Yagi on a Swing

Repairing yagis on the tower is not always easy. The parts you need to access may be far out of reach. With some well thought out choreography it is sometimes possible to twist and turn the yagi to bring the problem area within reach of the tower. In most cases you'll need to lower the antenna to the ground to work on it. That is often simpler and safer, at the expense of a larger time investment.

I decided to experiment with an alternative. It's one that few hams would ever need since it involves towers with multiple side mounted yagis. In this instance the purpose was to repair the XM240 40 meter yagi. Last fall the antenna was moved to one of the large towers and mounted on the rotatable side mount at ~75'. It replaced the TH6 which was raised a little higher and fixed to the south.

A mistake was made when the XM240 was reassembled for the tram ride to its new home and not noticed until a few weeks had passed. The boom clamp for the reflector element wasn't properly tightened. In high winds the element would rotate on the boom. 

A small amount of rotation is not a problem but a large rotation would not only affect its performance, it would not be rotatable since the element would collide with the TH6 above it. Damage would result since element tips are fragile. 

After an ice storm in early winter the element rotated so far that the tip was above the plane of the TH6 elements. Luckily there was no contact due to the direction the XM240 was pointed at the time. A week later a strong wind levelled it again! However I couldn't risk rotating the XM240 without first glancing out the window. That's difficult to do at night when 40 meters is open!

The XM240 boom cannot be tilted to access the reflector because of the TH6 above it. To access the reflector the yagi must be slid horizontally almost 10' (3 m). That's actually possible because there's a lot of tower above the two yagis. What I did was to build a "swing" for the yagi. The diagram shows the general idea. It is suspended from an anchor almost 40' further up the tower.

It isn't quite as simple as shown in the diagram for the following reasons:

• 
  The anchor should be at least 3× the height above the yagi as the horizontal swing distance. Otherwise the large swing angle will lift the antenna into an upward arc and the force needed to swing the yagi will be excessive.
• There can be no elements between the tower and the element being accessed since it will be blocked by the tower.
• The elements of the yagis between the anchor and the swinging yagi must be approximately parallel to the boom of the swinging yagi or the swing will strike the elements of those yagis.
• The swing can only work in one direction so it must be positioned on the side of the tower where the yagi will be swung outward.

I was fortunate that all of these constraints could be dealt with for the XM240. Pointing it west let the swing move between the elements of the TH6 and the lower yagi of the 10 meter stack. This orientation also reduced the swing distance to less than 8' (2.5 m) since the reflector is on the "back" side of the tower from the rotator and short mast. The support strut and legs of the 80 meter inverted vee legs are behind the swing and therefore not in the way.

In the picture you can see the TH6 above the XM240 and the 5-element 10 meter yagi up higher. A rope is dropped from an anchor a little above the 10 meter yagi to the driven element side of the XM240 mast clamp. My intent was for the boom to tilt down slightly on the backside to ease the repair and to help avoid bumping the TH6. The rope runs over obstacles on the side of the tower where the swing will occur. It is vital to avoid tangling the swing rope in any obstructions.

There are several precautions to keep in mind:

• 
  Do not tie the rope to the tower directly since the tower steel will cut into the softer rope material. The risk of failure is low for a rope in good condition but must still be avoided since failure can be catastrophic. I use a large shackle that has a smooth round surface for the rope to bear on. Loop the rope on the shackle several times to reduce tension on the knot. It will be much easier to remove later.
• Consider using steel cable instead of rope. Rope can be very elastic and it can stretch a lot over a long distance, making it difficult to tension properly. It can relax further while under load. Having the yagi come to rest below the mast and rotator can be a problem during the swing operation and when the time comes to remount the yagi on the mast. I used a large turnbuckle to add tension after tying off the rope and it wasn't enough. More on this later.
• Use a second method to support the antenna. Redundancy reduces risk and prevents the boom from tilting up or down too far when the rope is not attached at exactly the centre of gravity.

I installed the swing in late February when we had a period of unusually warm weather. Life then became messy for me and I abandoned most projects for several months. In mid-June I checked the swing and made adjustments in preparation for the operation. 

The boom truss was removed in February. There were high winds over the next 4 months but no ice storms. Boom trusses are partly aesthetic and partly structural support. For many medium size yagis like this don't fret over a brief absence. This XM240 is 10 pounds heavier than the original since the boom and elements were structurally upgraded by the previous owner.

We are looking toward to the reflector end of the antenna, the end I needed to access. A tow strap and shackle are loosely fit over the boom as the second restraint and to help guide the boom as the yagi is swung. The boom truss cable runs over, not within, the restraint.

