DIY Homebrew Linked End-Fed Half Wave (EFHW) Antenna With 64:1 Impedance Transformer (Unun) for 17M, 15M, 12M, and 10M

I am convinced that one of the major reasons that the higher frequency HF bands are relatively unused is because 17M and 12M bands are not resonant harmonics of the common 20M or 40M half-wavelength antennas that portable radio operators so routinely use in the field. I know some folks will decry “its the [prior] lack of sunspots, darnit!” And, who knows? They may be right. But I long suspect that “openings” in bands correspond heavily with how many people are actually on the air calling cq. And the fact of the matter is most portable operators stick to 20M during the daylight hours.

This past weekend, a bunch of us got together and set up five separate stations for a group Parks-On-The-Air activation. In the days leading up to the event, I knew the competition for bandwidth amongst us would be fierce. I decided to build an antenna that I could use on any ham band between 10 and 17 meters. Ok, truth be told, I tried for 6 meters too, but it did not work.

The design would be a 64:1 unbalanced-to-unbalanced impedance transformer (“unun”) attached to a linked radiator with wire segments corresponding to a halfwave lengths of 6, 10, 12, 15, and 17 meters. As I mentioned, ultimately 6M did not work on this design. The feedline would attach via the unun to the end of the radiator wire, thus making it a linked half wave end-fed.

64:1 UNUN

MATERIALS AND TOOLS

Polypropylene “poly” cutting board [9.24” x 6.87” x 0.3″]

One FT240-43 ferrite cores

Scotch 27 glass cloth electrical tape

14 AWG enameled wire (e.g., Thermalese), approximately 8′ in length

One 100pF 15kVDC ceramic capacitor

SO-239 (UHF female) chassis mount with screws, nuts, lock washers (size M2 worked for me and I did not use lock washers).

Hardware for antenna radiator wire and ground/counterpoise attachments posts, including two M8 bolts, two M8 thumb screws, several flat washers, nuts, and lock washers. Some of how these are arranged are personal preference.

Ring terminals for 14AWG wire, both for UHF chassis mount (i.e., size M2) and and for antenna radiator wire and ground/counterpoise attachment posts (i.e., size M8). Two needed for each size.

Drill with both standard bits for M2 and M8 bolts and step drill bit for SO-239 chassis mount.

Cable ties

Basic wrenches, pliers, screwdrivers

Wire cutters

Electrical crimping tool (I use a Klein Ratcheting Crimper)

Box cutter (for scraping enamel off of wire ends)

Paracord (for simulating and measuring lengths of windings on toroid)

Solder iron

Solder

INSTRUCTIONS

• (1) Wind the toroid. A 64:1 impedance transformer will have 2 primary windings (green) and 16 secondary windings (blue) for an impedance transformation of 50 ohms to 3200 ohms. Follow the diagram below. A couple of hints: 1. winding the toroid first with a string can give an idea of how much wire is required. 2. bending approximately 1 foot of the end of the enameled wire back onto itself and then twisting the bent segments together enough so that it can wrap around the toroid twice is effective. 3. Each time the wire goes through the center of the toroid it counts as a turn.

• (2) Now position and mount everything on to the cutting board.
• (3) Start by securing the SO239 chassis mount onto the board. The board I used i somewhat thick so not as much of the threads stick out as I would have liked.
• (4) Next, secure the M8 bolts. Don’t mind the fact that I accidently made extra holes; these were mistakes!

• (5) Before attaching anything else, you will need to short the twisted ground wires together. Technically, this is where the primary and secondary wires are shorted together. Refer to the drawing above and the picture below. All I did was scraped away enamel from portions of the twisted wire and soldered them together. I also trimmed one of the two wires. Finally, I added a ring connector at the end of the other one so I could connect it to the 8M bolt. It is imperative that you scrape off the enamel with a utility blade prior attaching the ring terminal! I both crimp and solder my ring terminals.

• (6) Now let’s take a closer look at what is happening at the SO239 connector.

• (7) The end of the wire of the primary winding after turn two needs to connect to the center pin of the SO239 chassis mount. Scrape away the enamel of the end of the wire, insert it into the center conductor and solder it together.
• (8) I used a short piece of enameled wire to create a jumper from the chassis bolt of the SO239 connector to the 8M bolt. Simply scrape the enamel off of the ends, crimp and solder on ring terminals, and attach.
• (9) I shortened the leg of the capacitor to the appropriate length, and then soldered and crimped a ring terminal on it. The other leg of the capacitor, I twisted and soldered on to the center conductor wire of the SO239 connector.
• (10) I scraped away the enamel and then crimped and soldered a ring terminal onto the end of the free wire of the secondary winding. This is the one that will go to the antenna radiator wire that we will be making next.
• (11) I secured the transformer to the cutting board by drilling adjacent holes on the inside and outside of the toroid and threading through cable ties.
• (12) Each M8 bolt binding post uses a bunch of different hardware — nuts, washers, lock washers, and wingnuts. It doesn’t really matter how you decide to secure everything together. Use whatever you have or whatever works for you!
• (13) That’s it! Your 64:1 impedance transformer (unun) is finished! Time to build the linked endfed half-wave radiator wire.

