Dan Koellen AI6XG
Trapped 20/30/40 Meter EFHW Antenna
Updated: Aug 2, 2021
An End Fed Half Wave (EFHW) antenna for operation on 20, 30 and 40 meters was built for SOTA operation. By using traps, the antenna works on all three bands with no operator interaction.
After acquiring a MTR3b for SOTA activations I decided I needed a different antenna than the End Fed Random Wire (EFRW) or the speaker cable 20 meter dipole I use with my KX2. I wanted an antenna that would not need an antenna tuner for multi band operation. A trapped EFHW antenna seemed to be a good candidate.
Indeed the speaker cable 20 meter dipole does not need an antenna tuner for 20 meters but I like to use it on 40 and 15 meters where the antenna tuner is required. For an EFRW an antenna tuner is essential. The MTR3b does not have an antenna tuner which restricts the use of the antennas I already have for SOTA activations.
There are several commercially available trapped EFHW antennas available but just a few that are lightweight enough for SOTA activations. There are a few designs for a trapped EFHW for 20, 30 and 40 meters by KT5X and K6ARK that I got ideas from. Basically you build micro-traps that resonant near but not in the segments of interest for 20 and 30 meters. These traps will isolate and connect the three wire segments of the antenna. Operation on 20 meters will use the first segment with the 20 meter trap isolating the rest of the antenna. Operation on 30 meters will use the first wire segment, the 20 meter trap, and the second wire segment with the 30 meter trap isolating the rest of the antenna. Operation on 40 meters will use the first and second wire segments, both traps and a third wire segment tail.
A small transformer to step down the high impedance of the resonant EFHW is used between the rig and the antenna. The transformer is constructed as part of a BNC female connector with a male to male BNC cable connection to the transceiver. A binding post is glued to the BNC connector to accept the antenna wire.
K6ARK, and others, have built the matching transformer directly on a male BNC connector for direct connection to the transceiver. That is a brilliant and very innovative build but I decided to deviate from that design. I prefer to have a short cable between the transceiver and the antenna so I can move my operating position around without changing the antenna and I am not required to hold the transceiver to keep the antenna feed point off the ground. The 6 foot RG316 cable also acts as a counterpoise, conforming very well to the suggested 0.05 wavelength counterpoise length at 40 meters. The binding post lets you attach different half wave antennas or do a quick repair on the antenna wire. I carry a backup 40 meter half wave that I can use with the transformer should the trapped EFHW have an issue.
Building The Transformer
Before winding the transformer, the BNC jack is prepared for mounting of the transformer coil. The BNC jack I use is a panel mount right angle jack (TE Connectivity 1-1337543-0). A standard insulated binding post is cemented to the jack. I recommend using a decent binding post because there will be stress on the binding post when the antenna is set up. The binding post like this one has survived a number of activations and has a nice flat on the threaded area. I haven't found a binding post on Amazon or eBay with a good flat and of known strength. Using super glue, attach the binding post to the side of the BNC jack that is nearest the common (i.e.
ground) connection. The solder post should be pointing in the same direction as the BNC jack solder connections. After the super glue is cured add a layer of five-minute epoxy for added strength, only apply epoxy to the binding post and the side of the BNC jack that the binding post is attached to. Let the epoxy cure overnight.
The transformer itself is wound as an autotransformer on a FT50-43 toroid. The winding starts with 29 turns of #26 enameled copper wire with a tap at 3 turns for the primary. In most cases 27 turns is optimal but I like to start with 29 and remove windings for matching. The tap will be the connection to the BNC center conductor, the antenna is connected to the top of the winding and the start of the winding is the common return for the rig and the antenna.
The leads of the transformer is stripped of insulation so it can be soldered to the BNC jack. To strip the insulation I used a lighter to burn off the enamel followed by a clean up with fine sandpaper. Position the coil on the back of the BNC jack so that the tap lead lines up with the jack's center conductor connection. The bottom of the winding will line up with the jack's common connection and the top end wire should line up with the binding post's solder connection. For now trim the common and tapped leads and solder to the jack's connection posts so that the coil rests on the back of the BNC jack. Leave the top wire untrimmed and unsoldered.
Now tune the transformer by connecting the top wire lead through the hole in the upper part of the binding post. Set up a 30 meter halfwave antenna that is resonant in or near the band and connect the antenna wire to the same hole in the upper part of the binding post. There are now two wires connected to the binding post's screw-down top, no connection to the solder post. Using an antenna analyzer find the resonant point for the antenna and look at the impedance. The impedance should be less than 50 ohms indicating that windings need to be removed to get close to 50 ohms. Don't trim the wire until you are sure of the best match. In most cases 27 turns works out best. Once the number of turns has been optimized, trim the top wire and solder to the binding post's solder post. This is a good time to epoxy the coil to the back panel of the BNC jack using five-minute epoxy; also apply epoxy to the coil to keep the windings in place and provide protection. Don't get epoxy on any of the connections that are soldered. Let the epoxy cure overnight.
