Improving the stability and performance of the FiFi SDR receiver

At the Northern Utah WebSDR link, among one of the several receiver configurations is that where a "SoftRock" receiver is used with a sound card.  This combination works very well - far exceeding in overall performance, especially dynamic range, almost any available "wideband" receiver including the SDRPlay, Red Pitaya and KiwiSDR ^{Note 1} with the caveat that only a bandwidth equal to the sample rate of a sound card - 192 kHz or so maximum - can be covered per "band".

Figure 1:
One of the three FiFiSDRs obtained for use at the Northern Utah WebSDR.
Click on the image for a larger version.

Problems with the USB Sound Cards:

Up to now, we have been using a combination of plug-in (PCI, PCIE) sound cards, using USB sound cards when the number of such receivers exceeded the number of plug-in slots on the computer.  Among the few affordable USB audio devices that can sample at 192 kHz are the Asus Xonar U5 and U7 (including the MK 2).

These devices work very well for the task - when they work:  After nearly 2 years, only two of the ten U5 and U7 devices that we had acquired over that period still work, the majority having failed when the USB interfaces would fail to negotiate at full USB 2.0 speed (if they negotiated at all!) after a few months of operation - often after a reboot.  Unfortunately, an Internet search revealed that this is not an uncommon problem and a plausible explanation as to the reason for these failures - or a fix - was not to be found.

An alternative:

Rather than spend more money on the unreliable Asus U5 and U7 USB devices - most of which we had gotten on EvilBay, originally in proper working order - we decided to switch to the "Fifi SDR" device from Box73.com in Germany.  Originally introduced around 2010, the current "Version 2" increased the bandwidth from the original 96 kHz to 192 kHz - and rather than just a sound card in a box, the FiFiSDR includes an entire synthesized "Softrock" receiver with decent performance and best of all, they cost about the same as a brand new 192 kHz capable USB sound card.  Several popular WebSDR systems - including KFS in Half Moon Bay, CA - use these devices and have reported good performance and reliability.  If they had been available when we were acquiring the equipment for the Northern Utah WebSDR, we would have started using them earlier.

Three FiFiSDRs were ordered - exactly enough for our needs - and upon arrival, I assembled them (a bit of soldering and mechanical assembly) and began to test them.  For whatever reason my Windows 7 machine at my workbench steadfastly refused to recognize the Fifi's sound card interface, but my Windows 10 laptop did and after a few missteps - mostly related to the program I was using to interface with it (HDSDR) having been previously configured for different SDR hardware - I got all three up and running.

Initial impression:

My initial impressions of the performance of the three FiFiSDR were generally good - ignoring the "elephant in the room" discussed below:  The receiver sensitivity, although varying by 2-3 dB between receivers, was within advertised specifications and once they had been powered up for several minutes the frequency was quite stable (within a few Hz).  Immediately, I noticed a few low-level CW spurious signals, but these were at or below the microvolt level and would likely be submerged in the noise floor - at least on the lower bands.

Because the sound codec was integrated within the receiver itself, the center-frequency (so-called "Zero Hz") noise was quite low because a possible pick-up point (e.g. a cable going from the output of the SoftRock receiver to a sound card) has been eliminated.  I did notice a fairly strong artifact at or near zero Hz - likely a DC offset with a bit of 1/F noise - but this is typically removed by a low-frequency high-pass filter in software and is not likely to be an issue.

I did notice two artifacts typical of "SoftRock"+sound card receivers:

• Under no-signal conditions, the noise floor would rise by several dB at "high audio" frequencies as manifest by a slightly "lighter" waterfall at the extreme low and high ends of the 192 kHz passband.  This is quite typical of sound cards and has been observed on nearly every sound card that I have used.

• Under conditions where the external (ionospheric) noise exceeded that of the receiver's noise floor, there was a bit of "droop" at the extreme low and high ends of the 192 kHz passband.  I've noticed this effect on nearly every softrock-type receiver and attribute it largely to signal drop-off in the audio chain at high audio frequencies.

I did see something that alarmed me:  Unstable spurious signals that drifted about (the "elephant" mentioned previously) but a "fix" for this problem is pretty easy and is described later.

Static sensitivity!

I'd seen a mention or two that the Fifi SDRs would occasionally "lock up" - but it didn't seem to be a common theme in the groups online - but once I got the them up and running, I saw two things that concerned me:

• When I touched the metal case and had no antenna connected the receiver's noise floor went way up.  This is bad news - particularly if it is to be installed in an electrically-noisy environment - like anywhere near a computer.

