Building a 3D-Printed EFHW Antenna for POTA and SOTA

Why an EFHW for Portable Ham Radio?

An end-fed half-wave antenna uses a wire radiator that is approximately a half wavelength long on its lowest intended operating band. In this case that lowest target band is 40 meters. End-fed antennas are popular for portable operation because you can feed the antenna at one end instead of having to run a feed line to the center of a dipole. That opens up several practical ways to deploy the wire: up a tree, as a sloper, or supported by a portable mast when the surroundings allow it.

One of the attractions of a 40-meter EFHW is multiband operation. A wire resonant around 40 meters can also develop useful resonances on harmonically related bands. That's why 20, 15, and 10 meters are familiar targets for a 40-meter end-fed half-wave design. But the fact that the harmonics exist does not guarantee a low SWR on every band with every wire, transformer, installation height, or feed-line arrangement. That's something I still need to measure on the finished antenna.

The other essential piece is the impedance transformer. At the end of a resonant half-wave wire, the feed-point impedance is much higher than the roughly 50 ohms expected by most transceivers and coaxial feed systems. A 49:1 transformer reduces that high impedance toward a value a radio can work with. It is a matching network, not a magic device that forces all four bands to resonate. The wire length, its surroundings, and the transformer together determine the actual result.

Portability is why I chose to put these parts in a printed enclosure. The radio, coax, support rope, and wire are already several moving pieces in a portable station. If I can keep the transformer and wire together, that is one less thing to unpack, untangle, or leave behind.

EFHW Antenna Parts List

Here are the parts and download links for building a similar portable EFHW antenna. I reused a 49:1 transformer from another project in the video, so the ferrite toroid link below is a source for a new build, not a confirmation of which core was inside my recycled transformer.

• 3D print files (N9DMT): QRP 40M EFHW Reel Style Housing on Printables. The combined wire winder and transformer enclosure used in this build.
• BNC bulkhead connector: BNC Bulkhead Connector on eBay. Panel-mounted coax connection for the printed housing.
• FT-140-43 ferrite toroid: FT-140-43 Toroid on eBay. A parts-list option for constructing a transformer; the video uses a previously assembled, recycled 49:1 transformer and does not establish its core type or winding details.
• Optional 100 pF, 2 kV capacitor: 2 kV 100 pF Capacitor on eBay. The linked part is rated 2 kV; the capacitor installed in the video was described as 100 pF, 3 kV. They are not the same voltage-rated part.
• 16 AWG speaker wire: Separated into a single conductor for the radiator, as shown in the build.
• Other supplies: Solder, heat-shrink tubing, jumper/lead wire as needed, basic hand tools, and a little adhesive for mechanical support.

The 49:1 matching transformer needs the appropriate winding arrangement; purchasing a ferrite core alone does not make a completed transformer. The video also does not document the final radiator length or demonstrate a measured SWR result on all four bands. Those remain part of the upcoming field test.

Printing the Reel and Test-Fitting the Transformer

The project begins with the spool. Once the printing was finished, I could see whether the internal compartment would actually work with the toroid I wanted to reuse. A drawing or photo of an enclosure doesn't tell you everything about how the wires will bend, where the connector hardware ends up, or how tightly everything will fit once it's assembled. I wanted to make sure the transformer could sit inside the case without forcing the connections against the plastic.

The smaller recycled toroid fit inside the housing with room to route its leads, which was exactly what I hoped for. I compared it with a larger transformer and explained why I favored the smaller one for a backpacking antenna: less size and weight to carry. This was a fit and portability decision, not a direct comparison of efficiency, heating, or power handling between the two cores.

I also started planning the orientation of the transformer before making permanent connections. That matters because the BNC body, its ground tab, the transformer leads, and the antenna wire all share a small amount of interior space. The routing needs to be comfortable enough that the lid can close and the solder joints are not supporting the mechanical load of the deployed wire.

The printed plastic also changes how I approach soldering. A hot iron held against the side of the housing can deform or melt it. During this build I worked carefully around the connector and kept the iron away from the printed walls. It is a small detail, but it becomes important when you're trying to use an enclosure as both a wire organizer and a finished electronics case.

Installing the BNC Connector

I went with BNC instead of an SO-239 or another larger connector because this antenna is intended to travel with QRP equipment. Several portable radios use BNC connections, and I already carry BNC adapters in my field kit. A simple connector choice can mean fewer little adapters to search for at a picnic table or on a trail.

