Resolving Amplifier Arcing

A vacuum tube high power RF amplifier is full of high voltages. The large physical size of the tubes requires high voltage between cathode and anode to function. The combination of high output impedance and high power results in a high RF voltage (by Ohm's Law). Prevention of high voltage arcs requires careful design, component selection and operation. Those arcs are potentially destructive: to the tube, power supply and your ease of mind.

Arcing is more common in older amplifiers due to chemical degradation of insulators, insulator cracks and component warping from thermal and electronic stress, and the oxidation and burning of the contacts in physical switches. Although arcing is rare in solid state amplifiers since they operate at relatively low voltages that is no panacea since they suffer from other ills. But this article is about tube amplifiers.

Not long after putting my recently acquired vintage Drake L7 to work it suffered from intermittent arcing at the high end of its power range. Because the high DC plate voltage is independent of power level the arcing was almost certainly at RF. Although the arcing has been eliminated I expect it to return because the faulty part requires replacement. It's on my very long to-do list and it'll take a while until I get around to a implementing a long term solution.

If you've never worked on a high power tube amplifier my relating of how I investigated and resolved the arcing may be of interest. Despite the large size and lethal operating voltages and currents tube amplifiers are actually quite uncomplicated. With attention to safety and the peculiarities of dealing with high voltage, current and power they are not too difficult to work on.

Locating the arcs

The case of the L7 has no opening except on the bottom and rear for forced air cooling. These are useless as windows into the amplifier's interior. The case must be removed. This is the first problem in any properly designed amplifier. There are interlocks to prevent accidental electrocution and burns to budding technicians who are out of their depth.

The Drake L7 has two interlocks: one shorts the 3000 VDC plate supply and other (well hidden) prevents the amplifier from being turned on. Both interlocks must be disabled to operate the amplifier without the case. Before you try this, or even consider doing so, you must educate yourself about what you're attempting to do. Better still, have a knowledgable friend help you out. If you get yourself killed don't say I didn't warn you.

Yes, that's really a brick! I needed something flat, heavy and non-conducting to safely disable the high voltage interlock. The mains interlock (not visible) on the bottom is disabled with a chunk of plastic wedged under it. In the picture the amp is on and idling with a plastic (non-conducting) LED desk lamp for added illumination. Arcs are so bright that no ordinary lighting will wash them out.

I mentioned my suspicion about the loading capacitor in an earlier article. I dutifully straightened the multitude of plates until they maintained a decent gap for their full rotation. Unfortunately that repair resulted in no improvement. Hence the deep dive into the amp's innards.

As I increased drive the amp arced as expected but not where I expected. The loading capacitor sat quietly when the fireworks began.

It was the plate capacitor that was arcing. Although the spacing was generally good (and much wider than the loading capacitor) a number of rotor plates didn't track well. I straightened them without needing to remove the capacitor from the amp. There is an unrelated problem that will require its eventual removal for repair but that can wait.

Straightening the plates did not fix the problem. Worse, the arc location was seemingly random. Each one occurred in a different location. The arcs were evidently due to a fault elsewhere that caused an excess voltage condition beyond the rating of the capacitor. The capacitor is working just fine.

This is interesting so let's take a detour to review the design of a tube amplifier's output network.

Amplifier pi-network

The simplified schematic below is that of a typical pi-network found in many tube amplifiers. It transforms the high impedance of the tube output to the low impedance of the antenna system. The operator adjusts the plate capacitor to resonate the plate circuit and the loading capacitor for a high efficiency match to the impedance presented by the antenna system.

The resonance condition is a typical feature of an impedance transformation network, as previously covered in this blog. The circuit also attenuates harmonics since it is a low pass filter.

The blocking capacitor keeps DC out of the antenna circuit and the choke keeps the RF out of the power supply. Band switching (not shown) alters the range of the variable capacitors and the inductor value. The T/R switch (input side not show) bypasses the amplifier during receive.

RF voltage is determined by the power and impedance in accord with Ohm's Law: E = SQRT(PZ). From the data sheet for a pair of 3-500Z tubes a little arithmetic suggests there is approximately 3000 volts across the plate capacitor at 1000 watts RF output. The recommended capacitor rating is 4.5 kV because the impedance, and therefore voltage, can be higher depending on operating parameters.

On the high bands the L7 places a fixed capacitor in series with the variable capacitor to reduce the capacitance. This also lowers the voltage across each capacitor since capacitors in series act as a voltage divider. Because of this arcing incidence is greater on the low bands. However it is not eliminated. Something more dire is going on to cause arcing when the RF voltage is low.

The voltage across the loading capacitor is easier to calculate since the antenna system impedance is nominally 50 Ω. At 1000 watts the potential is a less than 300 volts. The voltage will often be higher when the SWR is greater than 1, which is very common for most hams. If the SWR is too high the loading capacitor with its smaller plate spacing can arc.

Sequencing

Since the capacitors appear to be in good shape and the output impedance is well within the acceptable range the problem must originate elsewhere. Most likely is a fault that affects the impedance at the output port. The antennas, external switches and transmission lines were ruled out by additional testing. High power can aggravate weak components and loose connections in an antenna system to create intermittent and permanent impedance changes.

By this process of elimination I focussed my attention on the T/R relay. It is common for slow or faulty amplifier relays to cause plate capacitor arcing when transmitter power appears at the amplifier input before the output relay contacts have settled. Unsettled contacts cause a momentary high impedance (open condition) at the output port. Once an arc starts it can continue after the relay stabilizes since the path to ground is always lower impedance than the antenna system.

The solution is sequencing to ensure the amplifier relays settle before power is applied to the input port. This can be coordinated with the transmitter, signal source (e.g. PTT in advance of transmit) and even with the amplifier itself by having the output port relay close faster than the input port relay.

The open frame relay in the L7 is typical of many vintage amplifiers. It iss slow at best, and with age the contacts are suspect. Replacements can be found but better solutions are available.

While arcing at turn on was occurring its incidence was less than that of arcing during a transmission. That is sufficient evidence to rule out sequencing as the cause of my problem. It does not mean sequencing isn't a concern, just that it isn't responsible for the observed behaviour.

Relay woes

I had good reason to suspect the relay. I earlier had to clean the contacts on the input and output of the bypass side due to intermittent signal attenuation on receive. With an ohmmeter connected to the centre pins of the input and output SO239 jacks you clean the contacts until you reliably read 0 Ω. Since the relay arms can shift laterally it is important to test for this by manipulating the relay arms.

The bad contact can be isolated by connecting one ohmmeter probe to the bypass bridge seen on the right. The output port is at the bottom of the picture and the input port is at the top. The centre arm applies tube cutoff bias during receive. Just my luck that both sides of the relay were corroded.

I had to resort to aggressive cleaning with an abrasive when a deoxidizing contact cleaner and non-abrasive buffing were insufficient. Do this only when absolutely necessary since abrasives can easily damage the thin contact coating, assuming there is any left (usually silver) after several decades of use. This is discussed in more detail by W8JI. I used a thin strip of 3000x sandpaper to be as gentle as possible. It worked.

Checking the through amplifier relay contacts is more difficult since an ohmmeter cannot be easily employed to measure resistance. Deoxidizing cleaner and buffing didn't resolve the arcing problem, but it did seem to reduce its frequency. Having gone that far I resorted once more to the sandpaper. After thoroughly clearing the contacts of debris I did another test. This time the arcing vanished entirely.

Permanent solution

I don't know how long the repair will last. The relay needs to be replaced. In any case it is slow and loud. Not only is that very annoying it is cause for ongoing worry. I don't want to take the risk of it failing during a contest.

Designs and even kits to replace T/R relays in vintage amplifiers are available. Some are fast enough to enable QSK operation. I don't need QSK just a solution fast, reliable and quiet.

