Saturday, December 31, 2022

Anniversary and a Peek Ahead

The anniversary snuck up on me. It was almost exactly 10 years ago that I returned to amateur radio after being away from it for 20 years. I gave a few hints about how that happened in the very first article in this blog, which was published in February of 2013. There was more to it than what I wrote. No matter the circumstances, I am happy that I did come back.

The Elecraft KX3 that I assembled exactly 10 years ago, between Christmas and New Year's Day, was sold a few years ago. I had moved on and it no longer fit my operating style. Were you to peruse those early articles you'd notice how focussed I was on small stations and antennas, and how to get the most out of both. That's just who I am. Soon the DXCC countries piled up and then the reentry to contests.

With the resurgence of my radio ambitions, QRP no longer satisfied my appetite. I increased power and the antennas got bigger. A few years later I quit city life and moved to a rural QTH to give full rein to my ambitions. Although I certainly have gone far beyond what most hams would ever want to do, it filled a need in my soul. 

I am learning new things and achieving objectives I only dreamed about when I first ventured onto the air in 1972. I try to share that knowledge and enthusiasm in this blog. More importantly, I'm having tremendous fun. This is despite the many challenges of building and maintaining a station of this size. I continue to enjoy blogging so expect the pace of articles to be maintained.

The 530 blog articles (rate of one per week) since then follow my path and progression. That was my intent. Many hams make their sites a technical reference, updating and adding pages as they go. Mine is about the narrative: this is the story of my return and journey through the hobby. I let readers rely on the search function to find specific projects and other items of interest. The narrative doesn't end until I do.

I thought I'd end 2022 with an overview of what's in my station work pipeline. All of us have projects we're working on, and for me, at this time of year, most are indoors activities. Not all, but most. Many of these projects will get their own articles in the new year. This preview may be of interest to regular readers, or not. Whatever the case, here we go.

Station automation: first light

After several weeks delay I returned to software development for my home brew station automation system just before Christmas. This time I could put it to the test since I had the first phase of the switching system hardware assembled. For this first time the system would select antennas on band changes and alternative antennas for each band. I called this achievement "first light" in the same way a new telescope is put into service.

It's ugly but perfectly functional. Keep in mind that the switching system will be placed under the operating desk, out of sight. You'll hear relays clacking but that's all. In this picture the 20 meter stack is selected on radio 1. The second radio was not connected for the first test, and testing for that, and contention between radios was successfully tested shortly thereafter.

The first layer of relays controls the remote 2×8 antenna switch. I went with the same cabling as the manual switch for easy fallback in case of disasters. Since the picture was taken another layer of relays was added and is working. Despite the software being incomplete, the system is functional. The manual controller (seen in the background) handles functions not yet transferred to the new system. 

Cabling and connectors are a challenge but not nearly so bad as for the aborted design. The openness of the hardware eases maintenance and configuration changes. I hope to have the full system operational by mid-January. After it's complete and with at least one contest's worth of experience, expect an article on the project.

Operating desk

My current operating desk is very nice but not suited to effective multi-op contesting. Since that's my objective I built a new desk. This project was put on ice for quite some time when the pandemic ruled out multi-op contests. I am hoping to get to do one of those, with the new desk, later this winter.

The frame with the top removed (for painting) is shown at right. There's lots of legroom and shelving for peripheral equipment. The main support is at the rear where the heavy equipment, like the amplifiers, will be placed.

Installation was planned for the holidays but has been delayed until January. With so much progress on the automation system I didn't want to interrupt that project with several days of cabling and wiring work during the transition.

Related to this project is the purchase of PC for the second multi-op station. Only one PC is used for SO2R. It's a small desktop PC with an SSD and just 3 USB ports. It's lightweight, physically small and ideal for running N1MM. It has been set aside until the new desk is ready.

Beverage maintenance

The weather has been severe of late, with winds of 100 kph, heavy snowfall and ice. Add deer to the mix and the Beverage antennas are severely stressed. The northeast-southwest Beverage was bent out of shape by snow and ice laden trees and deer had chewed through a support rope. It was a mess. I spent a few hours repairing what I could. It works again but another round of maintenance is required.

The east-west Beverage is still unidirectional to the west since I ran out of parts to rebuild the lightning damaged head end. I hope to have that done in January. Other than that the Beverage system is working. With so much ongoing maintenance I have become more serious about considering a commercial vertical array. Perhaps not in 2023 but we'll see. I have other measures on my plan to reduce the system's susceptibility to lightning.

