Years ago I bought a Heathkit SA-2060 (non "A" version) 2kW-rated antenna tuner at a local swap meet for a good price. While not as heavy-duty as some of the venerable Collins or Viking tuners, it had a nice-sized roller inductor and a pair of large, wide-spaced variable capacitors, connecting in a typical "T"("High-pass") configuration.
I have used this antenna tuner for years, taking it to Field Day and other than having to tighten some screws and adding a bit of lubrication when I got it, it has served me well, capably matching the 200-something foot circumference "lazy loop" antenna at my home QTH that is fed with 450 ohm window line.
A month or so ago I was doing so rewiring after having my main electrical panel replaced in conjunction with the installation of a PV (Solar) inverter system and to do this work I had to "open up" some wall and ceiling spaces in the room containing my ham shack - but this also meant that I had to disassemble and relocate much of what was in the shack. While the "radio area" wasn't particularly disassembled for this task, I ended up piling a lot of stuff in that part of the room, essentially making the radio inaccessible.
One of the things that I did was to pull a brand, new 240 volt, 20 amp circuit for my Heathkit HL-2020 linear (really an SB-221 with a brown color scheme) and once I had the room more-or-less back together I reconfigured the amplifier for 240 volts (there were minor complications to this - perhaps another story) and I was ready to get back on the air.
For years now my tuner had been sitting on edge in the window with the 450 ohm window line coming through an insulated gap, past the vinyl frame, and connecting directly to it. In the rearrangement I'd needed to take the tuner out of the window and in the process one of the wires of the window line popped off - something that I noticed as I was preparing to test the amplifier under load. Happening to have the receiver on at the time, I reconnected the leg of the balanced line and...
No difference in the signal strength of the received signals.
Something was definitely wrong here! I would have expected that with one leg of the balanced line disconnected that I'd get at least an "S" unit or two difference in signal strength, but there was no obvious difference at all. Grabbing a screwdriver I shorted the balanced line and, again, could hear no difference, either, so I connected my antenna analyzer and noted that while there was a good match, it did not change much if there was one or two wires connected or if the "balanced" terminals were shorted together.
Hmmm...
Now, I was curious. It would appear that I'd been running the "loop" as in "T" type configuration with the downlead being (more or less) end-fed and the remainder of the antenna being a sort of distributed top hat. I've never really had trouble working other stations, nor had I really experienced any "RF in the shack" issues as I had a pretty decent, short ground with heavy decoupling of the HF coax feeding to the tuner via a large chunk of ferrite scavenged from an old computer monitor. In other words, I'd had no reason to question the operation of the balun itself or how it functioned.
The tuner's cover was immediately off and I was comparing the balun connection with that of an SA-2060A manual that I'd found online and the result was inconclusive: If the wires had been properly identified and taped at the time of initial construction, it looked correct, but if not, the only way to verify this was to remove the balun and check it out with an ohmmeter.
I regret that I didn't make a note of how the balun core was wired, but I do know that it wasn't at all right so I made the necessary changes and then tested the balun on the bench, with the antenna analyzer and the other end of the balun terminated with a 200 ohm resistor. According to the analyzer it was now working as it should, having a reasonable match to 50 ohms and going to infinity when the 200 ohm resistor was shorted or removed.
Putting the balun back in the tuner and reassembling it I had to readjust from my previously-noted settings to find a proper match (a good sign that it wasn't the same, actually!) and I then checked it out with 100 watts on 40 meters. Everything appeared to be fine, although the tuner seemed "touchy".
Firing up the amplifier I soon discovered that I couldn't tune it up without the "Plate" variable capacitor arcing over noisily. Grabbing a "Cantenna" 1 kW dummy load I verified that the amplifier itself was fine, but something else was wrong. Turning the power all of the way down and then slowly up again I discovered that around 200 watts of RF output the reflected power suddenly equaled the forward power. Popping the cover off the tuner again I confirmed my suspicion: The "output" capacitor in the tuner was arcing over.
What this meant was that the tuner was being asked to match something really awkward - but with my loop and given its length, I thought it unlikely that the feedpoint impedance would be really high, but rather it was more likely that it was "low-ish" - probably below 100 ohms.
The problem with this is that I now had a properly-working balun that provided an upwards impedance transformation. This meant that if I had a 50 ohm feedpoint resistance on my loop, the tuner would be "seeing" around 12.5 ohms: This is bad news as making a transformation from 50 ohms to 12.5 ohms implies a lot of current which also implies a high likelihood of a high-Q configuration of the tuner itself which, in turn implies high voltage and high current which implies the high probablity of loss!
