Reversible 40 Meter Moxon: Initial Model

I would like to retire the XM240 this year. It is not just because of its low efficiency due to the loading coils. When I side mounted it last fall, I did it with the full knowledge that I was impairing my flexibility on 40 meters since it can only be rotated through about 130°. It works well for working most of North America, and DX further afield to the south and west. But I miss having two antennas with full rotation.

I prefer to keep the limited-rotation side mount rather than replace it with a swing gate. The latter would allow 300° rotation, which is more than adequate for my needs. There is a 60° arc between 100° and 160° bearing that I can omit without serious loss of station effectiveness. When that direction is needed for long path contacts (e.g. Asia) and southern Africa, the high 3-element yagi is a superior choice, and it is fully rotatable. The offset mount of a swing gate requires robust construction and a strong rotator for the mechanical load of a 40 meter yagi.

After consideration of alternatives, I returned to an antenna design that I chose against several years ago: the W6NL 2-element Moxon. It's relatively small, has good F/B and broad SWR bandwidth, at the expense of modest gain and the narrow gain bandwidth inherent to every 2-element yagi, Moxon or not. However, it does not suit my application without one major modification: making it reversible. 

Reversibility on the side mount would permit 260° coverage and instant switching between, say, Europe and the US. The gaps are between 95°-145° and 275°-325°. There are few stations to work in those directions and, as already mentioned, there is the big 3-element yagi for those directions. The Moxon is small enough that my ancient (and multiply refurbished) Ham-M rotator can handle it.

The actual W6NL design works pretty well but it has a few unusual characteristics. The modelled SWR bandwidth appears to be narrower than a traditional Moxon rectangle, gain is slightly less and I worry about the capacitance hats striking each other when it's windy and their inconstant separation. The latter is a critical parameter of the Moxon design.

For these reasons I decided to explore and compare alternative approaches. I hoped to gain insights into their relative performance, both electrical and mechanical. Any 40 meter rotatable yagi is a large antenna and there may be good reasons to compromise electrical performance in favour of mechanical robustness.

The baseline model I developed is a symmetrical Moxon rectangle. It is not novel since others have made similar antennas, but I didn't have a model in hand that I was comfortable with. 

I proceeded by keeping the symmetric rectangle dimensions close to those of the traditional (asymmetric element) Moxon rectangle and placing a coil at the centre of the reflector. The coil lowers element resonance so that it has the proper reactance (phase shift) to be a reflector at the operating frequency.

In a real antenna the coil and feed point are switched to reverse the yagi but this is not necessary in a model since the switching system does not affect the antenna when properly implemented.  The switching system can introduce coupling and stray reactance that are not part of the model but must be dealt with during construction and testing of the antenna.

To meet my criteria the antenna must have these features:

• Symmetrical: two identical elements
• Critical coupling: element tips are placed near each other
  
• Switchable: driven element and reflector; the reflector has a coil at the centre
• Switching system: method for selecting which element is driven and which is the reflector
  

Exploration is a multi-step process. The first model uses constant diameter elements -- 25 mm in this instance -- with each element tuned to 7.0 MHz. That allows NEC2 to handle the antenna reasonably well since SDC (stepped diameter correction) is avoided, and EZNEC's SDC does not support bent elements well. Element dimensions and coil value were varied until the performance was approximately equal to that of a traditional (asymmetric) Moxon rectangle. 

When this reversible Moxon rectangle is optimized to this very good SWR, it looks as follows:

• 5.6 meter boom
• 14.5 meter elements and 2.65 meter inward legs
  
• 30 cm (12") gap between element tips
• 1.5 μH reflector loading coil
• Peak gain of 6.6 dbi at 6.955 MHz; Peak F/B of 23 db at 7.1 MHz
  

There may be slightly better solutions, but this is pretty typical for a Moxon rectangle. Every 2-element yagi with a reflector as the parasite, Moxon or not, has the maximum gain placed below the band edge so that the F/B and SWR are good across the band. This symmetric Moxon has the same attributes, which will be shown further below. This was a promising beginning.

There are commercial antennas similar to this, but they're rare due to their size. For example the one from Optibeam is electrically shortened. The performance penalty is modest, but there is one. It is rotatable but not reversible. 

Take note of the mechanical connection between the element tips. Sagging over the span of the boom length can be significant. Wind and ice demand a robust design for these large elements. I'll return to this later since it's an important structural consideration.

The W6NL design eliminates the sag problem by making each element horizontally balanced. The inner legs, that make it a type of Moxon, are one half of each capacitance hat. The element tips are extended which moves the hats inward. There is less stress at the ends of the elements. It is possible to design this style of yagi without element trusses if the elements are made sufficiently strong (and heavy).

However, the W6NL Moxon (whether built from scratch or as a modified XM240) is not symmetrical and it is therefore not reversible. For this modelling exercised I instead explored modifications of the basic design that are symmetric.