Prop pitch motor controllers

I have one old Hy-Gain controller that I will convert to a prop pitch motor controller. The transformers in the controller are not particularly suitable to making an op amp power supply so I will likely use the old controller's power supply. That and relays to remotely switch motor current from the Hy-Gain controller's levers will make a nice commercial looking solution. 

The project will wait until after the operating desk is replaced so that I don't have to do the wiring twice. For now I will, unfortunately, stick with the interim breadboard direction indicators.

40 meter antennas

The severe weather has taken another bite out of the big 3-element yagi. There are now two capacitance hat arms that have broken off, the latest one from the reflector element. The antenna continues to perform well but the problem must be dealt with. I have a couple of candidate designs to replace my poor choice for clamping the capacitance hats to the elements. 

I would also like to replace or complement the XM240 with a 2-element reversible and rotatable Moxon. I am at a preliminary stage of the design process to characterize the critical design elements. One objective is to eliminate all loads and their losses.

The elements must be symmetric for it to be conveniently reversible. That puts constraints on the mechanical design and switching electronics. I am unsatisfied by reversing methods that my brief research located, and I am working on alternatives. My initial model is similar in appearance to the Optibeam 40 meter Moxon.

The reason for making it reversible is to get 240° of coverage (2 × 120° of rotation) with a simple side mount rotator. I would like to avoid a swing gate for 300° coverage because the wind torque of a large antenna with an offset centre requires a more robust rotator. In particular, braking strength. I will most likely put this antenna where the TH6 currently resides and use the same Hy-Gain rotator.

No trap tri-band yagi

Following from above, the TH6 may go on the Trylon tower so that I have a rapidly rotating tri-bander for chasing multipliers. My longer term objective is to replace the TH6 with a trap-less yagi. I am looking at candidate designs. As time goes on I am increasingly unhappy with the unavoidable loss of traps. The Hy-Gain traps are pretty good but the design trade-offs are clearly noticable in comparison to my mono-band yagis.

Auxiliary antenna switches

The automation software has provision for up to 8 auxiliary antenna switches, and even for a 40 meter stack should I ever decide to do that. The immediate application is for 40 and 80 meters. I have two 40 meter antennas and I want to use a single port on the 2×8 switch plus an auxiliary switch to select between the XM240 and the 3-element yagi. I will do the same for 80 meters, using my old inverted vee for the second antenna. It's intended for short path contacts that are better served by a horizontally polarized antenna.

Using a switching hierarchy like this is common for contest stations. It makes the use of high power BPF (band pass filters) convenient and avoids over-subscription to the N×M antenna switch. The relays work fine on the low bands and may need compensation on 15 and 10 meters to keep the through impedance at 50 Ω. 

I hope to have these in place before spring, and when I do the 80 meter inverted vee will be installed on partway up the 150' tower. The feed line is already there. I need to buy enclosures for the switches, mount UHF jacks and install one SPDT relay in each. One of the existing Cat5 cables will be recruited to bring the control lines to the automated switch.

DX chasing

Yes, I occasionally get on the air! Although I sometimes wonder how I find the time. With one very rare entity currently active -- Crozet Island -- and another on the way -- Bouvet Island -- and improving propagation, this is a good time for DXCC chasers looking to reach the top of the Honor Roll. Rare DX is in my pipeline of things to do in early 2023.

It's karma that the Crozet call sign prefix is the same as the mode with the vast majority of the contacts to date. I hope that changes. The WSJT-X screenshot above was for my QSO with him on 30 meters. I have since done the same on 20. With these two contacts I now have a lifetime total of 3 digital contacts on HF. Until now I've restricted my use of digital modes to VHF and 160 meters.

I made the effort as insurance. If he has equipment failure or weather takes out the station I want to be sure I have Crozet in the log. With these insurance contacts in hand I was less anxious about working him on CW and SSB. He'll be active for some time and I expect to get what I want, eventually. (Note: Shortly after writing the above, I worked him on 20 meters CW. I'm happy.)

Soon we'll be bidding bon voyage to the Bouvet DXpeditioners and I wish them good fortune, for their sake and for ours. This one feels different because one of the team members, VE3LYC, is a friend. Although I worked Bouvet over 30 years ago, I restarted my DXCC count when I returned to the air 10 years ago. I don't believe in lifetime achievement awards.

