Sunday, April 30, 2023

Relay Phobia

I have a phobia about relays and I really don't understand why. In this age of solid state switching there is something messy or unwholesome about having electro-mechanical devices scattered throughout my station. 

Is my attitude entirely counter to the evidence or is there anything to support my fears? 

It is good that relays are as reliable as they are since they are everywhere in a modern ham station, more than you might realize. Yet they can and do fail. This is mostly due to two factors: quality and abuse. A little knowledge can help us to get the most from relays. Technology has not advanced so far that relays can be avoided.

As you might guess from my irrational phobia, I am not an expert on relays. I had to learn more about them to assuage my concerns and to choose and use relays in my many home brew projects. 

They are used in my station to switch:

  • DC and AC power
  • Antennas
  • Antenna switches
  • Antenna direction and stack controls
  • Matching networks
  • SO2R audio and keyer

Many of those are home brew projects. I even have relays that control other relays! Relays are common in commercial equipment to control power, route transmit/receive RF, inter-stage switching, ATU, BPF, antenna ports and more. One malfunctioning relay can ruin your day or contest. 

It's that critical role they play and the risk of failure that stokes my fears. I trust that relays are appropriately selected and employed in commercial equipment. This is not necessarily the case. One famous example is the Yaesu FT102. Every relay in that transceiver had a short lifetime and is responsible for far too many of these otherwise excellent rigs ending on the trash heap. 

Mine eventually landed on a flea market table. It sold for a low price that was, unfortunately, fair value. I could have salvaged the rig with replacement relays, which are available, but I deemed it not worth the trouble. It was a great rig in its day but no more.

Returning to my phobia, my lack of knowledge and therefore trust of relays leads to odd behaviour. For example, I have an irresistible urge to idle all the relays in my extensive antenna system when I step away from the shack for more than a few minutes. I don't yet have such a feature in my station automation software so I have move the rig to 6 meters to idle the 2×8 antenna switch and manually deselect directions and modes for the stacks and several other antennas.

Is this a feature I should build for my station automation system? Do relays deteriorate when they're energized? Is it better to leave them energized to avoid turning them off and then on again when I return to the shack? Relays do have lifetimes based on these metrics and many others as well. Let's see what the data can tell us.

Above is an extract from an Omron G2RL datasheet -- you may have to click on it to make it readable. It seems fairly typical of small, sealed relays. A fact that pops out quite clearly is that the mechanical life of the relay is far greater than the life of the electrical contacts. The mechanical lifetime is so long, even when switched continuously at a rate of over once per second, that it is effectively eternal in typical ham use.

The shorter electrical lifetime spec is misleading since it is for the rated contact voltage and current. When these are lower the lifetime increases. The chart at right is also from Omron. Clearly there is a benefit from choosing relays with higher ratings than what will be experienced. Better yet, by avoiding hot switching the lifetime can be greatly extended.

By avoiding hot switching and using high current contacts the lifetime is sufficiently extended that there should be little concern for the reliability and durability of the relays in my home brew projects and the commercial products I use. Relay lifetime can easily exceed my lifetime.

Hot switching can be avoided or mitigated in well designed equipment. Measures include:

  • Software features to prevent hot switching of antennas. I have this feature partly implemented in my home brew station automation system.
  • Sequencing of circuits so that relay closure occurs before current is applied, and the relay opens only after the current is cut. Sequencing of the input and output port relays is standard for the ports of high power amplifiers and in V/UHF systems with mast mount LNA (low noise amplifiers).
  • Step-start relay coil with a higher than nominal voltage so that closure is rapid. Arcing and contact wear are thus reduced when hot switching high voltage or current is unavoidable. 
  • Sequencing or ramping motors and other devices with a high initial current. In our shacks the most common application is high power motors in rotators such as prop pitch motors.

By avoiding hot switching entirely you can achieve relay lifetimes of well over 1,000,000 on-off cycles. The TE application note I linked to has a lot more on the topic worth reading, including contact materials, arc mitigation and other recommendation for selecting relays. 

An amusing story a friend told me related to the fourth bullet above was using relays to switch a prop pitch motor. Depending on cable resistance, the motor starting current can be 20 A. One day the contacts fused from excess arcing and he didn't immediately notice that the motor hadn't stopped. Luckily the damage limited to a shredded coax rotation loop. 

Now he sequences the power by first energizing the high current DC relay and then the low current AC mains of the 24 VDC power supply. I plan to do the same with my prop pitch motor; currently, I do the sequencing with manual switches.