Another problem with the existing T/R switching is the long lead lengths along the bypass path. On 6 meters it is enough (almost 0.03λ) to significantly raise the SWR. With 6 meter season in full swing I manually bypass the amplifier when I am not using it. Any T/R relay replacement will need to address this issue.

Vintage amplifiers are cost effective assuming you have the time and motivation to repair aging equipment and add modern features. My next amplifier will likely be a new purchase, one with a stiff power supply, silent operation and 6 meters. That way I can tolerate the quirks and faults of its older cousin at the secondary operating position.

80 Meter Stinger Version 2.0 (and Pipe Fitting)

You might think that because I write a blog about antennas and station building that I always do things right. I only wish! Perhaps I make fewer mistakes than some but I have my share of them. The original stinger for the driven element of my 80 meter array is one.

I was economical with the aluminum tubes and pipes I had on hand, saving the longer pieces for other projects. The stinger was 21' long, with 19' (6 m) projecting above the tower top. That was topped by over 3' (1 m) of 1" PVC pipe to get more height for the parasitic wire element support ropes. The total amount of aluminum and PVC above the tower was 7 meters long. That's a lot.

Although the stinger is guyed by the catenaries for the wire elements the tension produces a downward force (compression). It was obvious during setup that a butt joint between 1-½" x 0.095" aluminum tubes was not up to the stress. My hope was that it would last long enough that I could focus on other projects until I would have to replace it. All seemed well for a year as it survived one wind storm after another. Then the joint suffered a fatigue failure and the top half of the stinger fell down.

Stinger version 2.0

The stinger needs to be robust but not necessarily lightweight. The stinger must resist modest horizontal tension of the parasitic element catenaries. It must also be up to the compression force due to those same catenaries, especially with regard to high stress points where the yield point could be exceeded in a high wind and icing, or from tension imbalance among the 4 catenaries. It is laterally stabilized by those same catenaries which act as guys.

Being lightweight is beneficial when installing the stinger since it is long and must be lifted overhead to be dropped into the tower top. It can be assembled in pieces and raised from below at the price of more time and effort. As you will see I did a bit of both.

Rather than a butt joint between two lengths of 1-½" aluminum tubes the main improvement is a butt joint between two lengths of 1-½" aluminum pipe (1.9" OD). This allows me to use the existing 2" saddle clamps that secure the stinger to the tower plates. I also have a supply of these surplus pipes on hand and I know where I can get a few more at a good price.

Pipe fitting

My introduction to fitting pipes together for antenna construction was simply out of expediency: I had sources of cheap surplus aluminum pipe and tubes are expensive. I later came across the same idea in W6NL's book Physical Antenna Design (now out of print), which increased my confidence.

Although aluminum pipe is almost always 6061-T6 -- excellent tensile strength -- they have a seam, hard as it can be to find one on these aluminum pipes. They are theoretically weaker than seamless tubes although I have yet to see a seam failure. Indeed, some of my stock comes from commercial antennas that have survived harsh Canadian winters at great heights.

Aluminum pipe follows the same size schedules as steel pipe in the US and Canada. That's very convenient. Plastic pipe that is now in common use for water pipe and conduit -- ABS and PVC -- are similarly sized. I'll provide several examples of how pipes and tubes can be mated with respect to my 80 meter stinger, and additional ideas covered in earlier articles. There are other combinations of pipes and tubes that can work well together.

On the left are two schedule 40 pipes: a 1-½" pipe inside a 2" pipe. The inner pipe OD is 1.9" and the outer pipe ID is 2.067". The gap is 0.167", or 0.083" all around. Depending on the application the pipe can be simply bolted together. For improved rigidity a shim made of aluminum flashing can be used. An alternative is to increase the amount of overlap to reduce wobble, at the expense of greater weight.

The middle example is almost the same except that the outer pipe is schedule 80 with an ID of 1.939". That is a much better fit. Both are options to butt join two lengths of 1-½" pipe. I considered using the schedule 80 pipe until I found what is, to me, a more favourable solution. I prefer to save the schedule 80 pipes for my various yagi projects.

On the right is the 1" schedule 40 PVC pipe that is fit to a 1-½" aluminum tube at the top of the stinger as a non-conductive extension for the catenary attachments. The OD is 1.315" and the ID is 1.049". The 1" pine dowel purchased at a hardware store provides structural strength and fits well enough for the intended use.

This size PVC pipe will also fit well over 1" tubes. I may use PVC pipe as an insulator on the driven element 1" centre segments if I decide to use a beta match. Insulation is not needed for a gamma or T match.

Here are a few more pipe fitting ideas. When I used a 2-½" aluminum pipe to mate with 3" tubes for my first set of long yagi booms I had a machine shop turn down the pipe a few hundredths of an inch. The mast on my 150' tower is a 2-½" (2.875" OD, 0.25" wall) steel pipe slipped inside a 3" (3.068" ID, schedule 40) steel pipe used as the drive shaft for chain driven prop pitch motor. There the fit is so poor that I needed shims to prevent slippage caused by the large mechanical load. For one long yagi boom I fit 2" OD heavy wall tubes into both ends of a 2" schedule 40 pipe, which is a good fit. I bought several lengths of heavy wall 2" tubes to mate with these pipes to make booms for the 20 meter and 15 meter long boom yagis I am building.

There are other applications of pipes that I will discuss in future articles about those projects. Consult charts of pipe sizes and trawl through surplus yards for cost effective solutions in your antenna farm.

Stinger butt splice

In my junk box are Hy-Gain yagi parts that have been collected over the years. Hy-Gain booms are mostly 2" OD and spliced at the centre for the longer yagis. I have two of these surplus brackets. I tested a bracket on 1.9" OD pipes and was successful in achieving a secure fit despite the smaller size.

I butt spliced 10' and 7' pipes in the bracket. Holes were drilled through the pipes to make use of the bracket holes for that purpose in lieu of using the inner perimeter holes intended for a mast clamp.

I briefly experimented with plastic pipe as insulators to electrically isolate the pipes. I would need do this to attach a switchable coil to add 160 meters to the 80 meter array. I slit a scrap length of 2" white PVC pipe to slide over the pipe. It had to be slit since the ID is slightly less than 1.9".

At the end of the dressed pipe is a round insulator made of pressure treated lumber, cut with a hole drill, to provide mechanically robust isolation between the upper and lower pipes. The lower pipe does not need an insulating sleeve except perhaps to achieve a consistent diameter within the bracket.

I put the idea aside as not quite ready for implementation. It can be retrofit later. I first need to ensure high voltages between the pipes when the coil is active (not shorted) cannot jump the gap through the slit while maintaining high mechanical strength. It may be as simple as a wrap of thick polyethylene sheet and a seal to keep water out.

Upper stinger

Spliced to the upper pipe is a 7' (2 m) length of 1-½" x 0.095" aluminum tube. This is the only piece of the original stinger used in version 2. The 0.11" gap (the pipe ID is 1.61") is filled with a wrap of aluminum flashing coated in conductive grease. Stainless screws with nylocs hold it together. The 1" PVC pipe (1.315" OD) with its inner wood dowel are attached to the 1-½" tube (1.41" ID) in a similar fashion.

The original holes for attaching the rope catenaries are reused, and holes drilled through the wood dowel inside. The raw pine is protected by a cap of pressure treated wood, the top of which is sealed with caulk. The hose clamp adds tensile strength to the PVC and wood to better withstand the tension on the catenaries. The tension isn't high but I want to ensure years of trouble-free service.

The final stinger is ~2' (60 cm) longer than the original. This is intentional. I found that with my parasitic T-element design there is some slack on the vertical wire. The longer stinger removes the slack, and is easier than rebuilding the wire elements.