Disconnect the coax, if necessary. I had enough looped up that it wasn't necessary, which saved me the trouble of removing the weatherproofing. All I had to do was cut the tape and cable ties securing it to the tower. The boom truss must be removed from the mast. Place both where they will not interfere with the swing or become entangled. I left the truss cables attached to the mast plate and let the whole thing dangle below the antenna. If the cables must be separated, first tie them together so that you don't drop one accidentally and leave you with a truss cable dangling far out of reach.

With all the preparation done it was time to pull the bolt from the boom-to-mast clamp. I preferred to remove the mast bolts (saddle clamps) rather than those for the boom saddle so that the clamp plate remained on the boom. That made it easier to me to operate the swing. With the clamps removed, the boom came to rest several inches lower as the long swing rope stretched under tension. It leaned slightly on the reflector side just as I had planned. The boom restraint easily held it.

My original plan was to attach a long rope to the boom to be pulled by a helper on the ground. I scrapped that because the season is well advanced and the hay is high. It isn't fair to ask someone to risk the ticks by wading through the tall plants. I decided to do the job myself.

A lightweight rope is used to pull the boom. The swing is long so the force needed to move the boom is very managable by hand. The nylon rope is loosely looped several times around the boom and knotted. This is a common technique in tower work since it can be used to lift heavy pipe and perform other tasks where there is no available tie point. The rope can be easily slid along the boom in short steps as the antenna swings outward. You never have to reach out more than a foot or two during the entire operation.

Since I was doing this myself, I wrapped the pull rope around the mast for friction. It's like having a third hand as I manipulating the rope and boom and when I periodically tied the rope to the tower. I pulled the boom with one hand and the rope with the other. I would hold the boom while I slipped the knot backward as the swing progressed. The only serious obstacles were the splice bolts connecting boom sections. To get the knot past the bolts I held the boom with one hand to reduce tension on the rope while I lifted each loop of rope over the bolt heads and nuts.

This is what it looks like when the swing is at its maximum extension. It looks impressive from the ground (and on the tower!) since this is, after all, a 40 meter yagi that weighs 75 pounds with a 22' boom and 43' elements. The antenna isn't sitting flat due to the swing attachment point on the boom-to-mast clamp plate. The element tips are well out of the way of the TH6 elements above.

Once the reflector is accessible from the tower the pull rope is tied off. Only a minute was needed to tighten the element clamp bolts; they were really loose! I took another minute to move the boom truss clamp a few inches since it wasn't where it ought to be. At the time the antenna was raised last fall it was easier to add a second mast clamp than to tram the antenna back down to remedy the error. I knew exactly where to place it since there are small indentations on the boom from when it was properly positioned.

When swinging the antenna back to its usual position, take care not to let out the pull rope too quickly. You must do it in small steps, just as before, or the knot will end up out of reach. You will only be able to retrieve it by swinging the antenna out again. Avoid the temptation to rush the job. As the boom plays out, slide the knot backward, always keeping it within reach, easing it over the boom splice bolts as before.

As I hinted earlier, the rope had stretched several inches from the tension of supporting the antenna. That doesn't hurt the rope but it made the antenna difficult to remount on the mast. It wasn't possible to lift it the extra distance with one hand and install the bolts with the other. By then I was hungry so I added another rope to hold it in place and descended. An hour later I returned with a hand winch and attachments. Within minutes I had the boom winched upward and the bolts were reinstalled.

I cleaned the antenna of all the swing hardware and hung all of it on my climbing harness. The boom truss was installed and the antenna levelled, and then the coax rotation loop was returned to its previous state. I temporarily loosened the rotator mast clamp to confirm that the coax was properly dressed by turning the antenna through its full rotation. That done, I climbed up to retrieve the swing rope and anchor and, with the aid of gravity, brought all of the equipment down with me. I'm lucky that despite my age I remain strong enough to do this style of brute force work.

After stowing my gear I tested the antenna and rotator and put a check mark next to one more item on my long to-do list. There is much more to be accomplished this year.

In a way this was an unnecessary job. With contest season over and progress on my planned replacement for the XM240, it won't see much use for the next few months. I did it since the rigging was already in place and I wanted to confirm that the procedure would work as planned. I had also promised the station to a guest operator for one of the summer contests and I didn't want to disappoint him with antennas out of service or of limited utility.

