Part of our Building Your First Ham Radio Station guide.
The end-fed half-wave (EFHW) is a great antenna for portable ops or tight yards, no center support needed, just one high point and a run down to the radio. The catch is that the feedpoint impedance at the end of a half-wave wire is huge, typically 2,000-4,500 ohms, nothing like the 50 ohms your radio wants to see. That’s what the 49:1 transformer is for.
What you’ll need
- One FT240-43 ferrite toroid core (two stacked and glued together if you want to run more than 100W or heavy digital duty cycles)
- About 8 ft of 14 AWG enameled (magnet) wire
- One 100pF, 2-3kV NP0/C0G ceramic or doorknob capacitor
- A weatherproof enclosure (a plastic electrical junction box works well)
- An SO-239 or binding post for the coax side, and a bolt/eyehook for the high-impedance wire connection
- A ground lug or binding post for a short counterpoise wire
- Roughly 66 ft of wire for the 40m radiator (see below)
Step 1: Wind the transformer
This is an autotransformer, wound with a 2-turn primary and a 14-turn secondary (a 1:7 turns ratio, which squares to the 49:1 impedance ratio). In practice:
- Cut one length of 14 AWG enameled wire long enough to make all 14 turns around the core with some lead length to spare.
- Wind the wire through the core 14 times, spacing the turns evenly around the toroid. This is your secondary.
- Strip the insulation at the point on the wire that corresponds to 2 turns in from one end, and bring that tap out as your primary connection: this makes it an autotransformer rather than two separate windings.
- The two ends of the secondary become your “cold” (ground/counterpoise) and “hot” (high-impedance, to the long wire) connections. The 2-turn tap plus the cold end is your 50-ohm coax side.
Solder the 100pF capacitor directly across the primary (the coax side). This compensates for leakage inductance and noticeably improves SWR on the higher bands (20m, 15m, 10m) if you’re running a multiband EFHW.
Step 2: Build the enclosure
Mount the SO-239 for the coax connection, a bolt or eyehook for the antenna wire (this is a high-voltage point at power, so keep it insulated from the case), and a binding post or lug for your counterpoise. Seal the box well: moisture inside a ferrite transformer box is a classic cause of mystery SWR problems.
Step 3: Build the 40m radiator
Using the standard half-wave formula (468 ÷ frequency in MHz), a 40m EFHW cut for the middle of the band (7.15 MHz) comes out to about 65.5-66 ft. Cut it a couple feet long, connect it to the high-impedance terminal, and plan to trim from the far end during tuning.
Step 4: Add a counterpoise
Even though the coax shield does some of this job, a short counterpoise wire (roughly 8-17 ft, connected at the transformer’s ground point) makes a real difference in how well the antenna loads and how clean your signal report is. Lay it out away from the main radiator if you can, along the ground, in a tree, wherever it fits.
Step 5: Choke the feedline
Add a common-mode choke (several turns of coax through an FT240-31 or FT240-43 core, or clip-on ferrite beads) right at the transformer, or a few feet down the coax. Without it, the coax shield becomes part of the antenna, which distorts your tuning and can bring RF back into the shack.
Step 6: Deploy and tune
EFHWs work well as a sloper or inverted-L if you don’t have a single tall support for the whole run: the far end doesn’t need to be as high as the feedpoint. Hang it, check SWR with an analyzer, and trim a few inches off the far end at a time if resonance is low. A properly built 49:1 EFHW on 40m usually gives you a bonus: because it’s a half-wave multiple, it’ll often work reasonably well on 20m and 10m too with the help of your radio’s internal tuner.
A note on power and safety
A single FT240-43 core handles about 100W SSB/CW comfortably. If you’re running higher power or heavy-duty-cycle digital modes, stack two cores. The far end of an EFHW can carry surprisingly high RF voltage, so keep it well clear of where people can touch it while transmitting.