The diagram comes from Cebik's article that does a deep dive into the Moxon rectangle. The critical parameters are shown. There is the ratio of the length to the width and the gap between element tips. There is a dependence of k (ratio of wire diameter to wavelength) which I will skip over in this article since it isn't particularly relevant. Also, I use symmetric elements with loading rather than element length to tune them.

Element topology contributes to gain. The greater the length of the parallel element sections, the greater the gain; the tip radiation cancels in the far field due to their symmetry. The gap, C, determines the coupling, for which we need the best value to achieve the Moxon's particular advantages with respect to impedance, SWR bandwidth and F/B.

I explored these three variants. The one on the right is the most similar to the Moxon rectangle. On the left is the one most similar to the W6NL design; the element tips are short since I did a screen capture while I was playing with the model. In the centre is a hybrid where the capacitance hats are at the ends of the elements; it looks like a Moxon rectangle with outward arms at the 4 corners.

I found that the critical gap between element tips was around 30 cm (12") for all of these 40 meter variants. Moving away from that value in either direction had a large effect on the 50 Ω match, and a more gradual effect on the F/B bandwidth. In all cases the wire diameters are 25 mm (1") along their entire lengths. The choice is justified since this is a design exploration, not a construction article.

The symmetric Moxon on the right has a few interesting features. It works best when the boom length (width) is 5.6 meters (18'-4"), with respect to gain and F/B. After many trials, the same was found for the other two designs. The surprised me since the W6NL Moxon has a longer boom. Shortening the boom had the effect of making the elements longer and the capacitance hats shorter to compensate. When the booms are 5.6 meters, the capacitance arms are identical for all three: 2.65 meters. The outer arms for the two with T-hats are also 2.65 meters for mechanical balance.

Notice that the length to width ratios for the two on the left are lower than that on the right. The maximum gains are a little lower due to the shorter elements. That is due to loading by the hats. 

The element tips on the symmetric version of the W6NL Moxon proved to be a problem. The longer they were the worse the gain and match (I didn't closely monitor the F/B during the process). The problems mostly vanished when the tips were reduced to zero length, which is the T-hat version in the middle diagram. I can't give a definitive reason based on a cursory inspection of the models other than to say that the coupling between elements is less than critical due to the greater distribution of high-impedance points where capacitive coupling is under-utilized. 

I therefore discarded the design on the left and focussed on the remaining two. After coarse optimization I compared their performance.

Gain and F/B are sufficiently similar that we can declare them to be effectively equal. The gain of both is a little less than that of a traditional 2-element yagi with a reflector element and slightly longer boom. Both plots are continued below the band edge to demonstrate that gain increases, which is typical of all 2-element reflector yagis, Moxon or not.

The impedance matches are also very similar and quite good right across the band. The Moxon rectangle is on the top and the T-hat on the bottom.

Since their electrical performance is about the same we turn to the mechanical parameters. These can be as critical as performance considering the large size of 40 meter yagis. Regarding the electrical and mechanical performance of the W6NL Moxon, I suggest reading this paper by W8WWV.

There are several parameters to consider:

• Element length
• Element balance
• Weight
• Wind & ice load
  
• Maintaining the distance between element tips

First, let's compare the total linear lengths. For the symmetric rectangle, each half element is 7.25 meters and the inward sides are each 2.65 meters, for a total length of 19.8 meters. For the T-hat version, each half element is 5.9 meters and each T-hat arm is 2.65 meters, for a total length of 22.4 meters. The elements of the latter are shorter but the arms at the ends of the elements, where they are weakest are double that of the rectangle.

The T-hats are balanced on the element ends while the inward arms of the rectangle are not. The torque on the end of the element is a concern with the rectangle. It can be partially mitigated by a fibreglass rod to fix the gap distance and mechanically couple the arms of the opposing elements. However, the long span of 5.6 meters of, hopefully, lightweight tubing requires more mechanical strength than for the similarly coupled arms of the balanced T-hats.

The arms of the rectangle design require a stronger design than for the T-hats, and that may be a greater threat than the nominally lighter rectangle. Element trusses do not solve the problem. The T-hat design might not even need element trusses if the elements are made sufficiently strong. Since the T-hats are weight and load balanced, the 5.6 meter inner span between the elements might only need attention with respect to stress caused when the elements experience unequal loading in a strong wind.

It's a dilemma and I have no good answer at the moment. You could say the electrical performance is the easy part of the antenna design. Although I have enough aluminum in my stockpile to increase the strength of the elements, there is a greater risk of breakage at the centre since both elements must be split for feeding and for switching in a series coil. The elements must also be electrically isolated from the boom. The mechanical design is challenging.

That will be my next step. Until I have a robust mechanical design I will not make a final determination on whether to proceed with a rectangle or T-hat design. The basic mechanical design is common to both. 

There are alternatives like the NW3Z 3-element yagi and yagis with two V-shaped elements that do away with the high load at the ends of the elements, but those come with an additional performance penalty: they are not true Moxons and gain is reduced by the smaller effective element spacing.