I'll close this article with a cute picture. After climbing the tower I looked down and noticed that I had a visitor. A young fawn was hidden in the tall hay and calmly feeding. It wasn't at all disturbed by all the steel on steel racket I was making. I guess it didn't see me on the ground, or realized that I couldn't see it, and deer are not in the habit of looking to the sky for predators. It happily grazed less than 40' from the tower base. Clicking on the picture should bring up a higher resolution version of it.

My recent blog output is less than my usual pace and that is likely to continue throughout the summer. There are many demands on my time at the moment. Enjoy the summer weather as you work on your antenna projects.

12 and 17 Meter Dual Band 2-element Yagi

I have no antennas for the WARC bands: 30, 17 and 12 meters. They've been low priority since I began building this station in 2016 because they are not contest bands. However, I also love DXing so I have never lost sight on this objective. I have been reasonably successful on those bands by using antennas for other bands, judiciously selected for their performance rather than match. You can do well with compromised antennas when they're up high, really high.

For 30 meters, I will be happy with a single element antenna, as long as it's resonant with a reasonably omni-directional pattern and does not interact with contest band antennas. I have a plan but I won't talk about that in this article. Yagis for 17 and 12 meters are desirable for their gain. That's the subject of this article.

My objectives:

• Efficient: no coils or other loading devices
• Gain and pattern: "good enough" performance across both bands
• Small and uncomplicated: I don't need or want to deal with of large and complex antennas for non-contest bands
  
• Match: low SWR without an elaborate matching network

A dual band 2-element yagi seems to meet my objectives. Although the gain bandwidth for any 2-element yagi (and that includes Moxons) is narrow, so are these 100 kHz wide bands. F/B is similarly adequate since high performance on that metric is not that important to me. Gain and match matter more to me, though you may have different requirements. Cebik noted how a 2-element yagi is suited for the WARC bands. There are commercial 2-element yagis -- examples one and two -- for those that prefer to buy rather than build.

Developing a model of the antenna is quite easy. I began with constant diameter elements since I was not yet decided on the physical design. I used EZNEC, as I usually do. The included NEC2 engine is adequate to model the antenna, and its built-in SDC (stepped diameter correction) will be useful when I settle on the physical element design. Since the DE (driven elements) will be close together, the segment alignment guidance in the EZNEC manual should be carefully studied.

There were several decisions that needed to be made even for a small antenna like this:

• Dual driven elements versus one driven element (coupled resonator for the other DE)
• 12 meter DE on the inside or outside of the 17 meter DE
• Matching network, if any

Yagis are peculiar antennas when you stop to think about it. They rely on opportunistic coupling to create beneficial currents (phase and amplitude) in parasitic elements to create the antenna pattern. Beyond a few elements the relationships are so complex that they defy analytic solutions. Optimized designs had to wait for fast computers and algorithms to power numerical analysis and optimization. Even with just two elements there is much insight to be gained by playing with computer models.

The parasitic elements of a yagi determine the performance and frequency range of operation. It may surprise some to learn that the DE doesn't play a significant role in that. The role of the DE is to effectively couple energy to (and from) the parasites and to set the feed point impedance. 

For a 2-element yagi we therefore need to first focus on the one parasitic element. It is usually chosen to be a reflector since that typically makes for a better antenna. However, the difference between it and the director version is modest. 

As an aside, it should be evident that since the parasite determines the pattern that in a 2-element yagi there is a "mirror image" frequency range where the reflector behaves as a director. For example, for the 12 meter 2-element yagi, the gain and F/B peak in the reverse direction at 23.85 MHz (shown above). That attribute can be ignored since that occurs outside of any amateur band and a DE designed for 12 meters won't be provide a good match at the lower frequency.

Returning to the practical design of a 2-element dual-band yagi, the necessary element arrangement imposes constraints. The first is that the two DE must be adjacent. This is because they are either driven by one transmission line or one of the DE (the smaller) is excited by coupling from the other DE. That adjacency determines the placement of the two reflector elements.

Spacing between the 17 meters elements is 2.8 meters (0.17 λ). The 12 meter reflector is 0.7 meters inboard of the 17 meter reflector. For this configuration -- there are alternatives that we'll discuss -- the 12 meter element spacing is 2.3 meters (0.19 λ). Boom length is 3 meters (10'). DE spacing is 20 cm.

The few commercial designs I've looked at place the 12 meter DE on the outside and make it a coupled resonator. It seems reasonable to me to shorten the boom by putting it on the inside and to slide the 12 meter reflector back a little to compensate. Dual driven elements, connected by a short transmission line, are not difficult and would seem to remove the problem of fine tuning the DE spacing and length for proper excitation on 12 meters; coupled resonators (they go by other names as well) can be finicky.