After test-fitting the connector, I tightened it into the printed body using a wrench that barely cleared the case. The tight fit was a good sign for the layout, but it also meant there wasn't much room to work. I then bent the connector's ground lug slightly outward to make access easier for soldering. That gave me a place to bring together the transformer's ground connection without pushing the wire into an awkward corner.

Before soldering everything permanently, I checked where the transformer would sit relative to the BNC center contact. My goal was to keep the wire runs short, avoid unnecessary strain, and leave enough room for the capacitor that I planned to add later. The smaller the enclosure, the more these little routing decisions affect how straightforward the assembly is.

There is also an important difference between mechanical fit and electrical suitability. The fact that a BNC connector fits the printed opening tells me nothing about the antenna's match or power capability. Those questions depend on the transformer, the conductor and connections, the installation, and the tests I still need to run.

Using 16 AWG Speaker Wire for the Radiator

For the antenna wire I used 16-gauge speaker wire with the two conductors separated. It's inexpensive, familiar material, and I've used it for other antennas. There are lighter and more flexible portable antenna wires out there, but this was available on the bench, and it keeps this project focused on reusing materials wherever it makes sense.

I routed the wire into the enclosure so it could attach to the transformer output while still reaching the exterior winder. After getting the position right, I soldered the connection and covered it with heat-shrink tubing. Having a hot-air rework function on my soldering station made shrinking the tubing straightforward without having to put a flame anywhere near the printed housing.

One thing I kept coming back to is strain relief. When an EFHW is deployed, the wire might be stretched between a tree and a support point. I don't want that tension pulling directly on a soldered transformer connection. I looked at whether a small zip tie could fit inside and later added a little adhesive around the antenna wire entry to help hold it in place. Those choices are about protecting the assembled parts during routine handling.

Because the video stops after construction, it does not provide a final measured wire length. For a 40-meter EFHW, builders often start with roughly 65–67 feet, then adjust based on resonant frequency and the actual installed configuration. That's a starting neighborhood from common EFHW practice, not a claim about the exact length in this particular build. You should measure rather than cut to a number and assume the antenna will work tuner-free on every band.

Why Add a 100 pF Capacitor?

After completing the basic transformer wiring, I added an optional 100-picofarad, 3-kilovolt capacitor. Its two leads connect across the 50-ohm side of the matching transformer: between the BNC center contact and its ground/shield connection. I positioned it away from the toroid as much as the enclosure would allow and then improved the center-pin solder joint.

There's a real technical reason for experimenting with this part. The transformer isn't ideal across the entire HF range: leakage inductance, stray capacitance, winding arrangement, and the attached antenna can affect its high-frequency behavior. A small shunt capacitor across the low-impedance side is a recognized method of compensation, with the potential to improve matching on the higher bands, particularly 15 and 10 meters.

But it's important not to turn that into a blanket promise. A 100 pF capacitor does not automatically 'tune the 10-meter band' on every EFHW. Its effect depends on the actual transformer geometry and wire installation; on some windings it may make little difference or even worsen the match. ARRL's EFHW kit documentation discusses adding a 100 pF capacitor across the transformer primary as an optional high-band adjustment. It's a legitimate experiment, not proof of results until measured.

The voltage marking deserves context too. A 3 kV rating tells you about one capacitor specification; it does not establish the power handling of the toroid or the printed enclosure. The high-voltage end of an EFHW deserves care, even when the radio is running modest power, and the assembled antenna should never be handled while transmitting.

When I take this antenna out to test it, the sensible comparison is to measure the SWR response on each intended band with the actual setup and, if I want to evaluate the capacitor specifically, compare readings under the same conditions with and without it. In this video I install it, but I don't present those measurements.

Finishing the Housing and Adding Strain Relief

Once the solder work was complete, I added a small amount of Gorilla Glue near the BNC connector and where the antenna wire enters the housing. The intention wasn't to fill the box with adhesive. I wanted a little mechanical support so ordinary handling and winding the wire wouldn't put all the stress on the electrical joints.

I also made a point of not trying to seal the enclosure completely. This is a backpacking antenna. It will be set up for an operating session, then packed away afterward. It is not intended to remain outdoors in the rain all season, and the printed shell shouldn't be represented as waterproof. Moisture and condensation are reasons to think carefully about sealing, drainage, and storage, but leaving the housing unsealed isn't a guarantee against those problems either.

