Lifting Yagis: 15 and 20 Meter Stacks

 

This is the first of several articles on my (mostly) complete 15 and 20 meter stacked yagis. They were first put to real use in CQ WW CW and they worked well despite not easily rotated or configured. I will now back to talk about lifting and supporting the 4 yagis. There were 3 separate lifting operations:

• The lower 15 and 20 meter side-mount yagis in January
• The upper 15 meter yagi in May
• The upper 20 meter yagi in October
  

The lifts were progressively more difficult. Before each lift the gamma matches were tuned at a lower height. The first 3 lifts were done with a rope tram line which allowed the same rigging to be used to tune and lift the yagi. The upper 20 meter yagi was tuned with a rope rig and lifted on a steel tram line. You can't tune a yagi sitting on a steel cable for reasons that should be obvious.

Related items are mostly already covered in other articles: mast, tram line basics and prop pitch motor rotator.

Lower side-mount yagis

The wintertime lift of these antennas was briefly mentioned in a January article. I'll fill in the rest now. As mentioned above the lifting rig was the same for tuning the lower 20 and 15 meter yagis, and tuning the upper 15 meter yagi. The only modification for each was to move the tower anchor point for each lift. These antennas are not too heavy so we did the lifting manually. Two people are sufficient. Getting a car into a snow covered hay field was in any case impossible and I didn't have convenient access to a tractor.

The mounting brackets for the booms use pinch clamps. These were built by me from the original engineering drawings for this vintage LR20 tower. For convenience I used ¼" 6061-T6511 plate for the boom clamp and galvanized angles to pinch the tower legs. It's stronger than it looks. Not all of the weight bears on the bracket since a substantial fraction is taken by the boom truss.

The boom trusses are attached to a tower girt using short lengths of painted steel angles I pulled from my junk pile. One complication is that the centre of gravity (CoG) of each yagi is not at the boom midpoint. Tramming requires lifting at the CoG so that after mounting the boom had to be slid a short distance to centre the antenna.

Boom counterweights can be used to move the CoG to the boom midpoint. This isn't strictly necessary when the distance between those two points is small and I didn't bother to do so for any of the 4 yagis. It is still important that however it's done that the lower and upper yagis are in the same lateral positions for optimum stacking gain, otherwise the phase shift will slightly reduce stacking gain. 

When the lower yagi is rotatable (mine are fixed to Europe) there will in any case be a phase shift since the rotation centre of the side mount yagi is inevitably offset relative to the mast mounted upper yagi. The impact scales with wavelength and is rarely a concern at HF, even on 10 meters. A swing gate incurs the greatest offset.

The SWR of the yagis mostly survived the lift, as shown below. I was less successful with one of the upper yagis, as we'll come to later.

The 15 meter yagi either suffers from guy wire interaction or a shift in the gamma match. Since it is working well and as expected on air I suspect the latter. The risk of guy interaction was modelled and a picture of the potential difficulty is exemplified by the presence of a guy to the front and left of the yagi the photo below. SWR of the 20 meter yagi didn't shift at all. I took the picture of the 20 meter SWR in the shack because it was cold up the tower that day in January and I was in a rush to descend.

I connected the 20 meter yagi to the Heliax transmission line since it was in easy reach. It was for a few months until work on the stacks resumed. After initial testing the lower 15 meter yagi was left disconnected. The Heliax termination point was further away and I didn't want the bother of a temporary hookup.

Upper 15 meter yagi

We waited for spring to lift the upper 15 meter yagi. Lifting a large and heavy yagi to the top of a 40 meter tall tower and then to the top of a 3.5 meter mast is a major operation and carries risk if not done properly. Cold weather substantially increases risk because everyone is uncomfortable and uncomfortable people make mistakes.

The upper yagi of the 15 meter stack was raised in late spring just as the hay began its growth spurt and the ticks emerged. I don't like asking friends to work in those conditions. Trampling through the hay is unpleasant, makes the work difficult and risks the ticks endemic to the high vegetation. Lifting of the upper 20 meter yagi was deferred until late summer, after the harvest.

As for the side mount yagis we lifting this one manually. We added a third person to do the hauling because of the greater weight and vertical angle. I rigged the rope through a pulley so that the 3 people could work in a line and maintain COVID-19 physical distancing.

While all ended well the lift was not without drama. Despite a lot of experience with this operation I made a mistake in rigging the rope cradle for the tram pulley and haul rope. I would like to tell you what I did wrong but I don't know. The result was that the yagi gradually rotated from its carefully arranged orientation until the boom hung close to vertical rather than horizontal.