If you must switch transmit level RF and proper sequencing cannot be guaranteed, or the RF voltage is high, there is the option of vacuum relays. They are expensive, new or used. They can be found on many amplifiers. They are also useful when switches are needed at high impedance (high voltage) points of an antenna. The contacts can still be damaged by excess arcing but they should not oxidize.

The photo at right shows the effect of arcing on relay contacts. This relay has failed, will fail soon or the contacts might even fuse with continued abuse.

The relay contact protection measures described above are fine for high power switching but not necessarily for small signal switching. You might think that small signal switching is not worth worrying about, but it is. This is as true for relays used in receive arrays and control systems as it is for RF relays that must work well for both transmit and receive.

Contacts oxidize or undergo chemical changes due to current flow and micro-arcs. Open relays will accumulate grime. Sealed relays, like most of those I use (see above), are largely immune to the latter. There are mitigation measures described in the linked TE application note. However, placing components across the contacts that are suitable for DC and 60 Hz AC, it is not appropriate for RF applications.

The easiest way to deal with contact problems in sealed relays is to periodically switch them under load. In this case arcs are our friends. They burn off unwanted oxidation and other unwanted chemicals and restore low resistance conductivity. There is micro-arcing even for the small DC currents switched by the relays in my automation system.

Antenna relays that are usually not hot switched can be periodically switched while transmitting to clean the contacts. Typical power levels commonly quoted to do this range from 20 to 50 watts. Cycling 10 under load may be sufficient. Don't try this with your big amplifier! Those arcs can rapidly damage the contacts. In any case, amplifiers with fault protection will quickly go offline when an arc occurs. 

I occasionally did this when I had I was manually switching antennas, modes and directions and I suspected a problem. It didn't hurt to do it even when the problem might be elsewhere. In my case it always turned out to be a relay in the transceiver or amplifier. I can't do this with the automation system since it doesn't allow hot switching. Instead I would have to add a "cleaning" feature or temporarily connect the control cables to the manual antenna controller, which I keep as a backup. But I have yet to bother since there have been no recent relay problems to resolve.

Returning to the matter of relays used solely for receiving systems where the RF level is always small, dynamic cleaning methods don't work well since there is no arcing. If the contacts corrode there may be little remedy except replacement. Contact design and coating are particularly important. For example, using multi-pole relays in parallel for contact redundancy. Gold plated contacts are another possibility. 

An increase of contact resistance can weaken reception, especially at the low impedance 50 Ω side of the receive antenna switching circuit. Contact resistance will likely get worse over time unless the contacts can be cleaned. Sealed relays protect against environmental effects but also make it impossible to access the contacts to burnish them. I have had relay contact problems in my rebuilt Beverage head ends. Cycling the relays has served to reserve intermittent problems until now. It wouldn't surprise me if I eventually have to replace the reed relays in the remote Beverage switch (see below).

A trickle DC current can serve to wet the contacts to keep relay contacts clean in lieu of arcing. Receive RF is far too weak on its own for contact wetting. Combined DC and RF is most commonly found in bias-T circuits where the coax carries both, with signals combined on one end and separated on the other. This is done to eliminate a separate control cable run to the relays at the antenna end of the coax.

The only place in my station that I use a bias-T this is to reverse direction of my three reversible Beverages. There is a separate control cable to the remote Beverage switch, with one line putting +12 VDC onto the coax to the active Beverage via a bias-T circuit. The DPDT reed relays in the remote antenna switch thus carry both DC and small signal RF to the bias-T in the Beverage head end.

Does the reversing current help keep the reed relay contacts clean? I don't know. I've never had a problem with the contacts except when lightning struck the Beverage system last year. Those contact arcs destroyed the relays.

Contact wetting is in conflict with a bit of ham lore. There is a common belief that a DC bias on relay contacts can worsen contact resistance over time. Certainly I've seen contacts develop problems over years of use but I doubt whether that is the reason since it runs counter to my understanding. I don't know the truth of the matter so I thought to mention it since may encounter it from time to time. There could be a galvanic effect but it may be nothing more than a lack of arcing to clean the contacts.

Wrap up

Solid state switching has its own challenges despite not have moving parts and physical contacts. When employed correctly, relays will last a lifetime. There are inexpensive relay boards for Arduino, Raspberry Pi and similar controller that save the trouble of designing and building solid state control circuits. For switching high power RF, relays remain the easiest and most cost effective solution. That could change in coming years

In summary, relays are great. It's a matter of choosing suitable devices and following best practices to keep them healthy. They're inexpensive enough that replacing them isn't a great burden. 

My relay phobia is unjustified.

Thursday, April 20, 2023

You Don't Need an N Connector

N and UHF connectors have their pros and cons. The same is true of other connectors, be they BNC, SMA, F, DIN and the many other RF connector series. I use them all, by choice or by necessity when present on equipment I use. 