Wielding my antenna analyzer I connected it directly to the window line: Since it was hand-held and I was checking at only 40 meters I didn't think that it would really matter that it was "balanced" or not. The readings indicated a resistive component of around 10 ohms with a reactance of around 180 ohms inductive, but in tuning around to other amateur bands I couldn't make much sense out of the readings and was particularly suspicious when none of the resistance readings seem to go much above 80-100 ohms.
Suspecting that without the "bandpass effects" of a tuner that I was the victim of an AM broadcast station a few miles away being detected by the reflectometer bridge in the analyzer and causing false readings I dusted off my RX noise bridge and connected it to my FT-817 running on battery - this combination being comparatively immune to stray, off frequency RF and more-or-less "balanced" without any obvious ground reference. With that configuration I got a more sensible resistance reading of around 35 ohms and the reactance was in the area of 130 ohms inductive. If I took the 35 ohm reading seriously, that would mean that the antenna tuner was trying to match something under 10 ohms!
This brought to mind a discussion that I'd had with another amateur some years earlier. He pointed out that it seemed silly that most baluns had a balun that offered a 4-fold impedance up-conversion for ladder line since it was likely that a typical antenna was more likely to see a much lower impedance on most bands unless there was a configuration that was particularly prone to high impedance like a 1/2 wave end-fed wire or a full-wave dipole. What this meant was that for most purposes, the tuner was going to be matching at lower than 50 ohms - something that is likely to cause problems like loss - which is invariably accompanied by heating - and high voltages. What had been a reasonable hypothetical scenario was manifesting itself as reality!
The clear solution was to use a 1:1 balun, instead. I had the choice of reworking the existing 4:1 balun - which was now working properly - but I decided, instead, to get another balun and keep the internal balun intact as it could be easily removed from the circuit with a jumper on the rear panel. Because I was intending that I be able to use the tuner/balun combination with my amplifier which was capable of the full 1500 watts output, I also knew that it need to be both low loss and capable of handling very high voltages.
In perusing the various forums I looked at the possibility of making my own balun, but ultimately decided on the "1:1 ATU Balun" by a company called "Balun Designs." - link. The products of this company not only had good reviews, their web site was also impressive, explaining in good, sensible detail why one balun was better than another for a particular application and also outlining situations where certain baluns should not be used and why.
The balun that I chose (Model #1171) is a current balun which, unlike a more common "voltage" balun (one that typically resembles an autotransformer in the case of a 4:1 balun) it is essentially a common-mode choke that isolates one side of the balun from the other by virtue of the bulk inductance of the core. By suppressing the "common mode" aspects of the RF signals with a significant amount of inductance, the windings on the toroids effectively "choke" anything other than differential (balanced) currents and thus isolate one section of the feedline from the other - except for the equal-and-opposite RF that is supposed to be there!
While many of these baluns are wound with PTFE coaxial cable to preserve the 50 ohm impedance, this particular balun was wound with what amounts to parallel-conductor transmission line consisting of enamel wire covered with PTFE spaghetti tubing. What this means is that this "parallel transmission line" winding inside particular balun isn't particularly close to 50 ohms in its natural impedance (it's likely in the 60-100 ohm range) but that is of relatively little importance since it sits on the output of an antenna tuner: As long as it is low loss and can withstand the expected voltages it would have minimal effect on the overall efficiency!
When this balun arrived I connected it to the output of the SA-2060 tuner with a short (approx. 18") RG-8 style jumper and was easily able to tune the antenna with settings radically different than before - another good sign! Finding that everything looked good on the analyzer, I hit it with 100 watts - then 1500 watts and had no problems at all: I did notice that the window line became warm to the touch. The balun core and windings also became perceptibly warm, but by no means hot as the thermal image in Figure 4 depicts.
I also observed that if I disconnected just one side of the balance line together the signals on the band dropped by several S-units and the noise floor came up and shorting the two caused signals to all but disappear - exactly what I was expecting to happen in a circuit that properly rejected common-mode signal. When checking across the band at different times of day I also observed that the noise floor was 1-2 S-units lower than before and that the previously S-5 noise from the switching supply on the nearby DSL modem was barely detectable at the S-2 noise floor on 40 meters.
As far as the warming of the window line, I did some calculation and determined that it was likely that it was seeing a VSWR somwhere in the range of 8:1 to 20:1 or so, which meant that it was likely losing up to 0.5 dB along its 30 foot run - a loss of up to 11%, or in the area of 150 watts maximum: This is a small fraction of an "S" unit, but it would certainly explain the warmth!