Installation

The new stinger was raised in two steps. The lower 10' pipe with Hy-Gain bracket went up first and dropped into the tower clamps until the bracket rested on the top clamp. The nice thing about aluminum pipe is high strength-to-weight ratio. The 10' of pipe with bracket attached is only ~9 lb (4 kg). It is easy to hold it vertical over my head as I fuss with inserting it through the tower clamps.

The catenary ropes are tied to the top of the stinger and detached from the wire elements in preparation for the next step. I briefly considered leaving the elements attached until I realized that the lateral tugs of those small weights would prevent safe lifting of the 17' long upper stinger.

With everything in place I lifted the upper stinger and dropped it into the Hy-Gain bracket. Once that was secured the elements were reattached. The complete stinger was then pushed up through the tower top and clamped in place.

Back on the ground I tensioned all the catenaries. The test for tension equalization was to have the stinger straight and in line with the tower. That was after the adjacent picture was taken.

Although a simple procedure it is tedious. I took an overnight break at one stage when I felt that I was too tired to do the lift with complete safety. A brief delay is preferable to an unnecessary risk.

Matching network changes

The new stinger's mechanical length is 2' longer and its electrical length is 3' longer. The reason is explained below. But having done so the new electrical properties of the driven element must be dealt with. For a λ/4 monopole on 80 meters the approximate rate-of-change (dF/dL) is10 kHz/6 cm; that's ~150 kHz lower due to the 3' extension.

There is no reason to make the driven element resonant at any particular frequency. The low impedance still requires a matching network, both as an omni-directional vertical and in yagi mode. I am using a switchable L-network.

My next task is to remeasure the antenna's impedance across the band, in both omni-directional and yagi modes. I will then use TLW to determine what changes are required. They should be small. I'll describe the details in my final article about this antenna, which will be written once it is complete and fully operational.

Catenary rope length changes

Changing the stinger height presents an interesting geometry problem: to lengthen the catenary ropes in a manner that keeps the parasitic elements vertical and preserves yagi performance. My first inclination was to ignore the problem since the change is quite minor. However it isn't difficult to check this on paper so I took that precaution.

Since the two sides of the right angle triangle are approximately equal each increment in height lengthens the full catenary (including the T-top of the wire element) by 0.7 increments; that is, 1.4' for a 2' height increase. However we only need to lengthen A to B, the distance from the top to the vertical wire, since we can freely add rope at the bottom. With A and B only 40% of the total length -- 10.5/25.5 -- we need only 40% of 1.4' or 6" (15 cm).

By not lengthening the top rope the wire element will lean towards the driven element ~4" (10 cm). As confirmed by modelling (and as you'd likely guess) this is negligible. Since I had no slack to lengthen the upper rope section I built and installed long insulators between the rope and upper end of the T shaped wire element. These are made from PVC pipe. The height of the vertical component of each wire element was increased by 0.8' (25 cm) -- 40% of 2' -- which took up all of the slack. Mission accomplished.

Returning to work

With the stinger rebuilt, better and stronger than before, work can resume on the yagi. All the parasitic switch boxes are installed and working. Tuning of the elements is partially done. Then comes the final step: the main switching system at the base of the driven element (tower).

Progress on the antenna has slowed due to more urgent projects, especially the 20 and 15 meter stacked yagis. Not to mention 6 meter DXing and otherwise simply enjoying the warm weather. With my 80 meter interest being DXing and contesting the 80 meter yagi is not urgently needed. It can wait until late summer, but may be completed earlier depending on circumstances.

In consideration of the weather and my busy schedule don't be surprised by a slowed rate of articles through the summer.

Potential for FT4 on 6 Meters

The E_s (sporadic E) season is well underway. It peaks at the solstice, which is less than 3 weeks from now. Although the season is reasonably long it is less so for DXing, which requires multiple clouds and long path lengths. When it does occur the openings are usually fleeting.

Openings for any one signal are often too short for most stations to complete an FT8 QSO. Bigger stations -- power and antennas -- or those in excellent locations do better. There are in principle two factors to be overcome in achieving DX success on 6 meters E_s:

• Weak signals: The combination of multiple bounces and forward scatter keep DX signals in almost all cases quite weak, even for those with big antennas.
• Brief opportunity: For the same reasons each signal can be in and out in less than one minute, the minimum duration for an FT8 QSO.

In the first case the challenge can be overcome with more power and more antenna. Unfortunately that is not practical for most hams, especially when it comes to towers and and antennas. But if you can do it you will see a tremendous improvement in your results.

In the second case you can increase the window of opportunity with more power, a bigger antenna or both. However, for the majority the greater opportunity is with modes that allow for quicker QSOs. That way you can exploit the propagation peaks and not just their long heads and tails.

The diagram is copied from the article linked above since I believe it makes clear the QSO duration challenge. See the article for a description of the diagram.

Traditional modes are fast, but...

Although CW and SSB are faster than FT8 (and even the twice-as-fast FT4) you are rarely in the right place at the right time. I discussed this "discovery" problem in an article last summer.

It is a major reason why FT8 has been so successful on 6 meters that it reduced CW and SSB activity by at least 80% in just one E_s season. You can't argue with success. Well you can but arguing won't put DX QSOs in the log. It's the only reason I made the move to FT8. Trying CW in a recent opening saw me through only 3 QSOs, including one Caribbean station I have not heard on FT8. SSB activity was a little better.

TEP, tropospheric ducting and (we hope) F₂ benefit less from the speed of FT4. FT8 is fine but then so are CW and SSB for these propagation modes. When signals persist the comparison between digital and traditional modes is little different than for HF. To give you my take on this I will merely state that I do not use FT8 on HF.

Expectations

I am intrigued by FT4. As of its latest incarnation the time slots are half that of FT8 (7.5 seconds), promising QSOs in half the time. Indeed it may be even faster since there is the possibility of fewer message repeats due to QSB during a lengthy FT8 QSO.

Despite not having used FT4 and the few reports I've heard from those using it I am comfortable predicting that FT4 most likely will be a good fit for 6 meters. In particular to E_s DX openings.

• Loss of a few db of sensitivity is more than compensated for by speed
• Ability to squeeze more QSOs out of brief DX openings
• Favours the small station better than FT8; while this may seem surprising consider the ability to better exploit brief propagation peaks

I have not yet used FT4. I am content to wait for it to stabilize. This is scheduled for July. That leaves me with enough time to play with it at the tail end of this E_s season. Others are using the beta software on 6 meters at its proposed slot of 50.318 MHz.

But more than a choice of mode my main concern right now is propagation. DX opportunities have been slim lately on 6 meters despite a promising beginning in mid-May. DX has been worked though nothing new or of especial note.

I listen most days with WSJT-X monitoring 50.313 MHz while I go about other activities. Between 6 meters and chasing 3D2CR there is enough DXing to fill the gaps in my busy warm weather schedule.

Fallow Summer

My on air activity is limited at the moment. This is partly due to the arrival (finally!) of warm spring weather. There is so much to do that the weather makes attractive, both radio and non-radio related. The other part that makes me relatively inactive is a lack of antennas. That may sound odd when you consider the number of towers and antennas that I appear to have. But there are reasons.

The new big tower was raised to 120' in early May with the help of Eric VA3AMX and my regular assistant Don VE3DQN. The top 20' are waiting for my decision on a rotator. I will then fabricate plates, machine the mast and select bearings. There is no rush since the stacked yagis for 20 and 15 meters that will go on it are not ready.

In the background you see the 150' tower. The TH6 and XM240 up top have intermittent problems and so they must come down. I wanted to do this before the hay got too high (and the ticks emerged) but is delayed until the hay is harvested. I could not arrange a full ground crew and rig the tram line in the short window after the new tower work was completed.