There is no one correct connector. Each is designed to meet different engineering requirements. Connectors themselves come in many varieties -- chassis mount, PCB mount, screw on, solder, crimp, clamp, mechanical -- to suit different environments and applications.

The majority of hams use UHF connectors for RF connections. They are ubiquitous on HF transceivers, except those with very small enclosures. They perform well, are inexpensive and easy to use. 

V/UHF operators tend to use N connectors to avoid the impedance "bump" of UHF connectors that can be problematic at higher frequencies. UHF connectors do not preserve the Z₀ of the coax throughout their lengths, whereas N connectors do. There are different N connectors for 50 Ω and 75 Ω systems. There is no such differentiation for UHF connectors.

The reason for the impedance discontinuity in UHF connectors is that there is a short air gap between the end of the coax and from the mechanical structure of the termination, in particular the insulator surrounding the centre pin. The dielectric constant and conductor separation vary over that span. We have effectively inserted a short transmission line with a different nominal impedance. Mechanical design takes priority over impedance performance in UHF connectors. The impedance bump can be worse for coaxes smaller and larger than RG213. 

A typically quoted value for the bump is 30 to 35 Ω, which is believable from inspection of the transmission line equation for coaxial cable. Let's use the lower value even though there are differences across manufacturers and adaptors for large and small coax diameters. I'll further assume that the length of the discontinuity is 1 cm (10 mm or 0.4"), with a VF (velocity factor) of 0.7. Choosing these values should simulate a worst case for PL259 (male) UHF connectors.

I used SimSmith to insert a 1 cm long section of 30 Ω coaxial transmission line between a 50 Ω generator and a 50 Ω load. You can see that the effect is negligible at 50 MHz. It is marginally significant at 144 MHz and notable at 450 MHz. Of course there will be more than just one connector in a transmission line, with one at the rig, one at the antenna, two at each coaxial joint (e.g. barrel connectors) and two at each intermediate device such as antenna switches. The one at the antenna is usually of no interest since the feed point matching system typically compensates for that connection. The same is true of the transmitter or tuner at the generator side of the line.

SWR does not sum arithmetically. You can't simply add up the fractional quantities to discover the net SWR (mismatch) due to multiple connectors. That is, two connectors that exhibit a 1.05 SWR in a 50 Ω system when properly terminated do not give an SWR of 1.1. In select cases, insertion of a short 30 Ω transmission line can improve the match.

In a real system there are many variables that a simple analysis like mine cannot possibly contain. In each system you'd have to measure the complex impedances and coax lengths for each band of interest, along with the major effects of the load mismatch (no antenna is perfect!), generator action and other factors. My SimSmith model is illustrative but not universally applicable.

My usual recommendation to those who ask is to avoid N connectors from 160 to 2 meters. The performance impact of using UHF connectors ranges from negligible to modest, and is usually dominated by the antenna impedance, which is usually nowhere near 50 + j0 Ω. Other factors such as variation in coax impedance by manufacturer, application and age have impacts greater than that of the bump due to UHF connectors

None of this is a criticism of N connectors in general. It is reasonable to ask: why not use N connectors? Why not sweep away the uncertainties by avoiding UHF connectors wherever feasible? There are good reasons to prefer UHF over N connectors other than that of the dreaded impedance bump.


The typical N connector has several components. It is an unwise ham who breaks open the package without a plan. It is easy to lose one or more of them, especially the centre pin.

I opened one of my bags of used N connector parts so you can have a look. These are 50 Ω connectors that were salvaged from old coax. Most of the pieces are present, and I have other bags with different assortments. I have fewer pins than bodies, which is annoying. 

The main gasket is cut in two when first assembled, but they can be reused if you are very careful. Alternatively, do an especially good job weatherproofing the assembled connector. The gaskets in a pristine N connector provide good weather protection, but don't depend on it. Protect them as you would a UHF connector for reliable long life. Braided coax wicks moisture and will soon degrade.

Used N connectors can be frustrating unless you have a good eye for the slight size difference between 50 Ω and 75 Ω pins. I am not so worried about the impedance than I am about trying to mate mismatched pins. It doesn't work and you can't force it. Force fitting a male 50 Ω pin into a 75 Ω female pin will destroy it. UHF connectors don't have that risk.

Notice that I've shown soldered connectors. That is only for the centre pin since the braid is mechanically bonded when the external nut is tightened. However, you must be precise with the dimensions of the exposed braid, dielectric and centre conductor or the braid will be loose. It's a problem I've encountered many times with connectors I've assembled and with those that have followed me home.