I'll end with an additional concern for both designs: potential for interactions. The XM240 is not resonant on the third harmonic which would otherwise fall within the 15 meter band. I included capacitance hats on the 3-element yagi to accomplish the same. It was therefore important to know how the reversible T-hat and rectangle designs fare in that respect.

The SWR sweep of the rectangle is at the top and the T-hat is on the bottom. Both meet my objective of avoiding resonance on any contest band, and especially 15 meters. I was not really concerned since loaded elements (which includes the non-linear element profile of the Moxon rectangle elements) shift the third harmonic away from its simple arithmetic value.

ARRL DX CW - M/2 @VE3JM

When I was young and without a station or just a small one, the only way to do an effective multi-op contest operation was from others' stations. I don't do it very much these days. When my friend Vlad VE3JM asked me to join his team for ARRL DX CW, I welcomed the opportunity. I operated there several years ago for CQ WW SSB. His station has changed a lot since then.

As readers likely know, I have a "big gun" station of my own now, and that I've hosted multi-op teams for two contests so far: CQ WW SSB and CW. There are good reasons for operating from another station for a major contest despite having my own big station:

• Opportunity to team up with contesters I haven't operated with before
• See how other big guns design their stations
  
• Learn new strategies and operating styles
• Learn to use and assess other software packages, equipment and switching systems
• Less worry about problems arising and the stress of being the one to fix them
• Have fun!
  

There was one more reason: my station is experiencing several technical problems that would have seriously impacted a contest operation. None are major but it was impossible to deal with them in time for the contest. With luck and good weather I hope to be ready to try another multi-op for the ARRL DX Phone contest in March. If problems persist I may enter single-band single-op.

All of us were relaxed enough not to cause conflicts. Rather than five keyed up contesters fighting for the two operating seats, we were all happy to cede the seat when another wanted operating time. When an operator needed a break there was always another ready to step in. When problems arose we calmly worked around them. We were tolerant of our differing tactics for mixing running with hunting. All in all, we worked well as a team.

Antennas

There were several antenna problems. Since Vlad works and has limited time to attend to the station they could not be resolved before the contest. Even so we did well. Put enough aluminum up high and magic happens despite not always being able to turn the antennas or use all the yagis in a stack.

We had stacks on 20 and 15 meters, two tri-banders, a 10 meter yagi, two yagis on 40, an 80 meter 4-square and one of the big towers served as a vertical on 160 meters. There was one receive antenna.

More antennas would have been nice but, again, that has to wait until he has time. The picture shows his newest tower with only an XM240 at 140'. There is ample room for more antennas. With time they are certain to appear.

Equipment

Both stations had venerable Elecraft K3 transceivers and pan adaptors. I've only ever used the K3 during contests at other stations so there is a brief learning curve every time. The pan adaptor controls took getting used to since I had never used that K3 accessory. There were occasional receiving artifacts that may have been due to IMD or overload. 

The third station consisted of a Flex transceiver and a multi-band vertical that was hastily erected. This station was just for listening. It could have been used to work a few multipliers except that there was no interlock with the other two stations. Despite that constraint, it served very well for checking out multipliers and band conditions by one of the three otherwise idle operators.

In the picture (credit VE3JM) you can see me on the far right at the third station while Nick VE3EY (foreground) and Dave VE3KG operate the main stations.

Amplifiers were a manually tuned AL1200 and an auto-tune Flex PGXL. Despite the automatic band switching and tuning, the PGXL cannot be ignored entirely. It reacted to antenna matching difficulties such as one yagi that became intermittent, and again when we were hit by a snowstorm that altered impedances of the antennas. Although the AL1200 had to be manually tuned, it was easy to compensate for impedance changes and the grounded grid design was tolerant of imperfect tuning. 

High power BPF and tri-plexers are by Pavel VA6AM. They work extremely well. I have his prototype 6-band low power BPF in my station, which also work very well.

DXLog

I've never used DXLog before, but I've wanted to give it a trial run for some time. It was a necessity for this contest since the antenna control and automation are integrated with DXLog. VE3JM uses the same basic system developed by K3JO for the K1LZ super-station. Perhaps it was my bias due to my long experience with N1MM that I didn't really like it. That said, it did some things very well and has features that I have not (yet) considered for my home brew station automation.

The screenshot is that of the third station. The screen includes the Flex system and a browser window monitoring the contest scoreboard. The log entry window is at the bottom left. This is far busier than the two main stations that each has two displays. The lack of screen real estate wasn't a problem since the stations wasn't used for making contacts. The information display was all we needed to monitor our contest operations and to assist with the choice of operating tactics.

One of things I didn't like was the default colour schemes and fonts used in several windows that are not easy to read with older eyes. We made a few changes to increase the colour contrast. Most of the time it didn't matter since the operators were focussed on only a few windows. The window for available multipliers and contacts was easy to read and used to rapidly pounce on stations. I could run through the list and often work them faster than when running. One call was all it took in most cases.