Figure 2:
The sole "official" case-to-board grounding point is at the corner near
the 3.5mm jack
Copper foil was wrapped over both the top and bottom and soldered
to the board's ground on both sides with a bit of excess
folded over on the end to provide a connection to the end plate.
(The solder connection was re-done, but I didn't get a picture.)
Click on the image for a larger version.

• If I had even the slightest amount of static electricity on my body the Fifi SDR would crash when I touched it and refuse to come back to life until I unplugged the USB and plugged it back in again.

Upon observing either of these, my suspicion as to the problem was verified with an ohmmeter - the metal case was, in no way, connected to the internal PCB.

Inspection revealed why:  Not only was the case very heavily anodized, the two boards fit somewhat loosely in the slots inside - and there was only a single common ground in one corner of the main board.  Even if the board was snug, it probably would not have made electrical connection through the case's oxide coating.

Remedy:

Clearly, I needed to find a simple way to bond the board to the case.  The receiver's  main board's use of a single-point ground seemed reasonable - particularly when one pairs a computer with a very sensitive receiver as one must carefully avoid on-board ground loops - so I resisted any temptation to "bond it everywhere" - at least not without careful testing.

Figure 3:
The point where the copper foil makes contact with the end plate.  Note that
the ends of the drawn-aluminum case have had the oxide layer removed to
bare the metal:  The same was done on the end plates to allow the copper
to make contact.  The bared ends of the case and corresponding parts of
the end plates were coated with a light layer of anti-oxidant.
Click on the image for a larger version.

First, I used a rotary tool with a wire wheel to remove the anodization (clear oxide coating) from around the edges of the end panels as well as the very ends of the drawn aluminum case so that when the screws were installed, they would have metal-on-metal contact.

The next step was to provide a connection from that ground in the corner to the case - and I did this by wrapping the edge of the board in that corner  (Figure 2) with copper foil and soldering it, leaving a bit of excess to wrap around the end of the board (Figure 3) - the idea being that it would be compressed by the now-bare aluminum end panel and make connection to the rest of the case.

Because bare aluminum quickly forms its own insulator when exposed to atmospheric oxygen, a thin layer of anti-oxidant compound (e.g. "NoAlOx" or "DeOxIt" - both used in electrical wiring) was applied to the bared aluminum on the ends of the case as well as where the copper would press against it:  This would prevent re-oxidation and the loss of connection over time and with exposure to air and moisture.

Result:

Upon reassembly, the end of the board with the copper connection was tightly pressed against the inside of the end panel and there was a low-resistance connection between the boards inside and the case.  Because of this effort, not only does the receiver noise floor not go up when I touch the case, but I can give the unit a pretty good "zap" with a static spark and not have it affect the operation of the device!

If I'd had the time to do so, I would have tested the efficacy of additional board-to-case ground points, making sure that these additions did not reduce the performance of the receiver.

Comment:

Although not instructed to do so by the assembly guide, installing the nut on the 3.5mm audio connector may connect the board to the case (via the end-plates) at that point. As they are, the connector's body is not is not long enough to protrude very far through the end plate and only a few threads were presented.

Figure 4:
On the top, an 80 volt gas-discharge tube.  Holes were drilled on the top to
provide connection and mounting.  The tube is bent away
from the case to prevent it being shorted to the case.
When drilling, take care to avoid intercepting any traces on either
side of the board.
Click on the image for a larger version.

Because of the thickness of the end plates, the installation of the nut will prevent proper insertion of a cable into the connector - but since I wasn't planning to use that jack, I installed it anyway.

The "floating" antenna jack:

Presumably to prevent circulating currents between the antenna system and the "ground" of the computer (via the USB cable) the antenna jack is coupled to the receiver via a 1:1 transformer.  Having a "floating" antenna connector made me nervous:  If one were to connect a FiFiSDR to an ungrounded wire antenna, wind static could easily cause high voltage to appear on the antenna connector which might not only cause a shock, but if it arced to ground somewhere - possibly within the receiver - it could damage the receiver's RF amplifier and/or be conducted to the USB interface to the computer where it could cause the FiFiSDR and/or computer to crash or worse, cause damage.

Figure 5:
The 150k resistor on the bottom of the board to drain static.
Any value between 47k and 220k would suffice.
For another modification (described below) 0.1 and 0.001
capacitors were soldered across this resistor for RF bypassing.
Click on the image for a larger version.

