|Ready to rock 'n roll on the high bands|
The antenna in its final mounted position is show in the photo at right. The antenna is a Hy-gain Explorer 14. It's slightly larger than the type of antenna I had planned on putting on this tower. It weighs 45 lb (20 kg) and has a boom length of ~14 ft (4.3 meters). It can be lifted and manoeuvered by one person with some care in planning and safety procedures.
Considering its benefits of higher-performance and broadband match over a fully-trapped 3-element tri-bander I was willing to go for it. There is also the matter of chance in that I had a limited shopping period which limited my selection to what came available on the used market.
Super-stations have the art of lifting large yagis onto high towers down to an almost routine procedure. Some sort of tram is required to lift and guide yagis around and over guy wires. Trams, though simple in concept, can be a nightmare to build and operate by the inexperienced. The alternative of lifting the yagi in pieces is rarely recommended since it can be a difficult and dangerous procedure, even for small yagis.
A tram was required to lift this yagi. That the tower and antenna are small is no escape from this requirement. I had several planning choices to make.
- Element or boom first: In most tram systems the antenna is lifted with the elements pointed toward the tower top. This is necessary since the elements must fit around the tower and mast. Pointing the boom toward the tower is mechanically easier since the yagi bottom can rest on the ground along the back element and no cradle is needed to protect the elements from ground contact. But unless the yagi is lifted over the tower and mast it isn't possible for the yagi to approach the tower this way (you may need to visualize this in 3D if you are unfamiliar with the procedure).
- Starting position: With 3 guys there are 3 areas between guys from which to lift the yagi. The best one is free of overhead obstructions and has suitable anchor points for the rigging.
- Pulley and rope system: The pulley should be above the point where the yagi will be mounted, and leave enough room for the boom sling between mounting point and pulley. As with a gin pole it is necessary to consider the lateral and vertical forces on the tower, mast, pulley and rope since, during the lift, the yagi will be hanging some distance from the tower. A way to quickly and safely lock the lift rope is highly recommended.
|Yagi to be lifted through this gap. Rigging visible at tower top.|
Since the boom is 14' long and the elements are as long as 31' I made the sensible choice of lifting the yagi with the elements pointing up. There is less potential of tangling with trees and guys wires. The stress on the rig and tower is reduced since the yagi doesn't have to be moved out too far (laterally) from the tower. Unlike with the simple gin pole the tower, mast, rotator and northward guys are sufficiently strong to resist the lateral force of the offset yagi's weight due to the tram.
|Complete rig with tree limb obstacles|
First, I apologize that there is a dearth of pictures to show the steps in the lifting procedure. I had planned to take more. But when it came to take a picture or deal with the serious matter at hand I chose the latter. So I'll explain using a diagram superimposed on the picture of the rigging I designed and constructed.
The rig (or tram, if you prefer) is comprised of two ropes secured to the tower top (see picture above) and the ground. The ground attachment is a rope with anchor loops running between the trees I use for guy anchors. It is designed so that it is can be easily collapsed for the early steps of the procedure and then pulled tight to support the antenna. It is in the shape of a narrow triangle: 15 meters high and 5 meters wide at the bottom. This is similar to the type of tram that Hy-gain describes in some of their literature.
Step 1: The fully-assembled antenna is placed at the bottom of the tower. It is upside-down and the boom clamp (blue) faces outward. The tower goes in the widest gap between elements, in this case the driven element and reflectors. The rigging at this point is fully collapsed, with ropes stowed under the antenna.
Step 2: The antenna is lifted to a height that just exceeds the length of the longest half-element, or about 16' (5 meters).
Step 3: Climb up the short distance to the boom and rotate the antenna (counter-clockwise in the diagram) until the elements are not quite pointed straight up. This is to ensure the elements do not get inside the guy wires when the antenna is lifted.
At this point one of my neighbours from down the street came running over. He wanted to know if I needed help to lift the yagi. I chatted with him for a while, explaining the procedure and why it would best if he just watched. My plan was solid and would only be complicated by inexperienced hands. I do not want an injured neighbour on my conscience. For the most part my neighbours are neutral to actually supportive of my hobby, which might be unusual!
Step 4: The rigging is reassembled and pulled to not-quite full tension, pulling the antenna outward from the tower. I ensured that one side of the tram was inside the 10 meters reflector and the other side between the director and driven element. A couple of branches of the large spruce (visible in the picture part of the diagram) had to be guided around the reflectors and boom. This involved a combination of bending the branches and pushing the antenna around them.