The station automation and antenna selection is integrated into DXLog using APIs. I prefer to have the choice of whether to use DXLog or N1MM. For that reason I use RadioInfo UDP broadcasts for my station automation system. I eventually plan to enhance my software to accept RadioInfo messages sent by DXLog. They appear to be similar and rich enough to support the same functionality I have with N1MM. I don't want to be locked into either contest logger.

Contest scoreboard

This was the first time I've used the contest scoreboard. It never struck me before as useful or interesting. Vlad set it up for us to track our score against the other major M/2 competitor in Canada: VA2WA. I won't say that I'm hooked but I was impressed and it did indeed keep us motivated.

When our score was close to them or other M/2 you could feel the tension and motivation in the shack. I don't know if the rate increased because of it but we became more aware of available multipliers and chased them. We started slow on Friday due to technical issues so it took many hours to catch up to the competition. In the end we did and finally overtook them. Although that was our primary objective, we also watched how we did relative to our closest American competitors N2NL and K9RX. We had little hope of catching K9CT and W3LPL, the former with superior antennas and the latter also having a superior location. Nevertheless it was fun to watch.

Racking up the QSOs and multipliers takes time so it doesn't help to pay too close attention. We were always curious about which bands they appeared to be running or chasing mults. That made us rethink our tactics: were they the best or should we try something different. Most competitors reported band break downs which were very useful to understanding what they were doing in near real time. I enjoyed that aspect of the competition so much that I let others operate more so that I could spend time considering strategy. When advisable I would make suggestions to the operators. 

I can't say that the contest scoreboard helped us do as well as we did but it certainly made us pursue tactics that would boost our score. It is unlikely that I'll use the scoreboard often other than for multi-ops. I am less curious about the activity of others when I operate by myself.

Operating styles and tactics

The fundamental strategy for an M/2 entry can be summarized by one word: run. The only reasons not to run are to chase multipliers or to call other runners. The S & P sessions are brief and intense, and are usually interspersed with CQs. That is, when there is no response to a CQ you click on a spot, call and hopefully work them. 

Whether or not you work them, you immediately return to the run frequency and CQ again. Speed is of the essence. If you fail to work them, repeat the cycle.

S & P sessions last longer only when runs are particularly dry. A band change is often the more productive option. Band changes are otherwise only justified to run faster or to chase multipliers. Counting band changes is critical since each station is permitted 6 per clock hour (e.g. 1300Z to 1459Z); 8 are allowed in CQ WW.  Changing bands and back again counts as two band changes so you have to pay close attention to the countdown window (DXLog and N1MM both have this feature). We would use unused band changes late in the hour to chase multipliers just before they reset to 6.

The third operator, when there was one, monitors activity levels and available multipliers and makes suggestions to the operators on desirable bands and openings, and spotted multipliers. Runs from a big gun station can be intense so the operator might not immediately notice the spots. In all cases it was the operator that made the choice of what to do and when.

Other tactics were minor in comparison to the ones mentioned above. For example, which antennas to use, individually or in a stack, 

Those who were not operating did errands for the operators. This included delivering food, drinks and snacks. There was almost always someone on deck, ready to jump in when an operator needs a break. With 5 operators there was plenty of time to chat and sleep. Since I'm a nighthawk, I left most of the high band running to the others during the day so that I could operate overnight. My time to quit was after the gray line openings on the low bands were done soon after dawn.

Late in the contest we celebrated every multiplier and eagerly watched the scoreboard to see how it would change our position. The scores were often very close and every multiplier or brief run had a large impact. It was tremendous fun.

Conditions

This was one of those rare contests when propagation conditions were exceptionally good. What was particularly unusual was that all 6 bands were hot. 10 meters opened at sunrise and didn't close until hours after sunset. 160 meters delivered openings in all directions and the multiplier counts show it. We worked at least one QRP station in Europe. Even 20 meters performed well throughout the day. It is often suppressed midday by D-layer absorption that increases during a solar maximum.

The solar flux was high but not very high. Days of quiet geomagnetic conditions leading up to the contest appeared to be a major contributing factor.

A good example of the low absorption was the propagation on 40 and 80 meters. Our run rate to Europe continued for hours after their sunrise. We were astonished to be called by Europeans on 40 meters as far south as Italy a full 3 hours after their sunrise. The daylight openings were shorter on 80 meters but still exceptional. I haven't seen such quiet conditions in a major contest since the solar cycle minimum. 

Quiet geomagnetic conditions that last for days are unusual at a solar maximum. Sunspots are constantly erupting and sending radiation and particle streams our way. Nor were there any flares to cause radio blackouts during the daytime. 

Yet geography still matters. Stations south of us were able to reach a little further for a little longer. That raised their QSO totals relative to those of us further north. With the bulk of contest QSOs between North America and Europe (80% or higher), geomagnetic latitude matters. Multiplier potential was more equitable since those come from all directions. We were not disadvantaged in that respect.

Going home

After sharing a post-contest meal we went our separate ways. Leaving was not easy. Not because of the camaraderie but because of the awful weather and road conditions. We were beset by heavy snow squalls and high winds that made driving treacherous. Although the plows were doing their best on a Sunday evening, speeds were slow and the accident potential high. 

