Essentially, my analysis was that it comprises two 12t winds of two wire transmission line in parallel on the ferrite ring. The potential benefit was that the characteristic impedance Zo of each transmission line is probably close to 100Ω, and the parallel combination is probably close to 50Ω.
Online experts following fashion are opining that a low Insertion VSWR balun is better made with two wire line(s) than winding a single 50Ω coax line. They make these claims without evidence, I am not convinced.
In that vein, here is a variation on the TrxBench balun above.
The designer describes it:
Wound with 18 AWG PTFE, Solid Silver Plated Copper wire. By using that specific gauge wire with PTFE insulation tightly coupled in pairs results in a 100-Ohm transmission line. Two in parallel = 50-Ohms. The advantage using the wire over coax is it flattens and widens the bandwidth. I stress it is extremely important to pay attention to the small details. The spacing, twisting, orientation, neatness, and symmetry are extremely important.
He is quite correct regarding his last point.
His claim that the each pair of wires wound on the core has Zo of around 100Ω without supporting evidence, but it is believable based on my own experience of making and measuring similar line sections
The ‘tails’ not just two continuations of the 100Ω transmission lines (as wrapped on the core) paralleled at the connector and load resistor, they are two line sections of some other geometry and again without evidence of their Zo. I cannot call upon experience to inform me about likely Zo, I suspect it may be significantly higher than 100Ω.
Whilst the designer explained that the picture was of a test setup for measurement with a VNA, he did not give the results of such a measurement, an InsertionVSWR plot would be informative.
I wrote a series of articles that showed how a very small length of pigtails impacted InsertionVSWR of a balun:
So, we return to claims along the lines of If I recall correctly, that was also K9YC’s conclusion. His updated ‘cookbook’ moved away from coax to wire
which of course if fallacious if it attributes someone’s personal opinion to a well known author.
Where is the evidence of the InsertionVSWR of this ‘superior’ design? It is great to see thinking and experimentation, but a bit of scientific method would make it so much more valuable… if the claims are supported.
]]>A correspondent suggested that with a ferrite core, flux leakage is insignificant. This article calculates the coupled coils scenario.
Above is the ‘schematic’ of the balun. Note the entire path from rig to dipole.
Let’s use the impedance measurement with short circuit termination to find the inductance of the two coupled windings in series opposed.
Above is a plot of the impedance, R+jX. X at 1MHz implies L=8.6µH. Remember that this is the inductance of two series opposed coils, so it includes the effect of mutual inductance.
We can estimate reasonably by calculation that the inductance of one coil L1 @ 1MHz is 114µH.
Measurement of a SC termination gave \(L=(L1-M)+(L2-M)=8.6µH \) and since L1=L2 we can calculate \(M=114e-6-\frac{8.6e-6}{2}=109.7\;µH\) and from that the flux coupling factor \(k=\frac{M}{\sqrt {L1L2}}=\frac{109.7}{114}=0.9623\).
So, k is very high, there is very little flux leakage, but not enough to ignore… it has a huge bearing on the outcome.
]]>Above is the ‘schematic’ of the balun. Note the entire path from rig to dipole.
Above is a plot of VSWR from 1 to 51MHz. It starts off at VSWR=2.8 @ 1MHz, not good, and increases with increasing frequency to VSWR=500 @ 30MHz. (The marker label is misleading, it is a significant software defect, the values are not s11 as stated on the chart but VSWR.)
VSWR @ 10MHz is 96.
You might ask how is this different to the case where the two wires were twisted together and 10 turns wound onto the core. They both seem like coupled inductors… and they are, but there is a significant difference is in the extent of coupling, the extent of flux leakage.
A simple measurement of the input impedance of the balun with a short circuit termination gives us a low frequency inductance of around 8.6µH for 0.6m of two wire transmission line, that is around 14µH/m. That is 25 times the inductance if they were wound as a close spaced pair. The capacitance of the wide space wires is lower than if they were wound as a close spaced pair, so both of these and increases loss drive characteristic impedance Zo up to something of the order of 1400Ω, and velocity factor VF down.
Measurement of the short circuit section shows first resonance (antiresonance actually) at 44MHz which allows calculation of VF as 35%.
The combination of extreme Zo and very low VF causes much greater impedance transformation of a 50Ω load than normally desirable, as can be seen from the VSWR plot above.
Let’s compare that simple model of the balun with a simulation
Above is the measured data presented as a Smith chart. For a low Insertion VSWR balun, we would expect the trace to be entirely very close to the prime centre of the chart. This doesn’t even start off there, and just gets worse with increasing frequency.
Though a very simple model, the series transmission line section of Zo=1400Ω ohms and VF=0.35 captures most of the measured behavior.
A more complete model would indicate higher transmission line loss due to the inclusion of the ferrite based inductance in the transmission line distributed inductance. There is little point in measuring the transmission loss as the balun is impractical due to the extreme Insertion VSWR.
