Louis Varney, G5RV, described his multiband HF antenna in two forms.
The configuration pictured above using a hybrid of two wire line matching section and low impedance coax to the transmitter is very popular.
Whilst Varney did not recommend a balun, much has been learned about common mode currents in the intervening 70 years, and the use of an effective common mode choke is advisable. A common practice is to locate such a choke at the transition from the two wire line to the coax.
Common mode chokes for discussion
Let’s look at two different forms of common mode choke wound with two insulated wires.
Let’s look at 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, 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.
Application to a G5RV antenna system with hybrid feed
Which is more suited to the G5RV hybrid feed arrangement pictured earlier?
Some online experts are adamant that the right hand configuration is naturally more suitable since it is connected at a “50Ω point” (notwithstanding that Varney proposed 75Ω coax… hey, but we will use 50Ω coax so of course it is a 50Ω point.)
Of course their very strong position might hint less than sound thinking behind it.
Think about it?
For discussion in a day or three, have a think about it:
- does one appear better than the other for the G5RV application;
- are there disadvantages to one or other; and
- for what reason(s).