When near zero visibility whiteouts greeted us on the major highways, the semis took control and kept me safely on the roadway. Luckily we all got home without incident. It was not the best way to end the weekend. The photo at right gives you an idea what it was like, except that the highway was wider, the traffic heavier and it was dark with headlights only serving to blind drivers from the bright reflection off the heavy snowflakes.

Despite the commute challenge, the contest was well worth the trouble. As many have noted, conditions were outstanding on all bands and the activity high. Many records fell that weekend. If our score holds up we will have set a new Canadian M/2 record. That would be a nice accomplishment for a fun weekend with good friends.only show totals

GPIO Protection From an Op-amp Circuit

I have finally begun the design and implementation of a new controller for my two prop pitch motor rotators. The "legacy" controller is home brew by a previous owner. It is ugly and has problems, and the "temporary" outboard op-amp circuit and meter are especially unsightly. The time has come to eliminate it from my desktop.

The plan is for the power supplies to be placed out of sight under the operating desk, and operated from a lightweight and ergonomic controller on the desktop. The controller will be software based using an Arduino. There are several components:

• Two 24 VDC power supplies, with switching for the AC and the clockwise and counterclockwise direction wires; I have two power supplies but I want to rehouse them in a new enclosure
  
• Digital display of rotator direction using op amp circuits driven by the existing direction pots on the towers
• Manual activation via push buttons, and eventually supplemented by software controls
• Software limits and provision for greater than 360° rotation
• Fault detection and protection

There are commercial products that will do the job. However, as anyone that has followed this blog knows, I like to do it myself. Yes, it is less expensive to home brew, but I like the satisfaction of building what I can. This is not a particularly complex project so it is well within my abilities. There is ample learning to be done along the way, which is a major theme of this blog.

There will be further articles as the project progresses. In this one I will focus on one design challenge: protecting the Arduino processor from high and low voltage applied to its GPIO pins. The 741 op-amp circuits will employ ±15 VDC supplies and the microprocessor is acutely sensitive to out of bound voltages. Mistakes can happen during construction and during periodic adjustment of the op amp circuits and I want to reduce the risk of bricking microprocessors.

The maximum allowed range of voltages applied to the GPIO pins of a 5 VDC microprocessor is approximately -0.5 to +5.5 volts, whether for digital or analogue input. The range is narrower for a 3.3 VDC microprocessor, so you must confirm which you are using and choose a different value for D2, a Zener diode, in the following circuit.

One of my recent projects is to learn KiCad to develop custom PCBs. I have a long road ahead so I may resort to my usual use of protoboards for expedience in this project -- my time is limited. Its only purpose here is to draw the schematic of the protection circuit.

The purposes of the circuit include:

• Block negative voltage: D1 provides reverse polarity protection from negative voltages
  
• Limit the positive voltage: R1 and D2 (5.1 volt Zener diode) act as a regulator to keep the voltage well below 5.5 volts
  

The potentiometer in the test circuit is used to vary the source voltage from the 9 volt battery over its range from 0 to ~9.5 volts. The battery is reversed to test negative voltage protection. R2 emulates the high impedance of an analog GPIO pin of the Arduino. There are test points to measure the source voltage (A) the voltage drop across D1 (B), and the voltage presented to the GPIO pin (C).

The objectives of the test:

• Verify blocking of negative voltages and that positive voltages never exceed 5 volts
  
• Linearity of the circuit
• Voltage range for best linearity
• Component selection for best linearity

A push down protoboard was used for the test circuit. It is easy to substitute components. An inexpensive digital multimeter is used to measure voltage at the test points and to confirm resistor values. Readout precision is 0.01 volt ±0.01 count error. Multimeter accuracy is unknown.

Populating the other half of the protoboard is a circuit for testing the use of an LCD display with the Arduino Uno. It will be used for the controller software and display. I'll leave that component of the controller for another article, but I will mention that connecting and using the LCD display was far easier than I anticipated. I was even able to quickly construct the custom characters that I'll need.

I varied the source voltage from 0.5 to 5.5 volts in 0.5 volt steps with the potentiometer. I measured and plotted the voltage at test points A, B and C. Lower voltages are cut off by the silicon junction of D1 and higher voltages are rolled off by D2. The linear range, if it exists, must lie between those extremes. I repeated the test with R1 values of 220 Ω and 1000 Ω. A higher R2 may be desirable to limit current and heat dissipation during over-voltage conditions. The B curves are identical (as expected) so the yellow curve is hidden beneath the blue one.

The test went so well that I saw no need to substitute components for D1 and D2. Negative (reverse battery) voltage read 0.00 volts at all test points at all potentiometer settings up to the limit of the new 9 volt battery (9.7 VDC) the voltage at test point C never exceeded 4.9 VDC. There is a wide linear operating range for positive voltages. Although this is what I expected it is better to be certain. Those straight lines on data sheets can deviate depending on circuit design.

