so you must rely on the quality of the hardware.

The pulleys have seen lengthy service: lifting 300' of tower; tramming yagis; and a variety of other heavy lifts. Each has passed many thousands of feet (or meters) of rope and cable. No equipment lasts forever, not even when the working load capacity is never exceeded. Since an identical pulley is on the gin pole I brought it down and inspected it. Unlike the broken one there is no evidence of stress. Despite that the pulley will no longer be used at critical locations for heavy loads.

The 100 meter long polypropylene twist rope is approaching end of life, and that's okay since I planned to discard it once the tower was up. There is UV damage (polypropylene is very sensitive to UV) and fraying. Considering what I paid for it I have no complaint. I'll can buy another reel of it or a better product.

I subsequently spoke to a local tower pro. He showed me the rope and pulleys they use. These are very expensive and look awesome. I found it interesting that they never use steel cable, only rope. I will hunt for suitable products at a better price point.

For critical lifts I will be more diligent about using keepers. These are loosely bound ropes or cables that "catch" lifting ropes should the hardware fail. For example, a keeper for the recent failure would be a cable around the tower pillar that passes on the outside of the pulley. Even if the rope comes completely free the load will not fall. Unfortunately the mechanism to recover from a severed rope is not so easy to implement. The answer to that is to use better rope.

As to why the pulley wheel failed I have a likely failure mode. These pulleys are made for rope. As the loaded rope passes over the wheel it compresses and spreads the load over a large cross section of the wheel surface. Steel doesn't behave this way. Since it is thinner (for the same capacity) the force is focused on a narrower area of the wheel.

Second, aircraft cables are hardened steel that abrade the wheel surface, thinning it and reducing its load capacity. Steel cable should not be used on rope pulleys or the pulleys should be discarded sooner and inspected before each use. In future I will try to avoid steel cable for lifts and tram lines. The former I haven't done for a long time anyway.

None of us is perfect. By dissecting this mishap I hope to provide a lesson to myself and to others. Learning by direct experience alone can be deadly. Be safe out there.

Monday, October 14, 2019

Low Band DXing: You Snooze, You Lose

Those who have been reading this blog lately will know that I've been very busy lately with tower and antenna work. That time mostly comes from that devoted to operating so that the rest of my life isn't impacted too much. However there are many DXpeditions this month that do entice me to make an occasional effort.

One of those DXpeditions was ZK3A Tokelau. Although I have ZK3 worked and confirmed I wanted to log it on the low bands where it is a new DXCC entity for me. After working ZK3A on CW and SSB on 40 and 80 meters there was just 160 meters left to be worked.

I did try one time early in the DXpedition when I was awake during a sunrise enhancement. They were very strong for perhaps 10 to 15 minutes. Despite having a good antenna and running a kilowatt I was not successful. Indeed few in this part of North America had success that morning since East Asia, especially Japan, had the better signals.

As the sun climbed above the horizon they faded into the noise. Not being a morning person I tried only once or twice more. Each time they were not on 160 meters. I didn't fret since I was happily busy with other things and the DXpedition was scheduled to last until October 11.

On the 8th a friend called and told me how he'd worked them early that morning on 160. Rather than wait for sunrise he got up a little past 4 AM (0800Z), a time when it was daylight in Asia and most North Americans are asleep. The sun rises 30 minutes later in JA and the competition rapidly escalates. He planned it well and deserved the contact.

Our conversation motivated me to give it a try. A little after 4 AM the next night I got up and wandered in the shack. I turned on the rig and tuned around. About the only DX was a very weak T30GC making a few contacts with the west coast. There was no sign of ZK3A. In fact they were not spotted on any band.

Before giving up I checked one of the European DX news sites where the first item was that the DXpedition ended early. The boat arrived early for an unrelated medical evacuation so they tore the station down and jumped on board. I had missed my chance for good.

My friend commiserated with my bad luck by suggesting I'd have more opportunities in the future. But for him at his more advanced age, he explained, he might not have another shot at ZK3 on top band so he had to make the effort. It worked for him.

As the cliche goes: you snooze, you lose. Being a DXer isn't always comfortable not even when you have big antennas and power. The DX calls the shots, not you, and you must be flexible if you want to work them. I knew that yet I passed on the opportunity each night in favour of a good night's rest. Until next time.

Thursday, October 10, 2019

Aluminum Yagi Construction: Materials and Methods

Be warned: after reading this article you'll probably abandon your home brew HF yagi project and buy a commercial product instead. I am not that sensible. I'll keep building them despite the difficult experience.

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
All the local tubes were purchased in 20' lengths and cut to 10' lengths for transport in my vehicle. That explains the lengths of the 1" and ¾" tubes. The imported ⅝" tubes were purchased in 6' lengths to optimize shipping and cutting prices. Aerospace tubing comes in 12' lengths.