A related concern is the position of the centre pin. Get the dimension wrong by just 2 mm and the male and female pins either won't make solid electrical contact or will bottom out and bend when the connectors are joined. Again, this can't happen with UHF connectors.

There is another challenge with pin alignment when using RG213 and similar polyethylene dielectric coaxes. The braid can slide over the dielectric due if too much of the coax weight is taken by the connector. The same will happen by weather induced thermal expansion. It takes little movement to have a connector suddenly fail in the middle of a winter contest due to lost electrical contact or arcing. It is more common than you might imagine.

I had to deal with improperly aligned N connectors on a fellow ham's tower last year. If you have trouble properly preparing the coax and assembling an N connector on the ground, well, it's far worse doing it up in the air. The cables with the poorly aligned connectors were cut down for repair on the ground and then lifted back onto the tower and tied down to ensure there was no weight (stress) on the connectors. One of them failed again a few days later. This time we decided to cut it off and friction fit a UHF connector to the coax. Half a year later it's still working despite not being soldered.

Contact Area and fragility

Compare the size and area of the contacts in UHF and N connectors. You can see examples in pictures above and below. The small centre pins are particularly fragile and misalignment when joining connectors can bend the pin and distort the gripping flanges of the female pin. Pins of 75 Ω connectors are smaller and more fragile. Pressing a male 50 Ω pin into a female 75 Ω pin can easily destroy the female pin. It will never grip well after being overspread.

Alignment is especially problematic with solder connectors on flexible cables with a stranded centre conductor (e.g. RG213). Getting the pin centred must be done manually. A small deviation off centre can easily damage the pins when joining connectors. The hole in the female pin is very small and easy to miss since once your view of the pins is hidden when the connectors are brought together. You must be careful and go by "feel" alone.

Just 1 mm of linear insertion error due to the pins projecting too little or too much results in inadequate contact or bending, depending on the direction of the error. In the former case, the connection is unreliable either due to high resistance or arcing with high power. In the latter case the constant impedance of the N connector is lost and there can be a short or arcing when the pins are bent. The outer spring flanges of the male connector are usually not a problem because they are stronger and their position fixed by being bonded to the connector body. 

Threading ensures proper positioning of the male flanges against the body of the female connector. Since the threads are so fine -- ⅝"-32 UNEF -- crossing threads is easy, which can damage the threads and the connector flanges. Damage is easier with hard line coax since it is more difficult to manipulate the cables to ensure they're properly aligned when the threads are engaged. On a UHF connector, the large centre pin engages first and guarantees proper alignment of the contacts and thread engagement.

Above is a splice between a male N connector on buried LDF4 cable to my 80 meter vertical wire yagi and a female N on the long LDF5 run to the station. This week I discovered that the flanges are making intermittent contact, and that is the reason for the antenna's failure this winter. My guess is that I bent something slightly when struggling to align the connectors for thread engagement. I have it working again but I may replace one or both connectors this summer just to be certain that it survives next winter.

The centre pin and female spring flanges on a UHF connector are large and robust in comparison to those of N connectors. They are not easy to damage, yet there are hams who manage to do it anyway! If the flanges are overspread they can often be fixed, at least in an emergency, using a small flat blade screwdriver to bend them inward. The large contact area of the centre pin and threaded shell assure good electrical contact even in cases of minor damage. 

There are no gaskets in a UHF connector so it is mandatory to use external weatherproofing. Moisture infiltration will cause corrosion, and that leads to high resistance and arcing despite the large contact area. Never leave a UHF connector unprotected outdoors, even temporarily. Weatherproofing it after rain, snow or morning dew (condensation) will trap the moisture inside.


All is not doom and gloom! There are N connector styles that largely eliminate the problems of exact dimensions and pin alignment. These are so good that there is no reason to succumb to the temptation to buy or reuse solder N connectors. But it can be expensive if you have a lot of connectors in your station, as I do.

I am pretty well forced to use N connectors for the thousands of feet of Heliax in my station. They can be found surplus in quantity and at very attractive prices. If you know someone in the commercial wireless business you can often get them for free by dumpster diving. The Andrew connectors are very robust and I've had better than a 90% success rate reusing scrounged connectors. 

UHF Heliax connectors are rare. Newer commercial installation often use 7/16 DIN connectors, and they are not yet showing up in quantity on the used market. The DIN connectors are larger and hardier constant impedance connectors. The ring of thick flanges on the female 7/16 DIN connector in the picture at right are for the centre pin, not the outer conductor!