While I am not advocating using any antenna without appropriate grounding, I do know that it does happen and out of principle, I added circuitry to mitigate the risk:  The addition of a 150k resistor to prevent the accumulation of charge and an 80 volt gas-discharge tube between the RF and system ground.

Even if one does not add a gas-discharge tube, I would certainly advocate the addition of the drain resistor!

* * * * * * * *

Spurious signals in the receiver - the "elephant":

Having gotten all three receivers operational, I noticed something else that was alarming:  In two of the three receivers I could see, near the upper and lower edges of the passband (192 kHz sampling rate, 75 kHz and farther, symmetrical about the center frequency) some "ragged" signals that drifted about:  The third receiver also showed these same spurious signals, but they were much weaker, closer to the center frequency indicating that the same problem was evident, but likely farther out of the +/- 96 kHz passband of the receiver's sound card and showing up via aliasing.  Even if though the spurious signal on this third receiver was weak, I decided that was likely to mix with existing signals and cause additional, undesired signals to be produced within the passband, and would likely be the case with the other two receivers.

The spurious signals on the two worst receivers (example in the upper half of Figure 6) were fairly strong, about "S-9" in strength, making their existence unacceptable.  A quick check of the FiFiSDR wiki and trouble ticket system revealed that there were at least two tickets (#324 and #332)  reporting this issue - but both were years old and were still open with no suggested resolution.

Figure 6:
A screen shot from HDSDR showing the spurs before the modification (top half) and after the modification (bottom half).
These spurs are symmetrical about the center frequency (red line in the middle) indicating that they are NOT at RF, but rather at a point beyond the RF mixer in the audio chain.  The amplitude of these spurious signals make this receiver nearly unusable due to their strength.
Because of the rather low apparent signal strength, it is most probable that the actual frequency of these oscillations is not in the 0-96 kHz range, but much higher and being made visible because of the finite attenuation of the codec's low-pass filtering and aliasing of the A/D converter's sampling rate.  
Click on the image for a larger version.

 The symmetrical nature of this signal - and the fact that its nature was completely independent of the receive frequency - indicated that the origin of this signal was not at RF, but was within the audio chain or related to the FiFiSDR's power supply.

Careful observation showed something else:  As an applied signal within the passband of the receiver (e.g. +/- 90 kHz or so of the tuned frequency) increased in amplitude above approximately -70dBm, these spurious signals would start to  "noise up" and disappear - finally vanishing by the time the signal achieved -30dBm.  Because the signal was affected by signals in the audio chain, this observation took the onus off the likelihood of the power supply oscillating, pointing directly at the audio chain indicating that whatever was causing it was directly in the audio signal path.

Figure 7:
The addition of the 470k resistor to the "ADC1LP" pin of the codec.
The ground plane was scraped and the resistor soldered between it and
the capacitor(s) as shown, taking care to avoid shorting the "ADC1LP"
line to ground.  The corresponding (original) resistor on the "ADC1RP" line
is the upper-most resistor at the left marked "474", upside-down.
Click on the image for a larger version.

Wielding an oscilloscope, I started probing the audio chain - but I could see nothing obvious in terms of unusual signals - but I noticed that when I touched the probe to pin 39 ("ADC1LP") of the audio codec - an Analog Devices AD1974 - the frequency would shift slightly. 
Touching a voltmeter probe to this pin I observed that this spurious signal would disperse widely - as if frequency-modulated by the AC mains field on the workbench - but the same did not happen when I touched the voltmeter probe to pin 41 ("ADC1RP") indicating that the problem was only on the "left" channel of the codec.  Disconnecting the DC blocking capacitor from IC5, the audio amplifier, the amplitude of this oscillation remained the same, shifting frequency very slightly:  This implied that the problem was the AD1974 itself.

At this point I noticed something else:  The voltage on pin 41 ("ADC1RP") was a few 10s of millivolts lower than that on pin 39 ("ADC1LP") - something that should not occur as both were presumably biased from the same internal voltage reference.  I then observed that there was a 470k resistor between the traces connecting pin 41 and ground - but this resistor was missing on pin 39.

On a hunch I added a 470k surface-mount resistor to ground at the bypass capacitors connected to pin 39 and found that the spurious signal disappeared.  Apparently, the designers of the Version 2 of the FiFiSDR had observed a similar problem and added the 470k resistor to "ADC1RP" - but did not do so on "ADC1LP".

This "fix" worked on all three receivers.


