This article expands on the detail behind A low Insertion VSWR high Zcm Guanella 1:1 balun for HF with focus on InsertionVSWR.
Insertion VSWR is the VSWR looking into the balun with a matched load (termination) on its output, it is a measure of imperfection of the balun. It ought to be a specification item for low Insertion VSWR baluns, but it rarely given.
What is not mentioned in the above definition is the symmetry or balance of the load.
Above is a Smith chart plot of input Z of the balun with an isolated load of 50+j0Ω. Isolated to mean that there is no direct path from either load terminal to ground, it could be seen as a symmetric load with extremely high common mode impedance. All of the external connections use N type connectors with Zo=50Ω.
Let's remind ourselves of the internal layout.
Note the pigtails at each coax connector, they are a departure from Zo of the coax and connectors. They can be seen as short sections of transmission line with Zo perhaps 200Ω or more. The effect of these is to transform impedance and so cause the input VSWR to depart from ideal.
The departure is seen on the Smith chart above, almost no effect at 1MHz and Insertion VSWR rises to 1.15 at 29.5MHz.
To improve that, one really needs to use coax connectors that terminate the coax shield circumferential and preserve Zo from coax through the connector. That could have been done, but before deciding that it would be better in practice, let's look at the effect of load asymmetry.
Above is a Smith chart plot of input Z of the balun with a load of 50+j0Ω, and a strip of aluminium foil (low inductance) bonding the shield of the input connector to the centre conductor of the output connector. This is one extreme of an asymmetric 50+j0Ω load.
If Zo was preserved through the system, the response would be better, but not perfect because this connection makes the common mode impedance Zcm appear across one side of the load. This is the reason why Insertion VSWR is now worse than unity at the lowest frequencies, and would be even worse below 1MHz where Zcm is even less adequate.
Above is a Smith chart plot of input Z of the balun with a load of 50+j0Ω, and a strip of aluminium foil bonding the shield of the input connector to the shield of the output connector. This is the other extreme of an asymmetric 50+j0Ω load.
As a result of the asymmetric internal implementation, this response is almost identical to the first (the isolated load).
The Insertion VSWR of this balun is quite low, not perfect but sufficiently low for almost any practical antenna system implementation.
It is not instrument quality, for that one might make the balun internally symmetric. Chinese no-name 1-1000MHz Return Loss Bridge shows the internal detail of such a balun which has a second inductor to make the internal structure symmetric.