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

The inner diameter of the ¾" tubes is ½". Although the thick wall means I didn't have to nest a ⅝" tube full length inside the outer end must be widened to fit a ⅝" tube. Alternatively a coupler could be used -- " on the outside or " on the inside -- both are troublesome due to needing to import ⅞" tube (0.058" wall) or finding thick wall or solid ½" inserts, respectively.

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.

Mill versus mirror finish
With a sample in hand I made another trip to the machinist. He put his precision calipers on it and found the diameter to be 0.508". However the diameter is not consistent. That's what you get with a mill 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 sections

Apart 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.


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.


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.

Monday, October 7, 2019

Cutting Pipe Square

I cut a lot of pipes and tubes building towers and antennas. Most of the time I do rough cuts with a hacksaw or cutoff saw since the angle of the cut is not critical. It may look ugly but once it's up in the air no one will know.

For small diameter thin wall tubes I use a pipe cutter which is quick and makes a square cut. Except that a pipe cutter does not cut cleanly since rather than removing material it pushes it to either side forming a ridge, inside and outside, and the cut itself has a bevel. These must be filed off. The squareness of the cut can be distorted if the file is not kept level.

A hacksaw cuts more cleanly. However to make the cut square it requires a miter stand or the tracing of a reference line when cut free hand. Better is to use a metal band saw. Since my cutting needs are modest I have not invested in a band saw for my workshop. Instead I use a variety of manual cutting methods.

For large diameter pipe, steel or aluminum, the challenge of cutting the pipe square is multiplied. Manual pipe cutters of such a large size are rare and expensive. Even if available they require a lot of muscle. A hacksaw can be used if a square cut can be traced beforehand on the surface.

This is not difficult if the pipe has a known square end to use as a reference. Surplus pipe often does not have even one square face. A different technique is therefore required. I did this recently when I required a square cut on a 3.5" diameter aluminum pipe. I took a few pictures to show how it's done.

Draw a circumference

It's quite easy to scribe a circle around a pipe. You start at point A, travel orthogonal to the axis and come back to point A. But without a guide the line will almost invariably be a ellipse and not a circle. That is, it won't be square and the cut will be at an angle.

We need a straight edge that wraps around the pipe and does not crinkle, warp or that can lie askew. Guides I've tried and rejected include: tape, hose clamps, another bigger pipe, steel and fabric tape measures and drawing a line with a fixed marker while the pipe rotates. They fail because they are not rigid, rigid but not when stressed, a better but less than accurate edge or mechanical instability while drawing.

After considering the problem I found what is perhaps the most unlikely solution and one that covers half the space of my office desk: paper. Although it is easy to crumple and warp it is also easy to have it sit flat when wrapped around the pipe with a bit of tape and care taken to avoid "bubbles". The paper edges are a great straight edge when supported on a rigid surface such as a pipe. Paper is flexible with dimensional rigidity and is superior to straight wraps such as hose clamps because the sheet is wide: if it has a warp it will be immediately visible and can be corrected.

The pipe in the picture is 3.5" OD so an 11" paper edge (standard North American sheet height) almost exactly makes a circumference -- 3.5 × π = 10.9956. In this case the sheet is a little short since it doesn't quite reach around the pipe.

Masking tape on the pipe makes it easy to trace a fine line. The line doesn't have to be perfect provided there are no paper tears. If the pen or pencil wanders just fill in the gap and continue.

Making the cut

Starting the cut is the most crucial step.

Lie the pipe flat on a stand where it can't roll but can be easily turned by hand. I use an ancient Workmate (40 years old!).

With your gloved hand (for protection) precisely guide the blade along the drawn line. Cut with light pressure, taking care to keep the blade from wandering. Correct any deviation immediately.

Rotate the pipe a bit and continue. When you've gone around once you have a shallow cut that will help keep the blade on the line. The tape will tear or be marked if the blade jumps as you cut, warning you to correct the mistake. This is most likely while the cut is shallow.

Continue cutting while rotating the pipe a little every few seconds. As the cut goes deeper you can use two hands on the hacksaw if the stand opening is wide enough that the blade's force doesn't rock the pipe.

As you go deeper take care to align the blade so that it doesn't lean to one side. Many hacksaws, such as the one shown, seat the blade at a small angle for improved work visibility and to counter a natural inclination to lean the hacksaw towards the dominant hand. A straight cut means less filing to remove a bevel and less chance of warping the pipe edge from excessive filing.

Eventually the blade will penetrate the wall of the pipe. Avoid the temptation to speed up and not rotate the pipe. Keep rotating the pipe and the cut will be cleaner.


After completing the cut there is some filing to be done. Use a flat file across the full diameter of the pipe rather than filing one wall. This will keep the file flat and maintain the levelness of the surface. Burrs on the outside can be removed with the flat file and a half round file used on the inside.

The finished pipe stands perfectly vertical when on a level surface. When fit to the finished work the squareness of the cut was confirmed.

The use of this cut pipe will be described in a future article after I complete the new 140' tower. It forms part of the rotation system for the top mast. All the mechanical work to the top two sections is being done on the ground before being hoisted up.