Balun for Modified A3S

1:1 Balun Made Using Four Mix 52 Cores With 5 Windings of RG-58

Baluns, it turns out, are not all that simple. An important goal of a balun is to prevent common mode current on the feedline coax. Incidentally it is supposed to convert from the balanced antenna to the unbalanced coax. In the words of Bill Leonard, N0CU, “Only recently has the importance of Zcm been understood.” Mr. Leonard presented talks on baluns to the 285Tech Club, Conifer Colorado. His slides are here:

Mr. Leonard stated in the presentation that coax is a better choice for conductor than solid wire. The reason concerns the impedance of the conductor. With coax the impedance stays at 50 ohms from beginning to end. With solid wire the impedance could be something other than 50 ohms and may influence the effectiveness of the balun.

If common mode currents radiate from the feedline coax it will act like a vertical antenna and that radiation will alter the yagi antenna’s pattern. The balanced to unbalanced conversion is apparently very easy to accomplish and almost any style or type of balun can do that. The common mode rejection part is not so easy. After lots of reading on the subject from the ARRL Antenna Book to articles on the web and YouTube, this is what has been decided upon. Five turns of coax thru a stack of 4 cores of mix 52 toroids. Here is how that combination was determined. First the requirements. The balun must function well from 17 meters to 10 meters. “Function well” is defined as having a high common mode rejection ratio and be non-reactive in the frequency range being used. It must have only resistance and no reactance. A typical problem in many baluns is they function well up through 20 meters then fall off above that. Commerical baluns are not a solution. Why not a commercial balun? No manufacturer publishes specifications of either common mode rejection or reactance. One does not know what one is getting from a commercial manufacturer other than glowing product claims. That problem could be easily overcome if there was a way to measure a balun. Measuring HF Balun Performance by Ron Skelton, QEX, Nov-Dec, 2010 explains the issues involved.

Mr. Skelton discusses that a balun is a 3 port device and thus needs a three port VNA to adequately perform measurements. A nanoVNA like at W0QL is only two ports. Some examples of using a two port VNA have been shot down as only testing the differential mode. Switching the leads around and repeating the tests will work but the results must be run through a complicated algebraic process to get an answer. Depending on the measurements taken by others is the obvious next step. ARRL Antenna Book 24th Edition contains graphs and pictures of practical transmitting chokes on page 24.50. (Earlier editions have the same information).

Antenna Book 24th Edition

In the chart above notice that all baluns fall off above about 14 MHz except one. That one is 3 turns on a stack of 7 cores. Not ready to build a stack of 7 cores the search continued. Another deal killer is the lack of mention of reactance. Next a chart was found by G3TXQ, Steve Hunt (SK). His charts include reactance as well as common mode rejection ratio.

G3TXQ (this link works intermittently)

In this chart we are looking for a balun that will work in the 18 to 30 MHz range and have resistance higher than reactance. Two meet that bill. First is 12 turns on one core. Second is a more manageable 5 turns on 4 cores. That is our choice and is the balun pictured at the top this posting. Mr. Hunt explains in his articles why the mix is 52. Mix 31 is for the lowest bands like 80 meters. Mix 43 is ok for the mid bands. Mix 61 is primarily used for the upper bands and vhf. Mix 52 is rather rare but a perfect match for 17 meters through 10 meters. These charts visualize baluns are not broadband as some literature states. They are optimized for certain bands.

The one big problem remaining is how to measure the performance of baluns . Please post a response if you have a solution for measuring balun common mode rejection ratio, Zcm, or any other parameter important to common mode current.

This has been my path of determining how to construction a balun for a modified Cushcraft A3S. Next will be a follow up on how well it works.

Followup 8/19/2020: The balun is installed and the antenna is up in the air. Contacts are being made and the antenna analyzer measurements look good. As for the performance of the balun the pattern of spots of our station is the only indicator available. If the pattern is what would be expected from a 3-element yagi then the balun is doing it’s job of keeping current off the outside of the coax.

The only spots from DX are from Europe. That’s a good indication that the beam is working because it’s pointing toward Europe. Also that’s a good indication the balun is working.

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