Building The Traps
Two micro-traps need to be built, one for 20 meters and one for 30 meters, consisting of a smd capacitor and toroid coil in parallel. Each trap is a similar build but with a different capacitor value and coil winding. For my traps WS0TA boards, available from OSH, were used. The WS0TA board has solder pads for the smd capacitor, coil leads and the antenna wires. They do not have strain relief for the antenna wire though. I have not had an issue with antenna wire connection failure but my future builds will use the K6ARK modified protoboard that provide strain relief. In that video K6ARK covers the building and tuning of the traps as well, it is a good reference for this build.
The traps were built using the recommendation by KT5X for capacitor and inductance values. The 20 meter trap uses a 15 pfd ceramic MLCC NP0 SMD 1206 capacitor and 39 turns of #26 wire on a T50-2 core. The 30 meter trap uses a 33 pfd ceramic MLCC NP0 SMD 1206 capacitor and 37 turns of #26 wire on a T50-2 core. The capacitor was soldered to the two inner pads on the board and the coil soldered to the adjacent pads.
Each trap assembly needs to be tuned by using an antenna analyzer to check for dip in SWR at the desired frequency. A wire through the toroid is attached to the antenna analyzer and the coil turns are bunched or separated until the trap resonates at the desired frequency. You may want to do the tuning in two steps. The first step with the coil soldered to the board but external to the board in case you need to add or remove turns. Then finalize the tuning by inserting the board through the core in its final configuration and re-soldering the leads. The 20 meter trap was made to resonate at 13.9 MHz and the 30 meter trap resonates at 10.0 MHz .
After the trap is tuned, apply five-minute epoxy to the windings and the board; be sure not to get epoxy on the two outside solder pads where you will attach the antenna wires. If you have large enough diameter heat shrink tubing, a short length around the coil will be added protection. Let the epoxy cure overnight.
Antenna Assembly and Tuning
Assembly and tuning of the antenna follows similar steps as the K6ARK video. Use the setup that you will use when operating. I use a carbon fiber mast on SOTA peaks so I used that as well as my trekking pole to hold the antenna during assembly and tuning.
Cut your antenna wire for a 20 meter half wave with some added length. Solder one end to the 20 meter trap and attach the other end to the transformer's binding post. Be sure to have a piece of heat shrink for the connection to the trap on the wire ready for shrinking at the end of tuning. Adjust the length of the antenna wire until it resonates in your desired portion of the 20 meter band. Next add another length of wire to the other side of the 20 meter trap and then adjust the length for 30 meter band. Slip on two additional pieces of heat shrink and solder on the 30 meter trap. Finally add the final length of antenna wire to the other side of the 30 meter trap and tune to the desired portion of the 40 meter band. Add another piece of heat shrink tubing to cover the end of the 30 meter trap. The traps will have some affect on the antenna tuning so you may have to do a bit of fine tuning to get coverage. I was able to get under 1.6:1 SWR on the 30 meter band and about 1.2:1 SWR on the 20 and 40 meter bands. Once you are satisfied with the tuning, shrink the pieces of heat shrink over the antenna wire to trap connections.
For my build I ended up with the following antenna wire lengths, your lengths will vary depending on trap frequency and other setup parameters. transformer to 20 meter trap: 30 feet 10 inches (9.398 meters) 20 meter trap to 30 meter trap: 3 feet 10.5 inches (1.181 meters)
30 meter trap to end: 6 feet 9 inches (2.057 meters)
For the antenna wire I use BNTECHGO 26 Gauge Silicone Red Flexible 26 AWG Stranded Tinned Copper Wire
The antenna has been used for a summer of SOTA activations with good results and reliability. My typical setup is with the far end of the antenna suspended from a carbon fiber mast, the end of the antenna wire is about 12" from the mast. The transformer end is fed to the rig with 6' of RG316 cable; the transformer is suspended from my trekking pole or from a line hung from a tree. The antenna wire is suspended from the binding post. The binding post has proven to be reliable connection even in high winds.
Alternative Transformer Support
As originally built the binding post is anchoring the antenna wire. This has proven to be reliable through over a dozen activations, some in high wind.
As an alternative, I added support for the transformer and coax using an insulator. The insulator is also the strain relief for the antenna wire and the line support. Zip ties and a plastic S carabiner holds the coax, the antenna wire is routed through a few holes in the insulator and held in place with a zip tie. There was no change in SWR with this configuration.