New matching transformers

The matching transformer is key to the performance of the Dual Beam Pro and I-Pro Home antennas. It provides a balanced to unbalanced connection between the antenna and the feeder and also transforms the native impedance to provide a better match to the 50 Ohm coax cable.

In 2023, the supplier of the ferrite cores raised the minimum order quantity making them ridiculously expensive so I worked with Andrew of G-Whip to come up with an alternative design. We eventually settled on a direct replacement 1000W PEP version and a simpler 400W version.

Sadly, Andrew suddenly succumbed to a long standing illness before he could make the first batch. Andrew and I had worked well together and he had become a good friend so this was a big personal and professional loss.

Going forward, I decided to take on the production task myself and this project includes the design, testing and production of the new matching transformers.

Design

The original matching transformers used a very compact ferrite core that was 52mm diameter and 15mm high. I wanted to use the same enclosures for compatibility but it was very difficult to find an equivalent core that would handle 1000W and fit in the box. The 400W version was relatively easy: I used a small powdered iron T200-2 toroid that fitted neatly in the box and began production. The 1000W version took longer and I eventually settled on 2 ferrite FT240-31 cores stacked together. Due to the larger size (60mm diameter) I had to change the orientation of the box and mount the connector on the shorter side so that the core would fit. This was an acceptable compromise as the same mounting and connection could still be used and there was an added benefit of making it easy to distinguish between the 2 versions.

The Dual Beam Pro and I-Pro Home use a 4:1 voltage transformer and this is based on the Ruthroff design.The number of turns was determined by testing with a Rig Expert analyser and a 200 Ohm load across the antenna terminals to give the desired 50 Ohm impedance on the transmission side of the transformer. The target was to find the lowest SWR across the HF band for each of the core types.

The 1000W version was optimised with 6 turns and the 400W version with 18 turns.

Winding connection diagram and schematic (number of turns is indicative only)

The previous matching transformer design used a potting compound to hold everything in place inside the box. This was very effective but the compound was equivalent to adding a small capacitance across the connector which I wanted to avoid. The potting compound also made it impossible to fault find, repair or recover components in the event of an issue with a matching transformer.

I decided to use custom formers to hold the toroid in place and designed and 3D printed the parts for each type of matching transformer. These have the added advantage of holding the windings in place during construction and keeping the spacing even between the turns.

All the 3D printed components were created in OnShape and are available in the public folder. If you want to make your own matching transformer you are welcome to view the files in OnShape.

Search for MT1000 or MT400

You can also download a more detailed construction guide or contact me using the contact page for assistance.

Assembly and Testing

Once assembled each matching transformer is tested and allocated a serial number for reference back to the test results if necessary.

400W

1000W

400W

1000W

Typical SWR test sweep with 200 Ohm resistive load across the antenna terminals