Formal 2023 plan

2022 was a year when I took a break from building and installing large antennas. Expect a return to antenna work in 2023. I'll be setting project priorities for the year in the waning months of winter and be ready to go in the spring. As in previous years I'll have an article on my 2023 plan along with how well I kept to my 2022 plan.

Have a happy new year and I'll see you on the other side.

Monday, December 26, 2022

CW Contest Sending Speed

The speed of the CW we send in a contest is strongly correlated to our CW skill level. It's true even though we rarely use the paddles, by relying on computer software to do the sending for us. Is this sensible or is another strategy more effective? In a contest, effective means a higher score, and we score higher by having more QSOs, and that typically involves completing each QSO as quickly as possible. High speed CW depends on the skill and tactics of both stations in each QSO.

This is an important question since in a contest it is routine for highly skilled CW contesters to work less skilled operators. Indeed, many operators in this era of no-code licenses only learn the code in order to contest. There is often a wide discrepancy of ability that everyone needs to understand and deal with.

Since many CW contesters are old timers who learned the code decades ago, the difficulty newcomers face learning the code, and especially at the high speeds common in a contest, is not always appreciated. It takes time and practice, sometimes quite a lot. Few of us became proficient overnight! We forget that and become impatient when we are forced to QRS or are delayed by slow sending and repeats.

Hams with a natural talent for CW are fairly rare. I am not a member of that select group. Even today I sometimes struggle to send and receive at high contest speeds, and more when I take to the paddles. Years ago, memory keyers helped and today computer software takes over much of the effort. I built my first contest keyer in my teen years soon after I gravitated to contests. What a pleasure that was for a clumsy CW hack like myself!

Contests are not about having conversations. It is very possible to score well with modest CW skill. The computer does almost all the sending and copy is easier since the format is predictable and short. Although the sending speed you ought to select should be intuitive after spending an hour or two in a contest, I am always surprised by how many make choices that impair their results. 

Many casual operators do not care, and that's okay. Those that do want to increase score potential should reflect on the points I raise in this article.

For those who can't copy at contest speeds

A prevalent "mistake" by casual CW contesters is to send no faster than they can copy. When I ask why, they tell me that's what they're comfortable with. If that's 20 wpm, well, that's what they set the computer to send. This is odd because a large majority of contesters can copy much faster. Both operators are delayed and it isn't necessary. Go ahead and crank the speed up to 30 or 35 wpm when you call someone. They'll copy you just fine.

Consider S & P (search and pounce): You already know the call of the station being called, and that's probably the hardest information to copy. The exchange may be fixed (e.g. CQ WW) and if you use the call history feature of your logging software, the exchange may be pre-filled, or you already have it from their previous QSO. You need only verify that it is correct and make changes where necessary. 

For variable exchanges such as serial numbers, well, that you have to copy. But, again, you may already know what to expect based on their previously sent number. In any case, numerals are long characters (5 elements) and there are rarely more than 4 of them in a serial number. That isn't too challenging. Cut numbers are another matter which I won't get into (e.g. A for 1, E for 5, T for 0, etc.).

One thing you can be certain of is that the running station will not QRS to your sending speed. Not only does it throw off their rhythm and require extra work, they won't see the point since you obviously copied their call sign before calling them. Also, you probably already copied all or most of the exchange. Sending slowly does not help anyone in this most common circumstance. Be more aggressive; that is, be a contester and get out of the slow lane.

I know CW contesters who are not very competent at high speeds. They may use a CW reader application on their computers as an operating aid. They'll glance at the display to confirm what they copied or to fill in the gaps. A glance at the super check partial (SCP) window of known contester call signs is also useful as a sanity check: if it isn't in there you may want to listen again or request a repeat. 

There's no shame is using these aids as you climb up the lower slope of the learning curve. Check the contest rules since your entry class may be affected by the use of code readers.

Competition in the pile up

Another danger of sending slowly is that you will lose to the faster competition when other stations are calling. I deal with this quite often. Let's say that I am running at a good clip, sending ~35 wpm, with 2, 3 or more stations calling at a time. That's contest heaven: lots of callers but not so many that I can't pull one full call sign out of the bunch. 

I don't always pick the loudest or the one who starts sending first. Often it is the fastest that goes into the log. The reason is that I can copy it easily in a second or two. Time is of the essence. When I am operating SO2R it is critical that call sign reception take no longer than my transmission on the other radio or I will find myself having to copy on both radios at the same time, and that isn't easy. I pick the fastest since it is to my advantage.