Improving common-mode (longitudinal) isolation:

(Sorry, no pictures at this time.)

One thing I noticed about this receiver was that its antenna input was transformer-isolated from the case.  Ideally, this is a good thing as it can reduce any ground loops that may contain circulating AC or DC currents which, unless everything is well-bonded to a common ground, may cause problems (e.g. hum on audio devices, potential USB instability).  Unfortunately, no transformer is or can be perfectly balanced - and this could be demonstrated on the FiFiSDRs by simply touching the outer shell of the BNC connector and observing a slight noise increase when in an indoor RF-noisy environment - even after the case grounding issue discussed above was solved.

To quantify this imbalance I took some measurements, applying an amount of signal between the FiFiSDR case after bonding it to the circuit board as described above and the shell of the BNC connector, observing the amount of signal that was required to achieve an "S9" reading.  Doing this at both 5 MHz and 29 MHz, I obtained the following results in terms of common-mode (longitudinal) isolation:

• 5 MHz:
• Isolation = 29dB unterminated
• Isolation = 50dB terminated at 50 ohms
• 29 MHz:
• Isolation = 24dB unterminated
• Isolation = 35dB terminated at 50 ohms

Whether or not this is acceptable in your situation is something that you will have to decide - but I chose to make a minor modification:  The addition of a 0.1uF and 0.001uF capacitor in parallel (two capacitors being used to provide low impedance from low to high frequencies) with each other (this was in parallel with the 150k resistor depicted in Figure 5) to bridge the "RF Ground" and the system ground:  Unlike connecting the two grounds together with a jumper, this would still provide low frequency DC and AC isolation.  The result was that there was no longer a significant difference between the readings when the BNC connector was terminated or unterminated.

With the addition of the capacitors, the isolation improved to about 50dB on both frequencies - terminated or not.  I was hoping for even greater improvement than 50dB, but I suspect that because the case-to-board mounting occurs in only one place, in a corner away from the RF connector, circulating currents were flowing across the board.  It is possible that bonding the system ground to the case near the antenna connector would have improved this - but I did not have time to test this and make sure that it did cause significant degradation.

* * * * * * * * * * * * * * * * * * * * * *

Note 1:

Unlike some of the inexpensive "Wideband" receivers based on the RTL chips (e.g. RTL-SDRS - which have just 8 bit A/D converters) higher-end receivers that have greater simultaneous bandwidth (SDRPlay, Red Pitaya, KiwiSDRs) have greater bit depths - typically 12-14 bits, offering greater dynamic range.

Even a higher bit-depth wideband receiver can be at a disadvantage compared to the combination of a SoftRock and sound card:  Not only does the 16 bit depth of a sound card offer more dynamic range, but the lower operational bandwidth (192 kHz maximum for a sound card based receiver) means that there is less overall "energy per Hz per bit" impinging on the A/D converter than a MHz-bandwidth A/D converter.

* * * * * * * * * * * * * * * * * * * * * *

Comments:

• An attempt was made to post the solution to the problem of the spurious signals to the FiFiSDR ticket system, but the post was rejected by the system.  I have not had the time to register with the site.

• We were able to obtain three more FiFiSDRs of earlier manufacture and ALL THREE exhibited the same spurious signal response.  Fortunately, the "fix" - adding the resistor - worked in all three cases.  These receivers were also modified (case grounding, RF bypassing of the transformer, etc.) as well with the same, good results.

This page stolen from ka7oei.blogspot.com

[End]

A 1:1 balun was the best overall choice for feeding the horizontal loop...

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 in a typical "T" ("High-pass") configuration.

Figure 1:
The Heathkit SA-2060 tuner and (now) 1:1 balun feeding the 450 ohm
window line.  It no longer sits, on edge, in the window, as it had previously -
a much more convenient arrangement!

I have used this antenna tuner for years, taking it to Field Day and other than having to tighten some screws and adding thread-locker as well as a bit of lubrication of the moving parts after I got it, it has served me well, (seemingly) capably matching the 300-something foot circumference horizontal "lazy loop" antenna at my QTH that is fed with 450 ohm window line.

Discovering the problem:

A month or so ago I was doing some 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 to accommodate that which had to be moved out of the way.  While the "radio area" wasn't disassembled for this task, I ended up piling a lot of stuff in that part of the room, making it inaccessible.

One of the things that I did during this work 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.  From what I'd read, the combination of the higher mains voltage and the Peter Dahl transformer would provide a higher plate voltage under load along with higher output with slightly less drive - and upon testing with the dummy load, this appeared to be true.