Once they are down and repaired the XM240 may end up in its original position atop the Trylon tower at 21 meters height, below the 6 meter yagi. However this depends on whether I can build and raise a new 40 meter yagi onto the big tower before winter. It will be a challenge. The TH6 and TH7 will be stacked at a low height on one of the big towers to cover North America.

The 160 meter vertical is disconnected and the radials rolled up, again in consideration of the haying. To simplify redeployment this year I left it tied to the tower top and secured the wires to a guy anchor to keep them from tangling with farm equipment.

My 80 meter yagi project suffered a setback when the stinger for the driven element broke and the top half speared into the ground. After putting it up I discovered that the column (compression) strength of the stinger was insufficient to withstand the tension on the rope and wire supporting the parasitic elements.

My hope was to delay replacement, but it must now be dealt with before I can resume work on the yagi switching system. The job is not difficult just annoying, especially since I was making good progress on the direction switching system construction and installation. In any case the low bands are not attractive during summer due to the high noise level.

Inside the shack there are also problems. One is that the recently acquired Drake L7 amplifier has a couple of faults. One is the T/R relay (a common failure point) which has intermittent high contact resistance on receive. My gentle cleaning temporarily cured the problem but now I must take more aggressive action. Eventually I will replace the open frame relay with something faster and quieter.

The second fault is the loading variable capacitor. Occasional arcing on the low bands led me to discover that several rotor plates are not straight and get too close to the adjacent stator plates. With the capacitor pulled pulled from the chassis I found that it had been previously worked on, and not very well. The repair is easy but time consuming. Again, there is no urgency since I am relatively inactive during the summer.

Last weekend was CQ WPX CW. Although it is not one of my favourites I wanted to play around for a while with high power. Since that wasn't possible and because of the antenna situation I opted to enter as 20 meter low power. It is perfectly possible to have a few hours of fun with a crippled station.

The one antenna I do care about right now is the 6 meter yagi. At least that antenna is trouble free. If only DX conditions were better. Sporadic E season started well but has entered a lull in this part of the continent.

I'll have more to say about yagi construction when I'm further along in that project. The bulk of the required aluminum is in hand and machining of the tubes has begun. I want the yagis ready to fly in August for testing and adjustment after the hay is harvested and it is easy to move around the fields. This project will keep me busy since with my lack of antennas (and sunspots) there is little incentive to spend time in the shack. Well, at least when 6 meters isn't hopping.

Reflections on Doing Business

Building a middling large contest station necessarily involves lots of spending. It adds up quickly. Consider the following (partial) list:

• Station equipment: rigs, amplifiers, switching, rotators, antennas
• Electronic components
• Materials: steel, aluminum, concrete, wood, PVC
• Fasteners: clamps, screws, guy hardware
• Services: welding, machining, heavy equipment, steel work
• Cables: coax, wire, electrical

That's a lot of business! Even when you build much of it yourself as I am doing there is an endless list of items to be purchased. For most of us who are not wealthy it is important to develop and work to a budget to avoid frustrated ambition.

Let's look at a few examples of doing business while building my station. There are lessons for us all, positive and negative.

Tower services

The commercial tower business is thriving. Most is done by large enterprises that have multiple locations and remote management. They are the least flexible in working with hams. Small local outfits are far easier to deal with. When I first moved to this QTH I made a point of dropping in to see the local tower service company to get to know them and see where they might be able to help me with my station. I knew a few hams who had hired them and were happy with their performance.

You might not be so lucky but unless you take the trouble to look them up you'll never know. They put in the foundations for both my big towers. Although there were a few problems they took the extra effort to resolve them. Some of these were due to miscommunications, such as their expectation that I knew more about dealing with heavy equipment better than was the case.

They have become a good source of surplus equipment and material from decommissioned towers and communications systems. I make a point of recommending them to others and I continue to give them them my business.

Aluminum

Buying antenna aluminum in Canada is a challenge. Although we have a huge aluminum industry the selection and availability of many types of aluminum tubing is often not friendly to hams. The 0.058" wall tubes that assist with telescoping yagi elements are almost unheard of despite being widely available in the US. These are considered aerospace material. Importing long tubes is far from economical.

On the other hand aluminum tubes and other shapes are widely available and economical if you are willing to improvise with more common sizes. I've modified my yagi designs in accord with local availability. All I'm then left with is finding the best company to work with.

My first experience several years ago was barely adequate. The company that seemed best had a limited selection and they only dealt in full lengths, typically 20'. Cutting to size was expensive. They catered to industry and barely tolerated retail customers. But the prices were excellent.

Several months ago I went to another firm that makes a big deal of their selection, cutting to small quantities and walk in retail customers. Again, the result was not great. For my second order I selected a small quantity of short tubes to test in my workshop for suitability to build yagis. Every tube in that order was wrong, either OD, ID or alloy. I gave up on them for good.

I returned to the first company. On the phone they were far more receptive than I remembered so I dropped in to see them. What a change. They worked hard for my business including hunting down non-stock items and discounting the price for the quantities I needed. The order was filled to perfection.

When I had difficulty loading the tubes into my car the company president offered to deliver it to my place, despite the bother and the 100 km distance. It was delivered in good order the very next day. On being asked to think of them in future I told him I certainly would.

Like people, companies change. Don't be afraid to try again long after a poor experience. Companies that survive must change to stay competitive.

Wire and cable

In my station there are ~500 meters of coax, 2000 meters of wire, 1300 meters of guy strand, 500+ meters of control cable, plus guy grips, junction boxes and much more. I buy very little of it at the better known retail outlets. Some was bought used, most was bought new and I've acquired quite a lot for free or close to free.

After shopping around I do most of my buying at a local branch of a province wide supplier of electrical equipment to trades and industry, including power companies. Their prices and selection are good and they make an effort to special order what they don't stock locally or their central warehouse. They are happy to sell small quantities to retail customers, although they prefer large quantities.

When there have been errors and other problems they always try to make good, albeit grudgingly at times. I'll keep going there for as long as the good experiences predominate.

Welding and machining

There are welders everywhere. In rural areas like mine there is a sign hanging out front every few kilometers where small operators work out of their homes. There are almost as many commercial enterprises with 2 or 3 people. Then there are the large companies that cater to industry. Machinists tend to be collocated with welding shops since they are closely associated despite being distinct trades.

I went by recommendation rather than hunt one on my own, choosing the shop that did the welding for the guy anchors on my first big tower. The proprietor is a machinist and his partner is a welder. This is where I went for my custom guy yokes, tower load bearing plate and trimming aluminum pipe for booms.

A misunderstanding led to incorrectly bent plates for the yokes, which they corrected by making new ones from their own metal stock. Aside from that error the quality of the work is excellent and the prices reasonable. I intend to return to them to fabricate steel plates for the mast bearings and rotator needed for the new tower.

Specialty markets: retail and swap nets

The specialty amateur radio market in North America is going through a secular shift as older generations age out and the type and quantity of business changes. In Canada several ham market retailers have closed down in recent years and the ones that continue have branched out into other lines of business to survive. It's a tough sector to be in.

At a flea market this spring it was obvious that our hobby is approaching a crisis. A friend and I surveyed the crowd and saw an almost uninterrupted panorama of male grey heads. That isn't sustainable. Among the items on my table the old stuff interested old people. More recent technology drew in younger hams. By younger I mean in their middle years.

It won't be long before the smaller ham flea markets fade away. That's a shame since they can be very enjoyable events.

Online swap shops suffer from the same ills of all online fora: swindlers. This was far less common when swap shops were in print or on air. The bad actors are mostly small time criminals and scammers that infest all branches of online person-to-person commerce. A few are hams. Due diligence is required on every deal, whether you are a buyer or a seller.