Slow sending and long call signs upset my rhythm. It slows everything down and puts more burden on my short term memory, where your call resides until I type it in. When I pick the slower station, the subsequent delay discourages other callers since they don't want to wait the extra 10 seconds. They click on another spot and disappear. I not only lower my immediate rate and I might never hear that other station again. In my experience, slow stations don't immediately QSY when I answer a faster caller.

If you're not a contester, this reluctance to wait a few seconds might seem ridiculous. It is the reality we deal with and it affects scores. Send faster and you will become more competitive. If you have a weak signal or there is QRM or QRN the station you're calling will request a repeat. That may be a good time to QRS a little, but only then.

Help the other station by being fast. Unnecessary repetition of a call sign or exchange halves the effective speed. If you're already at 20 wpm, that takes you down to 10 wpm. That's breeds impatience and you may not get through at all. 

When there's QSB, QRQ is recommended because when it takes several seconds to send your call or exchange there is a strong possibility that one or more characters will be missed during a signal dip.

Running? QRQ could be a problem

Your speed does matter if you are running to attract callers. Many will QRS a few wpm. This is about more than courtesy: they want to move on quickly and if you ask for a repeat because they're sending too fast for you to copy the QSO takes more time. 

Of course there are those that never QRS, and that's a problem for both parties. Valuable time is wasted on repeats. I hear this all the time, and it's only funny because I am not one of the parties in the QSO. Operators who persist in calling slower stations at high speed often fail to rank high in the contest standings. That's not a surprise! The smart operator will QRS to get you in the log quickly so they can move on to the next contact.

Let's say you have to ask for a repeat of the QRQ stations' call sign or exchange. The effective speed of their sending is less than half of their actual sending speed. After all, the information is being communicated twice (or more) plus the time for the request to be made and the multiple turnovers in transmission and reception. 

Unrelenting QRQ slows the QSO rate for everyone. The slow runner is perfectly justified to ignore the persistently crazy fast caller.

Effective speed

The common method for calculating sending speed is by measuring the duration of character elements (dots and dashes). Character and word spaces are included in the calculation, using assumptions for character mix and average word length. A machine can do it easily, and they do.

CW Skimmer software measures speed quite well. Turn on skimmer spots on your favourite DX spotting network and soon your band map will be filled with skimmer spots. Watch the data in the Telnet window or (easier) hover your mouse over the call in the band map. The screen shot shows how it's done in N1MM Logger+.

That's only half the story. We are not machines and context matters. What we measure as CW speed (wpm or words per minute) is actually bit length: the time to send a dit. However, the true information rate is that of characters and words, and there are important variations.

Let me give an example of two call signs I worked in a recent contest: EE5X and OM0M. Despite both being 4 characters long there is an obvious difference between them. Using the common specification for CW keying, the number of dit lengths in the first call sign is 31 versus 53 in the second. That is, one of those calls takes almost twice as long to send at the same speed.

For many contesters, the one that is faster to send may seem attractive since it saves time. But therein lies a problem. The operator copying the call must work at twice the mental speed since the 4 characters come in half the time of the other call. That is the cause of many copying errors, and errors spawn repeats and busted calls and exchanges.

The Morse character set is deliberately optimized this way, similar to Huffman coding, by assigning short codes to letters that appear more frequently in typical English language text. Being less common, numerals and punctuation are given longer codes. Thus E is the shortest letter, at one bit, and J and Q are the longest, at 13 bits. It is no coincidence that E is worth 1 point and J and Q are worth 10 points in English version of the game Scrabble.

To bring us back to the point of all this, when you send at contest speeds, the message content matters. For call signs with short characters it can be very worthwhile to send more slowly. Conversely, call signs with long characters can be sent more quickly. It is quite easy to do so in most contest logging applications by altering the speed before and after call signs and to insert padding between characters.

Dealing with dits (again)

Following from the previous discussion, it is no surprise that strings of dits cause problems despite our best intentions. Ears and brains must work more quickly than for other element sequences. If your call sign has one or more of these strings, it is worthwhile giving some thought to how fast you send it.

This is a subject I've mentioned before, and more than just once. As much as I like my call sign it is a problem on CW. The VE3 prefix may seem to be a problem with its strings of dits, yet it rarely is. The prefix is so common that almost everyone recognizes the sound of it and can copy it correctly, no matter how fast it is sent. A few mistake it for VE2. For me the trouble lies with the second V in my call. 