For years my tuner had been sitting on edge in the window with the 450 ohm window line coming through an insulated gap, past the vinyl window frame and connecting directly to the balanced wire connection on the back panel.  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...

There was 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 balun's balanced line output (with one of the wires of the window line still connected) and, again, could hear no difference, either.  I connected my antenna analyzer and noted that while there was a good match through the tuner, it did not change much if there was one or two wires connected, or if the "balanced" terminals were shorted together.

Hmmm...

What was happening:

Now, I was curious.  It would appear that I'd been actually running the "loop" in a "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 results 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 with an ohmmeter.   

Rewiring the balun to specs - and a problem:

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 after "buzzing" the wires with an ohmmeter to identify them and tested the balun on the bench with the antenna analyzer and the other end of the balun terminated with a 200 ohm resistor.  Unlike the original configuration of the balun, according to the analyzer it was now working as it should, having a reasonable match to 50 ohms across the HF spectrum and going to infinity when resistor was shorted or removed.

Putting the balun back in the tuner and reassembling it I had to readjust from my previously-noted tuner settings to find a proper match (a good sign that the settings weren't the same, actually!) and I then checked it out with 100 watts on 40 meters.  Everything appeared to be fine, although the tuner struck me as a bit more "touchy" in its adjustments as compared to before.

Firing up the amplifier I soon discovered that I couldn't tune it up without its "Plate" variable capacitor arcing over noisily.  Grabbing a "Cantenna" dummy load I verified that the amplifier itself was fine at full power, but something else was wrong.  Turning the power all of the way down and then slowly up again I discovered that at around 200 watts of RF output the reflected power went up, suddenly equaling the forward power.  Popping the cover off the tuner again confirmed my suspicion:  The "output" capacitor in the tuner was arcing over.

When the typical 4:1 balun probably isn't appropriate:

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 well 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 the loop had, for example, a 50 ohm feedpoint resistance, 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 the likelihood of a high-Q configuration of the tuner itself which, in turn, implies high voltage and high current which further implies high losses!

Wielding my antenna analyzer I hooked its BNC connector directly to the balanced line:  Since the analyzer was hand-held and I was checking at "only" 40 meters I didn't think that it would really matter much that it was properly "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 values seemed to go much above 50-80 ohms.

Suspecting that without the "bandpass filter" effects of the 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 Heathkit HD-1422 RX noise bridge and connected it to my FT-817, running on battery - this combination being comparatively immune to stray, off frequency RF and like the analyzer configuration that I'd used, more-or-less "balanced" without any obvious ground reference.  With that configuration I got a more sensible resistance reading of around 35 ohms and a reactance measurement 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 via the balun!

Most antenna tuners match "up" better than "down":

Figure 2:
The exterior of the Balun Designs Model 1171t 1:1 "ATU" balun.
This model is equipped with studs on the top of its weatherproof case
for connection to a balanced feedline.

This brought to mind a discussion that I'd had with another amateur some time earlier.  He pointed out that it seemed silly that most baluns with tuners offered only a 4-fold impedance up-conversion, but it was likely that a typical antenna fed with balanced line was more likely to see a 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 (e.g. a "Zepp" antenna), a rhombic (don't most of us wish we could have one of those?) or a full-wave dipole.

What this meant was that in most cases that the average amateur would encounter, the tuner was going to be matching to substantially lower than 50 ohms resistive through the 4:1 balun - something that is likely to cause problems like loss - which is invariably accompanied by heating - and high voltages, internally.  What had been a reasonable hypothetical scenario was manifesting itself as reality!  In reality, most antenna tuners will more efficiently match to impedances higher than 50 ohms than lower because of these potentially high currents, so having a balun that tried to make my antenna's somewhat low impedance even lower (at least on 40 meters) was probably not a good idea.

The solution:

The clear solution in this case was to use a 1:1 balun, instead to avoid the tuner from seeing an "awkwardly low" impedance.

I had the choice of reconfiguring the existing 4:1 balun - which was now working properly - perhaps by rewinding it with some PTFE 50 ohm coax - but I decided, instead, to get another balun and keep the internal balun intact in the event that it would be needed (it's nice to have options!) as it could be easily inserted or removed from the circuit using a jumper on the rear panel.  Because I was intending to use the tuner/balun combination with my amplifier - which was capable of the full 1500 watts output - I also knew that it needed to be both low loss and capable of handling very high voltages and high currents.