A few days later I had the opportunity to check into a round table with a group of friends across the western U.S. and conditions were abysmal, but not only could I hear all of the stations pretty well, one of the stations with the weaker signals reported that they could hear my just fine with signals comparable to another station across town from me running about the same power - a reasonable indication that I wasn't burning up too much power in losses!
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| Figure 1: The Heathkit SA-2060 tuner and (now) 1:1 balun feeding the balanced line. |
I have used this antenna tuner for years, taking it to Field Day and other than having to tighten some screws and adding a bit of lubrication when I got it, it has served me well, capably matching the 200-something foot circumference "lazy loop" antenna at my home QTH that is fed with 450 ohm window line.
A month or so ago I was doing so rewiring after having my main electrical panel replaced in conjunction with the installation of a PV (Solar) inverter system and to do this work I had to "open up" some wall and ceiling spaces in the room containing my ham shack - but this also meant that I had to disassemble and relocate much of what was in the shack. While the "radio area" wasn't particularly disassembled for this task, I ended up piling a lot of stuff in that part of the room, essentially making the radio inaccessible.
One of the things that I did was to pull a brand, new 240 volt, 20 amp circuit for my Heathkit HL-2020 linear (really an SB-221 with a brown color scheme) and once I had the room more-or-less back together I reconfigured the amplifier for 240 volts (there were minor complications to this - perhaps another story) and I was ready to get back on the air.
For years now my tuner had been sitting on edge in the window with the 450 ohm window line coming through an insulated gap, past the vinyl frame, and connecting directly to it. In the rearrangement I'd needed to take the tuner out of the window and in the process one of the wires of the window line popped off - something that I noticed as I was preparing to test the amplifier under load. Happening to have the receiver on at the time, I reconnected the leg of the balanced line and...
No difference in the signal strength of the received signals.
Something was definitely wrong here! I would have expected that with one leg of the balanced line disconnected that I'd get at least an "S" unit or two difference in signal strength, but there was no obvious difference at all. Grabbing a screwdriver I shorted the balanced line and, again, could hear no difference, either, so I connected my antenna analyzer and noted that while there was a good match, it did not change much if there was one or two wires connected or if the "balanced" terminals were shorted together.
Hmmm...
Now, I was curious. It would appear that I'd been running the "loop" as in "T" type configuration with the downlead being (more or less) end-fed and the remainder of the antenna being a sort of distributed top hat. I've never really had trouble working other stations, nor had I really experienced any "RF in the shack" issues as I had a pretty decent, short ground with heavy decoupling of the HF coax feeding to the tuner via a large chunk of ferrite scavenged from an old computer monitor. In other words, I'd had no reason to question the operation of the balun itself or how it functioned.
The tuner's cover was immediately off and I was comparing the balun connection with that of an SA-2060A manual that I'd found online and the result was inconclusive: If the wires had been properly identified and taped at the time of initial construction, it looked correct, but if not, the only way to verify this was to remove the balun and check it out with an ohmmeter.
I regret that I didn't make a note of how the balun core was wired, but I do know that it wasn't at all right so I made the necessary changes and then tested the balun on the bench, with the antenna analyzer and the other end of the balun terminated with a 200 ohm resistor. According to the analyzer it was now working as it should, having a reasonable match to 50 ohms and going to infinity when the 200 ohm resistor was shorted or removed.
Putting the balun back in the tuner and reassembling it I had to readjust from my previously-noted settings to find a proper match (a good sign that it wasn't the same, actually!) and I then checked it out with 100 watts on 40 meters. Everything appeared to be fine, although the tuner seemed "touchy".
Firing up the amplifier I soon discovered that I couldn't tune it up without the "Plate" variable capacitor arcing over noisily. Grabbing a "Cantenna" 1 kW dummy load I verified that the amplifier itself was fine, but something else was wrong. Turning the power all of the way down and then slowly up again I discovered that around 200 watts of RF output the reflected power suddenly equaled the forward power. Popping the cover off the tuner again I confirmed my suspicion: The "output" capacitor in the tuner was arcing over.
What this meant was that the tuner was being asked to match something really awkward - but with my loop and given its length, I thought it unlikely that the feedpoint impedance would be really high, but rather it was more likely that it was "low-ish" - probably below 100 ohms.