The linear range is from 1.0 to 5.0 volts (A), within ±0.02 volts at test points B and C. Linearity is critical for tracking the linear direction pot, assuming linearity of the op amp circuit. Non-linearity, had it been present, can be remedied by an improved hardware design, software compensation based on the circuit's operating curve, or by restricting the voltage to a narrower range where it is found to be linear.

In my application the target voltage range will be 1.5 to 4.5 volts, representing directions from 0° to 360°. I leave a small amount of the linear range at both ends to exploit the lack of mechanical limits on the prop pitch motor to extend clockwise and counter-clockwise rotation past 180° (due south) by a small amount. The rotation loops can accommodate at least 20° of additional range in each direction.

Although the choice of R1 did not affect linearity in this test, the value still matters. As mentioned earlier, a higher value serves to limit the current when the source voltage is high. However, a higher value reduces the usable resolution due to voltage division with the load (GPIO pin). Notice the slopes of the red and green lines. I did not measure the current through R1 and D2 during the test.

For the source voltage range of 1.5 to 4.5 volts, the corresponding range presented to the GPIO pin (C) is 2.8 volts when R1 is 220 Ω and 2.66 volts when 1000 Ω. Thus the centre (north) positions are 2.9 and 2.83 volts, respectively.

The Arduino has a 10 bit ADC with 1024 values from 0 to 5 volts. We're using a little more than half that, say, 60%. The direction readout is therefore approximately 360 ÷ 600 = 0.7°. The difference may not be so pronounced when connected to an actual GPIO since the input resistance is typically higher than the 10 kΩ used for testing.

While not a concern for HF yagis, a higher resolution may be desirable for VHF/UHF antennas, but only if the resolution of the direction pot is better than the software resolution. High resolution can be useful for monitoring any antenna system rocking in the wind and to ascertain whether the rotator is moving in the moments after power is applied.

Too simple?

After reading this article you might wonder whether the circuit and its characterization are too simple to require exposition. Simplicity is in the eye of the beholder. For those that live and breathe electronic circuit design, this article might elicit nothing more than a shrug. 

But I'm a ham, not an electrical engineer, although I do know software design. Understanding simple circuits like this is well worth the effort. Walking through the process is likely to be of use to other hams like me. It might even inspire experimentation and home brewing, and that's always a good thing in our technical hobby.

Next steps

The next step is to connect the circuit to the Arduino. The software to convert the voltage to a direction and display it isn't complicated. The op amp circuit to convert the direction pot resistance to the Arduino ADC will be designed to centre its output on the protection circuit's linear range. 

It will be slightly different from the prototype I am currently using since a meter reads current and the ADC reads voltage. The op amp circuit will be adjustable so that no software correction will be needed to derive the correct direction.

Reflections on CQ 160

Single band contests like the CQ 160 Meter contest have a unique character. This goes beyond their particular appeal to aficionados of those bands. Once you've worked a station, that's it; you don't get to work them again. There are exceptions, such as the ARRL 10 Meter contest where you can work stations on CW and SSB if you enter the mixed mode category.

As the contest progresses there are fewer stations to work and the rate drops. The decline can happen quickly. In last week's CQ 160 contest, my first hour rate was over 130. By the time I quite on Saturday evening the hourly rate was between 30 and 40. Stations that put in a full time effort saw their rates plummet further. If rate is what draws you to contests, these are rarely where you want to be after the first few hours. I took several breaks on the first evening at my convenience and I doubt that my score was impacted despite the high rate that I interrupted -- the stations were almost all there to be worked later.

Most contesters drawn to single band contests will only stick with it if there are other things that appeal to them. For example, I operated QRP in the ARRL 10 Meter contest. QRP introduced a self-imposed handicap that made every QSO more challenging, especially DX and DX multipliers. Since a high rate was unlikely, the rate (such as it was) didn't decline as much as it did for higher power stations. I've done this as well in ARRL Sweepstakes for the same reason since despite being a multi-band contest, you can only work a station once.

For me, the appeal of the 160 meter contests is DX. That is never easy on top band. Outside of contests, CW activity is very low. While it is nice to see the same DX stations every night, there is little novelty. Watching the ebb and flow of top band propagation is interesting but not enough to keep me there. I am always eager for top band DX since I am relatively new to having an effective station.

Contest weekends significantly raise the activity level, and the CQ 160 Meter contest does it best. The activity level on 160 during CQ WW CW is high but not as high. The difference is that in CQ WW many stations activate rare countries and zones. The potential country haul during CQ 160 is lower but it is interesting to see more activity from home stations.

With limited DX opportunities and little rate beyond the first hours I chose a non-serious entry in this year's CQ 160 Meter CW contest. I took maximum advantage of the opportunity by operating high power and assisted. I quit the contest when the rate was painfully slow and there was little prospect for improving DX propagation. 

If you want to read more on my thoughts about single band contests like this one, you can find them in several past articles. For example, this one about the CQ 160 Meter contest.