Above you see my trusty ground crew puzzling over the predicament. They are (left to right) Alan VE3KAE, John VE3NJ (holding a tag line) and Don VE3DQN, all veterans of tower work at my QTH. The lower elements in this picture are tangled in the lower 15 meter yagi. I went up the tower to wrestle with the yagi to push and pull it around the lower yagi, guys and the tower itself. Lowering it and starting over would have been no easier. It took an hour of frustrating and exhausting effort until it was finally sitting atop the tower.

Mistakes happen, unfortunately, regardless of experience. Happily no damage was done although there was some banging about, including the fragile gamma match.

I was relieved to measure a perfect SWR curve when the yagi was on the mast. It's only slightly little different than when tuned closer to the ground. Modelling agrees that the sharply inclined guys below have little effect.

After twice the time it should have taken the yagi was secured to the mast. I was pretty tired after the marathon effort, as were my friends. We had a late lunch and called it a day. A week later the yagi was lifted to the top of the mast. The tram winch was moved to the haul rope and operated by ground crew while I pushed it up the mast, moving the pulley upward in stages. 

The picture above gives you an idea how messy the rigging can get! Despite the mess it was all nicely arranged. There is also a length of Heliax visible that was raised soon after.

The design and construction of the air core coax choke for this yagi is identical to that for the lower yagis. The difference is that this time, rather than use 400UF (ultra flex) I used LMR400. I considered using RG213 (higher loss at 21 MHz) or ordering more 400UF but instead went with what I had. There are several rolls of LMR400 from 40' to 150' in my stock that I purchased as reel ends at a good price. 

Shown below are the coax choke and gamma match of the upper 20 meter yagi. A continuous 22' length of LMR400 comprises the choke and boom lead in. The choke can be built as a separate unit but I prefer to reduce the number of inaccessible coax connectors and therefore points of potential failure.

Making a choke from LMR400 requires care. The foam dielectric combined with a solid centre conductor can deform when improperly formed into a small diameter coil. Read and understand the mechanical specifications of the cable before proceeding. LMR400 and its various clones do not necessarily share the same specs. These chokes use LMR400 made by Times Microwave.

The minimum bend radius depends on how the cable is used. It is smaller when bent once than when bending is repeated. That should be unsurprising. The 15 meter choke has a diameter of 6" (15 cm) for a bend radius of 3" (7.5 cm). Since 3" is less than the 4" (10 cm) value for repeated bending it is advisable to get it right the first time you wind the coax. There is risk of damage despite being well above the 1" minimum radius for a single bend event.

Damage may not show up immediately. The stress within the thick centre conductor will relax with time or it may apply pressure to and migrate into the pliable foam dielectric. Impedance and power handling will suffer in the latter case. Measure the impedance before and after making the choke. Let it sit for several days and check it again. Doing so can save you grief later when the yagi is high in the sky and the choke is inaccessible out along a long boom.

Because the yagi is perched at the top of the mast the boom truss requires a separate support. It is a pipe several feet long that is clamped to the boom-to-mast plate. For stability there are 4 large clamps holding the plate to the mast. Only two were used while pulling it up the mast to reduce "grabbing".

The truss is adjusted on the ground then fine tuned on the mast to level the boom. Only then is it lifted to the top of the mast where the turnbuckles are no longer accessible. The SWR was also checked to confirm it was good before raising it up the mast. You don't want to discover problems after it is lifted to the top of the mast. The procedure to lift it up the mast was discussed in a previous article.

Upper 20 meter yagi

The upper yagi of my new 20 meter stack has 5 elements, a boom length of 12.5 meters (41') and weighs at least 55 kg (120 lb), including mast clamp and boom truss. This is the largest antenna to date that I've raised at my station. The tram line had to be engineered for the task. Constraints included suitable anchor points, lack of a mast back stay (to counter tram tension) and a forest of guys to navigate. 

The yagi had been sitting on the ground for 2 months awaiting the completion of mechanical work, good weather and the availability of friends. As summer turned to autumn the weather turned colder and unstable.  The first order of business was to adjust the gamma match and confirm that the SWR curve was as expected.

The general approach is the same as for the first batch of yagis. The difference is that due to the proximity of its twin on the tower the tuning location had to change. Another factor is its significantly greater weight due to the more robust boom. A strong rope runs between the towers at about 21 meters height. The towers are 60 meters apart. A winch tightens the rope and the tension pulls the yagi upward. The rope harness reduces droop and stress due the antenna's weight when it is turned and pulled during repeated tuning trials.