My solution (first link in the previous paragraph) is to program a function key that sends the suffix slowly and with extra spacing. When the inevitable occurs I can correct the other operator with minimum effort and time. Unfortunately this only works for S & P; when running, few operators send my call sign and I can't correct them. If one of them spots me as VE3UN (the most common error) I am inundated with dupes. 

The error is so common that VE3UN not only appears in the SCP database, it usually appears above my correct call. When an operator "yanks" a call sign from the database it is often the wrong one. Thus the error persists, perhaps forever. Yet I have never heard VE3UN in a contest. Too many operator trust SCP without reservation. If you make use of SCP to cope with fast CW, be suspicious of what it recommends.

CW Skimmers don't often make this mistake, but they do make mistakes that are uncommon to human operators and apply to all call signs regardless of speed or content. Smart operator treat skimmer spots with extreme caution.

I envy other VE3 contesters with suffixes that are more easily copied because they're not full of dits. Perhaps one day we will be allowed 2×1 contest call signs. Until then I'll have to live with the problem. It is unusual to hear me sending faster than about 34 wpm in a contest because of this, even though I am comfortable at higher speeds. Meanwhile my friends can send closer to 40 wpm and be copied incorrectly less often.

Is there a speed limit?

A few contesters speed recklessly. They rely of spotting networks to distribute their call sign, or are blatantly unconcerned with whether others copy them correctly. All they want is your call in their log. What goes into others' logs, right or wrong, doesn't impact their scores. There are exceptions in a few contests where both parties must copy correctly but they are not the most popular events.

For those in rare multipliers, QRQ works to their advantage. For everyone else it often does not. Don't emulate them. Many operators will be scared off because they are unable to copy north of 40 wpm. How fast the speed demon can copy is irrelevant. The objective should be to be correctly copied without repetition. Moderating the speed delivers results even though each QSO, considered individually, may on average be quicker with extreme QRQ.

There is a tenet of effective communication that applies to all writing, and applies equally well to CW contests:

There are two ways to write. One is to write in a manner that is convenient to the writer. The other is to write so as to be read and understood. To be successful, be the second kind of writer.

When too many potential callers can't copy you or copy you correctly, whether in good conditions or bad, everyone pays the price. As in driving: speed kills; drive according to the prevailing conditions.

I find it both sad and amusing when an operator persists in sending at 40 wpm to a station sending at 18 wpm. There are endless repetitions and frustration on both sides of the QSO. I see it in contests and in everyday DXing. The keyer has a speed control and it's there to be used.

Enjoy the holidays. I can't say whether I'll have time to complete another article before January, so I may end December with one less article than is typical for me.

Tuesday, December 13, 2022

Overhead Cable Run: Version 2

I bury cables every time it is practical. This includes both transmission lines and control lines. I have thousands of feet of buried cable. This keeps cables safe from farm machinery, yard work, antenna maintenance, most critters (including human!) and other mishaps. Of course the cables must be suitable for direct burial. I do not use conduit.

There are several places where I run cables overground. For the Beverage system in particular there are long runs of RG6 that lie directly on the ground. I can get away with that in the bush where no one goes but me. Tree roots and other obstructions can be major impediments to burial, but it is convenient to go overground for long runs where it is not unsightly and the risk of damage is low.

The most important overhead cable run is from the edge of the hay field adjacent to the house yard to the Trylon tower where most of the switching and cable interconnections is located. The overhead run was a temporary measure until I had time to bury them. Since then I have come to realize that overhead is better. Maintenance and changes are easier. The mat of tree roots a few inches below grade make burial a formidable challenge.

The original overhead run was not sufficiently robust to survive for many years. Construction was admittedly rushed to leave more time for major tower and antenna projects. This year that had to change. Sagging and anchor woes coupled with the addition of more cables spurred me to act. The first step was to replace the ground anchor with one that is better able to support the messenger cable and the weight of the many cables. Once the concrete cured I got to work.

It is helpful to describe the problem and the underlying theory before diving into its construction. Let's do that using the following diagram.

The messenger cable is anchored to the tower and the ground. The post outside the stone wall keeps it at a safe height to walk beneath. Tension is needed to resist sag due the weight of the messenger cable and the cables it supports. The greater the tension the less the sag. This is a critical design parameter since there is stress due to tension on the tower and on the ground anchor. 

