"The Simple Build" 64:1 EFHW antenna transformer (UnUn)

 

 

 

 

By Tim Wallace, VA7PD

Let's Get started!

Whether your building an EFHW antenna for 80 meters or for 20 meters, this transformer will do the job for you very well. Firstly you are going to need all components in the parts list, everything can be found on Amazon.ca and Mouser.ca. Check out the photos in detail to get a better understanding on how to build this cool project.

Step 1: Drill holes for and install the SO-239 or BNC connector, the stainless steel machine screw antenna connector and the ss machine screw ground connector.

Step 2: Take the 6 feet of 18 gauge enameled copper wire, measure 11" from one end and fold it back. Twist this portion of wire together, this will be the primary section of the coil. Once you start winding the core, you will want to leave a couple inches of twisted primary sticking out to connect to the ground terminal and also a couple inches of single wire at the other end of the primary to attach to the SO-239 center. Wind 3 turns primary and 21 turns secondary for a total of 24 turns. Remember, the wire has to go thru the core to count as a turn. The far end of the secondary will be attached to the machine screw antenna connector. Scrape off the enamel at wire ends and remove the red insulators before soldering to ring crimp terminals. I also solder the ring crimp terminal to the screw head for a more positive electrical connection.

Step 3: Install the 3000 volt or higher rating 100pf capacitor between the primary ground and the primary center lead. Don't forget to scrape the insulation off of the wires where you will be soldering to the leads of the capacitor. The reasoning for using this capacitor is to improve SWR performance at frequencies above 20 meters -- so, its your choice whether to install the capacitor or leave it out.

I hot glued the toroid in two places to keep it sturdy as well I hot glued the two silica desiccant pouches to the inside corners. You may also want to drill a couple drain holes in the bottom of the box (SO-239 end).

Here's the math behind the winding ratios: 3 turns primary + 21 turns secondary = 24t total. 24/3 = 8. 8 squared = 64:1

The next project that I will publish will be to make a shorty 80-10 meter end fed half wave antenna using a coil for the 80m portion making the overall length less than 100'. The coil will also use 18ga enameled (magnet wire), and I will be using 14ga insulated speaker wire for the antenna.

I will also be building a 9:1 UnUn and random wire end fed antenna, and a multi-band HF hex beam sometime in the near future, so please stay tuned for future articles in "Project Corner".

73 de Tim

Disclaimer: These are projects that I build for myself and don't mind sharing with like-minded hobbyists. Comments, feedback, etc. please contact me at va7pdz@gmail.com

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