When I placed the 12 meter DE inside the 17 meter DE, although the gain and F/B were fine it was difficult to achieve a good 50 Ω SWR on 12 meters. This was true with both dual driven DE or the 12 meter DE as a coupled resonator. When placed on the outside of the 17 meter DE, a good match was easy to achieve with either feed method.  As already mentioned, the "found" attributes of yagis have to be worked with or worked around as determined by the mutual coupling of elements in any particular design.

When dual driven, a short transmission line connects one DE to the other. While it doesn't matter which is connected to the feed line there are two differences to consider. One is tuning. The short transmission line inductively loads the second DE because its nominal impedance is typically 150 Ω to 300 Ω. The DE at the far end of the line must be shortened to compensate. The second is that it is mechanically and electrically awkward to connect coax to the outer DE. You can see his in the Bencher (DXE) Skyhawk yagi where the coax has to loop backward to connect to the outer DE.

Since the coupled resonator version of the antenna works so well it is no surprise that the commercial designs chose a coupled resonator. Only one DE needs to be isolated from the boom, simplifying construction. I recommend spacers on each side of the DE to prevent the close-spaced DE from touching in windy conditions.

Achieving the excellent SWR in the model required careful adjustment of the DE spacing and the length of the 12 meter DE. In practice it will be necessary to make physical adjustments during construction and tuning. Although I achieved the best 12 meter SWR with a DE spacing of 20 cm, I strongly suspect that they will be closer after tuning.

I have deliberately avoided reporting the element lengths since that depends on tube diameters, lengths and steps, and the method of mounting them to the boom. If you are not comfortable with computer modelling you may be able to adjust lengths during initial tuning since the antenna isn't complex. However, modelling is strongly recommended since it can be extraordinarily accurate using EZNEC with NEC2, its built-in SDC, W6NL boom coupling adjustment and segment alignment of the close spaced DE as documented in the EZNEC manual. If and when I build this antenna, I will create a precise model of my chosen physical design.

It is worth noting that the SDC algorithm in EZNEC can only be applied with a narrow frequency range. When setting the frequency in the model to one band, the SDC cannot be done for elements of the other band. That restriction does not apply for the NEC4 and NEC5 engines. However, the SDC constraint has little effect on a dual-band yagi design of this type in which the non-resonant element lengths of the other 2 elements have a limited role. But be aware of the restriction when you model a multi-band antenna.

6.6 dbi free space gain for both bands is quite good. Although the theoretical maximum gain of a 2-element yagi exceeds 7 dbi it is very difficult to achieve in practice. It is more difficult for a dual-band design. F/B is not great but that is not unusual for a 2-element yagi since the frequencies of maximum gain and F/B do not coincide. Improved F/B is possible with nested Moxon rectangles at the cost of greater complexity. I don't require a high F/B so I am happy with these azimuth patterns.

I didn't bother to plot gain and F/B across the 100 kHz of both bands since there is so little variation. F/B varies about 2 db and gain less than 0.2 db. Tune these antennas at the centre of the bands and they will work well over the full bands. The same is true of the 30 meter band which is twice the wavelength but half the size (50 kHz).

Now we come to the big question: will I build it? My difficulty is less with the construction than where to put it. My towers are crowded and I want to avoid destruction interactions with my large contest band antennas. Side mounting is not an option since this directional antenna has to rotate. It will be necessary to develop models of it next to my other antennas to see where they may be problems. I am no willing to build and maintain another tower for this antenna.

Even without a placement plan I may still go ahead and build it. It's a small project and offers an opportunity to compare it to the model and see how it tunes and performs. Until then, I will continue to use my non-WARC band antennas on the WARC bands

Age and Delayed Gratification

One of the measures of maturity is the ability to delay gratification. When a small child wants something, he or she typically wants it immediately. Denial can lead to tantrums, sulking and similar behaviour that we label childish. As we mature into adults we learn to accept delay while never losing sight of our objectives. Adults plan.

As the amateur radio population ages, I've seen how delayed gratification can work against us. Children have their whole lives ahead of them so they can afford to wait for what they desire. Yet they often won't. On the other side of life, when we're old, waiting can too easily result in permanently lost opportunities; delayed gratification, which once served us well, becomes a curse.

Pink Floyd said it well in their song "Time":

The sun is the same in a relative way, but you're older
Shorter of breath and one day closer to death

If you're an older ham, are you putting off replacing that old rig with a far better new one? How about that Windows 7 computer, or with an even older operating system? When that ancient yagi is missing elements, hardware is rusting and the coax became water contaminated years ago, do you hesitate to replace it?