Lessons learned

It is well worth the effort to find and stay with companies that are reliable, fair and willing to take the trouble to keep customers happy. For most of these companies the business from a ham is small in comparison to their industrial clients. Since many can't be bothered with small retail customers it is important to appreciate the one that do.

Although ugly stories abound most people are reasonable and reliable. Doing business can be enjoyable with the right person or company on the other side of the deal. Don't let yourself be ruled by cynicism: many people are their own worst enemies. When you mostly have bad experiences doing business it's probably you not them. Don't be one of "those" customers.

Strive to build strong long-lasting business relationships. Treat a first deal like a first date. Trust builds over time. Excuse a few lapses since we all make mistakes. But move on when it becomes a habit.

Do you need a recommendation for a supplier or service? Ask the people you've come to trust doing business with. One of the reasons they're good is that they rely on other good companies. They'll lead you to the good ones. That's how I've had success finding the dealers and services I've come to rely on.

Prime Time 6 Meters

Summer is coming.

The solstice is less than 6 weeks away and E_s (sporadic E) season is well underway. As I tell everyone I know with a passing interest in 6 meters mid-May is when the DX starts to roll in. Indeed the DX has been heard here since early in the month.

This is the DX I've heard so far on 50.313 MHz and 50.323 MHz FT8:

• Europe: EA, F, CT
• Africa: EA8, 5B
• South America: PY, YV
• North America: VP9, J8, KP4

Some signals have been excellent and in for several minutes at a time. Here is one example:

I have not worked any DX as yet. The interesting stations have been weak or fleeting while the strong ones I've worked before and did not want to get in the way of the many trying to work them, perhaps for a new one. Further east in W1, VE1 and VO1 the openings have been better than ours.

Chances are you have an HF transceiver with 6 meters. Try loading up any HF antenna with the ATU and give it a try. It doesn't take a lot of power or a lot of antenna to have some fun. This is prime time 6 meters. It will continue until early August, peaking in late June. Go ahead and give it a try. If you are already active on 6 meters be on the lookout for those elusive DX openings.

Have fun and I hope to see you on the magic band.

Yagi Elevation Angle Nulls: Tilt & Fill

A common source of ideas for this blog is other hams. When a recurring question came up recently I thought that it would make for an interesting article. The question is: can you fill in those elevation angle nulls in a yagi pattern by tilting the antenna up? The short answer: no.

More interesting is why it is true since it can teach us about how antenna patterns are what they are. In this article I'll dig a little deeper and discuss its implications, including how to go about filling those nulls. I believe it is worth the effort since you can be held back in the achievement of your operating goals if you ignore pattern nulls.

Yagi tilt in free space

Ground is responsible for the existence of elevation nulls. Before we go into that we should have an understanding of a yagi's free space pattern where ground plays no role. Only then should we bring ground into the picture.

I am using EZNEC to do the models and pattern generation. I am use its features to easily rotate antennas and to compare patterns by overlaying them on one plot. Since I have 6 meters on my mind these days I returned to my optimized A50-6 model to provide the examples. It has enough gain that the main lobe is not too narrow nor too wide, helpful in illuminating the current topic.

In free space the main lobe (looking forward along the boom) is a bulb shape. With no ground reference the 15° tilt has no effect on the pattern. Of course the same is true for any amount of tilt, for any direction or boom rotation. Just as for astronauts in space there is no up or down.

Ground reflection

The pattern of a horizontally polarized antenna over ground is the sum (interference pattern) of the sky wave and the ground reflection. The relative phase and amplitude of the two determines the flux in every direction. Since we cannot transmit a signal into the ground we can safely ignore the half sphere of the pattern below a plane tangent to the ground. EZNEC automatically trims it.

We can now return to a comparison of the two antenna patterns. Because height affects how the ground reflections sums with the sky wave I am using the actual height of my antenna, which is 24 meters (80'). I've made the elevation plot large and used a 1° step size so that detail can be seen.

There are a few features of the pattern comparison that stand out. First, gain of the tilted yagi decreases at low elevation and increases at high elevation. This should not be a surprise. The antennas have equal gain at 35°. The second and perhaps most important feature is that the elevation angles of the nulls are identical. All that has changed is the depth of the nulls, which are shallower for the tilted yagi. The third and equally important feature is that the nulls of the tilted yagi are increasingly shallow at high angles.

To understand what's going on we need to refer back to the free space patterns. Gain is equal for equal positive and negative elevation angle deviations from 0° elevation due to pattern symmetry. Therefore when ground is parallel to the yagi ground reflections are therefore of similar amplitude for equal positive and negative angles. Reflection gain (nominally 6 db) and null depth depends on ground quality which determines reflection loss and, at low elevation angles, reflection phase shift.

Maximum gain and maximum null depth occur when the amplitude of the reflected wave is equal to the sky wave. Lobe peaks are at elevation angles where the phase difference is 0° and nulls occur where the difference is 180°. The higher the antenna is above ground, as measured in wavelengths, the greater the number of minor lobes and nulls. The 0° null is due to phase reversal of reflections at low incidence angles over imperfect ground.

Tilting the antenna does not affect behaviour of ground reflections. What does change is the amplitude of the reflections. Since the yagi main lobe is no longer symmetric with respect to 0° elevation the sky wave and ground reflection amplitudes are no longer equal for equal positive and negative elevation angles. For the modest 15° tilt being examined the inequality is greatest at high elevation angles.

Suffice to say this is not what we want. Tilting a yagi upward not only doesn't fill the nulls, except at the less useful high elevation angles, the gain at low angles is reduced.

Terrain

This analysis assumes flat, uniform ground. Complex terrain introduces complex ground reflections that shift the positions of the lobes and nulls, and their heights and depths, respectively. However, tilt still has no particular advantage since the lobe and null elevation angles remain as they are. Ray tracing tools such as HFTA can provide insight into how the terrain affects the elevation pattern.

Ground quality determines the amplitude of reflections, including the elevation angle below which reflections are phase reversed. The latter is why gain is zero at 0° elevation. For horizontal yagis like yagis the reflection amplitudes even over poor ground are reliably strong. Not so for vertically polarized antennas, but we're restricting this discussion to horizontal yagis.

Stacking

A common technique for increasing gain is to stack two or more yagis. Power is split among the antennas with phase set to achieve the operating objective. In almost all cases the yagis are fed in phase.

Another desirable characteristic of stacks is to move or reduce elevation pattern nulls. This is done by taking advantage of the different heights of the yagis (not applicable to side-by-side yagis). The lobes and nulls are different for each yagi alone and when two or more yagis are used. We'll keep it simple by restricting the discussion to two yagis in a vertical stack.

I've kept the first yagi at 24 meters height and added an identical one at 18 meters height, which is 1λ separation, the same as the boom length. Equal separation and boom length usually works well, and in any case we are interested in the general pattern rather than maximum forward gain.

Green is the upper yagi, red is the lower yagi and black is the stack (BIP: both in phase). Stack gain is as expected at about 3 db. The higher yagi has more minor lobes due to the greater height. For the lowest inter-lobe null (marked) the difference among the three is only very small at a little over 1°. The difference increases at higher angles. Unfortunately that first null is pretty stable for all configurations of the stack, so our objective is not met since the lowest null is the most critical for DXing.

The reason for the lack of movement of the lowest null is that the ratio of yagi heights is only 1.3. Small height ratios are typical at VHF and above, while at HF larger differences are the norm. The small ratio is good for gain but not for moving nulls. If this is important at VHF it is desirable to have a second antenna at a lower height. For example, to work single hop E_s at high angles and DX at low angles, as covered in a previous article.

Space

Now then, what about antennas that must tilt upward to target satellites and the Moon? I rarely hear talk about the effects of ground reflections in these communications modes, yet they can be important in some situations.