Step 5 (not on the diagram): Lift the antenna until the upper end of the elements fully clears the lower set of guys (guy station at 23' height).
Step 6 (not on the diagram): Pull the rigging to its full tension and continue lifting to the top of the tram. The added tension is needed for this step since the tram is closer to the upper guys than the lower guys. The bottom halves of the antenna's outer elements ride over the upper guys as well as the tram in this step.
|The yagi left dangling while I took a lunch break|
It wasn't only hunger that drove me to take a break. All that work will tire anyone. As a rule I will not approach a tower or other demanding work without being physically and mentally alert. An hour's break made sure that I was ready for the next set of tasks.
As detailed below in "what went wrong" there were some unanticipated tasks to be done. Although it was annoying I did leave ample time in the schedule for that sort of thing, and I knew the fair weather would continue throughout the day.
The remaining tasks are in point form, since there are so many of them (and perhaps more than you care to read about):
- Rotate the yagi back to horizontal, but this time right side up. This was easily accomplished now that the yagi was well above every obstruction.
- Position the boom-to-mast clamp and tighten it down. I had placed the clamp on the mast some days earlier so, again, this was quick.
- The 4 bolts to secure the boom clamp to the boom-to-mast clamp (threads pre-greased) were installed. The bolts are Grade 5, not the weaker stainless ones that came with the antenna. This took a little longer since the clamp faces had to be almost perfectly parallel and the holes lined up to drive the 5" bolts through. The most difficult was the second bolt. After that they went in easy.
- Lift the coax and temporarily hook it up to the antenna. Antenna direction was set towards Europe.
- Test the antenna in the shack across all 3 bands. I made a few QSOs, including an OD5 with a small pile-up on 15 SSB. SWR and gain comparison with the inverted vee were good.
- Seal the coax connector and wire lugs on the beta match and driven element.
- Increase slack in the coax in preparation for lifting the yagi. At this time the coax is held up by only its lift rope.
- Untie the boom sling and remove the pulley from the mast.
- Loosen the boom-to-mast clamp and push the antenna up to its planned height. It is bolted tight without (yet) worrying about its direction.
- Fit the plastic cap to the top of the mast to keep water from getting inside of the mast.
- Loosen the rotator clamp and turn the antenna north, which is the centre of rotation and where I'd earlier positioned it. Tighten the clamp and test by pulling/pushing on the boom.
- Untie and drop all ropes (except the one holding up the coax) to the ground.
What went wrong
Can you admit an error? Acknowledging and investigating mistakes is at the very core of achieving future improvement. This is done when a airplane crashes, a train explodes or someone is injured or killed in a work-related accident. Tower work is no different. I made some mistakes and underestimated the difficulties with some steps in the antenna-raising procedure. Let's use my experience as a learning exercise.
- Rigging: The rope rigging was constructed the day before antenna raising. It was all carefully measured and tested, ready for the big day. I then separated the sections to stow the ropes out of the way for safety. The next day I put it all back together (see Step 4 above), paying special attention to the marks I made on the rope to properly place them. But I didn't check the reconstituted rigging before proceeding to lift the yagi. It was only afterward that I noticed the rigging was laterally offset by 1.5 meters. I had misinterpretted one of the rope marks. That explained why the yagi kept skewing to one side (and increased tangling with tree branches) during lifting. Luckily this mistake didn't cause any serious difficulties beyond costing some time.
- Boom sling: In the picture of the feed system below you can see the thick rope of the boom sling. The sling must allow easy rotation of the boom during the lifting operation yet not encourage unintentional rotation. My first sling used thinner rope. When I lifted the antenna (see Step 3 above) the thin rope could not be easily manipulated. Down went the yagi and on went the new sling with thicker rope. The second one worked fine.
- Snags: To avoid snagging of antenna hardware (especially tube clamps) with the rope rigging or guys I made sure that all the hardware on the underside of the antenna (the side that would rest on those lines) was rotated to avoid snags. For example, the open end of hose clamps. Snags occurred despite my precaution. The pressure of the antenna on those lines made it possible for even the tapered trap end caps to snag. The 1" of boom protruding past the outside elements and those element-to-boom clamps also snagged on the rigging, guys and tree branches. This cost me time but no serious problems. The heavier the antenna the worse the problem of snags.