As we'll see, it takes a lot of tension to reduce sag to an acceptable amount. Even with high tension, sag is unavoidable. Consider the picture at right of the top guy on the 150' tower. The static tension (pre-load) is more than 1000 lb, yet it sags a foot or two. The guy is ~200' of 5/16" EHS guy strand. We won't come anywhere near that tension on the messenger cable!

This brings us to the catenary. It is a common subject in the ham literature since our antennas bend and sag due to wind and gravity, affecting performance and survival. Catenaries are more discussed with respect to wire antennas where the effect is particularly noticable. The physics and mathematics of catenaries is deceptively complex, and that is annoying when we want a quick answer to what appears to be a simple problem.

My "simple" catenary question: how much tension is required to have no more than 2' of sag in a 60' horizontal messenger cable and its cable load? Finding the equations that provide a reasonably accurate answer wasn't quick. I found the answer first by measuring the tension and the sag. The required tension is more than I can tolerate for the constraints I am faced with. 

Before diving into the mathematics I'll describe my practical approach. I ran the new ¼" EHS messenger cable above the existing overhead run, anchoring it to the tower with galvanized chain, to a replacement 10' steel post, and down to the recently installed ground anchor. I used several rubber tie-down straps to tie the cable bundle to the new messenger cable. A come-along at the tower anchor was used to increase the tension and measure the sag. As tension increased, the old messenger cable went slack and the cable load was taken up by the new messenger cable.

The old ⅛" aircraft cable messenger and aluminum post remain tied to the cable bundle. Had there been a mishap it would be there to retake the load. The old cable was temporarily transferred to the new ground anchor.

At right you can see a yellow rope that runs from the old post to the 150' tower 100' behind the photographer. I did that when the screw anchor failed. There is a steel peg under the post that provides good lateral support, but that is not reliable enough on its own. 

That peg gave me the time I needed to install the rope support. Whenever possible never have a single point of failure in a critical system.

My Loos cable tension gauge was mounted on the new messenger cable to monitor the tension during construction. I did not do this for the original cable, relying on "feel" to keep the tension within reason. 

To meet my sag target on the original system I had to install two intermediate 8' posts. They divide the span into 3 approximately equal sections which greatly reduced the sag for the amount of tension I was comfortable putting on the messenger cable, post and anchors. My hope was to eliminate the intermediate posts with the improved messenger cable. They can be a nuisance.

The top picture immediately above shows detail of the old and new posts during the project. The one on the bottom shows the Trylon anchors, old and new, with grips for the messenger cable and for attachment of the come-along. Look closely and you'll see that that plated chain for the old tower anchor is rusted. 

Galvanized hardware is more durable than plated hardware. This is especially important on smaller hardware since there is less base metal. I recommend spraying metal (rust) or cold galvanizing paint on plated hardware every couple of years. Galvanized hardware will eventually wear and also need to be painted.

When I increased tension to 700 lb tension (measured tension includes the weight of the cables) there was still too much sag. I couldn't safely walk beneath the cable. The cable and ground anchor can handle that tension but I am not so sure about the Trylon tower. Increasing the tension was out of the question since it rose alarmingly for little reduction in the sag. I lowered the tension and hit the internet to dig deeper into catenary mathematics.

Catenary theory is fascinating. For a mathematically minded person like myself it is easy to slip into the depths. Easy but unwise since I had an immediate problem to solve. Much of the material I encountered was far too general and required effort to simplify the equations to fit my project. More digging was required before I found what I needed.

There are many resources on the internet and I am sure there are better ones that what I found during the time I spent on researching the question. There are also spreadsheets to do the calculations Here is the link to the page that I found most helpful. It isn't perfect but it gets to the heart of the matter: the relationship between sag, tension and weight for a uniform horizontal catenary.

S = (WL²)/(8H)

S is the sag, W is the unit weight of the catenary (including the cable) and H is the tension. Any units can be used but they must be used consistently. I'll use English units in this discussion: feet and pounds. The only difficult variable to determine is W, which is the weight of the messenger cable per foot. From the known weights of the EHS, Heliax and heavy control lines that comprise a large majority of the weight, W is estimated to be 5 lb/ft. That comes to 300 lb for the 60' span.

For a tension of 700 lb the calculated sag is 3' at the midpoint of the 60' span. Since the ends are about 9.5' above ground and the bottom of the cable bundle hangs about 6" below the messenger cable, the ground clearance at the midpoint is 6'. This is surprisingly close to what I measured; it brushed the top of my head. Either I did the calculation correctly or I was lucky.

