DIY 49:1 Unun Impedance Transformer For End-Fed Half Wave (EFWH) Antenna (Step-by-Step Instructions)

February 14, 2022 by KM1NDY

The following is a list of materials required to make a 49:1 unun impedance transformer for an end-fed half wave antenna. You will also need a half-wavelength length of wire (14 or 12 gauge typically) in order to construct an antenna radiator for whichever ham band you choose. Please note that I will not be describing the construction of the antenna wire radiator here. I included the tools I used to analyze the transformer as well; this is only necessary if you are interested in analyzing your project.

MATERIALS NEEDED FOR TRANSFORMER CORE:
Two FT240-43 ferrite cores
Superglue
Scotch 27 glass cloth electrical tape
Two pieces of 14 gauge enameled wire (e.g., Thermalese), one short piece (about 1 foot in length) and one long piece (about 7 feet in length). The short piece is for the “primary winding” of the transformer, and the long piece is for the “secondary winding” of the transformer.
One 100pF 15kVDC ceramic capacitor

MATERIALS NEEDED FOR HOUSING:
4 x 4 x 2 electrical junction box (available at big box hardware stores)
One PVC DWV Test Cap (available at big box hardware stores in plumbing)
2 eyehooks with washers and nuts (I used #8 diameter, but size is up to you.) One is for the top of the box so that it can be hung up. The other is to provide strain relief for the antenna wire).
1 bolt for antenna hook up (with nuts, washers, lock washers, and possibly thumb screw; I used #8 diameter, about an inch or so in length.)
SO-239 (UHF female) chassis mount with screws, nuts, lock washers. A small ring terminal for 14ga wire that fits these screws would be helpful too.
Cable ties
A few ring terminals for 14ga wire.

TOOLS FOR CONSTRUCTION:
Basic wrenches, pliers, screwdrivers
Wire cutters
Box cutter (for scraping enamel off of wire ends)
Drill with appropriate bits
(Helpful) Step drill bit for SO-239 connector
Paracord (for simulating and measuring lengths of windings on toroid)
Solder iron
Solder

TOOLS FOR ANALYSIS
Antenna analyzer for HF frequencies (I use the RigExpert Zoom AA-55)
Resistor(s) totaling about 3000 ohms (I used 1K and 2K ohm resistors in series)
A way of connecting the analyzer to the transformer. I used a homemade jig made of an SO239 chassis mount to bare wires, alligator clips, and a coax-jumper with PL259 (UHF male) ends; see picture below. I did not want to put the transformer in the housing until I tested it.

BUILD INSTRUCTIONS FOR 49:1 UNUN IMPEDEDENCE TRANSFORMER:

1. PREPARING THE TOROIDS AND WIRE
- Superglue 2 toroids together. Wait a few minutes for glue to set.
- Wrap toroids with glass electrical tape.
- Twist the short and long wires together starting at an end of each wire. Continue twisting the entire length of the short wire.

Two FT240-43 toroids superglued together

2. WINDING THE TOROID
- Turns 1 & 2 are primary windings (the short wire). Turns 1 through 14 are secondary windings (the long wire). Each winding is counted when the wire enters through the center hole of the toroid.
- Start winding the toroid with the twisted wires at loop #1 (see schematic below). Make sure the beginning few inches of twisted wire is not involved in the winding (i.e., just hanging down from the toroid).
- After winding #2 is made and extends down the backside of the toroid, untwist the wires. Let the now free end of the short wire hang down from the toroid.
- Continue on winding the long wire through the toroid to complete loops 3 through 14 as shown in the schematic below.

3. PROCEDURE FOR ANALYZING THE 49:1 UNUN
(Please note, I am not an electrical engineer, and I may be explaining this incorrectly here. But I believe the overall concept is correct. I am happy to read your comments on this.)
- The goal of the impedance transformer is match the 50 ohm impedance of the feedline (coaxial cable) with the potential 3000 to 4000 ohm impedance expected from an end-fed half wave antenna wire radiator. This is to avoid the antenna radiator from acting like a resistor, and thus instead of radiating radiofrequency energy produced by the transmitter, sending it back down the coax toward the transmitter. This undesired reflection of RF power back toward the transceiver is identified as a high voltage standing wave ratio (VSWR or SWR).
- Placing 3000 to 4000 ohms of resistance between the free end of the twisted portion of wires (the gound) and the free end of the long wire (the wire that connects with the antenna radiator after the 14th winding), can simulate adding an antenna radiator wire to the unun impedance transformer. See the picture below. I used 2000 and 1000 ohm resistors in series connected to the proper unun wires with alligator clips for a total of 3000 ohms.
- I connected the free untwisted end of the short wire to the center conductor of my homemade testing jig (see tools list) with an alligator clip.
- I connected the free twisted end of the short and long wires (the ground) the chassis wire (ground) of my homemade testing jig with alligator clip.
- I swept the VSWR of the unun and resistors as described from <1 to 55MHz.