Competition

I can't win. This is not a defeatist attitude. I am realistic about the competition in this contest. 160 meters is a tough band to successfully work DX and every decibel counts, and so does geography. Ultimately this contest is one of attrition since there is little difference of contacts and multipliers between similarly equipped top-ranked stations in the same area and entry category.

The winners have antennas with gain. A typical 4-square or K3LR parasitic array has 3 to 4 db advantage over my single vertical. Power is another area where I fall short. I stay pretty close to the Canadian legal limit. That's a 2 db disadvantage to the US legal limit. Many stations don't respect their countries' power limits. There is little risk of enforcement in either country for stations in rural locales where EMI is low risk.

When DX signals are riding the noise, those decibels convert to QSOs and multipliers that are out of my reach. I'm not complaining since it is an incentive to pursue phasing my towers for top band gain. I can already hear well with my Beverage system. Indeed, I can hear many DX stations that cannot copy me. Not everyone can live in a quiet location.

Since I am not in contention to win I can only compete against myself. That is, to improve my score over preceding years or to reduce the gap to the big guns on top band. I prefer to instead chase DX in the contest and do as well as I can without upsetting my life by staying awake for two nights. My objective may change when/if I improve my transmit antenna.

The short version: I can't win and the contest is long, so I decided to have fun by working DX and running NA stations at other times. Mission accomplished.

Capabilities

My transmit antenna is very good but has no gain. Recently I doubled the number of radials for higher efficiency which, although it helps, is an improvement of no more than 1 db. I have little trouble working DX when conditions are cooperative. Late Friday evening when the propagation to Europe was good, I did well at running European stations.

My receive capabilities are better. With three reversible Beverages that are from 150 to 175 meters long, I can copy most anyone. I could build a vertical receive array with a higher RDF, but hearing well is not my problem. I hear better than almost everybody else hears me. Most hams have a higher local QRN level. 

An imbalance between receive and transmit capabilities can be a problem. Usually it's a station running high power and unable to hear due to their local noise. We call those alligators: big mouth and small ears. I'm in the opposite situation. I something think of myself as an SWL (short wave listener) when I can't work many DX stations that I hear very well. Alligators and SWLs both have difficulty filling the log.

The only antenna work I performed in preparation for the contest was to repair an intermittent F connector on the northeast-southwest Beverage. I pulled off the twist-on connectors, removed corrosion and lightly coated the threads with dielectric grease. It was an easy job in the cold winter weather. I considered myself lucky that recent wind and ice storms didn't damage the Beverages.

DX prospects

An important difference between a major contest like CQ WW and CQ 160 is the number of contest DXpeditions. There are many in the former and few in the latter. Don't expect to work many countries in CQ 160. That said, there is a lot of DX to be worked when propagation cooperates. This year that was mostly limited to contacts between Europe and NA. That's quite good on top band for the peak of the solar cycle. I logged 51 countries, which is pretty good for the hours I put in. The highest I saw from eastern NA was 70.

There is an ebb and flow to propagation on 160 meters. It isn't like the HF bands. Signals from an area can rise for 10 or 30 minutes before falling back down. You need to be on at those times to work DX. Since these events are largely unpredictable, you must be diligent during the contest. I stayed up late the first night and was rewarded with lots of contacts with Europe and the west coast of NA.

Stations located far from the major global population centres have little incentive to be active in CQ 160. They can only work stations during periods of propagation enhancement. Many of those stations are in the southern hemisphere where it's summer and the atmospheric noise is very high. That's less of a hindrance in multi-band contests since you need only check 160 occasionally. For a 160 meter contest it can be very dreary indeed. 

When there is propagation to those distant parts of the globe there may be no activity at all. Several times during the contest I saw weak but consistent RBN reports from a CX, yet throughout the contest I heard no stations from South America further south than Venezuela.

Propagation extended towards the west and northwest to KH6 and KL7. I worked two of each but nothing farther. Japan and other east Asian stations were active but not heard here. It was the same for them except for a few of the top band big guns. Based on their reports, ZL and VK stations worked mostly western NA stations, and few of those.

I enjoyed working the DX on offer, and that's all you can ask for on 160 meters. Many times the propagation during 160 meter contests is far worse.

Conflict

With so many stations packed into a fairly narrow spectrum there is bound to be conflict. There were several examples of it during the contest.

When DX conditions to Europe are marginal, we can hear each other although QSOs are difficult. The reason is that most stations do not hear well, either due to local noise and poor or no receive antennas. NA and EU stations operate right on top of each other without noticing. Try to work one of those Europeans and conflict arises. I tried a few times and it wasn't worth the trouble, even when I did my best to avoid QRMing the NA station when calling the DX station.

One workaround that some stations use is to run high in the band. The idea is to be decoded by skimmers and attract stations to QSY from the lower end of the band where activity is greatest. It works, to a degree. One reason is that many skimmers use antennas that are poor for receiving DX stations and fail to decode weak signals. Another is that many of the stations drawn to the spot can't copy the weak DX station. Of course, there are many unassisted stations that won't see the spots.

