SUPER OCFD (Hybrid) BALUN

N4CY is privately building some of the best baluns on the planet for OCFD antennas.  One of the challenges has been building a Legal Limit (USA) BALUN for running digi-modes on OCFD antennas.

With a maximum output power limit of 750 Watts here in Germany, and having no personal interest in digi-modes, I have not even looked into this.   I'm grateful to Everette for providing this information and happy to share it with the ham radio community.

Mother of all Baluns

Everett Sharp N4CY everettsharp@aol.com 10/17/2021

Recently I purchased an Elecraft KPA1500 linear amplifier, replacing a Elecraft KPA500.

My antenna is an OCF Dipole, fed at 28.9% which had a 4:1 DIY Hybrid Balun. The Balun was a Rothroff design using a FT240-61, the Choke was a FT240-43 wound with 17 turns of RG400 and all enclosed in one box. What I found when operating at 1500 watts, on 80 and 40 meters, is that I was getting SWR creep after operating for 30 seconds, or so and some RFI issues, at 700 Watts and lower there were no issues.

Prior to changing out the balun I took a current measurement with a MFJ-854 current meter on 80 meter at 1000 watts out, the current was 120mA. After installing the new balun I took a current reading again and it was 50mA.

After seeing the problem I did some research to see what balun/choke design I might use to eliminate this problem. One of the major sources of information was Jim Brown’s “A New Choke Cookbook for the 160–10M Bands”, which was useful in helping with the choke design.



4:1 Hybrid BALUN

Finished Balun weighs 3 pounds, 3oz and is housed inside 3” PVC pipe.

I replaced the Hybrid Balun with the newly designed Hybrid Balun as described above. After making the change I made several CQ calls on both 80 and 40 meters FT8 at 1500 watts for around 10 minutes on each band and there was no SWR creep, or an RFI issues. I like it when a plan comes together.