If you point a high horizontal yagi well above the horizon there is no problem. Ground reflections are negligible because the main lobe has a narrow beam width and little radiation is directed towards the ground. However there are ground reflections for moderate and low gain yagis that enter the picture for satellite passes less than ~30° above the horizon.

Circular and vertical polarization are immune from deep nulls. But many small satellite antennas are linearly polarized, especially those that are hand held. Hand held horizontal yagis are actually less of a problem on low passes because they are close to the ground and the nulls are at higher elevation angles.

So, not a big problem overall. When a null is encountered it tends to be a transitory phenomenon (the satellite is in motion) that may be mistaken for bad aim or incorrect polarization. By the time you adjust the antenna the satellite would have already moved out of the null.

Anyway...

Tilt seems a simple solution to dealing with nulls. Like many simple solutions to difficult problems it does not work. Besides which it isn't easy to accomplish. Do you really want to spec the mechanical design of an elevation rotator for a long boom 20 meter yagi? Leave those elevation rotators where they do something useful: satellite and space communication.

FT8 Is Not a Low Power Mode

Early this year I gave a talk on 6 meter DXing with FT8. It was well enough received that I was twice asked to repeat my performance to two other clubs. If the interest shown is any indication expect to hear a few new DX seekers from FN25 and nearby grids on 6 meter FT8 this year.

I will not echo that presentation on the blog since many of the ideas therein were covered in 2018 blog articles. Instead I will dig deeper into one topic: power. How much is enough? Does it make a difference? Is it the right thing to do with FT8? With the summer E_s (sporadic E) season on the doorstep and FT8 the go-to mode for 6 meters (personal preferences aside) this is a good time to review this sometimes controversial subject.

There is a widely held view that FT8 is a low power mode. I will state right up front that this is not true, or at least no more true than for other modes. Most of the time we all use more power than is necessary to complete a QSO, whether it is with CW, SSB, RTTY, FM or other modes.

We do this most often because it is a bother to fuss with power level and we don't really know how little is enough since we don't know the listening conditions at the other end of the QSO. Many hams contend with EMI in our dense urban areas in addition to QRM and QRN. Of course there are those who run QRO for no better reason than they can and want to do so.

Those are very general statements and I want to restrict this discussion to the unique propagation conditions on 6 meters. Operating FT8 on 6 meters is not the same as on HF, and we need to do it differently to increase DXing success. E_s is particularly challenging and most of the time that is the propagation by which we work DX.

E_s propagation for DX

From DXMAPS.com

E_s openings tend to be spotlight openings in which, for example, your neighbour hears the DX but you do not. This is because the area of E layer ionization that supports 50 MHz refraction back to Earth is typically small and both stations need that ionosphere patch to be between them and "visible". Extend this to lengthy multi-hop path and the effect is accentuated.

The sample plot of 6 meters QSOs in a randomly selected opening is illustrative of this effect. Don't despair if you are not on one of these lines since the area and position of ionization patches that support 6 meter propagation constantly change. Like the aurora it is not the ionized patches that are moving but rather that the pattern of E layer ionization varies.

This variability is a part of the challenge of 6 meter DXing. When your neighbour has propagation, or someone midway along the wished for DX path, you need to keep monitoring since your turn may come next.

With a big enough station you don't always have to wait. It is possible to work DX when the MUF (maximum usable frequency) between you and the DX hovers below 50 MHz. This is due to forward scatter, a diffuse refraction of signals downward from the main signal path, which itself doesn't bend downward enough to be usable. Unfortunately the signal level of scattered signals is well below that of signals below the MUF. The lower the MUF the weaker the scatter.

Power and big antennas are needed to exploit forward scatter. This is an important advantage the big guns have over everyone else. With an amplifier you will have more success with forward scatter by increasing the duration of DX openings. Of course the other station must be sufficiently equipped or although they will hear the big gun they are not able to be heard. This is often my fate when calling the big guns.

Fitting FT8 into brief openings

The following fictional sequence is the shortest possible FT8 QSO with a DX station. Counting the first CQ it is 60 seconds long. It is reasonable to add the 15 seconds for the second CQ since it confirms that the other station believes the QSO is complete. Add another 15 seconds if you don't omit Message #1 (grid square). Real 6 meter DX QSOs rarely go so fast, often requiring 2 minutes or more.

CQ QQ8ABC LL19
  QQ8ABC VE3VN -10
VE3VN QQ8ABC R-14
  QQ8ABC VE3VN RR73
CQ QQ8ABC LL19

This has implications since DX openings are typically marginal and brief. Frequently QSOs are not completed due to QSB. Another signal peak may occur but often doesn't, and even if it does the other station may be busy working someone else. With 6 meter DX E_s there is no tomorrow: you might not get a second chance.

Power plays a role in this, or more precisely ERP (effective radiated power). This is a measure of the flux in the direction of the opening (azimuth and elevation), which is a combination of power delivered to the antenna and antenna pattern. For example, the ERP of a 100 watt transmitter through a transmission line with -3 db loss and an antenna gain of 11 dbi in the (ionospherically determined) DX path's azimuth and elevation angles would be a little over 600 watts.

Power helps the other station hear you but doesn't help you on receive. The antenna and transmission line gain and loss apply to both transmit and receive.

Now let's look at a fictitious but representative DX opening in diagram form. The horizontal axis is time, 15 seconds (FT8 interval) per vertical bar. The vertical axis is received signal strength with an arbitrary scale. The labelled lines are the noise floors for a variety of stations. The blue curve is the signal strength of the DX station.

Station A never hears the DX. This can be due to antenna, topography, noise or all these reasons. The better equipped B has just enough time to complete a 60 second FT8 QSO requiring a large dollop of good luck.

As the station capability improves you can see that there is more time to complete the QSO, and a reduced risk of decode failures and the necessity of repeated messages. We may aspire to be E but few stations are at that level. They are the 6 meter big guns.

Power and asymmetry

At right is a screen capture of my attempted QSO with 4X4DK last summer. I copied him quite well for several minutes yet he decoded me just once. Notice how the trend line of his signal strength mirrors the arc in the above diagram. Also notice the difference in reported signal strengths: it is more than 20 db.

This is a classic asymmetry. If it were due to power alone it would imply his transmit power was ~20 kW in comparison to my 100 watts. It's unlikely he was running more than 1 kW, leaving an unexplained 13 db difference. It may be due to noise at his end. WSJT-X uses his higher noise floor to calculate my signal strength. For me this situation is quite common since I live in a quiet rural setting whereas most hams are in noisy urban areas.

No matter the reason I need more ERP to be reliably heard. Either that or I wait for a better opening which may never come. I estimate that I missed about 20 DXCC countries last year, ones that were heard but not worked. Although 56 countries on 6 meter FT8 last year is pretty good, 75 would have been very welcome indeed.

"You can't work them if they can't hear you"

The immediately preceding example turns on its head the classic saying: "you can't work them if you can't hear them." More precisely said, the wording of the cliche depends on which side of the QSO you are on. To complete a QSO both stations must copy each other. However you can only improve your own station, and that means ERP must be considered.

There are a limited number of ways to improve your ERP. One or two of these should be attainable by anyone:

• Power
• Transmission line
• Antenna gain and height
• Move to a hilltop or the tropics

The last is not an option for the majority other than by portable operation or international travel. Moving is rarely feasible when DXing is the sole justification.

Single hop E_s propagation can have a relatively high elevation angle. In contrast DX paths require multiple hops and perhaps ionospheric ducting, and sometimes joining the ends together with TE (trans-equatorial) propagation across the equator. These paths are most often at low elevation angles and that requires height or a hilltop.