I have several new and used DIN connectors in my stock. They are useful for splicing sections of Heliax but not at the ends of the feed line where you'll likely need an adaptor. I have just one N-to-DIN adapter in my stock and it wasn't cheap. I have never shopped for a 7/16 DIN connector for LMR400 to connect to Heliax DIN connectors. If they exist they are certain to be expensive.

These are my most recent flea market finds. A paid a modest price for these used LDF5 Heliax connectors for the convenience of just picking them up and walking away. Dumpster diving is free but there is effort involved. I've already started giving them away to friends. I have enough on hand and more will undoubtedly appear in the coming months and years. Surplus and "reel ends" Heliax is available at good prices if you are fortunate enough to have contacts with commercial tower service companies.

Adaptors for joining N and UHF connectors are common and inexpensive. The kind shown in the picture are often found surplus for a dollar. I also buy the less common female-to-female adaptors for when the Heliax connector is an N male.

I use many of these adaptors on the tower and on the ground to interconnect RG213 and LMR400 running to antennas and antenna switches. One good feature of adaptors is that the N side is always perfectly positioned. Alignment is never a concern.

An N connector I particularly like for LMR400 coax is one with a "captivated" centre pin. It is also advertised as a clamp connector. Like the adaptor, the pin is fixed and can never slide out of alignment. On the inside of the connector, the coax centre conductor is press fit into flanges that grip it from all sides. The outer conductor is held in the usual manner, be it by a nut or crimp. There is more latitude with dimension errors since there is room for axial motion within the flanges.

Fitting the coax can be frustrating if you don't closely follow the installation instructions. The centre conductor must be chamfered with a file so that it can fit inside to lift the flanges as it is pressed in. Without the chamfer the conductor won't fit and no amount of pressing will help. The coax must be very straight for this operation. Any deviation must be corrected since there is little wiggle room to align it with the flanges once it's inserted, and you must do it blind.

Captivated connectors are not cheap. I purchased several at a good price when I was first building this station. I thought it would be easier and cheaper than using UHF connectors plus an adapter to the Heliax N connectors. I was wrong on both counts so I stopped using them. However they are excellent connectors and they are in wide commercial use for their reliability. 

I couldn't find a good picture of an LMR400 captivated connector online so I took one of a single piece Heliax connector in my stock that uses a captivated centre conductor. The difference is that the Heliax centre conductor is hollow so the flanges grip the inside rather than outside of the conductor.

There are restrictions on the application of captivated connectors. They fit one and only one type of coax. You must buy the correct connector for the coax. Captivated connectors don't work on coax with a stranded centre conductor like RG213.

Two-piece Heliax N connectors are easier to install correctly than those for LMR400 or RG213. One reason I like using LDF5 in my station is because the cable and its connectors are widely available on the surplus market and the connectors are easier to install than on smaller Heliax (LDF4) and larger Heliax (LDF6 and LDF7). All it takes is a hacksaw, knife, file and wrenches to make a perfect termination.

More alternatives

Does all this information make your head spin? Do you really hate fooling around with coax connectors, but you still would like N connectors? There are many companies that will fit any length of coax with the connectors of your choice as a complete custom assembly. The prices I've seen are reasonable, though more expensive than doing it yourself. It is also no guarantee against future problems. The choice is yours.

With regard to UHF connectors, you can also use commercially prepared cables. Many fear damaging the coax through excess soldering heat and cold solder connections. If you do it yourself, I recommend a silver plated connector which takes solder with less fuss and therefore a lower risk of damage. Some like K3LR solder the braid to the outside of the connector body. But if you do it wrong the shell won't slide into place. The impedance bump is longer with this method so it is best to avoid it on VHF systems.

There are many inexpensive UHF connectors on the market that do not meet spec and are difficult or impossible to use. Buy from a supplier with a reputation for quality. If soldering is too much to handle, use crimp UHF connectors. Cost for the crimp tool is worth it when you have many connectors to prepare. You may be able to borrow a tool from a friend if you only have a few connectors. 

I stick with conventional silver plated connectors from reputable dealers, soldered through the holes, and I rarely go wrong.

Some hams convert Heliax N connectors to UHF. You can do this because the exterior threads are the same for both connector series: ⅝"-32 UNEF. See the proof in the above demonstration. I prefer the risk of future mechanical woes of Heliax N connectors over the work to do the conversion. Consider it food for thought.


In addition to the myth than UHF connectors do horrid things to your SWR at VHF, and even HF, there are others. Two examples are that N connectors can't handle a kilowatt or a high SWR. Both are untrue, or at least not all that different from the performance of UHF connectors.