There is a simple explanation for the very poor Insertion VSWR of the N6THN balun, it uses a loaded transmission line section with very high Zo and low VF.
If you want low Insertion VSWR in a Guanella 1:1 balun, ensure that Zo of the transmission line section is close to your load impedance.
]]>In this case, it is described in the referenced video as part of a half wave dipole antenna where you might expect the minimum feed point VSWR to be less than 2.
Apologies for the images, some are taken from the video and they are not good… but bear with me.
Above is the ‘schematic’ of the balun.Note the entire path from rig to dipole.
To the experienced eye, it immediately raises questions.
Above is the implementation.
Cursory analysis suggests this will have very poor Insertion VSWR. When used with a low VSWR(50) load like a half wave dipole, the VSWR looking into the balun will be very poor.
Let’s check it out with the ubiquitous nanoVNA.
Since Insertion VSWR is the initial concern, let’s measure Insertion VSWR from 1 to 51MHz. The original video used a #31 core, I have used a #43 as I have them on hand. Not exactly the same, but the same issue arises either way.
The balun was hooked up with an accurate 50Ω load (two tiny 1% 100Ω SM resistors at the left of the balun), and connected to the nanoVNA with a transformer to allow the balun balanced drive. The nanoVNA with the attached transformer is OSL calibrated at the terminal block on the transformer board, so we can measure the DUT with 50Ω termination.
Above is the test configuration.
Above is a plot of VSWR from 1 to 51MHz. It starts off at VSWR=2.8 @ 1MHz, not good, and increases with increasing frequency to VSWR=500 @ 30MHz. (The marker label is misleading, it is a significant software defect, the values are not s11 as stated on the chart but VSWR.)
Above is the same data presented as a Smith chart. For a low Insertion VSWR balun, we would expect the trace to be entirely very close to the prime centre of the chart. This doesn’t even start off there, and just gets worse with increasing frequency.
Above is a plot of the impedance, R+jX. For a low Insertion VSWR balun, we would expect that R would be very close to 50Ω over the whole range, and X would be very close to 0Ω over the whole range. This plot starts off with R=50Ω, X=55Ω @ 1MHz, and R just increases way off scale.
It is hard to find an adjective to describe how bad the Insertion VSWR is, it is clearly a total failure on that count alone.
Read widely, be critical of what you read on social media. In respect of balun designs, look for relevant measurements, think about them, analyse the offering.
]]>Above is the prototype 2631540002×2 wound with 3.5t of RG316.
Above is the plot of R and X components of Zcm from 1-30MHz. Self resonant frequency SRF is 5.4MHz.
|Zcm| is very high from 2-14.5MHz and high from 1-26MHz, and this should make an effective choke for most reasonable scenarios.
Having measured the SRF, we can calibrate the predictive model.
Above, the calibrated model is quite close in form to the measured, allowing for the rather wide tolerance of ferrite.
A follow up article will report thermal tests on the prototype balun.
]]>
Above is the prototype 2843009902 binocular wound with 3.5t of RG316.
Above is the plot of R and X components of Zcm from 1-30MHz. Self resonant frequency SRF is 8.75MHz.
|Zcm| is very high from 3-22MHz and high from 1.8-30MHz, and this should make an effective choke for most reasonable scenarios.
Having measured the SRF, we can calibrate the predictive model.
Above, the calibrated model is quite close in form to the measured, allowing for the rather wide tolerance of ferrite.
A follow up article will report thermal tests on the prototype balun.
]]>
This article presents the workup of a balun with similar design objectives using a low cost Fair-rite 2843009902 binocular core (BN43-7051).
Above, a pic of the core.
The design is a variation on (Duffy 2007) which used RG174 coax for the choke to give low Insertion VSWR.
For low Insertion VSWR, the choke uses 50Ω coax wound around a pair of ferrite tubes. The coax is a miniature PTFE insulated cable, RG316 with silver plated copper centre conductor (be careful, some RG316 uses silver plated steel and it is unsuitable for HF).
Matched line loss in the 350mm length of coax is 1.2% @ 30MHz, 0.4% @ 3.5MHz, and could be higher or lower with standing waves.
PTFE coax is used for high voltage withstand and tolerance of high operating temperature.
Above, an insulation test of the RG316. It withstood 7kV peak (5kV RMS) from inner to outer, and the jacket also withstood 7kV peak at a knife edge. Voltage breakdown is more likely to occur somewhere else in the balun.
For this design, the cores need to be large enough to accommodate 4 passes of RG-316 coax, but no larger.
Above, the cores will accommodate four round conductors of diameter 2.6mm, so they will comfortable accommodate the four passes of the RG-316 coax (2.45mm each). (For the mathematically minded, the minimum enclosing circle diameter for four equal circles is 1+√2 times the diameter of the smaller circles.)