As I said, 700 lb of tension is too high, so the sag must increase. That is unacceptable. Even were higher tension permissible, the required tension rises rapidly. Notice that sag is inversely proportional to the tension. For example, to halve the sag to 1.5' (which would meet my objective) the tension is 1400 lb.

I concluded that the intermediate posts must remain. That makes 3 spans of 20' each. If I cut the tension in half to 350 lb the calculated sag for the shorter span is less than 1'. Since the posts are 8' high the bottom of the cable bundle at each of span midpoints should be almost 7'. And, indeed, that is what I measured. I think I'm getting the hang of catenaries!

In this picture the tension is less than 300 lb. Lots of rubber straps have been installed to hang the cable bundle; more were added after the picture was taken. I didn't use them for the old messenger cable, opting for plastic cable ties each time cables were added to the bundle. Although I no longer need the ties they might as well be left there until cables need to be removed. It's messy but not a problem that needs fixing.

Notice that straps are not placed close to the posts. When I tried, the cables were pressed hard against the post. It is possibly that could damage the more fragile cables over the coming years. By the time I was done there were about 15 rubber straps spaced an average of 4' apart. They're cheap and effective. I've heard that the rubber is prone to UV damage but the ones I've had outdoors for a few years do not show signs of deterioration. They'll be inspected at regular intervals.

There is one last item to discuss: lateral stability. The overhead run present a large surface area for the wind to press against. For a 60' run witha 4" cylindrical cross section, the total projected surface is 20 ft². That's a lot! For our wind zone it is recommended to build for a wind speed of 135 kph (85 mph). At that speed the wind load is 20 lb/ft², which adds up to a total force of 400 lb on the overhead run.

The actual force will be much lower. The overhead run is close to the ground and there are many obstacles that block wind that is orthogonal to the cables. The overhead run does lean to the leeward in high winds but not enough to be at risk. 

Snow and ice increase the unit weight but, again, it hasn't proven to be a problem. What is a problem is snow on the ground which reduces walking clearance by a foot or more in late winter. I am not too concerned by that since few people other than myself are likely to be walking there when that happens.

There are a few more steps left to complete the project. Now that the strength and reliability of the new structure is confirmed, the old messenger cable, post and tower anchor will be removed. After that is done, the new post will be fitted to the ground peg the old post still occupies. The new post leans a bit since it had to be placed several inches from the old post. I should be done by Christmas.

Monday, December 5, 2022

Prop Pitch Direction Pot Updates

When I raised the first of my big towers in 2017 there was no direction indicator for the chain-driven prop pitch motor rotator. I made one using a Bourns 10 kΩ 10-turn potentiometer, a device used by other hams of my acquaintance. 

These components are not hardened for outdoor use and require protection. I built a plastic enclosure on a bracket dangling below the motor platform and hoped for the best. It lasted 5 years, far longer than I expected.

It was time to replace the pot on that prop pitch rotator. The pot had water damage and there might also be internal mechanical wear. The resistance jumped around a lot during rotation and there is evidence of bald spots on the resistance wire.

I went through at least two plastic "hoods" for the pot. The container seen above replaced the original, which was too small to adequately protect against wind driven rain. I repurposed food containers to save cost for what I expected to be a temporary measure . The work great when they're new. Of course they are not resistant to UV and thermal cycling also takes its toll.

As the plastic cracked the silicone sealant around the gaps couldn't block water incursion. Notice the rust at the base of the pot shaft. It was always a danger of this design because the pot shaft points upward, and open to rain and snow without excellent water protection.

An underappreciated problem with ordinary pots in rotator service is wear due to rocking of the mast and antenna system. This is directly coupled to the direction pot, with the pot wiper moving back and forth over the resistance wire where it is resting. 

The wire in a typical pot is not designed for this amount of abuse. They have a limited service life in this application. In contrast, the pot in a Hy-Gain rotator (photo credit: MFJ) is much larger with thicker wire and a wide wiper. It may help to vary the direction of the yagis when they are not in use.

The multi-turn linear pot has its advantages and it is not too much of a burden to replace them every 5 years or so. To better protect the Bourns pot from the elements I undertook a redesign. I had planned this for some time but put it off until the pot failed.

