Side Mount for a Limited Rotation Yagi

For the past few years I have had a Hy-Gain TH6 side mounted at ~72' (22 m) on the 150' tower, fixed south to W4/W5, the Caribbean, Central and South America. It has been used as a multiplier antenna in contests, and it has proven to be very useful in the intended application. That said, having it fixed is less than ideal. I would to use it to cover more of North America to earn more contest points working the US when my bigger antennas are pointed to more distant DX locales.

I have written previously about different methods of side mounting yagis on towers for partial compass coverage. This year I decided to do it for the TH6. A rotation of a little more than 120° allows coverage from 150° (Brazil) to 285° (VE7). That's perfect for my purposes.

I have more to say about the TH6 that will be deferred to a future article. It had to come down for reasons other than the new side mount system: to permanently repair a wire bond that could not be done on the tower, and to flip the boom to mast clamp since the rotatable side mount is on the opposite side of the tower from the fixed side mount bracket.

It is amusing to note that the antenna and rotator were manufactured not long after I was licensed almost 50 years ago. I acquired them secondhand in 1985 when I bought a house and could put up a tower after moving Ottawa as a young man several years earlier. Both went into storage when the tower came down in 1992, where they remained for over 20 years. 

The rotator, a CDE Ham-M, was refurbished and used when I returned to the hobby and put up a tower and tri-bander in 2014. In 2016 they went back into storage.

Looking around at my stock of metal and hardware I formulated a plan. I have a large stock of scavenged material that tends to find its way into projects or gets passed on to others. Nothing needed to be purchased for this project other than a few bolts. 

The tasks to be accomplished included:

• Take down the TH6 and its side mount fixture for service
• Service the rotator
• Select a mounting geometry that would achieve the desired rotation and not interfere with climbing and the numerous cables affixed to the tower
• Build a side mount system that is sturdy, servicable and meets the objectives
• Design the cabling system (about 230' long) to use the minimum amount of new cable and provide adequate current capacity
• Raise the refurbished TH6 and test the system

Getting the TH6 down was straight-forward. For simplicity I used my lawn tractor as the tram line anchor. You back up until the rope is taut, then block the wheels. I can do this for antennas up to around 70 lb, and more if the slope of the tram is low. For heavier antennas and the requisite greater tram line tension, the wheels lift off the ground. Lawn tractors aren't very heavy.

There is some difficulty rigging the antenna for proper balance since, unlike going up, you really only get one chance. The TH6 was a little unbalanced and a couple of tubing clamps snagged a guy. Jiggling was enough to free the snags. The entire job was quickly done with the help of just one friend as the ground crew. Less muscle is needed to tram antennas down, and is further helped by the shallow slope (under 25°) of the 200' long tram line and a height of 75'.

The new side mount went up the same day that the TH6 and the old fixed side mount bracket came down. It had been fabricated over the previous weeks in my workshop where I have a spare tower section to use as a jig. At 10' tall the tower section rests against a roof rafter. I use a step ladder to comfortably work at the top of it.

Laying the tower section horizontally makes it easier to for work on it in some respects. The negatives are that floor clearance is needed for tower projections, visualization is more difficult and levelling and supports can behave unexpectedly. Keeping it vertical help with visualization, and levelling and alignment are the same as when it is on the tower. I shim the wood plate under the tower section to make it level and vertical. Garage floors are sloped for drainage.

On the left is the basic structure. The angle aluminum (⅜" × 3") attaches to the tower girt with two grade 5 ½" bolts. Although A325 hardware is a better choice, the shear force in this application is well within the rating of bolt, nut and tower girt. Large fender washers provide a larger bearing surface for the bolt heads, which helps to reduce point stress on the tower girt and aluminum arm. The aluminum angle was cut to a length that is long enough to mount the rotator, and not too long for excess bending stress due to the weight of antenna, rotator and mast balanced at the end of the arm. 

Not seen in this picture is a vertical support cable for the arm, running from next to the rotator to a tower strut. You can see it below after it was installed on the tower. The cable reduces stress and the turnbuckle allows for exact levelling of the system when it is under full load. Even without the cable the support easily handles my weight when I stand where the rotator is located.

That 3" width of the aluminum angle is a tight fit for the Ham-M bolt pattern so I attached a short length of steel angle to the aluminum. The rotator is centred on the joint between the two to keep it in line with the tower face. Levels were used to keep everything aligned. On the right a pipe (mast) goes from the rotator through the upper support arm affixed to the next higher girt. Detail can be seen in the picture below.

The multitude of holes in the aluminum angle were already there. There are a couple of holes directly under the rotator that are used to fish the 8-wire cable to a terminal strip on the angle, where it is partly sheltered from the weather. The aluminum angle was rescued from a dumpster for free. I had no particular project in mind when I claimed it, but I was sure that it would find a use. There is more in my stock for future projects.

The support girt is at the 75' level, which is 5' above the second set of guys. The upper support arm is at 80'. These are nominal figures that ignore tower section overlap, and the actual values are closer to 72' (22 m) and 75' (23 m). This is a good height to minimize guy interactions and to match the elevation angles needed for the intended coverage area.

