Users of some ATUs may have noticed particular sensitivity to hands on the capacitor adjustment knobs. It is a common problem with cheap implementations of the T match as the capacitor rotor is usually at high RF voltage and if that shaft is extended to the adjustment knob, under certain circumstances tuning becomes very sensitive to hands on the knobs.
In some of these implementations, if the users hand touches the metal grub screw in the knob, or the metal panel bushing behind the knob they may get a significant RF burn.
Let’s use the MFJ-949E as a discussion example. It is a T match, and the metal capacitor shafts in the knobs and panel bushings carry RF voltages.
I mentioned in my article WIA 4:1 current balun that the use of a single toroidal core in the above graphic compromises the balun. This article presents some simple measurements and analysis that question whether the balun works as so many users think.
The popularity of the balun derives from the work of VK2DQ and is often known as the VK2DQ 4:1 current balun (though probably not his invention).
Analysis at the limits
Often, analysis of a network as frequency approaches zero or infinity can simplify the analysis whilst allowing a reasonable test of the sanity of the design.
Above is a conventional transformer schematic of the WIA 4:1 current balun on a perfectly symmetric (balanced) load. At frequencies where the electrical length of each winding is very short, we can assume negligible phase delay along or between windings, simplifying analysis greatly. Continue reading WIA 4:1 current balun – further measurements
Correspondents have informed me that the balun dealt with in article 4:1 current balun – review and fix and variants are very common. This article gives a checklist of common issues and some basic measurements using an antenna analyser that should reveal some issues without breaking into a sealed assembly.
Baluns are commonly employed to obtain nearly balanced feed line currents (ie negligible common mode current) in two wire lines or negligible common mode current on coaxial feed lines. This article discusses baluns for that application.
A very common 4:1 current balun is Guanella’s 4:1 current balun, but there are others including pretenders.
Three common 4:1 current baluns
Guanella 4:1 current balun
(Guanella 1944) described a 4:1 current balun in his 1944 article, he did not show the winding pairs coupled by a magnetic core as shown above.
Above is Guanella’s circuit, and he does not show coupling between the two winding pairs.
Properly implemented, this device is known to work very well.
Sevick’s single core 4:1 current balun
Let us look at Sevick’s device because it underlies so many failures.
If you look at it very carefully, you will see that the two output wires emerge from opposite sides of the core, the left hand wire exiting under the core was wound from front to back of the core and the right hand wire exiting on top of the core was wound from back to front of the core. Continue reading 4:1 current balun – identifying bad ones
This article reports tests on two 4:1 current balun configurations – a collaboration between Bruce, VK4MQ, and myself.
Purported current balun on a single magnetic core
Above is an attempt at a 4:1 current balun on a single core. Note that this is NOT wired in the insane series opposed connection of the WIA 4:1 current balun. Note also that this is NOT a Guanella 4:1 current balun (see below).
Lets measure the Insertion VSWR by placing a good 200+j0Ω load on the output terminals and measuring input VSWR over the range 1-30MHz. This load is what we will call an Isolated Load meaning it has only two terminals, and the current that flows into one terminal must flow out of the other terminal… in other words, the current MUST be balanced (ie equal magnitude but opposite phase currents in the two terminals)… we will come back to the Isolated Load later.
Above is a diagram of the so-called “cable balun”.
My evaluation essentially showed that it was not effective in an example practical scenario where one might want to use a balun, and that of itself, it was not likely to significantly reduce common mode current in most scenarios.
Radcom Mar 2020 published a letter in “The last word” from the author defending the device citing a NEC model of one scenario, curiously though without explanation, a different topology to the diagram above from the original article. Note also that it is a structure in free space with no discussion of how that is relevant to real world antennas near ground. Continue reading Radcom Feb 2019 “cable balun” – comment on Radcom “The last word” letter