Early in the contest few stations tread on the FT8 watering hole at 1840 to 1843 kHz. Over time, more and more stations went there. To avoid adding to the potential QRM I avoided calling those stations. As the rate slowed and FT8 operators abandonned the window (probably because of the QRM), I called a few of the CQers in the window. The damage had been done. 

There was similar interference with phone operators (AM and SSB) during the evening hours when they are most active. Due to the different mode band widths some "sharing" can be tolerated but it is never pleasant for either operator. Conflicts of this type are common during contests and some pile ups on rare DX stations since there are no firm mode sub-bands on 160 meters. I recall tuning to 160 meters during NAQP SSB and deciding not to bother searching for DX due to the large number of phone signals.

SO2R/SO2V operators that vacate a frequency to call a station (S & P) elsewhere risk losing their run frequency. There were several times when I came across a clear frequency and heard no reply to a couple of "QRL?" requests. So I called CQ to start a run. Soon the absent station returns and resumes CQing. Who's frequency is it? Since I wasn't serious in the contest I never bothered to assert my claim to the frequency. In other circumstances I might react differently.

Now it's time to talk about signal quality. That means key clicks. When everybody is packed together, key clicks interfere with stations up to 3 or 5 kHz away. Mostly it's just annoying while at other times the interference is a problem. I wonder how many of those stations realize how poor their signals are. They are glaringly obvious on the 7610 spectrum scope. I feel a little sheepish mentioning it since my FTdx5000 is notorious for key clicks. However, in this contest I used the far cleaner Icom 7610.

Prospects for improvement

Doing well in 160 meter contests requires more than a quiet location and good receive and transmit antennas. In a single band contest it is necessary to maintain a constant presence by running almost full time. Yet it is also necessary to search for and work stations S & P.

One of the methods that the most competitive stations use to increase their score is a second receiver (SO2V). Some go so far as a second station (SO2R). If the signal of the transmitter into the second receiver is too strong -- which is almost always the case -- the second receiver is disabled while transmitting. That is highly disruptive to effective operating, especially since the primary VFO or radio should be almost always running.

The solution is a remote receive antenna. It can be far enough to allow the second receiver to perform well during transmissions, or have a notch in its pattern directed at the transmit antenna. Using a receiver with a high blocking dynamic range allow continuous reception on the second radio, often quite close to the transmit (run) frequency. When a station is found, the VFOs or antennas can be swapped to call them with only minimal interruption to the run radio. This is ordinary SO2R behaviour but with both rigs on the same band.

SO2R for 30 hours is not easy, and is made more difficult by the paucity of new stations to work during the second night. There's a lot of effort for little return. Assisted operation using human spots and skimmers (RBN) eliminates the tedium of spinning the VFO and typing calls. Many top band operators dispense with assistance and embrace the challenge of doing all the work themselves.

I kept it simple and relaxed by operating SO1R with assistance. I felt no temptation to turn on the second VFO to call stations while I was running. SO2V isn't technically difficult if you can deal with constant interruptions due to the transmit cycle. I have no plan for a separate remote receive antenna to make SO2R/SO2V more viable on 160 meters. My only plan for improvement is to eventually phase the two big towers, as described earlier.

Scoring equity

It is a rare contest where the scoring equalizes the playing field among stations in diverse locations. There are two rules in particular that skew the points awarded to a QSO:

• The 5 points awarded to QSOs between US and Canadian stations favours Canadians because there are so many more Americans. US to US and Canada to Canada contacts are worth 2 points.
• QSO between countries are worth 5 points while those between continents are worth 10. That rule benefits stations just beyond the continental divide. For example, south Italian islands in Africa, Cyprus, Africa islands near Europe, and north South America.

The scores of Canadian stations are much higher than those of Americans with similar numbers of QSOs and multipliers. The difference is greater for African stations adjacent to Europe who typically win the overall. Of course, stations farther afield fare even worse since they can never make many contacts in a 160 meter contest. An inter-continental QSO is 10 points whether the distance is 100 km or 10,000 km.

Do these scoring differences matter? Solutions exist such as the distance and power based QSO points used in the Stew Perry top band contests. In CQ 160, if you restrict comparisons between stations in the same country, region and category, the scores are comparable. But many of the contests top awards aren't awarded on that basis. I don't expect to see scoring changes anytime soon. I'd be happy to be proved wrong. I say this despite the benefit to Canadians of the extant rules.

There is also the matter of hemispherical bias. A top band contest in January favours the northern hemisphere when nights are long and the atmospheric noise low. Southern hemisphere stations have short nights and high noise. Moving the contest to coincide with one of the equinoxes would improve hemispheric equality. That, too, is unlikely to change since the majority of 160 meter activity is in the north.

Recordings

I'll close with an interesting development. Several stations in Europe sent me recordings of our QSOs. Recording contests is common, and not only for the top competitors for whom it may be mandatory. I suppose some enjoy doing it for whatever reasons they might have. It was interesting to hear what my signal sounds like at the other end of a 160 meter DX contact.