Dimensions for the gamma match were almost exactly that of its side mounted twin. That is the ideal to strive for. Later I attached the coax choke and began work on the tram line. It required some thought to a suitable design because, including mast clamp, truss and the rigging the tram line would be under considerable stress.

Rather than rope I opted for a steel tram line and rigging that is more rugged and less prone to the type of mistakes that can happen with ropes and knots, as it did with the upper 15 meter yagi. Steel has the advantage of a small MoE (modulus of elasticity) so that less tension is needed to support the antenna for an equal amount of sag in a rope tram line. That can be very helpful to keep yagi elements from snagging guy wires and other obstructions.

I purchased snatch blocks designed for steel cable rather than ever again risk failure of a rope pulley. This is not a place to skimp as I discovered to my dismay. Although not the ideal choice I have thousands of feet of guy strand on hand so I chose a 200' coil of ¼" EHS as the tram line. Aircraft cable or wire rope are better choices for their flexibility since the stiffer EHS can weaken from the stress of a large load at a single point as in this application. 

Use of multiple pulleys or snatch blocks spreads the load for safe lifts of even heavier antennas using EHS. However, under no circumstances use EHS for hauling. The turning radius around any pulley will be too small for EHS, and that will damage the pulleys and cable. For that job I use rope.

The initial test of the EHS tram was successful. The snatch block I seslected is large enough to ride the guy grips at the mast and ground terminations. That is important to put the yagi close to the mast.

The other tower base is the tram line anchor. A winch tightens the tram line and a guy grip is fitted to a coupled length of EHS to support the load during the lift; the grip is wrapped when correct tension is reached. The upper end is a tow strap looped around the base of the mast and tied to the thimble on the tram line with a shackle. 

Once wrapped the winch is left in line for backup and to take up the tension later when the tram line is disconnected. A second backup is a rope that doubles as an aid to guiding the steel tram line around the lower yagis both before and after the lifting procedure.

I did not anchor the tram line high on the mast since there is no convenient anchor on the opposite side of the tower to support a temporary back stay. With a tension of several hundred pounds I wanted to avoid stressing the mast. My trusty tension meter was used to monitor tension on the tram line and the upper set of tower guys.

Tension on the tram line with the yagi off the ground is less than 400 lb. Tower deflection was slight and not a problem at all. The tension on the two opposite top guys each rose about 100 lb.

At right is the pulley arm at the top of the tower, shown after the antenna was raised and most of the rigging removed. The shackles provide freedom of rotation so that tension on the haul rope aligns the pulley with the ground anchor and the lower pulley. The tower face does not quite point in the needed direction. The pulley at the base of the tower enables a vehicle to provide lifting power with horizontal travel.

For additional safety I put keepers on the pulleys. These short loops of chain protect against falling yagis and tram line should a pulley suffer a catastrophic failure. At this height the heavy weight would destroy the lower yagis and can damage or even sever a guy. The latter would be catastrophic. The precaution is well worth the trouble.

When the big day finally came -- warm and sunny for early October -- my friends gathered for the lift. Unfortunately we were cursed when the wind speed rose too high for a safe lift. The yagi remained grounded and we instead undertook other important jobs on my list. 

The wind load on a 5-element 20 meter yagi makes it too unwieldy to steer and stabilize when the ground wind speed rises above 40 kph. The wind is almost always stronger 40 meters or more above ground. It isn't worth the risk. Had there been two of us up the tower we might have proceeded. 

Two weeks later we tried again, this time successfully. A shorter delay would have been better if only I could coordinate my friends' time with the weather forecast! As the saying goes: beggars can't be choosers.

Once it's all rigged the lift goes smoothly. The challenge is getting to the point where it looks easy! The driver (VE3DQN) is just beginning the haul in the picture. I am always begging cars from friends for these pulls since an automatic transmission is strongly recommended for smooth torque control and the ability to quickly "feel" and react to snags. Any vehicle has sufficient power since it is rare that a tower section or antenna weighs more than one adult.

There are a couple of attachments near boom centre. The turnbuckles for the boom truss are tied together and to the boom. The bit of rope tying the two turnbuckles is very important. If you accidentally let go of one while attaching them to the mast it can't fall out of reach. Mistakes happen, so plan for them. The other item is a come-along and tow strap. They are for lifting the antenna into position once it was at the top of the tram line. Doing it this way was easier than climbing encumbered with the awkward weight.