- Boom bolts: The boom is in two pieces which are secured in the boom clamp (see the picture below). The bolts of the clamp, and the boom-to-mast clamp squeeze the boom sections to hold them tight. There is also one bolt per boom half that pierces the boom and clamp for added security. You can see the head of one of these bolts peeking around the left side of the boom-to-mast clamp. I installed these on the ground with the heads on the other side of the boom. The bottoms of the bolts protruded far enough to prevent connection of the boom-to-mast clamp. I had to remove these tight-fitting bolts at the top of the tower and reverse their direction. That was not fun. I should have checked this on the ground.
- Tilt: When mounted and tightened onto the mast the elements were not horizontal, even though they were all in the same plane. The tilt was only a few degrees, which although it does not affect performance it is unsightly. I wanted the cleanest appearance so that neighbours would wonder why the antenna was crooked. Again, I should have checked this on the ground. I added a few washers between the boom clamp and boom-to-mast clamp to correct the problem.
- Final lift: After completing antenna testing I temporarily secured the antenna to the mast. I then removed the boom sling and the mast pulley. I then needed to lift the antenna ~50 cm (20") to its final position at just under 15 meters above ground. This small difference has negligible performance impact but does minimize interaction with the top ends of (future) low-band wires antennas. The 45 lb antenna is easy to lift on the ground and even up the tower. It is less easy when you are perched at the top of the tower and somehow have to push it upward, hold it in position and tighten the bolts of the boom-to-mast clamp. The pulley and sling had to be removed since there was no room for them in this operation. It took some minutes but I finally found a way to brace my feet, bend my knees and push it up with my shoulder. Once lifted in this way it was (relatively) easy to tighten the bolts. It worked but was too ad hoc for my liking. In the past I have usually worked on larger towers on which this procedure is easier.
- Cable dressing: The coax is dressed to the boom and tower with a combination of vinyl tape and UV-resistant cable ties. The ties are for security and the tape is to spread the pressure and weight of the dressing to avoid crimping the coax. The cable ties were too short for the task. I corrected the error by chaining together two cable ties.
The stock Explorer 14 comes with a BN86 balun: coax goes in one end and the other (balanced) end connects to the driven element and beta match. But this is a voltage balun which is not the best choice to choke common mode current on the coax outer shield.
I removed the BN86 and made a not-quite-as-bad choke by coiling the coax. There are 9 turns of 4.25" diameter. It is an acceptable choke though not ideal for the 20-15-10 range.
|Modified feed for the Explorer 14 yagi|
The picture shows the incomplete feed system atop the tower, temporarily connected for on-air testing. The rigging is still attached since it would be needed if a problem required the antenna to be moved. It worked fine. I completed the feed system by installing a plastic block (one side concave) under the aluminum "L" with the SO-239 (quickly made in my workshop) and securing it to the boom. This is necessary to isolate the coax from the boom to ensure good electrical balance.
The rubber boot (a discarded bicycle tube) over the PL-259 is for weather protection. Unlike most yagis the Explorer 14 has the driven element closer to the director than reflector so the yagi points to the right of the picture. The slack section of coax to allow rotation (sometimes called a drip loop) is placed to the left of the boom-to-mast clamp to keep it away from any antenna elements.
The benefit of the yagi over the inverted vee was immediately apparent. I plan to spend some time comparing them over different band and paths so that I can better quantify their respective attributes. Small stations don't often have the ability to compare antennas so this is an opportunity to learn. What follow are my observations from the first 48 hours with the yagi.
Seemingly quiet bands like 10 and 15 came alive with signals with the yagi. In a way it's subtle and sneaks up on you. It's like travelling from the city to country at night and suddenly the sky is full of stars. In particular 10 meters became a far more interesting band. Europeans poured in each morning. It took some time but I eventually cracked the ZD9XF pile-up on 10 CW. On the inverted vee he was barely audible over the local QRN, and I wouldn't have bothered with my 10 watt signal. Polar paths were poor the last two days but from the signals I did hear it bodes well.
SWR across the bands is mostly very good. It is low in the CW segments and slightly higher at the top of the bands, which is the opposite of what the manufacturer specifies. On 15 the SWR rises well above 2 higher than 21.3 MHz but still works well. SWR is below 2 across 20 and 10. Gain seems good everywhere but is difficult to quantify, as gain always is. F/B ranged from good to fair.
I am looking toward contests and future DX opportunities with tremendous optimism. I should be able to improve my contest placing in the QRP category. DX pile-ups suddenly seem far more crackable. The neighbours for the most part seem comfortable with the look of the tower plus yagi, which is a welcome bonus.
Next come the low-band wires for 40 and 80 which I'd like to get in place over the next 3 weeks.