Another Rotatable Side Mount - 15 Meters

One lack in my station is a sufficiently strong signal on 15 meters into Asia, especially east Asia. While we cannot run Asian stations like our friends further west there is a rich vein of multipliers to be mined during major contests. I was reminded of this during the last All Asia SSB while running Japanese stations by how incredibly weak most were, barely a whisper above the noise. And this is from a quiet rural QTH.

The 5-element stack for 15 meters has a rotatable upper yagi at 43 meters and the lower yagi is fixed on Europe at 32 meters. Individually and together they are very effective antennas. In daily operating the stack can easily break pile ups with just 100 watts. To achieve stacking gain the lower yagi must become rotatable. That is never easy for side mounted antennas.

There are two possibilities: a swing gate or ring rotator for close to full 360° rotation, or a fixed side mount for about 130° of rotation. The first is not really necessary in my station since I have other yagis that can fill the gaps. It is also the more mechanically challenging of the alternatives. 

The latter method can work well if the 130° of rotation includes both Europe and east Asia. That I can do. The plot shows the approximate coverage for the 15 meter yagi's rotatable side mount.

Unlike the 40 meter Moxon, the 5-element yagi can't be reversed to double the azimuth coverage. That's okay for my situation on 15 meters so I followed the same design as for the first rotatable side mount. Deviations from that design are minor, to accommodate differences on the mounting location and the coverage objective. 

Last fall I picked up a used Ham-IV rotator and controller. It looked clean on the outside and proved to be in good condition on the inside. That's more than enough rotator for the weight and wind area of the 15 meter yagi. A swing gate would require a far heftier rotator to deal with the high torque of an offset load.

Since the weight of the yagi, mast and rotator bears on the lower strut, I used 4" × ¼" steel angle. It was scrap and rusted but perfect for the application. I cut a rectangle of ¼" steel plate with conveniently located holes that lined up with slots in the angle stock. This made a sturdy platform that put the rotator far enough from the tower to allow rotation from about 60° (through north) to 280°, just as I wanted. The slots permit a small adjustment range to align the rotator and mast bushing

The strut is strong enough to support my weight and even jump up and down on it, which I did to test it after installation -- I should point out that I don't weight much so be very careful about trying this yourself! Unlike the aluminum strut that I used on the other rotatable side mount, there is no need for a support cable to keep it from deflecting under load. If it becomes a problem later I can easily add it.

The LR20 guyed tower is popular in Canadian big stations while rare elsewhere. Mechanical details will therefore be of little interest for most readers. Nevertheless, the design and construction process can be applied to other towers. I keep a section of LR20 (10' high, 120 lb) in my workshop as a jig for jobs like this.

The first challenge is that I had to use a different tower girt for this side mount. There are two guy/support girts on each section. Due to the yagi's position the girt for the lower strut has to fit between the guy yokes and section splice bolts. Pictures will help so here are a few.

On the left you can see the two cutouts for the splice bolts. I cut the angle stock with an abrasion wheel on a circular saw. Since it's out of sight high on the tower it doesn't have to be pretty! I regularized the slots as well as I could and removed all sharp edges and points. The strut was descaled and painted.

The strut couldn't be centred on the girt due to the guy yokes. A short length of steel angle stock seats the strut on top of the girt. 3" ×⅜" aluminum angles are used as fore and aft clamps to the girt to keep the strut stable under the stress of the weight of the yagi, rotator and mast on the far end of the strut. The clamps are cut with peculiar angles to fit the insides of the tower legs. 

The rotator plate will be shown in a later picture. It is outboard of the strut to maximize the rotation range. They are joined by ⅝" grade 5 hardware.

The upper strut only has to deal with lateral forces. The 1.9" O.D. 6061-T6 pipe mast runs through a 2-⅜"O.D. schedule 80 pipe section, which is used as a bushing. This is identical to the other rotatable side mount that is currently used for the 40 meter Moxon. I considered a polymer bushing, which I have in stock, but that is more difficult to clamp to the strut without crushing the polymer. I took the easy path rather than fuss with a more elaborate design to accommodate the polymer cylinder. 

