Hybrid-Balun-II Project
A "Hybrid-Balun" (a.k.a. "Combi-Balun") is a dual-balun consisting of a transformer-balun and a choke-balun, connected together as one balun. The transformer can be on top, the choke on the bottom, or visa-versa.
Why: In antennas where impedance transformation is required, but a dual-core 4:1 Guanella balun does not have sufficient Common Mode Impedance to fully block ALL Common Mode Current, the Hybrid-Balun is the best alternative. It assigns each of the two tasks (e.g., transforming the impedance, and impeding Common Mode Current) to two separate baluns. It has twice as much Common Mode Impedance as can be achieved with a dual-core 4:1 Guanella balun.
Where: The Hybrid-Balun is the preferable balun for use in Off-Center-Fed Dipole antennas. Often it is the only type of balun that is capable of removing ALL of the Comment Mode Current from the coaxial feedline.
I developed my first Hybrid-Balun in 2015 during my Battle of Baluns Field Test.
That test led to developing a Hybrid-Balun with a 4:1 Ruthroff (voltage) Balun connected to the OCFD antenna and a 1:1 Guanella (current) Balun between the Ruthroff Balun and the Coax.
Other Hybrid-Baluns (i.e., by Andrew Roos, ZS1AN) place the choke balun on top, connected directly to the antenna with the 4:1 transformer inserted between the choke and the feedline.
Some commercial versions use 3 cores; a dual-core 4:1 Guanella transformer and a 1:1 Guanella choke-balun. The "theory" is, the dual-core Guanella transformer provides additional Common Mode Impedance to assist the 1:1 Guanella balun.
Although many "claim" that a dual-core 4:1 Guanella (current) Balun is sufficient, I proved beyond a shadow of a doubt that there is still residual Common Mode Current remaining on the coax unless additional choking impedance is provided.
In my 2015 Battle-of-Baluns Field Test, I proved that the 1:1 Guanella Balun has sufficient choking impedance to fully block Common Mode Current. There is no need for the transformer to provide additional choking impedance. Thus, a 4:1 Ruthroff transformer is sufficient for the task and requires only one Toroid.
This saves complexity, weight, and cost.
So Why the Need for a Version-II?
While developing my 40m End-Connected Off-Center-Fed Dipole (ECOCFD) in 2023 - similar to the OCFD, but with the 4:1 Ruthroff and 1:1 Guanella baluns separated by about 20%, I was incurring slightly high SWR on 20 and 15m and about 6:1 SWR on 40 and 10m. I suspected it was caused by the reactance of the 4:1 Ruthroff balun.
I designed a new 4:1 Ruthroff balun using two BN43-7051 Binocular Ferrites glued together. After replacing the Toroidal 4:1 Ruthroff with the Binocular 4:1 Ruthroff, the SWR was excellent on all 4 bands of the ECOCFD Antenna.
See: ECOCFD Antenna.
The Binocular version worked significantly better in the ECOCFD antenna than the Toroidal version. In this project we shall find out if it also works better in the OCFD antenna.
In addition, I was also made aware of a different type of Toroid that requires far fewer turns of coax for a choke with the same Common Mode Impedance.
The manufacturer is "VAC - Vacuumschmelz" Advanced Magnetic Solutions. I want to test the Vitroperm Toroid in my Hybrid Balun and compare it to the traditional Amidon/Fair-Rite Toroids.
Here is a chart comparing the Common Mode Impedance (CMI) of chokes wound on #31 ferrite to the Vitraperm. My target is a CMI of 8k Ohms on 80m. I will probably need 10 turns, but I will begin with 11 turns.
Lately (2024), the Internet Antenna Groups are full of comments from people raving about this "Dual-Transmission-Line" 1:1 Choke-Balun. Is it the greatest thing since sliced-bread?
When I simply look at its design, I immediately see a big problem with this device. My hunch proved to be correct.
We will evaluate this choke extensively, in 3 different field tests.
And Finally: THE HYBRID BALUN
Once I have determined the best 4:1 Ruthroff balun and the best 1:1 Guanella choke-balun, I will combine them into a Hybrid-Balun and compare to my original design I developed in 2015.
I originally designed 3 sizes of my Hybrid-Balun, using 3 different sizes of Toroids:
- FT-240-xx (for 1kW SSB/CW)
- FT-140-xx (for 600 W SSB/CW)
- FT-114-xx (for 200 W SSB/CW)
My original Hybrid-Balun shown on the right is the mid-size version.
THE PLAN is to FIELD TEST a Hybrid-Balun with various combinations of 4:1 Ruthroff baluns (on the top) and 1:1 Guanella baluns (on the bottom).
To facilitate easy swapping of either of the two baluns used in the Hybrid-Balun, I have built a balun backplane on a piece of Epoxy-board (FR4). It is large enough to mount any of the multiple baluns I will be testing.
The 4:1 Ruthroff mounts on top, the 1:1 Guanella mounts on the bottom.
The two baluns are joined at the three tiny pins in the center.
The larger holes flanking the 3 pins are screw connections for the wire elements.
The wire legs have a few short layers of heat-shrink tubing on them where they attach to the balun. These are strapped to the backplane with wire-ties, to facilitate strain relief.
NOTE: Throughout this section of my web, whenever I state the value of the Common Mode Impedance of chokes or balun, it is based on the values shown in the color charts by Steve Hunt, G3TXQ (SK), published on his web, here: G3TXQ: RF-Chokes
The next page will introduce ALL of the components to be used in this project.