LINKED ANTENNA RADIATOR WIRE FOR 6/10/12/15/17 METERS

MATERIALS AND TOOLS

30′ Coleman 12AWG 600V flexible wire (or equivalent)

(#8) Anderson Powerpole connectors (30 amp). Click this link if you want instructions on using these.

Many Large Cable Ties

(#1) Crimp Ring terminal for 12awg wire and size 8 bolt

Heat shrink tubing (assortment)

Wire cutters/strippers

Powerpole crimping tool (I use the Tricrimp tool)

Electrical crimping tool (I use a Klein Ratcheting Crimper)

Antenna analyzer (I use a RigExpert AA-55 Zoom)

>25 feet of RG8X or equivalent/preferred coaxial cable with PL259 ends

INSTRUCTIONS

• (1) Don’t make it the way I did. I cut the wires exactly the correct length, attached powerpoles, tied bowline knots at the ends for strain relief, and ended up with an antenna that was too short electrically. I did compensate somewhat for this, but overall you should start with wire segments that are longer than suggested and then cut them back as you tune it.
• (2) Refer to the following figure for wire segment lengths. I used a dipole calculator at a frequency within the phone portion of each band to determine an estimated length. Remember that it is the entire length of the dipole, not just the length of each individual leg, that constitutes a half wavelength.

• (3) Start with a 12AWG wire more than 18 feet in length. Note it will be cut back to closer to 16’3″ as shown in the above schematic. This will be for the first 10M segment. There is no need to make a 6M segment — it does not work.
• (4) Crimp the ring terminal on to one end of the wire. I like to add heat shrink tubing underneath the plastic sheath of the ring terminal.
• (5) Tie a bowline knot in the antenna wire close to the ring terminal. I also like to use heat shrink tubing in the loop of this (and all the other bowline knots) in order to protect the insulation of the wire. See below for how to make a bowline knot. And the picture below that for how the proximal end (end attached to the unun) looks with strain relief in place. You can see the additional white heat shrink tubing near the yellow plastic of the ring terminal and the bowline knot with clear heat shrink tubing zip-tied to the unun frame.

• (6) Now is the time that if you want to make heat shrink labels on each wire segment, you should do so. Or at least make sure the heat shrink is in place. I used one layer of light colored heat shrink which I wrote on with a sharpie marker and then covered it with a clear tube of heat shrink.

• (7) Tie a bowline knot in the end of the 10M wire opposite the ring terminal. Make sure you have the heat shrink tubing in the loop of the knot, but do not apply heat yet as this may not be the final location of the knot.
• (8) Attach the ring terminal to the output bolt of the unun. Attach the bowline knot at the ring terminal to the unun using a zip tie.
• (9) Attach 25 or more feet of 50-ohm coaxial cable to the unun and hoist both ends around 15 feet or more in the air.
• (10) Test the SWR with an antenna analyzer, preferably one with a graphing function like the RigExpert Zoom models or the NanoVNA. Most of these graphing analyzers will have increasing SWR on the Y-axis and increasing frequency on the X-axis. Both the RigExpert Zoom and Nano-VNA have the capability of showing the boundaries of a given ham band. When you sweep the antenna with the analyzer to check SWR, a line will appear showing the SWR compared to frequency. That curve will “dip” toward a 1:1 SWR (closing in on the X-axis) where the antenna is resonant (a noted simplification for those keeping track). If the SWR dip is at to the left of the ham band (i.e., at a lower frequency) like the yellow curve, then your antenna is too long and will need to be cut back. If the SWR dip is to the right of the ham band (i.e., at a higher frequency) like the pink curve, then the antenna is too short and you will have to add a link (as will be described) to lengthen it. You will have to keep taking it up and down from the trees, cutting back a little at a time, and re-analyzing it to make sure you do not cut the wire too short. It is a tedious process.