The lid sits over the electronics compartment and screws into place. At this stage I deliberately didn't permanently glue it shut, because the next step is to test the antenna. If I need to adjust a connection or make a change based on the measurements, I want to be able to open the housing without destroying the print.

With the lid fitted, I wrapped the speaker wire around the outside of the reel. This is the feature that makes the design useful for me. When I get to a park, I can unwind the radiator from the same unit that houses the transformer, rather than digging out a separate winder and an extra box. The reel might weigh a little more than the simplest possible wire-only arrangement, but the convenience is the whole reason I picked it.

What Still Needs Testing Before I Call It a Multiband Antenna?

At the end of the video I have an assembled antenna, not a completed antenna evaluation. My design goal is operation on 40, 20, 15, and 10 meters without a tuner. To establish whether this particular build meets that goal, I still need to deploy it in a realistic configuration and measure it. That distinction matters: a 49:1 transformer and an appropriately long wire are the ingredients for a multiband EFHW, but the finished product needs to prove itself.

The first thing I want to see is the resonant point and SWR on 40 meters. If it's not where I want it, wire length and surroundings will need attention. Then I can inspect the harmonically related bands. A good 40-meter reading alone doesn't tell me that the antenna is ready to operate on 20, 15, and 10 meters, especially at band edges. The results also depend on where the antenna is installed and how the feed line is routed.

I'll want to check for behavior beyond a pretty SWR trace, too. Does the matching unit heat noticeably at operating power? Does changing the coax arrangement alter the response? Does the speaker wire handle repeated winding and unrolling without damage? Is the printed reel easy to use when I'm trying to get on the air quickly? Those are the practical questions that determine whether this becomes something I keep in my POTA bag.

There is also the question of actual contacts. A low SWR is useful because it shows a reasonable match at the measurement point, but it does not measure radiated efficiency, antenna pattern, or how well it will work in a particular location. A portable antenna earns its place by being deployable and usable as a complete system, not just by looking good on a bench.

For now I'm leaving the housing accessible so adjustments are easy. The following field session will determine whether the wire length and transformer produce the tuner-free multiband coverage I'm aiming for.

Is This a Good EFHW Design for POTA, SOTA, and Backpacking?

As a physical package, I like what N9DMT's design accomplishes. A printed reel provides a tidy place for the radiator, and the interior gives the transformer a home without needing a separate enclosure. The BNC connector works with the style of portable radio equipment I carry. And using a toroid from a previous project and wire that was already on the shelf keeps this from becoming an unnecessarily expensive experiment.

There are tradeoffs. A 16 AWG speaker-wire radiator is not the lightest possible choice for a summit hike. An open, non-weatherproof printed case needs sensible handling. The reused transformer's power rating is not established by this build, and the antenna still needs measurement before I can say how well it matches on the target bands. Those limitations don't make the idea bad; they simply define what the video actually demonstrates.

For a POTA outing where I can get reasonably close to the operating spot, fast setup and neat storage are especially attractive. For SOTA and longer backpacking hikes, the total weight of the radiator and accessories may matter more, and I may eventually try a lighter wire. Either way, the integrated reel concept is worth testing because it solves a problem I encounter every time I deploy a portable wire antenna.

If you're building one yourself, use the creator's actual print files, confirm the toroid and connector physically fit, and verify the wiring before attaching a transmitter. Don't treat a shared housing design as proof that a different transformer is rated for the same power or will match identically. After the mechanical assembly, measure and adjust the complete antenna in the configuration you plan to use.

Final Thoughts: Built for the Backpack, Ready for the Field Test

I'm pleased with how the reel-style housing came together. The toroid fits, the BNC connector has a solid mounting point, the radiator winds neatly onto the spool, and the lid can be removed if anything needs adjustment. Best of all, I was able to reuse a transformer and ordinary speaker wire rather than start from scratch with an expensive parts list.

The next step is the important one: take it outside, measure the SWR on each target band, adjust the wire if necessary, and see what happens when the antenna is connected to a radio during a real activation. Until then, this is a completed physical build with a clear goal, not a proven four-band, no-tuner performance claim.

If you've built an EFHW using a printed reel, or you have a must-have 3D print for portable ham radio, let me know in the YouTube comments. I'm interested in projects that make getting on the air easier, not just projects that look good on the workbench.