Since going higher incurs more transmission line loss it is worthwhile to use the lowest loss coax you can acquire. Stacking yagis for gain is far easier on 6 meters than on HF. If you go this route I recommend vertical stacking for 2 or 3 yagis so that the gain comes from narrowing the vertical beam width rather than the azimuth beam width. It will make aiming less challenging and ensure your beam covers the maximum population possible.

Antenna selection in a stack can help you deal with elevation pattern nulls between all those minor lobes. The EZNEC elevation pattern above is for my A50-6 (optimized) up 24 meters. The lowest lobe is very good at below 5° but there are many nulls due to its being 4λ above ground. It is the ionosphere that determines the elevation angle, not your antenna, so if you have a null in the wrong place you'll attenuate the DX signals. Nulls in a stack are far less deep, or you can move the nulls by selecting just one of the yagis in the stack.

An amplifier is the easiest path to a bigger signal. By all means do so whether on its own or in combination with better antennas. Since you may cause AGC derived receiver desense to your ham neighbours within the FT8 window be judicious in your use of the amplifier. It is especially important to transmit a clean signal when you are QRO so adjust it for maximum linearity and have a friend check your signal for distortion products.

My preparation for E_s season

I have no time for 6 meter antenna work in this year's busy schedule. A 6 meter amplifier is also low priority. My sole improvement has been to replace the transmission line. Following re-cabling work last fall the transmission lines to the outdoor switching system from both operating positions are LMR400 in the house, LDF5-50 underground and short LMR400 jumpers to the antenna switch.

The 40 meter long run of 35 year old RG213 I used the last two years has been removed and given to a friend for use on the low bands where its loss is low. A new run of LMR400 goes from the antenna switch at the base of the 21 meter tower up to the rotation loop. The small loss in the total run should be ~2 db better than before. Eventually I intend to use Heliax end-to-end. But not this year.

Every decibel counts on the marginal openings found on the longest DX paths. While I cannot guaranty a modest 2 db improvement will help it surely won't hurt. QRO would be even better, as would bigger and higher antennas. Perhaps in the future as time and interest permit.

A development I will be following this year is the emergence of FT4 since it promises shorter QSO times. This could be a good fit for 6 meter DXing. The price of lower sensitivity than FT8 may be well worth paying to best exploit signal peaks. Lower sensitivity can, in part, be solved with QRO.

Going QRO

It's finally happened: I purchased a kilowatt amplifier for my station. This checks off one more item on my plan for this year. It won't stop here since I need a second one for SO2R and multi-op. That may wait a while longer until I have sufficient filtering protection in my station to deal with high power.

I'm starting simple, with an old style amplifier of an earlier generation: the Drake L7. This amp is capable of over 1000 watts CW and several hundred watts more on SSB peaks (PEP). It is a grounded grid class AB (linear) amplifier utilizing a pair of 3-500Z tubes with a combined plate dissipation of 1000 watts. Initial checks show the power and electrode currents are within spec and power output is as it should be. With a stiffer power supply it could do more but it already reaches the legal limit in Canada.

An amplifier like this is not entirely straight-forward to install and use. There are many considerations to get the best from it during contests. I have not decided how far to pursue these objectives with this amplifier and may instead make this a temporary resident in my shack. For my present purposes it is acceptable. A more modern amplifier is in my future.

The limited use it has seen in the short time it's been here has been positive, both in operation and on air results. Because it has been many years since I've run QRO there are things to relearn. There is no rush and I am not easily seduced by transmit power as my ongoing use of QRP should impress. For me QRO is about entering contest categories where I will compete with the big guns to discover how much my antennas (and the operator!) can accomplish against the best. Successful QRO contesting requires a different style of operating to be competitive.

In this article I'll briefly cover what I believe are the most important aspects of integrating an amplifier into a contest station. There is more to it than finding room on the shack desk and plugging it in.

Electrical service

An amplifier that puts out 1000 watts of RF consumes substantially more than this. Going by amplifier efficiency alone the heat generated is ~800 watts. For amplifiers utilizing vacuum tubes count on ~150 watts for the filament(s). Since power supplies are not 100% efficient there is at least another 100 watts of heat generated. The rest of the circuitry consumes comparatively little power and can be ignored for this calculation.

My favourite electrician: Geoff VE3KID

The total power consumption is ~2200 watts. That's approximately 9 amps for a 240 VAC mains branch circuit. Peak consumption is higher on SSB but the amperage impact depends on power supply design.

Let's assume that we need 12 amps at 240 VAC. The branch circuit should be capable of 20 amps to allow a safety margin and to avoid inadvertently tripping the circuit breaker. I put two branch circuits into the shack. Receptacles are on the floor behind the operating desk to avoid fussing with fishing wire and punching holes in the wall.

My friend Geoff VE3KID, a retired electrician, did the hookup and installation of a 50 amp sub-panel and branch circuits for the amplifiers. He previously installed the service into my garage and workshop (shown in the picture). Running cables and putting in boxes was my job, which he inspected before doing the panel work and hookup. Geoff is also the fellow who installed the circuit for my 30S1 way back in 1985 when we were both much younger.

Although many hams are comfortable working with lethal voltage and current, hiring an electrician is strongly recommended. Where required have your utility inspect and approve the work. Aside from the immediate risk there is potential for future electrical failure or fire and a declined insurance claim.

Noise

Here I am referring to audible noise, not RF, although that too can be a problem (see below). Audible noise sources include fans, open-frame relays and power supply hum. Some can be managed by positioning the equipment. In other cases the amplifier may need modification.

L7 fan noise is modest. The stock L7 antenna relay is very loud and the power supply (a separate unit) hums when in SSB mode (high setting of the plate voltage). Neither is unusual with "classic" equipment of this vintage. Both can be cured with some work. I have not decided what I will do or if I will do anything. Since the amplifier may not be here for the long term I may let it be.

Headphones that totally enclose the ears reduce the noise to a barely acceptable level. This was never a problem with QRP! It takes some getting used to, or in my case re-acclimatizing myself to what I experienced many years ago when I had a Collins 30S1. It, too, was quite loud.

RFI

Since urban and suburban properties are small most hams have their antennas close to the house. The antennas may be directly above the house and shack. Should you have RFI problems with low power it will only get worse with high power. A previously RFI free shack can easily sprout many problems when you install an amplifier. Be prepared for it.

Your computer and peripherals are especially susceptible, exhibiting faults that can be difficult to identify and isolate. Keep a supply of RFI suppressing toroids and use then on unshielded (and even shielded) Ethernet and USB cables, power cords, video cables and so forth. RFI in appliances outside the shack can cause greater grief since your family will exert pressure on you to fix it or QRT.

RFI problems in the house and with neighbours have happily diminished over the years due to changes in technology. The ones that do appear can be more difficult to deal with since they involve digital technology. Proceed with caution.

My major RFI fear is with respect to SO2R and multi-op contesting. Until I have band pass filters and automated band switching the impact of 1000 watts on an adjacent receiver is not to be contemplated. Until then my QRO operating will be strictly SO1R.

Heat

An idling tube amplifier dissipates in excess of 200 watts. Waste heat increases to well over 1000 watts during operation. Although solid state amplifiers have much lower idling power because they have no filaments their operating heat dissipation is approximately the same.

Heat fills the shack and can make it uncomfortable in warm weather. An open window or air conditioning can remove the heat. In winter the shack can be made cozier by the additional warmth, and can also make you drowsy. These problems are exacerbated during contests since high power stations tend to run all the time, resulting in more on time for the amplifier(s).

If you live in a rural area like I do the cost of the electricity can be a burden. Less so in winter when the heat removes load from the furnace, even if electricity as fuel is typically more expensive than what is used by the furnace: mostly fossil fuels in Canada. My house has a ground source heat pump that runs on electricity but with a 5:1 ratio of heat output versus input. Space heaters and amplifiers produce heat at a 1:1 ratio.