If you ask a ham about coax connectors you will almost always get an answer, and it will be delivered with supreme confidence. Sometimes the answer will be correct. Dig deeper by asking why and what their experience is with those connectors. You should be able to quickly spot the pretenders. Be especially wary of listening to those who tell you what you want to hear.

Uncertainty leads to extreme behaviour: aiming for perfection or taking an anything goes approach. The first can be a poor investment of time and money. The second is asking for trouble. Make decisions based on solid knowledge, not mythology.

Wrap up

My guiding philosophy can be summed up pretty simply: use UHF connectors when I can, and use N connectors when I must.

Unless you have a particularly good reason to use N connectors you are better off sticking with UHF connectors for HF and VHF. My reason for using N connectors is all the Heliax in my station. If not for that there would little need to deal with the many challenges of N connectors. 

Most hams don't need N connectors.

Wednesday, April 12, 2023

Spring is Coming

The title is an unapologetic riff on the "Game of Thrones" tag line: winter is coming. It's appropriate. With our cold and snowy winters, most tower and antenna work is halted for several months. Winter is a time to operate and work on indoor projects. As the snow thaws and the temperature rises in early spring, there is a looming sense of dread despite the more clement weather.

Sure, spring and summer are glorious times, which I love, but that's not all. The station develops problems during the harsh winter months when it's too cold to deal with them. The list of new projects also grows. Springtime signals a rapid transition from relaxation to frenetic activity. Insects wake up and hunt for victims. There are disease-carrying ticks, black flies (the kamikaze pilots of the insect world), and the slower moving but voracious mosquitos. They must be braved while working down the long to-do list.

My lawn is very large and there is much to be done in the spring before the grass (and weeds) begin their growth spurt. That alone will consume many days and it all has to be done in April. What I can't complete will have to be deferred to October when the growing season comes to an end.

In May the hay starts its serious growth and I am largely kept out of the fields and away from major tower projects. The growing hay is difficult to work in and there is a limit to how much of it I can mow to make room for that work. The hay is also where ticks lurk and wait for passing deer and careless hams. 

Hay season from late May to early August is prime sporadic E season. I spend some of that time to chase DX on 6 meters. On days when the band is closed I am often found in my workshop building antennas and other contraptions, or out cycling or other summer activities. Late summer and autumn is the second and longer period for tower and antenna work.

A fox almost blundered into me while I standing on the rock wall while working on the overhead cable run. I heard noise in the leaf litter, looked down and it looked up. I'm not sure which of us was more startled. The fox bolted. It looked back a few times as it went, wondering what I'd do. I simply shrugged and got back to work.

We had an ice storm a week ago. A large tree limb fell, luckily away from the overhead cables. This is another peril of spring: lots of precipitation when the temperature hovers near 0° C. Antenna damage was minor (one of the Beverages) and quickly repaired. Unlike in other storms, the vulnerable rural distribution system survived and we had no power outage at all. The storm damage and power failures were worse elsewhere.

By the time the fox arrived the fallen tree had been removed. I was on the wall finishing the work on the overhead cable supports. It was well supported when winter swept in so there was no urgency. This week I finally cleared away the old supports. The new post gingerly winched out of the ground. It had been pushed down 6" by cable tension. It must have slipped off the wood plank without my noticing.

With the winch and a second steel post, I lifted and set it on the steel pin and bolted it for lateral support. I couldn't do it in the winter so the old post occupied the support frame. The job took a day and a half, which is 3 times what I estimated. There were complications due to the need to support the cables during the work.

Winter took its toll on a couple of antennas. The 160 meter vertical developed an intermittent during high power transmissions. Operating on 160 meters without high power pretty well kept me off the band for a few weeks. All I could do in the cold was to quickly test the many mechanical connections, twice. This turned an intermittent problem into a permanent outage. Once the weather warmed up I did a more thorough investigation.

It turned out to be a wire that was squeezed out from between a pair of washers and was barely contacting the capacitors of the gamma match. As noted at the time, the voltage at the gamma capacitor is very high and prone to arcing. It was easy to fix once found.

The problem with the 80 meter yagi could not be found at all in the cold weather since several key connection points to the switching system and radials were encased in ice at the base of the driven element (tower). That has all thawed during this week's unusually warm weather. I will be out there in the coming days to work on it. I am hopeful that it is something simple.

During the cold of winter, tower jobs are limited to those that don't require fine work. Climbing in winter isn't the problem, it's having to take my gloves off to fiddle with hardware and wires. Once your fingers are chilled they don't warm up too quickly and it's cumbersome to continue work and then climb down with stiff and partially numb fingers.

In the coming days and weeks there are several jobs on my list that I will attend to. Others are less urgent.