In perusing the various web sites and forums I looked at the possibility of making my own balun - which I could have done for $20-$40 in parts - but ultimately decided that I didn't want to take on yet another project right then and settled on the "1:1 ATU Balun" by a company called "Balun Designs." - link.  The products of this company not only had good reviews, but their web site was also impressive, explaining in good, sensible detail why one balun was better than another for a particular application and it also outlined situations where certain baluns they sold 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 described by Joe
Reisert (W1JR) in the September 1978 issue of Ham Radio magazine - that
is, the second "half" of the windings cross over to minimize coupling
between the input and output to provide best isolation and to
minimize the "one turn" effect inherent with "normal" toroid
winding techniques.

The balun that I chose (Model #1171t) is a current balun which effectively operates in series with the signal path (unlike a more common "voltage" balun which typically resembles an autotransformer as in the case of a typical 4:1 balun) and 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 over which a transmission line is wound.  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 current 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 enameled copper wire covered with PTFE spaghetti tubing - a configuration that has potentially lower loss and higher power-handling capability by virtue of its larger conductors.  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 70-100 ohm range) when terminated with a 50 ohm load the apparent match, when viewed with an antenna analyzer seems to degrade as frequency increases - likely a result of the "transmission line" comprising the balun's winding causing some impedance transformation.  This property is of relatively little importance for this application since it is designed to sit on the output of an antenna tuner, anyway:  As long as it has low loss and can withstand the expected voltages and currents, it would have minimal effect on the overall system efficiency despite a mismatch under most conditions.

When this balun arrived I connected it to the output of the SA-2060 tuner with a short (approx. 18", 50cm) RG-8 style jumper to minimize losses 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 - and then 1500 watts and had no problems at all.  I did notice that the window line became warm to the touch and the balun core and windings also became perceptibly warm, but by no means "hot" as the thermal image in Figure 4 suggests.

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 their 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."   (The "warm" UHF
connector at the bottom appears thus as it is reflecting heat from
elsewhere in the shack.)  Considering that over 1kW
of RF is flowing, this amount of heating represents negligible loss - likely
less than that occurring within the tuner itself.
Of course, the amount of heating will depend both on the power level
and the amount of current flowing through the balun, and this depends
on the matching/impedance conditions encountered.

I also observed that if I disconnected just one side of the balanced line, the signals on the band dropped by several S-units and the relative floor relative to signals came up but when shorting the balanced line, this caused signals to all but disappear - exactly what I was expecting to happen in a circuit that properly rejected common-mode signals.  When checking across the bands 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 (just a foot or so away) was now barely detectable at the S-2 noise floor on 40 meters:  Thus are the benefits of common-mode rejection in the prevention of electrical noises from the shack and the house's electrical system from being conducted onto the feedline/antenna and into the receiver.

As far as the warming of the window line, I did some calculation and determined that the feedline was likely seeing a VSWR somewhere in the range of 8:1 to 20:1 or so, which meant that, when it was dry, it was losing as much as 0.5 dB along its 30 foot (10 meter) run - a worst-case (20:1 VSWR) loss of up to 11%, or in the area of 150 watts at the test power of 1100 watts.  This is a small fraction (approximately 1/12th) of an "S" unit, but it would certainly explain the warmth!

A few days later I had the opportunity to check into a 40 meter 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 my signals being 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!

Final comments:

• One of the first things that I do when I get gear at a swap meet - be it commercial, a commercially-made kit or homebrew - is to check it out, making sure that all hardware is tight and electrical connections are solid, but I will admit that it never occurred to me when I got this tuner to check to see that the balun was wired properly!
• While the implementation of the new balun reduced my 40 meter noise floor from about S-5 to S2, I could still hear some low-level noise related to the AC mains and a small switching supply in my shack.  After a few weeks it finally occurred to me that where the new balun was hanging by the feedline placed it squarely over a small switching supply in a wall-mounted outlet strip.  When I moved the balun away from the power supply the noise floor went from S-2 to S-1 or so and I now hear only atmospheric noise:  For a while I had a metaphorical palm-shaped dent in my forehead!
• There is an interesting article written by G3TXQ - "Tuner Baluns:  4:1 or 1:1" - link here.  This article analyzes the impedances typically found and makes suggestions about how to determine whether a 1:1 or 4:1 balun is most appropriate when used with a tuner/antenna/band combination.   Spoiler alert:  He suggests a 1:1 balun for the general case - in fact, the same Balun Designs #1171 as depicted above.

    [End]

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