The problem with this is that I now had a properly-working balun that provided an upwards impedance transformation. This meant that if I had a 50 ohm feedpoint resistance on my loop, the tuner would be "seeing" around 12.5 ohms: This is bad news as making a transformation from 50 ohms to 12.5 ohms implies a lot of current which also implies a high likelihood of a high-Q configuration of the tuner itself which, in turn implies high voltage and high current which implies the high probablity of loss!
Wielding my antenna analyzer I connected it directly to the window line: Since it was hand-held and I was checking at only 40 meters I didn't think that it would really matter that it was "balanced" or not. The readings indicated a resistive component of around 10 ohms with a reactance of around 180 ohms inductive, but in tuning around to other amateur bands I couldn't make much sense out of the readings and was particularly suspicious when none of the resistance readings seem to go much above 80-100 ohms.
Suspecting that without the "bandpass effects" of a tuner that I was the victim of an AM broadcast station a few miles away being detected by the reflectometer bridge in the analyzer and causing false readings I dusted off my RX noise bridge and connected it to my FT-817 running on battery - this combination being comparatively immune to stray, off frequency RF and more-or-less "balanced" without any obvious ground reference. With that configuration I got a more sensible resistance reading of around 35 ohms and the reactance was in the area of 130 ohms inductive. If I took the 35 ohm reading seriously, that would mean that the antenna tuner was trying to match something under 10 ohms!
 |
| Figure 2: The exterior of the Balun Designs Model 1171t 1:1 "ATU" balun |
The clear solution was to use a 1:1 balun, instead. I had the choice of reworking the existing 4:1 balun - which was now working properly - but I decided, instead, to get another balun and keep the internal balun intact as it could be easily removed from the circuit with a jumper on the rear panel. Because I was intending that I be able to use the tuner/balun combination with my amplifier which was capable of the full 1500 watts output, I also knew that it need to be both low loss and capable of handling very high voltages.
In perusing the various forums I looked at the possibility of making my own balun, but ultimately decided on the "1:1 ATU Balun" by a company called "Balun Designs." - link. The products of this company not only had good reviews, their web site was also impressive, explaining in good, sensible detail why one balun was better than another for a particular application and also outlining situations where certain baluns should not be used and why.
 |
| Figure 3: Inside the 1:1 balun. It is wound with parallel, highly-insulated enameled copper wire in the "Guanella" fashion - that is, the second "half" of the windings cross over to minimize cross-coupling between the input and output to provide best isolation and to minimize the "one turn" effect inherent with "normal" toroid winding techniques. |
While many of these baluns are wound with PTFE coaxial cable to preserve the 50 ohm impedance, this particular balun was wound with what amounts to parallel-conductor transmission line consisting of enamel wire covered with PTFE spaghetti tubing. What this means is that this "parallel transmission line" winding inside particular balun isn't particularly close to 50 ohms in its natural impedance (it's likely in the 60-100 ohm range) but that is of relatively little importance since it sits on the output of an antenna tuner: As long as it is low loss and can withstand the expected voltages it would have minimal effect on the overall efficiency!
When this balun arrived I connected it to the output of the SA-2060 tuner with a short (approx. 18") RG-8 style jumper and was easily able to tune the antenna with settings radically different than before - another good sign! Finding that everything looked good on the analyzer, I hit it with 100 watts - then 1500 watts and had no problems at all: I did notice that the window line became warm to the touch. The balun core and windings also became perceptibly warm, but by no means hot as the thermal image in Figure 4 depicts.
 |
| Figure 4: A thermal image showing the heating of the balun and transmission line. As can be seen, the closer to the "output" of the balun, the warmer the windings got, but after approx 20 seconds at 1100 watts of RF on 40 meters the temperature stabilized. The image above depicts a maximum temperature inside the balun of less than 120 degrees F (49C) with the feedline at approximately 105 F (41C) both being warm, but not "hot." Considering that over 1kW of RF is flowing, this amount of heating represents negligible loss - likely less than that occurring within the tuner itself. |
As far as the warming of the window line, I did some calculation and determined that it was likely that it was seeing a VSWR somwhere in the range of 8:1 to 20:1 or so, which meant that it was likely losing up to 0.5 dB along its 30 foot run - a loss of up to 11%, or in the area of 150 watts maximum: This is a small fraction of an "S" unit, but it would certainly explain the warmth!
A few days later I had the opportunity to check into a round table with a group of friends across the western U.S. and conditions were abysmal, but not only could I hear all of the stations pretty well, one of the stations with the weaker signals reported that they could hear my just fine with signals comparable to another station across town from me running about the same power - a reasonable indication that I wasn't burning up too much power in losses!