  • Inspect towers, antennas, wires and mechanical fixtures; this is a semi-annual chore
  • Inspect and make temporary repairs to the mast coupling system to the upside down prop pitch motor rotator for the 15 and 20 meter stacks; I'll have to replace it eventually but hopefully not this year
  • Complete modifications to the pulley system driving the direction pot for the same prop pitch motor; despite the high tension there is still occasional slippage that requires re-zeroing the indicator
  • Build, install and test new capacitance hats on the driven element of the 3-element 40 meter yagi; if they do well I hope to undertake the far more difficult job of replacing the hats on the reflector and director elements (see early construction phase above)
  • Ground all the Beverages in preparation for lightning season; this will not be an inconvenience since I must roll up the radials of the 160 meter vertical in preparation for the summer haying
  • Find and install better straps that bind the mass of cables to the overhead cable run; the new rubber straps I installed in the fall rapidly deteriorated, while the far older ones are fine: quality matters (see below right)

With all of these jobs, yard work and other activities there has been little time for operating. As I write these words, I've made less than 10 contacts this month! I usually monitor 6 meters (50.313 MHz) when I'm busy elsewhere, but even that has been pretty quiet due to the low solar flux. We are in the doldrums between March equinox-enhanced north-south propagation and the start of sporadic E season a few weeks hence.

There are also a growing number of requests from friends to help out with their tower work. That doesn't happen as often as it did years ago with elderly hams holding fast with what they have and not undertaking new projects. I help out where I can. Others have also been making their lists in preparation for spring warmth and they reach out to me.

With all of the maintenance work to be done and the many new projects in my 2023 plan I expect to stay busy this year. If you enjoy QSO parties, please come out for the Ontario QSO Party on April 15 and 16. I will again be hosting one of the bonus stations so my presence in the contest is guaranteed.

Thursday, April 6, 2023

Auxiliary Antenna Switch

Woe to those with too many antennas! No, not really, but it does create a switching nightmare that isn't present in most stations. I have finally reached the capacity of the switching hardware in my station.

The core of my switching system is the 2×8 Hamplus antenna switch that is at the base of the closest tower. Putting it there means there are only two coax runs into the house, one for each operating position. Eliminating that multitude of coax runs into the house simplifies cable management and keeps the holes in the wall quite small. 

The cost of the remote switch is ~18 control lines from the shack to the switch (3 × Cat5), weatherproofing (and insect-proofing) and occasionally having to stomp through the snow drifts for mid-winter maintenance. Few contesters of my acquaintance do it the way they have. They prefer staying indoors to work on their switching systems.

I've been able to stay within the 8 antenna capacity of the switch until now mostly by luck. I freed one port when I moved the VHF antenna (6 meters) to a separate run of Heliax into the house. The Acom A1500 amplifier has 3 antenna ports, and I do the switching there. But the 6 meter antenna can't be accessed by the other operating position unless I add an antenna switch inside the shack. There is little point since the second station's L7 amp doesn't have 6 meters.

When I add a 2 meter antenna, I may switch them at the top of the tower with a remote switch. I have no great need to access both antennas at the same time. How I switch the coax within the shack will depend on what I settle on for 2 meter equipment. For now it is enough that I have removed VHF antennas from occupying ports on the 2×8 switch. 

Before installing the additional transmission line, I could plug either the 6 meter or 160 meter antenna into the 2×8 switch. I did it once in the spring and once in the fall. It's good that I no longer have to do this since 6 meters has become a year round band with the rising solar flux. I am also investigating methods to make the 160 meter vertical a year round antenna rather than having to roll out and roll up the radials before and after the winter season.

The addition of the 80 meter inverted vee spells the end of seasonal port swapping as a viable strategy. The 80 meter vertical yagi had to be disconnected. That's not a problem right now since it developed a fault this winter and isn't working properly. But I'll fix it soon and I need an antenna port to plug it into.

The 8 ports are currently for: 160, 80, 40 (2), 20, 15, 10 and the TH6. The time had come to build auxiliary antenna switches for 80 and 40 meters. There is rarely a reason that more than one antenna for these bands would be in use at the same time. By freeing up a port from the two allocated to 40 meters (3-element yagi and XM240) I can use it for another multi-band antenna or perhaps antennas for the WARC bands, when I have them. 

One antenna switch port per contest band makes it possible to place single band high power BPF at that point in the switching system. No matter which antenna is selected for each band, the BPF will be the correct one. That can greatly simplify use of BPF since no switching is required. I am happy with my 6-band switched low power BPF and there is no plan to spend big on high power BPF.


I used the smallest sheet metal aluminum enclosure that would comfortably accommodate all the parts and leave finger room to manipulate the connectors. Small size keeps the lead lengths short, and that minimizes impedance "bumps".

