G3LNP described a 4:1 balun for HF antennas in Radcom Nov 2017.
Above is the schematic supplied by G3LNP. He describes the dashed link at the bottom as optional, but uses it in his prototype so this analysis is with that link installed. The prototype used equal lengths of coax (1m PF100, an RG-6 like coax), and the toroidal choke appears to be 8t on a T130-2 powdered iron core.
Exploration of behaviour of baluns on extreme asymmetric load often reveals whether they work properly for asymmetric loads.
Lets use a load that is a series connection of 200Ω and 0Ω resistors to the top and bottom output terminals respectively and junction of the resistors grounded Such a load is very easy to analyse, there are three parallel paths from the input connector:
- the top 1m PF100 transmission line has a load of 200Ω;
- the bottom 1m PF100 transmission line has a load of 0Ω; and
- a toroidal inductor formed by the outer surface of the coax and core.
Lets solve the toroidal inductor impedance and admittance.
I will use RG-6U characteristics, it is very similar to PF100.
Lets calculate the three parallel branches appearing at the input connector, we will use admittance so that we can simply sum all three branches:
- Input admittance of 1m RG-6U transmission line has a load of 200Ω is 0.005030+j0.001026S;
- Input admittance of 1m RG-6U transmission line has a load of 200Ω is 0.004405-j0.148520S;
- Admittance of the toroidal induct or 0.00-j0.0646S.
Total admittance is 0.009435-j0.2121S.
Lets calculate the input VSWR given that total input admittance.
Above, input VSWR is extreme at 241, the device is not suited to asymmetric loads.
Though not declared as such, the prototype G3LNP balun is a voltage balun. The circuit is a small extension to Ruthroff’s balun in the addition of the transmission line delay element in the non-inverting output path.
By way of contrast, a good current balun would deliver an input VSWR close to 1.0 for such a load.