The elements are level in this pull and that was a mistake. Usually I tilt them upward, perhaps as much as 30° to better clear guys. Since the upper 15 meter yagi is already attached to the mast and the elements tend to tilt further upward near the top of the tram when the haul rope takes all the weight I wanted to avoid tangling the elements of the two antennas. My improved rigging kept the elements from tilting and due to the droop in the long 20 meter elements directors 1 and 2 (the ones on either side of centre) did slide underneath the upper set of guys.

When I saw it happen we lowered the antenna (easy to do with a car providing the muscle) and I adjusted the rigging and attached a tag line for steering. This got us past one guy but not the other. Up the tower I could not swing the element out of harm's way since the antenna is too bulky. Instead I leaned out and removed the element. I hung it on the tower until later. With that last tangle corrected the yagi was lifted to the side of the tower.

At this point things get complicated. However it's a complication that I planned. To avoid bending stress on the mast by anchoring the tram line a few feet above the tower (discussed earlier) the tram line is anchored below the top of the tower. I attached the come-along between the boom truss clamp and the boom. A cable and shackle to attach the come-along was installed a few days earlier. It is visible in the picture above.

Once I took up the weight the tram line was detached at the ground anchor and walked in to the tower. Next, I removed the snatch block, detached the cable from the mast and tied it off to a previously positioned shackle located on the tower below the mast. The space above the yagi was now free of obstacles.

With the 1 ton come-along I pulled the yagi the 2' or so to the side of the mast where it was to be mounted. The only difficulty was that the bolts on the boom-to-mast clamp hooked under the top tower girt and the boom hooked under the pulley arm (see picture above). I had to push the boom outward with one hand while stretching my other arm to crank the lever on the come-along. Other than the muscle required the lift was quick.

In case you're wondering, the clamp plate was made in my workshop from ⅜" 6061-T6511 plate. The u-bolts on the boom are the big stainless ones from DX Engineering with the textured solid aluminum saddle. The u-bolts on the mast are the galvanized Cycle 24 clamps, also from DXE. The bolts are not inexpensive but this is not the place to be cheap! I prefer the Cycle 24 clamps for the mast since they are rugged, easy to work with on the tower, don't have the galling risk of stainless steel and resist motion due to wind torque better than the textured saddles and round stainless u-bolts.

By this time sunset was close and my friends had families expecting them back home. The antenna was left untested with the boom truss not yet attached and a missing director. The next day I slid the boom so that boom centre was at the mast. The truss was attached and I levelled the boom. Director #2 was attached a few days later.

Sad to say, all was not well electrically despite no mechanical problems and no antenna damage. The SWR was about 2 across the band. Although the yagi worked perfectly well as a yagi somehow the tuning of the gamma match shifted before the lift, perhaps when I installed the coax choke or when I weatherproofed the components of the gamma match. 

The season is late and this is not an easy antenna to lower, tune and lift again. Besides which, the yagi works, on its own and phased with the lower yagi. It just doesn't have a low SWR and power division between the yagis is not optimal. I took a full set of measurements and sat down to study them on the computer. There is a solution that does not involve taking the antenna down. More about this in a future article.

I'll end with a picture taken at sunset the next day after the boom was trussed and level. The fall colours are golden and sporting a missing director the yagi looks like a child grinning after scoring big with the tooth fairy. But it is pretty.

On a closing note, you may have noticed that despite the length of this article I am vague about numerous details. That is deliberate. From years of corresponding with readers of the blog, too often these articles are taken as gospel rather than a simple description of my adventures in amateur radio. That is, for some out there they feel that if I, a ham, can do these things so can they. The reality is that this is often not true.

Before embarking on a project the size of this one you need to reflect on the challenge, the danger and, yes, the cost in money, time and maintenance. These are serious considerations. Thank you for reading and, please, be careful out there.

Coming up...

Expect articles this winter on the complete stack switching system, operator control and planned integration with station automation. You can also expect to hear more about the anomalous SWR of the upper 20 meter yagi. As already noted the stacks played well in CQ WW CW so the SWR is not a deal breaker. 

The prop pitch motor suffered another failure after the bearing was repaired and that was why the stacks were not quite ready in time for the contest. The motor is working fine for now but it will need more work next year. I may have to write another article on prop pitch motor repair when that job is done.

In 2021 or later further improvements to the stacks will be investigated with the intention of improving their effectiveness during contests. There will also be a stack for 10 meters to take advantage of the rising cycle 25.