There are 4 different styles of UHF chassis sockets. I used what I had, and one I had to purchase at a local flea market. Extra holes were drilled to increase the number of sheet metal screws. That is to improve the seal against water and insects. There is no electrical reason to do that at HF; the wavelengths involved are too long to "leak". I was sloppy about it but there are no bonus points for attractiveness!

The +12 VDC control line terminals are #4 screws through plastic flanges that isolate the screw from the chassis. It's adequate and avoids the trouble of mounting a multi-conductor connector of some kind to the enclosure.

The 4 antenna ports point down for additional weather protection. The angle bracket on the other half of the enclosure is for screwing the switch to a wood panel alongside the 2×8 remote antenna switch.

There are no surprises inside the switch. AWG 18 wires carry RF. The SPST relays are positioned to minimize the lead length. Solder lugs ease wiring of the relay coils and suppressor diodes (1N4007). The relays are mechanically supported by the RF wires. The TE System high current relays are used in antenna switches throughout my station. They easily handle a kilowatt provided the SWR is not too high and hot switching is avoided.

Performance measurements

This is the kind of switch that hardly rates testing beyond a continuity test with the relay turned off and on. RF at 40 and 80 meters is very forgiving of sloppy layout and wiring. Nevertheless, it is worthwhile to take a few measurements for peace of mind and to see how it does at higher frequencies. I will likely need one or more similar auxiliary switches for the high HF bands and VHF as I continue to add antennas.

Testing the switch does not require a 2-port VNA when you have an accurate single port antenna analyzer. Two ports to measure insertion loss is pointless for this circuit, and port isolation is likely to be quite small. There is no requirement for high isolation between the antenna ports of each switch since only one of those antennas is used at a time. High isolation is almost certain between the two switches at low HF frequencies. I considered and rejected putting a shield between them.

A sweep of the SWR from 2 to 30 MHz is about what I expected. At 30 MHz the impedance is approximately 52 + j3.5 Ω. This is about j2 Ω higher than the 50 Ω load alone. That is due to the inductive reactance of the internal wiring and relay. Both ports of both switches have identical impedance curves(within measurement error).

The impedance at 50 MHz is higher at about 53.5 + j5.5 Ω. The 1.15 SWR is low enough for most applications. I experimented with TLW to design an L-network that would compensate for the stray inductance. As for the 10 meter stack switch and its longer wires, a shunt capacitor is sufficient since the calculated series L is no more than 0.05 μH. A value of between 20 and 25 pf works well to correct the small mismatch on both 10 and 6 meters. Addition of the capacitor hardly disturbs the match at lower frequencies. 

I'll eventually want a switch to share one run of Heliax on 6 and 2 meters. Compensation for the stray inductance would be needed at VHF even if I make the switch more compact with less stray inductance since the path through the relays can't be shortened. The present switch for 40 and 80 meters needs no compensation for excellent performance.


Port sharing requires additional conflict management in the recently completed station automation system. The Arduino software and switching software already has 8 auxiliary control lines for controlling devices such as this. Each antenna has pair of values specifying the port on the 2×8 antenna switch and auxiliary control line (or none). However the Arduino software does not yet prevent conflicts. That job is left for the hardware lockout in the 2×8 switch.

The UI (user interface) software running on the PC has been enhanced to check for port conflicts. If another radio is using the same port as that for a selected radio, the antenna is unavailable. In almost all cases this should only occur with multi-band antennas since, for each port with two or more antennas, those antennas are for the same contest band. 

Antenna selection for mono-band antennas is by auxiliary switch, like the one described in this article, or stack switch (currently only for the 10, 15 and 20 meter stacks). In a contest there would rarely be a port sharing conflict for radios on different bands.


The auxiliary switch has been installed and connected to the 80 meter port of the 2×8 antenna switch. The photo shows it mounted under the rain and snow shield before weatherproofing was completed. Both 80 meter antennas -- the inverted vee and the vertical yagi -- are connected to the switch. 

The control lines are not yet connected so only the inverted vee is currently accessible -- I made it the "default" for the time being. I used a permanent market to label the NC (default) port and the NO port, corresponding to the relay contacts. The letters are barely visible in the photograph.

The software and hardware additions to the automation system are in place and partially tested. Final testing has to wait until I patch the auxiliary switch Cat5 cable to one of the spare cables running to the tower base. That job will be completed in the coming days now that warmer weather has finally arrived. I will only take the opportunity to diagnose and repair the fault with the 80 meter yagi. The 40 meter antennas will be moved to the auxiliary switch later this month.