On the left you see the support cable and the level used to adjust the turnbuckle. It has to be tightened a tad when the antenna is mounted. The rotator cable and terminal strip are visible below the level.

The steel angle arm supports a mast bushing. A bearing is not really necessary for horizontal wind thrust when there is no vertical force. Also, a bushing is cheap to fabricate. For the 1.9" diameter of the mast the bushing is a short length of 2" schedule 80 aluminum pipe with an ID of 1.939". The fit is so close that the bushing must be carefully aligned. A burr present on the mast had to be filed flush to the pipe surface so that it would slide through the bushing. The mast diameter is probably closer to 1.91" due to the galvanizing for an even closer fit. 

Time will tell how the bushing wears in service; it is aluminum so that it will wear rather then the steel mast. The bushing is easy to replace if necessary.

The thrust arm is steel angle (⅛" × 1-½"). It pivots on the rear bolt to the girt for one degree of adjustment (there are 2 bolt holes on girt that can be selected) and another degree of adjustment is made by moving the nuts on the ⅜" threaded rod between the arm a short and heavy steel angle bolted to the girt face (not shown). The adjustment range was tested on the jig and determined to be sufficient.

Once everything is tightened the arm is very stiff. It will be periodically inspected it to check that it doesn't shift. I will add lock washers and other hardware if the need arises.

Hy-Gain rotator clamps are not very adjustable and a mast has to be about 2.1" diameter to be properly centred. The 1.9" OD pipe can be shimmed to meet that spec. I don't see the need since the distance to the bushing is far enough and the restricted rotation angle small enough that mast misalignment is unlikely to cause binding at the bushing.

After the TH6 was repaired it was trammed to the new side mount. The picture contains several items worth noting. First, notice that the mast clamp is low on the mast, close to the rotator. Although the bushing prevents bending stress on the rotator, an extreme wind can "bow" the mast. That increases bending stress and it is maximum at the midpoint of the mast. It's not a major concern but why take chances. A second benefit is that a shorter mast can be used to secure the boom truss. In this case the mast clamp for the boom truss is just below the bushing.

For a swing arm you must use thrust bearings above and below the swing arm for support and to protect the rotator. In my side mount article the diagram I drew of the swing arm is incorrect; I know better but the mistake crept in anyway.

Geometry and utilization of the tower faces dictated that the side mount would be placed on the tower's one climbing face. One of the other faces is dedicated to cables, which excluded it from consideration, and the other face was not ideal for the desired compass coverage and there is the risk of the antenna boom striking the cables. Putting it on the climbing face raised the problem of having to climb over the wide aluminum strut (see above). In practice the 3" projection from the tower face turned out not to be a problem. Not only do I not accidentally bump into it, it is comfortable to stand on when I'm working at that level.

The pulley for the haul rope is suspended directly above the mast with an arm attached to the tower. Depending on the geometry of the tram line this may be necessary to ensure the mast clamp aligns with the mast when the tram line is slacked. It is easier to bolt together the clamp halves without having to use muscle to align them to insert and secure the long bolts. Alternatively, have a friend join you on the tower. Yagi wrestling isn't fun.

The longer the side mount the further the rotator is offset from the tower. This can be tempting since you can eke a few more degrees of rotation. With my mount I have approximately 135° of rotation available. Placing the boom on the outside of the mast adds another couple of degrees. However, go too far with most antennas there will be a problem: the elements on either side of the mast can strike the tower. That's a bad idea.

Rubber cushions will be put on the boom so that it softly strikes the tower at the rotation limits. The Hy-Gain rotators have too little torque to do any damage and the motor will halt when contact is made.

Side mount rotation, whether a fixed mast or a larger-rotation swing arm, requires an antenna with elements spaced far enough from the centre so that it is the boom and not the elements that limit rotation. Smaller yagis and those for higher frequencies are more problematic. 

The Hy-Gain Explorer 14 that I recently sold is not compatible with a side mount because the driven element is close to the mast (boom centre). The TH6 has adequate space for this system (see diagram, above, copied from the TH6 manual). I opted to use the TH6 rather than the TH7 because there is inadequate space between the two driven elements of the newer antenna. The mono-band yagis I built for 10, 15 and 20 meters are compatible with rotatable side mounts, should I choose to do that in the future.

In addition to mechanical clearance there is electrical interaction of the innermost elements and the tower as they come into proximity during rotation. In most cases it is minor, and when it's the driven element the effect is mostly on the feed point impedance (SWR) and not the antenna pattern. The benefits of rotation typically far outweigh the negatives of pattern distortion. I ran several models to judge the potential magnitude of the interaction for the side mount article, and I reported my general findings. Exact determination of pattern distortion is difficult to do in a software model.

In a future article I'll say a little more about the side mount rotator and rotation. As of this writing the rotator is not wired back to the shack. Indeed, the TH6 is oriented so that the tram can remain in place to lift and adjust the 10 meter yagis that are now built. I should have everything working by early October.