The upper strut pivots on the rear bolt to the girt. The outer end is adjustable with a threaded rod that runs through the strut and a bracket bolted to the back girt. While not very visible in these pictures the design is almost identical to the one for the other rotatable side mount (I provided a link to that article above). By swinging the upper strut and sliding the rotator plate on the lower strut slots the mast can be vertically aligned and rotated without binding. 

On the right you can see the boom of the yagi still supported on the original plate that is attached to the tower with pinch clamps. A chain holds the boom to the tower. That becomes the pivot for swinging the boom over to the mast. A pulley and rope support the boom on the rotator side of the tower (centre photo). With the chain and rope lightly supporting the boom, the boom truss and the saddle clamps of the existing fixed side mount are removed. 

With the yagi's weight fully supported by the swing the mast is lifted out of the rotator with one hand and the boom is pushed to the outside of the mast with the other hand. Then the mast is dropped back into the rotator, as seen in the left photo. Although a second pair of hands would have come in handy it wasn't difficult to do.

While not too heavy, it was still awkward to lift and level the yagi to permit attachment of the boom to the mast clamp. The front rope was pulled to raise the boom up to the clamp and tied off to the tower. A second rope was used to lift the boom within the confines of the chain. There is no risk of the yagi escaping during the operation.

Unfortunately the saddle clamps that I selected didn't fit the 3" boom. That was quite a surprise! Since it was time for lunch I left the yagi to bounce in the wind and climbed down. On examining the clamps on the ground I discovered my mistake: I had selected clamps to fit 2-⅞" O.D. pipe. The u-bolts fit but not the saddles.

Since I was out by a small amount I tried to fit the u-bolts through the 3" saddles taken from the old fixed mounting system. To my relief they fit. After lunch I took the mix of u-bolts and saddles up the tower and completed the installation of the yagi to the mast. I was lucky that I didn't need to machine a new mast clamp to accommodate the 3" saddle clamps.

With the yagi attached to the mast it was time to quit for the day. The rain clouds were getting very close. I kept the antenna pointed at Europe and reattached the coax. The clamps were not too tight so the yagi swung on the mast a bit in the high winds that accompanied the rainstorm. It was almost a week until the cold, wind and rain subsided and I could resume work. The remaining tasks included: boom truss, protective rubber bumpers on the tower, coax rotation loop, and alignment with the upper yagi for maximum stacking gain.

Wiring of the rotator was completed last fall when the struts were installed -- this and several other projects were supposed to have been completed before winter struck us early and more severely than expected. The motor phasing capacitor is encased in UV-resistant plastic and a barrier strip matches the 8 wires required for Hy-Gain rotators. 

Wire #1 for ground/common (on the left) is connected to the tower, thus eliminating one of the 8 wires. The phasing capacitor is connected to #4 and #8 to eliminate the need for those wires. Cat5 cable is used for the direction pot wires #3 and #7. The high current wires for the brake (#2) and the motor windings (#5 and #6) are 14/2 electrical cable. 

This is my preferred wiring method for Hy-Gain rotators with long cable runs, well over 100 meters in this case. It gets the job done for the least cost. Copper isn't cheap. Extra cables to support future projects such as this one were thrown into the trench when it was dug during the early days of COVID.

I am still experimenting with the rotation loop, as I will explain shortly. The photos show its current configuration. When the phasing lines from the stack switch were made I included a few feet of slack to ensure adequate length for routing around obstacles and other unforeseen issues. For the lower 15 meter yagi there is approximately 3' (1 m) extra, with most of that coiled at the stack switch.

To make a sufficiently long rotation loop the full length of the LMR400 phasing line was pulled down through the cable ties. Since this is semi-rigid coax the amount of flex during rotation must be limited. The present configuration accomplishes this. It took a few tries to get it tracking properly and ensuring that there was very little stress on the cable.. 

Note: In these and a few other photos you'll see a few seemingly out of place wires and ropes. Those connect and support the two halves of the boom truss during the project. It's no fun having to lean far out from the tower to retrieve a dangling cable and turnbuckle that can easily escape from one's hand. For most of my yagis I tie a short length of rope between the two turnbuckles in case of turnbuckle or cable failure. It's cheap insurance.

