The article Antennas – disturbing the thing being measured – open wire lines #5 demonstrated an inconsistency between the notion of a balun CMRR property and a complete NEC model for predicting common mode current behavior.
In that case, two scenarios were modelled with only a change in the feed line length, yet they showed quite different currents near the same balun.
A common metric bandied around is the Common Mode Rejection Ratio (CMRR) and the definition is a bit rubbery, but it tends to come down to the ratio of the magnitude of common mode current to the magnitude of differential current in a test scenario (usually a lab workbench… with intention that the metric is then applicable more generally). (Anaren 2005) gives a popular explanation.
It is worth noting that the conventional meaning of CMRR in relation to op amps is that it is the ratio of differential gain to common mode gain and large +ve dB values are goodness, and it makes sense. Common use in terms of baluns is the opposite, Anaren gives the expression CMRR=S1c/S1d which will give large -ve dB values as goodness. The balun ‘crowd’s’ use of a -ve rejection ratio seems a bit tautological, as if they haven’t really thought this through, it is a bit like the hammy thing of talking about the attenuation of a length of coax as -xdB.
I don’t think CMRR is a useful property of baluns per se, certainly not as a component of practical antenna systems, so I have written this article to report common mode ratio (CMR) (being the ratio of common mode to differential mode current at the point of interest). CMR is not a property of the balun, it expresses the relationship between the magnitudes of the components of current at a point of interest.
Keeping in mind that the differential current and common mode current distributions are usually both standing waves in the general case (usually with different phase wavelength and therefore relative phase), another dimension of the antenna problem is to look at the current distribution on the feedline of the NEC model scenario used for this series of articles. The model used here is the 20m feed line height and current balun with Zcm=1130+j1657Ω.
Above is a plot of |Ic|, |Id| and CMR in the NEC-4.2 model scenario. Segments are numbered from the lower end to upper end of the 20m long feed line. Continue reading Antennas – disturbing the thing being measured – open wire lines #6
The articles Antennas – disturbing the thing being measured – open wire lines #3 Antennas – disturbing the thing being measured – open wire lines #4 demonstrated an inconsistency between a partial linear circuit model and a complete NEC model for predicting common mode current behaviour.
One of the oft proposed solutions to characterising a balun is to find the Common Mode Rejection Ratio (a term carried over from other applications, eg voltage driven operational amplifiers). (Anaren 2005) explains a method of finding balun CMRR. (Skelton 2010) goes so far as to say
The Common Mode Rejection Ratio (CMRR) of a balun is defined in professional literature as the ratio of wanted to un-wanted transmitted power. As rejection of common-mode transmission is the primary purpose of a balun, it follows that CMRR should be the key figure of merit.
Let us take the model scenario used in Antennas – disturbing the thing being measured – open wire lines #4 and lower the height of the dipole 10 10m, and compare the model ratio of Ic/Id.
Again, we will use Python to do the complex maths for the without and with scenarios at 20m height, and without and with at 10m height. Continue reading Antennas – disturbing the thing being measured – open wire lines #5
The article Antennas – disturbing the thing being measured – open wire lines #2 did a simple analysis of current flows in the model scenario using ideal voltage balun drive with a current balun.
It was mentioned that solution of the lumped values network is only a first approximation and not as good as the NEC solution which properly models the coupled conductors, and their mutual effect on the distribution of currents in the system.
This article reports the NEC current solution decomposed into differential mode and common mode components.
Ideal voltage balun drive
Above is a zoomed in view of the feed point with a balanced pair of voltage sources feeding the line against the ground electrode. Continue reading Antennas – disturbing the thing being measured – open wire lines #4
This article demonstrates the use of a nanoVNA-H to measure the response of a low pass filter designed to pass 7MHz frequencies but attenuate harmonics. The inductors and capacitors make a seven element Chebyshev filter as designed by G3CWI for use in a 50Ω system.
Above, the filter is assembled on a piece of matrix board with two BNC connectors. The inductors are fixed with hot melt adhesive, and the whole thing served over with heatshrink tube. It is not waterproof. Continue reading nanoVNA-H – measure 40m low pass filter for WSPRlite flex
The article Antennas – disturbing the thing being measured – open wire lines #2 did a simple analysis of current flows in the model scenario using ideal voltage balun drive.
That begs the question, what difference would a good current balun make?
We can get a good approximation of what happens by inserting the current balun’s Zcm in series with Z3. Let’s take Zcm to be 1130+j1657Ω (11t on a FT240-43). Continue reading Antennas – disturbing the thing being measured – open wire lines #3
The article Antennas – disturbing the thing being measured – open wire lines illustrated the pitfalls of a simplistic model of an antenna presented on two terminals of an open wire line.
A more complete representation of the antenna can be formed by making three impedance measurements (Schmidt nd). Continue reading Antennas – disturbing the thing being measured – open wire lines #2
A common question in online forums relates to inability to reconcile analyser measurements of an antenna system with the transmitter system antenna facing VSWR meter.
The cause is often that the antenna system was changed significantly to connect the analyser.
Seeing recent discussion by the online experts of how the measure the impedance of an antenna system looking into a so-called balanced feed line gives advice that is likely to cause reconciliation failure.
I will make the point firstly that the line is not intrinsically balanced, it is the way the it is used that may or may not achieve balance of some type. I will refer to that type of line as open wire line.
Let’s explore the subject using some NEC models.
I have constructed an NEC-4.2 model of an approximately half wave dipole at 7MHz, it is 20m above the ground, and fed slightly off centre with open wire line constructed using GW elements. At the bottom, I have connected a 2 segment wire between the feed line ends, and two sources in series. Continue reading Antennas – disturbing the thing being measured – open wire lines
I wanted to modify a soldering iron to insert brass threaded inserts into holes drilled in plastic parts, and for this application looked to eBay for an inexpensive temperature controlled soldering iron that could be adjusted down to around 200°.
Well first check was of its temperature when set to 200°.
Ouch, that is a fail. The Chinese cheats have supplied product that does not comply with its description. Continue reading Soldering iron – temperature control failure
RFPM2 – current probe described a current probe for use with a power meter calibrated in dBm (eg RFPM1 and RFPM2).
Progress has been slow (too many projects?), but the CNC Router project matures and it is time to try making some components of the current probe.
The PCB was designed in Kicad.
Electrically, the current probe is a current transformer with 10t secondary wound onto the toroidal core and terminated on the PCB which as 2x4R7 1W 1020 SM resistors providing the CT burden and a SMA end launch coax connector for a cable to the RFPM2. Continue reading RFPM2 – current probe – #2
An oft cited advantage of the nanoVNA are choices:
- hardware (several clones of the basic thing, the ‘improved’ -H series, the coming -H with bigger screen, the -F with bigger screen… and the future v2);
- firmware (lots and lots of forks, some hardware targeted);
- external clients (PC clients, web interfaces, Python / Octave / Matlab code etc).
There is not necessarily interoperatibilty between all instances of each level of this tree. For example, nanovna-F may not share firmware images with the original nanoVNA and its clones, and vice versa due to a different display protocol.
Some PC clients support features not implemented in all current firmware versions, eg screen capture. Continue reading nanoVNA – a surfit of choices