Placement

Amplifiers are large and heavy. This can make them difficult to place on the operating desk. As you can see in the above picture I put the L7 on top of the FTdx5000MP. I can do this because the transceiver is structurally robust and the power supply is in a separate enclosure under the desk. A thick cable interconnects them, including a heavily insulated lead for the 3000 VDC plate supply.

The less weighty RF deck of the L7 is an easy reach for tuning and monitoring and has lots of space around the cabinet for forced air ventilation. For smaller, more fragile rigs the amplifier is best placed on the desk outboard of the rig. There is more reaching involved but then the rig is typically narrower (e.g. K3) or entirely absent (e.g. FlexRadio).

Broadband amplifiers with automated switching can be hidden away from the operating position, taking up none of the valuable desktop. You might notice this in pictures of big gun stations where there is no more than LCD displays, keyboards, VFO knobs and paddles. That's very nice but also very expensive, at least at present.

Tubes

Have you checked the prices of high power RF tubes lately? They're high and getting higher. One reason I chose an amplifier with glass envelope tubes is that they are less dear than their metal-ceramic cousins. However they're larger, requiring a larger cabinet, and more fragile.

When run properly both styles of tube can last for many years, but one mistake can be costly. Tetrodes in particular are very sensitive to abuse since the screen grid is typically unable to dissipate any heat at all. That's unfortunate since tetrodes typically have lower distortion. Over-driving grounded grid triodes is also a recipe for early failure. Too many hams push their amplifiers hard, reducing lifetime and polluting the bands with distortion products. Many learn their lesson the hard way when they need to replace an 8877.

Suffice to say it is important to operate amplifiers with care, retuning as necessary when changing frequency, band or antenna. Diligence can ensure that you'll never have to replace those expensive tubes. Solid state amplifiers are no panacea since they have other concerns, such as intolerance of mismatch and, in many cases, greater distortion products. On the positive side these newer solid state amplifiers have safety features to protect you from faults and operator error.

Ceramic tubes such as 8877, 4CX1500B and many others require a 2 to 3 minutes warm up before they can used. This can be aggravating for DXers. Don't bypass the safety feature that enforces the delay or you'll face a drastically reduced tube lifetime.

Tuning

Speaking of tuning, it is worth mentioning a few of the challenges involved. Most hams are accustomed to broadband transceivers that handle frequency, band and antenna changes without operator intervention, including an ATU with memory. That is rarely the case with a kilowatt amplifier. The impact on contesters such as myself can be significant.

Tube amplifiers in particular almost always require manual tuning of the output tuned circuit. The output impedance of a tube or FET amplifier is higher than 50 Ω, while bipolar transistor amplifiers have a lower impedance. In all cases impedance transformation is required. Broadband transformers can be effective for most solid state amplifiers, but these can be large and expensive at the kilowatt level, and are usually impractical for tube amplifiers due to the large impedance ratio.

Broadband transformers also require that antennas be close to 50 Ω if they are to perform well. Manual tuning of tube amplifiers can match a wider impedance range at the price of operator effort. Having your antennas exhibit low SWR makes operating any amplifier much easier and safer.

Contesters and other active hams will happily deface their amplifiers with stick on labels for penning marks for various antennas and frequencies so that tuning can be accomplished quickly. Fewer contest QSOs are lost and DXers are less likely to miss out on rare DXpeditions. It can also help to avoid tuning errors. If you do use labels try to find those that are easy to remove and do not leave a permanent mark on the panel, otherwise you will have an eyesore with a lower resale value.

We can't forget the amplifier input which must also be tuned since the input port is rarely 50 Ω. Untuned amplifier inputs are found on lower end equipment and may require use of an ATU on the transceiver. Even with tuned inputs it may be necessary to use an ATU on the WARC bands, depending on amplifier vintage and quality. Since my amplifier has tuned inputs I have to turn off the rig ATU when I go via the amplifier to an antenna with a poor SWR. That's one more detail to worry about until all my antennas are good 50 Ω matches.

You may notice that if the rig and amplifier could be integrated that the transmitter output network and amplifier input network could be collapsed into one. Since that it rarely the case, and indeed the rig is often operated alone, an impedance of 50 Ω between the units is sensible.

Another thing: did you ever notice that many modern rigs don't have a button to transmit a carrier for adjusting an external tuner or amplifier? Something needs to be rigged up to implement this feature. On my FTdx5000 there is a jack on the rear panel (Tx Req) that you supposedly ground to do this, but mine doesn't doing anything. I'm working on it.

Sequencing

Although not often thought about in the typical ham's station, once you add an amplifier there may be a need to deal with sequencing. For our purposes the only signal of significance is when you go key down. Problems can arise depending on where the signal comes from: keyer, computer, transceiver or other peripheral controller, and the speed of the switching circuitry in the rig and amplifier.

Most high power amplifiers use relays at the input and output to switch the amp in and out of the circuit; during receive the amp is bypassed. The output relay tends to be of the large and slow type to handle the power. If RF appears at the input port before the relays fully engage hot switching can occur. This can damage the relays, cause arcing in the amp and truncate the first transmitted symbol. This is mostly a problem on CW, on that first dit or dah, and on digital modes, but typically not on SSB unless VOX is used.

When using a "smart" keyer such as a Winkeyer or contest software the sequencing problem can be mitigated with some experimentation. When playing memories -- these are the majority of contest transmissions -- delay can be added between assertion of PTT (Tx enable) and the first message symbol. On SSB it is even easier since it is unlikely that the message begins at full volume. The trouble is with VOX on SSB but especially with CW sent with paddles. In the latter case there is no way to advance PTT assertion, and delaying symbol transmission will adversely affect the operator since audible feedback will be out of sync with the fingers.

In initial testing of the L7 the serial assertion of PTT from Winkeyer to rig to amplifier does not affect the initial CW symbol. Monitor the bands during a contest and you will find instances where the first symbol is short or entirely lost. The problem gets worse as CW speed increases. If you have any instances of arcing in the amp (relays, capacitors, etc.) stop immediately and deal with it.

There are two alternatives to serial chaining of PTT. One is to key the rig and amp in parallel from the computer or Winkeyer. Before trying this ensure that the voltage and current are within the capability of the Winkeyer or computer circuit you are using. Those big relays in older amplifiers can draw more current than you expect.

The second alternative is to modify amplifiers with electronic switches and fast relays. These are far quieter as well (see 'Noise' above). Vacuum relays at the output port of the amplifier may be required. The best systems are QSK compatible.

What all of this means to me

From the foregoing discussion you may appreciate why I have been slow to include an amplifier in my station. Aside from trying it out on the bands during ordinary DXing I have yet to put it to the test in a contest. I am in no rush since the contest season is winding down with the arrival of spring. Good thing, too, since my antennas are in rough shape.

Soon enough I will try it out in a minor contest. That will give the amp a good workout and will force me to exercise my QRO contesting skills and tactics. It is very different in comparison to low power and QRP contesting. I will also have to practice rapid band changes.

With the arrival of spring and sporadic E season it is a shame that this amplifier does not work on 6 meters. At some point I will need more power on that band for optimum DXing performance. That will shortly be the subject of another article for the blog.

That I now have an amplifier does not mean I will no longer operate low power or QRP. I enjoy all power levels and will continue to be flexible in my operating choices. I prefer to delay QRO contesting until I have made more progress on low band receive antennas. Otherwise I will not be able to copy many of the smaller stations who will call me. I do not want to be an alligator.

My second amplifier, when I get one, will likely be more modern than the L7. I would like one with better operating features, in respect to many of the considerations raised in this article. Broadband tuning would be especially welcome. When a suitable amplifier becomes available I will make my move. I can afford to wait.