This is a follow up article to G5RV baluns – a comparison of two options – part 1 and will make more sense if that article is read first.
Louis Varney, G5RV, described his multiband HF antenna in two forms.
The configuration pictured above using a hybrid of open wire matching section and low impedance coax to the transmitter is very popular.
G5RV baluns – a comparison of two options – part 1 asked the question of whether one of the following two balun options was superior.
Two options from hfkits.com.
Option 1 choke is quite traditional. The two wires form a transmission line wound 21 turns around a ferrite core. If a pair of PTFE insulated 1.0mm silver plated copper conductors close spaced are wound, differential characteristic impedance would be about 100Ω.
Option 2 choke is effectively two such transmission lines wound 12t in parallel (see (How) does this balun work). The differential characteristic impedance of the combination is around 50Ω. The intention of the more complicated implementation is that InsertionVSWR50 will be lower that the simpler choke.
The design was possibly originated by DG0SA?
NEC-4.2 model
An NEC-4.2 model was constructed of a basic G5RV at 10m above ‘average ground’ (σ=0.005, εr=13) to explore the VSWR response looking into the two wire line section. The model assumes a totally effective common mode choke, ie zero common mode current.
Above is the modelled VSWR response for Option 1 choke.
Above is the modelled VSWR response for Option 2 choke.
Can you tell the difference?
Above is one graphic overlaid on the other for comparison. The feint curve is Option 2. There is not much difference, and neither is clearly better than the other.
Given the variations likely to be encountered in your own application given soil type, nearby buildings and vegetation, variations in conductor lengths and choke implementation, I would suggest there is no significant difference… except that the choking impedance is likely to be significantly different (the number of turns is different, and the geometry is different).
Choking impedance
The optimum number of turns for a core type and application is not simply the most that will fit on the core, but the Option 2 winding method can fit fewer turns of given wire on the core that Option 1. This is even more relevant if lower permeability cores are chosen.
Let’s look at an example posted recently online.
Above my model of an example in Option 2 style described online recently with 6t a side on what appears to be a FT240-52 core.
Above my model of the same core in Option 1 style with 11t on a FT240-52 core.
The Option 1 style model has significantly higher choking impedance.
Summary
So, the Option 2 choke has:
- more complicated to build;
- possibly higher cost of materials;
- likely poorer choking impedance;
- has greater risk of incorrect wiring.
So you might ask why is it so popular? Perhaps it is the appeal of the TRX Lab video propelled though social media?