The chain-driven drive shaft is 2" OD with a wall thickness of about ¼". I projects less than 1" below the bottom of the housing that sits on the side of the tower. There is just enough room to fit the Bourns pot. So that's what I did. As you can see it's completely enclosed and well isolated from the elements. There is no opening above the drive shaft where water can enter.

The difficulty with this design, and the reason I didn't do it this to begin with, is that with the pot fixed in place it is the shaft that turns. The wires must turn with the shaft. Alignment of the shaft is critical to avoid bending force that can bind the pot during rotation. As for the first version, the pot is mounted on thin aluminum flashing that flexes to protect against a minor misalignment.

I tied down the wires to prevent tugging on the pot terminals. The 3 wires are coiled around the coupling. The wires are AWG 24 scavenged from stranded Cat5 cable. The plastic insulation is very pliable but it is stiffer than the wire when used in this application. Disciplining the wire coil is therefore not easy. It looks ugly but it works. Now that winter has arrived we'll see how it does when coated with snow and ice. A snow shield might be necessary.

On the other prop pitch motor turning the 15 and 20 meter yagis I encountered a different problem with the direction pot. A few months ago I replaced the temporary rope belt with higher quality ⅛" nylon weave rope. Since little friction with the mast and pulley is needed to drive the pot I kept the tension low. The rope loosened only slightly but that was enough for it to slip. Worse, it preferentially slipped in one direction. 

At first this could be dealt with by adjusting the zeroing pot on the op-amp direction indicator. With the continual back and forth rocking of the mast the pot soon hit the end of its 10-turn rotation and could no longer be adjusted. I climbed the tower and confirmed the nature of the problem. I centred the pot and pulled out the adjustment bolt to increase tension on the rope.

With the higher tension the mechanism has been working well for the past few weeks. I hope that it survives the winter. Another design may be necessary. One example I've considered is a chain drive. It would provide positive engagement without fine adjustment of chain tension.

I have discarded bicycle chains and cogs (½" pitch) to work with. The challenge is that there is no way to slip a cog over the mast. The cog would have to be cut in half and machined and attached to a collar that wraps around and is affixed to the mast. The pulley on the pot shaft would be replaced by a small chain cog. A master link solves the chain wrap problem. It's straight-forward work that I hope to avoid. Time will tell.

The direction indicator circuit described in an earlier article works perfectly well but it is more complex than currently needed. The goal was a circuit to drive either a physical meter or an analogue Arduino GPIO. The needs are different because a meter responds to current and the GPIO ADC responds to voltage. A second gain stage provides the required voltage range without risking the linearity of the differential amplifier.

Since my immediate plan is to use physical meters the second stage has been eliminated in the schematic shown above. I used the freed space on the breadboard to build two of the circuits. A few days before CQ WW CW I discovered that the meter movement of the old direction indicator (for the 40 and 10 meter yagis) had malfunctioned. The single op-amp circuit is a temporary fix since I needed it for the contest. As these things go, I expect it'll be in service for a while yet.

The circuit provides a minimum 10 kΩ load for the op-amp to keep it stable. The pot is tapped with a current limiting resistor to protect the meter movement. The resistance range was selected to support a large range of current requirements and drive ratio at the direction pot. The circuit is simple and works well. Calibration is performed in the same way as for the original circuit. Substitution of the current limiting resistor may be needed for meters other than the ones I have tried.

Because there's just one meter, one of the meter wires must be moved between circuits to monitor the direction of each prop pitch rotator. An SPDT switch would make this job easier. The arrangement is far from ideal but acceptable in the short term while I decide on a final design. 

I may do what some others have done, by repurposing an orphaned Hy-Gain rotator controller. The meter indicates direction and the levers operate the motor. Unused circuitry in the controller can be removed or left in place in case it is later returned to service as a Hy-Gain rotator controller. It also looks good on the operating desk. A home brew professional looking controller is a lot of work.

There is RFI, which shouldn't be a surprise considering the layout. You can see one RF choke at left on one of the pot wiper lines. This was proof of concept to see how it would help. It suppressed most of the RFI but not all. When properly built the lines will be choked and bypassed to suppress RF getting into the circuitry. Lightning protection must also be included, but probably at a connection point far from the shack for maximum safety.

My next step is to find a used orphan rotator controller and build permanent direction indicator circuits. The ±12 VDC for the op amps is not easily built with the stock transformers in Hy-Gain rotators so I may use the existing power supplies and wire them up to the controllers. The motor controls can be implemented at the same time or later.