Al (10kHz) is about 9µH.
The main contribution to loss and heating will be the ferrite core losses, and they are dependent on common mode current.
Above is a first estimate of common mode impedance of 3.5t (4 in one hole, 3 in the other – an approximation) assuming an equivalent shunt capacitance of 2pF. The latter is an experienced guess, and will be adjusted upon measurement of a prototype.
Implementation will be described in a follow up article.
]]>
This article presents the workup of a balun with similar design objectives using a pair of low cost Fair-rite 2631540002 cores (FB-31-5621) which are similar in size to the LF1260 and have higher µi (1500 vs 1000).
Above, a pic of the cores from Amidon’s catalogue.
The design is a variation on (Duffy 2007) which used RG174 coax for the choke to give low Insertion VSWR.
For low Insertion VSWR, the choke uses 50Ω coax wound around a pair of ferrite tubes. The coax is a miniature PTFE insulated cable, RG316 with silver plated copper centre conductor (be careful, some RG316 uses silver plated steel and it is unsuitable for HF).
Matched line loss in the 350mm length of coax is 1.2% @ 30MHz, 0.4% @ 3.5MHz, and could be higher or lower with standing waves.
PTFE coax is used for high voltage withstand and tolerance of high operating temperature.
Above, an insulation test of the RG316. It withstood 7kV peak (5kV RMS) from inner to outer, and the jacket also withstood 7kV peak at a knife edge. Voltage breakdown is more likely to occur somewhere else in the balun.
For this design, the cores need to be large enough to accommodate 4 passes of RG-316 coax, but no larger.
Above, the cores will accommodate four round conductors of diameter 2.6mm, so they will comfortable accommodate the four passes of the RG-316 coax (2.45mm each). (For the mathematically minded, the minimum enclosing circle diameter for four equal circles is 1+√2 times the diameter of the smaller circles.)
The main contribution to loss and heating will be the ferrite core losses, and they are dependent on common mode current.
Above is a first estimate of common mode impedance of 3.5t (4 in one core, 3 in the other) assuming an equivalent shunt capacitance of 2pF. The latter is an experienced guess, and will be adjusted upon measurement of a prototype.
Implementation will be described in a follow up article.
]]>
Some unexpected ‘bumps’ on the measured response of a short SC transmission line section were concerning, there was no apparent explanation.
The bump around 80MHz had no obvious explanation, and appeared to be an artifact of the measurement fixture, or the instrument. The s11 values from 70-150MHz are suspect.
The expected s11 response can be gleaned from a Simsmith simulation.
The line section was then measured using a VNWA3E calibrated with the same fixture.
Above, no sign of the bump, the response is quite as expected.
My baseline config of ttrftech firmware v0.8 and nanoVNA-mod client was calibrated with the same fixture and the same measurement made.
No sign of the bump there either. Of concern though is the low end results, |s11| should not be greater than 0dB.
So, same VNA, same fixture, same DUT and different results for the oneofeleven suite, not just different but an unexpected / unexplained bump.
I did create an issue on github re a bunch of other errors with computed ‘info’ values in the application, no response yet, but it is early days.
Some further testing on this issue is detailed at https://github.com/OneOfEleven/NanoVNA-H/issues/4 .
Update 15/03/2021: two issues listed at https://github.com/OneOfEleven/NanoVNA-H have quietly disappeared and new issues cannot be added, perhaps a sign that the software has gone unsupported.
]]>Self-SWR is commonly known as Insertion VSWR. The article contains several errors in definition of SWR, Return Loss and Insertion Loss… but suffice to say that he uses \(InsertionLossdB=-|s21dB|\).
In the example given above, VSWR=1.2 means ReturnLoss=20.8dB (and |s11|=-20.8dB).
InsertionLoss in this case is due to input MismatchLoss (failure to capture the maximum power available from the source) and Loss (conversion of some energy to heat).
Let’s check the last statement in the quote by calculating the scenario using Calculate Loss from s11 and s21.
InsertionLoss of 0.5dB in the presence of input VSWR=1.2 (|s11|=-20.8dB) has Loss=0.4637dB. We can calculate the power lost in the switch itself
as heat as \(P=50 (1-10^{\frac{-0.4637}{10}})=5.06 \: W\), somewhat less than Gable’s 5.8W. (Since the calculator gives loss as a numeric value of 1.113, a simpler calculation is \(P=50 (1-\frac1{1.113})=5.07 \: W\).)
Note that the inferences of these measurements with a VNA apply to sources that are well represented by a Thevenin equivalent circuit with Zth=50+j0Ω. Most ham transmitters do not comply with that requirement, and the power output under mismatch is best assessed with a directional wattmeter.
It is difficult to understand why QST would publish an article that purports to drill down on these effects when it is not based on sound concepts and theory.