Why?

I see this a lot since I do tower work for others and visit many of their shacks. Whether they are hams I've known for decades or they are recent acquaintances, they have a ready excuse. Here are a few of them:

• "Well, it's kind of okay. I jump into the pile ups and I get through eventually with the amp."
• "You get used to all that noise (or distortion or ringing). It doesn't bother me much any more."
• "I don't want to bother anybody to climb the tower (or up to the roof)."
• "I don't keep up with all the new technology and products so I never know what to buy." 
• "It seems like a lot of money. Can it really worth it? I'll think about it some more."
• "Yeah, there's lots of DX on the digital modes, but I'm too old to learn that computer stuff."
  
• "Why bother? I could drop dead tomorrow."

I bet that many of you could easily extend the list. I've heard so many spurious justifications for delay. I've used a few of them myself!

There is a modern saying that provides the antidote: YOLO -- you only live once.

No matter how much time you have left, why deprive yourself? Except in rare situations, none of us can know how long we have, whether you're 20 or 80. There is no advantage is deciding that it's all over and engage in acts of self-deprivation. There is always something new to learn or experience. Don't forgo the many available opportunities.

Indeed, I argue that the older you are the more you must pursue your dreams. Make a ham radio bucket list and get started. Do it today. The young have time to delay gratification. The old no longer have that luxury. The urgency should be escalated.

Dispose of the ancient and dusty stuff now -- don't saddle your heirs with it. Buy new and better equipment. There really is a difference!. As much as we complain about prices and complexity, the quality and performance of electronics and antennas is superior to what fills so many shacks. Nostalgia is nice but it doesn't do much for us on the air when we dive into the pile ups. The shack will be tidier, the operating more enjoyable, and the new equipment will fetch a good price for the estate when the time comes.

Yet there are obstacles. Just make sure they are approached as obstacles and not as excuses. 

• Money: You likely have the money, or at least enough to outfit the shack with new equipment. Perhaps you believe that you are spending the inheritance you plan will leave, or that you've developed a habit of thriftiness, and so you won't spend what you can. Reconsider. A few thousand dollars is minuscule in comparison to the large assets you will bequest, real estate in particular, and the resale value can be very good.
  
• Labour: When it comes to tower and antenna work, there are options if you are no longer able to do it on your own. Almost every club has members who will do at least small tower jobs for others -- hams helping hams. At worst, hire a rigger with experience working for hams. The expense is modest.
  
• Learning: Old dogs can learn new tricks, if you persuade them to get them off the couch. We tend to relax and let life wash over us as we age even though we are perfectly capable of mentally and physically exerting ourselves. You can learn to use a PC, digital modes, novel features of modern equipment and sophisticated test equipment, and so much more. All you have to do is, well, do it.
  
• Overreach: Indulge, yes, but don't go so far that you create a burden for yourself and others. It would be difficult to overdo by upgrading the shack, but so easy once you plant towers. You don't need to build a big station like mine to have a lot of fun and operating success. Don't alarm your family and neighbours by putting up multiple 100'+ towers and large yagis. One reasonably high tower, a multi-band yagi and low-band wires can do wonders. It will not be expensive to dismantle when the time comes. My towers will almost certainly be cut down to save the expense. My large property makes this possible.
  
• Cost-benefit analysis: A common objection to investing further in the hobby is that there may be little time to enjoy the investment. How can you know? We can die tomorrow or not for many years. The assumption of impending demise (fatalism) is merely an excuse to deny ourselves. If it's only a year, well, who cares? You won't be around to do the final sums, but you will have had a year of enjoyment. Fatalistic estimates should not unreasonably constrain your choices.
  
• Burden on survivors: I have a friend who incrementally downsized his antenna systems as he aged. When he was longer confident doing tower work he took down his tower. When he could no longer climb onto the roof, he resorted to ground mounted antennas. He went QRT when that became a maintenance difficulty. He was more comfortable doing it that way even though I offered to take down his tower when the time came. I wanted him to enjoy the hobby for as long as possible. Make a plan and you can eliminate the worry of leaving behind a burden for your survivors.
  

Have I motivated you to act? Sadly, probably not. That's been my experience from the many conversations I've had with elderly hams of my acquaintance. It can seem a major undertaking at a time of life when we increasingly spend our time looking to the past rather than ahead.

I think that's a shame. You can all too easily rationalize delaying gratification beyond the point of no return. Try this: replace that old equipment with a modern digital transceiver. It isn't all that much money. There's nothing easier you can do to appreciate the value of looking towards the future with optimism. Don't deny yourself.