The yagi in the previous pictures is pointed at a bearing of approximately 45°. The yagi further down the tower is the lower yagi of the 20 meter stack pointing to Europe at a fixed bearing of about 50°. Due to the offset of the rotator the clockwise stop for the 15 meter yagi is approximately 55°. In the photo immediately above, the yagi is at its counter-clockwise stop, pointing almost west at about 275°. This is the rear of the yagi (facing east) with first director adjacent to the tower. The rubber bumper protects the tower and boom when the operator over-rotates the yagi. The Ham-IV has limited torque and so is easily stopped when it hits a solid obstacle without the risk of damage to the yagi or tower.

Yagi de-tuning with the director so close to the tower is minimal. Coupling interaction is minimum at element centre and maximum when near the element tips. There is in fact more interaction as the yagi is rotated towards north. The measured SWR barely budges. Gain is difficult to measure or model due to the structure of the lattice tower. Modelling that I've done in the past of similar tight coupling is encouraging so I choose not to worry about it.

Of greater concern is stacking gain over the rotation range. Previously, the stacking gain was optimized with the lower yagi fixed towards Europe. Since the rotator is offset from the tower centre, where the upper yagi is centred, the yagis can only be in phase in one direction. In all other directions the feed points are not vertically aligned. For the rotatable side mount the phases are also aligned when both yagi point to Europe. The question is then how much the stacking gain is reduced in other directions due to the phase error.

The top-down diagram demonstrates how the analysis was done. The red dot in the triangular tower's centre shows the position of the upper yagis' mast and rotation centre. The blue dot is the lower yagi's rotation centre. The red dot adjacent to the tower is the 0° phase point that is in alignment (main lobes in phase) with the upper yagi when both yagis point to Europe (northeast).

The circle shows how the phase centre of the lower yagi moves in relation to the tower and upper yagi as the rotator is turned. The phase offset is quite small when the lower yagi points north. Modelling shows that the gain reduction is less than 0.1 db. That's negligible. 

The misalignment becomes quite large when the lower yagi points west. The offset is approximately 5' (150 cm) or roughly 38° at 21 MHz; 150 cm is 0.105λ for the 14.3 meter wavelength; the phase calculation is approximate, not exact.

The elevation plots are overlaid to show the effect. Since the resolution is limited, making it difficult to see small details, I'll mention that main lobe's gain is reduced by 0.5 db. That's not bad. The greater impact is that the nulls between the multiple elevation lobes are shallower, pretty well disappearing at high elevation angles. In a contest that can be helpful since fewer signals will be attenuated when the arrival angle falls into one of those nulls. It is inconvenient to fiddle with the stack switch (BIP, lower and upper by mouse control) for every contact. 

While not shown, the lateral offset of the booms has negligible impact. This is unsurprising since the vertical alignment of the boom (when pointed in the same direction) has little to no impact on the formation of the far field pattern. I took a few minutes to confirm it in the model.

The lower yagi could be slid backward to improve phase alignment to the west. However that would unbalance the wind load, require adjustment of the boom truss, and require greater extension of the coax rotation loop. The last will be difficult. For my style of operating the improvement is not worth the effort. It is rare that I'd benefit from stacking gain to the west. When I do I can live with the 0.5 db deficit. 

Another potential benefit of sliding the boom back is that the first director would be farther from tower when the yagi point west (see above). On the other hand, the front of the yagi (third director) would come closer to the upper guy that is due west, as shown in the adjacent photo. The top guys descend at a sharp angle and this is a large yagi. That interaction could be worse than the one between the first director and tower.

As I finish writing this article there is still some tidying up to do. The most important is the forward boom truss for which the cable isn't long enough to reach the mast. Pieces of the turnbuckle unscrewed and fell down over the winter months and I haven't found them all. 

I am also contemplating replacing the aluminum mast with steel. While not serious, it abrades a little in the rotator mast clamp and bushing. I'll check it again before winter and decide whether to replace it.

Other than these minor items the rotatable side mount is done and I've already tested it to Asia on air. The results are encouraging. As with any combination of antennas like this, especially so they're so high relative to wavelength, there are times when the lower yagi is superior. The reason is that with many elevation lobes there is an increased risk of a signal's arrival angle falling into a pattern null. The more expansive stack arrangement gives me greater ability to deal with those signals.