• (11) Redo the bowline knot so that it is within a few inches of the end of the 10M wire. Shrink the heat shrink tubing within the bowline loop.
• (12) Add a 30AH Anderson Powerpole connector to the end of the wire. It does not matter which color you pick, but I chose to mate red and black connectors along the length of the wire for consistency.
• (13) It’s time to build the 12M portion of the antenna next. If you refer back to the measurement graphic above, you’ll see that you need to add 2′ 6.1″ to the 10M wire in order to have a wire resonant on 12M. So, start with a wire that is quite a bit longer than this. Add a Powerpole connector on one end and tie a bowline knot with heat shrink tubing in the loop next to it. Go ahead and shrink the tubing.
• (14) Connect the new length of wire onto the 10M portion which should still be attached to the unun, and hoist it into the trees again. Measure the SWR and trim until the resonance is solidly in the 12M band.
• (15) Add a Powerpole connector to the remaining free end of the 12M segment, tie a bowline knot with heat shrink tubing a few inches from the end, and shrink the tubing.
• (16) Connect the two loops from wire segments together with a zip tie. The zip tie loop should be shorter than the total length of the wire connected between the bowline knots. This is so that there will be no strain on the Powerpole connectors when the antenna is actually deployed. See the picture below for how the linkage mechanism should look.

• (17) Repeat steps 13-16 for the 15M band. Refer to the measurement graphic for the target additional wire length needed for resonance on 15M (3′ 2.5″). Start with a wire length significantly longer than this and trim it back as has been described.
• (18) Repeat steps 13-14 for the 17M band. Again, refer to the measurement graphic for the target additional wire length needed for resonance on 17M (3′ 10.1″). Start again with a wire length significantly longer than this and trim it as needed.
• (19) The difference for the 17M segment is that there is no need to attach an additional Powerpole connector at the end. There are many ways to terminate the end of a wire antenna; refer to the graphic below to see the way I do it. I make a loop in the wire at the end, with heat shrink tubing within the loop. I attach a cable tie to hold the loop together (shown in yellow). I then back track the free end of the wire up over the cable tie and secure a second cable tie around the base of the loop and now include the free end of the wire (shown in pink). Finally, I secure one more cable tie at an angle across everything (transparent gray). Add a loop of cable tie to act as an insulator between the wire and the rope, and you are good to go! A completed EFHW for 17/15/12/10!

TESTING THE 64:1 UNUN WITH LINKED EFHW RADIATOR

The antenna was tested at a group POTA out in the Boston Harbor Islands. The unun was less than 15 feet in the air, with the antenna radiator running horizontally into another tree limb. Keep in mind that I did not follow the procedure I described above to build this radiator, but rather cut the wire to the lengths described on the diagram without testing it. I believe that once I added in the bowline knots, I electrically (and physically) shortened my antenna. I compensated by adding in an extra two foot link of wire. I do not recommend that you build your antenna in this manner, rather stick to what I described above. You will see that overall, my antenna is too short and required the use of an antenna tuner to provide a reasonable match.

In general, the unun actually hung better than I expected. I thought the positioning of the SO239 would create a lot of strain on the coax cable hanging from it, but that did not seem to be the case.

Below are the antenna analysis of each of the lengths of wire. As I expected, the results showed that the radiator wire links were electrically short. The results across each band were close to 2:1 SWR, and perfectly usable with a tuner. Had I built the radiator wires to the correct size, it appears that the SWRs would be around 1.5 across each band except 10M. In the pictures below, each blue vertical marking represents an amateur radio band. A pink star marks the band referenced in each analysis. A vertical line marked at the top by a carot is an indicator line and not actually a ham band. The minimum SWR for each graph is noted on the bottom. SWR is on the y-axis and frequency is across the x-axis.

And after all that, did I manage to make any contacts? Given the group setting and the fact that there was an operator working FT8 on 15M, I was never able to get on that band. I did spend a small amount of time on 10M, however did not make any contacts. The 12M and 17M bands however were both successful. I was only able to use 17M intermittently and there was a tremendous amount of RF interference given our operating conditions with five separate HF stations running simultaneously and mostly on frequencies higher than 20M. More testing is certainly warranted, and rebuilding the linked radiator to lengthen it electrically would be useful, however the I consider the antenna a success! The QSO maps for 12M and 17M are below.

Bottom line? This linked endfed halfwave antenna with a 64:1 unun is an interesting antenna build that has the distinct advantage of being multi-banded for the higher HF frequencies, including 17M, 15M, 12M, and 10M. It is relatively short and easy to deploy with only the unun and free end of the radiator needing attachment points. Overall, I think this could be a good field antenna particular with a better cabinet to house the impedance transformer.

Catch you on the airwaves!

KM1NDY