4. RESULTS OF 49:1 UNUN ANALYSIS
- I used the RigExpert Zoom AA-55 to test VSWR of the 49:1 unun impedance transformer. The vertical (y-) axis shows the VSWR and the horizontal (x-) axis shows the frequencies. Each vertical blue line represents an amateur radio band. Beginning from the 3rd band from the left, the vertical blue lines are: 160M, 80M, 60M, 40M, 30M, 20M, 17M, 15M, 12M, (11M), and 10M bands.
- The 49:1 unun with 3000 ohms of resistance shows a VSWR under 10:1 from 160M to 15M, and less than 3:1 from 80M to 20M.

SWR sweep of 49:1 unun (without capacitor).

5. ADDING A CAPACITOR TO EXPAND ANTENNA BANDWIDTH
- For reasons that I do not understand, adding a capacitor between the coax conductor wire (untwisted free end of short wire) and the ground (twisted free ends of long and short wire) can increase the functional bandwidth of the antenna on the higher HF frequencies.
- A VSWR sweep of the unun after adding a 100pF capacitor shows an SWR <5:1 from 160M to 10M.
- In theory, my LDG Z100 Plus autotuner should be capable of matching the impedance of this 49:1 transformer with a end-fed half wave length antenna radiator across all amateur HF bands.

100pF 15kVDC capacitor between center conductor and ground.

6. CONSTRUCTING HOUSING FOR TRANSFORMER
(Note: I am not satisfied with this part of the project, so I will describe it here briefly. Ultimately, this housing works fine, but it is heavy and clunky.)
- Drill out holes in the designated bottom of the electrical junction box for the SO239 chassis mount. I only used two screws instead of four.
- Drill a hole in the top for an eyehook (to be used to secure the unun to whatever it is you want to hang it from.)
- Drill a hole in one side for the antenna radiator wire mount (for a #8 bolt in my build). The free end of the untwisted portion of the long wire (past the 14th winding) will be secured to this bolt as well on the inside of the box, while the radiator will be secured to it on the outside.
- Drill another hole in the same side as the antenna radiator wire mount hole. This will be for an eyehook in order to provide strain relief for the radiator wire.
- There probably should be some holes for ventilation. I have not done this yet.

7. SECURING THE TRANSFORMER INTO THE HOUSING
- I attached the toroids to a PVC DWV test cap with 3 cable ties. While I have not done so yet, I plan on gluing the test cap to the back of the junction box.
- The following wire connections do not need to be made in the order I described. Do what works best!
- Cut the antenna radiator wire (free untwisted end of long wire) of the unun to the correct size, scrape the enamel insulation off of the free end with a box cutter, add a ring terminal, and secure to antenna radiator bolt with washers and lock washers.
- Cut the center conductor wire (free untwisted end of short wire) of the unun to the correct size, scrape the enamel insulation off of the free end with a box cutter, and solder it to the SO239 connector. I actually did this first and before the connector was bolted to the box.
- Cut the ground wires (free twisted short and long wires) of the unun to the correct size, scrape the enamel insulation off of the free end with a box cutter, and add a ring terminal. Connect this ring terminal to one of the SO239 chassis bolts.
- Solder a the 100pF capacitor between the center conductor wire and ground wires.
- I will also find some sort of heat resistant packing material to help hold the transformer in place.

The 49:1 Unun Impedance Transformer secured in electrical junction box with all proper connections assembled.

100pF 15kVDC capacitor soldered at center conductor of SO239 connector and twisted ground wires.

Antenna radiator wire from Unun secured to bolt on inside of junction box (right side of picture) with antenna radiator secured to the outside of the junction box (left side of picture).

Completed 49:1 unun impedance transformer secured to a fence with antenna radiator and coaxial cable attached.

REAL WORLD DIY ANTENNA TEST

WX1MAR and I brought the DIY 49:1 unun with a borrowed antenna radiator wire cut for 20M half-wavelength to Ashland State Park K-2417 in Massachusetts for a Parks-on-the-Air (POTA) activation. We hoisted the free end of the antenna wire approximately 30 feet into a tree. The unun was floating at about 15 feet above ground. A 50 foot run of RG-8X coax connected the antenna into the LDG Z100-plus tuner and then a Yaesu FT-891A transceiver.

VSWR analysis of the antenna in the field showed that VSWRs between 3-5.5 from 40M to 10M. In total 29 contacts were made from British Columbia to Texas to Florida in 40 minutes on 20M. A success!

Each vertical blue line represents a ham band.

Some important and helpful references I used for this work:
https://elginradio.wordpress.com/2017/10/17/end-fed-half-wave-antennas/

Steve Ellington’s YouTube channel

(Originally published February 22, 2021).