For portable radio operation, I like End-Fed Half-Wavelength Antennas (EFHWA, pronounced “EF-WAH”). This type of antenna is similar to the common half-wavelength dipole, but with one significant advantage. A dipole has its feedpoint (where it connects to the radio) in the middle of the antenna, but an EFHWA’s feedpoint is at one end. This makes it very convenient to throw the antenna up in a tree and connect the bottom of it to your radio. Here are instructions for making a multiband end-fed half-wavelength antenna that works on 17, 20, 30, and 40-meter bands.
Materials (pictured) A/B. Solid Core Wire (two colors, 24-gauge recommended) C. 5000-ohm resistor (recommended for testing) D. 2 Binding posts E. 2 BNC connectors F. T94-2 iron powder toroidal core G. 4 jumper wires with alligator clips (you may need more) H. Air variable capacitor, ~10-200pf (e.g. MFJ 282-2005) I. 200 feet of antenna wire. (e.g. from thewireman.com) J. Coaxial cable K. Radio (I use the Yaesu FT817) Tools (not pictured) Soldering iron Solder Helping hands Wire cutter Wire stripper Antenna analyzer (recommended, but not necessary. I use a MiniVNA)
1. Start by wrapping the wire around the toroid. Wrap the first wire around the toroid 27 times (I used a black wire for this). Make sure the first wire is wrapped evenly around the whole toroid. On top of the first wire, wrap a second one around the toroid three times (I used a red wire for this). Also on top of the first wire, wrap a third wire around the toroid four times (I used a black wire for this).
2. Connect a binding post to each end of wire 1 (these are not polarized, so it doesn’t matter which connector is connected to each wire). Connect wires 2 and 3 to the BNC connectors.
3. Solder two short pieces of wire to the terminals of the air variable capacitor.
4. Connect the air variable capacitor to the binding posts. Once again, these are not polarized, so it doesn’t matter which binding post is connected to which terminal.
5. Cut your half-wavelength antenna wire(s). To determine the wire length, use the formula 468/frequency. Choose a frequency that’s in the middle of the band you wish to operate on. For example: The 20-meter band covers 14.0 to 14.35 MHz. 14.2 is a frequency approximately in the middle of the band, which gives a length of 468/14.2 = 33 feet.
6. If you have an antenna analyzer, you can test the antenna at your workbench to check that everything is properly connected before going out into the field. Connect the 5000-ohm resistor across the binding posts. This will simulate the impedance of a vertical antenna. Connect the antenna analyzer to the red wire BNC connector. Adjust the air variable capacitor to get the SWR to be as low as possible at your desired operating frequency. This should be about 1:1 in the middle of the band, and 1.5:1 at the ends of the band. If you’re not able to get this, check your connections.
7. Now you’re ready to go out into the field. Attach one end of your antenna wire to one of the binding posts, either directly or using a jumper wire. Using the technique of your choice, support the other end from a tree or other tall support. Be careful of power lines and other hazards!
8. Next, create the counterpoise. How to create a counterpoise for an EFHW antenna is hotly debated. Different sources tell you to make the counterpoise wire different lengths, some even say that you don’t need a counterpoise. I decided to just use a few jumper wires chained together to create a counterpoise, with a total length of about four feet.
9. Connect the antenna analyzer or radio to the red BNC connector.
10. Tune the antenna by adjusting the air variable capacitor. If you don’t have an analyzer, you should put your radio on the lowest possible power setting and transmit a CW tone while monitoring the SWR meter (following proper procedures for identifying your transmissions and avoiding causing interference to other stations). You want to get the lowest SWR possible on your operating frequency. Make sure not to transmit when you’re touching any part of the antenna or the matching network, or you could get an RF (Radio Frequency) burn.
11. If you couldn’t get the SWR down to 1:1, try connecting the antenna tuner/radio to the other BNC connector and repeat the previous step. Your radio expects a 50-ohm impedance, but the impedance of an end-fed wire is much higher, on the order of thousands of ohms. The exact value can depend on many factors and will change depending on how and where you’ve hung the wire. The toroidal transformer’s job is to reduce the impedance, and the two BNC connectors on it give you some flexibility in finding a good match in the field. The red connector has a turns ratio of 9:1 (27 to 3), which corresponds to an impedance reduction of 81:1. This would be ideal for an antenna with an impedance of ~4000 ohms. The black connector has a turns ratio of 6.75:1 (27 to 4), giving an impedance reduction of 45:1, ideal for an antenna impedance of ~2000 ohms. One of these options should be good enough for most configurations.
12. If you’re still not able to get a good SWR, try adjusting the length of the counterpoise by adding and removing jumper wires, then repeat the previous two steps. When you’ve found a good length, you can replace the jumpers with a piece of antenna wire cut to the same length.
13. When you have your antenna tuned and working, put the tuner in a housing (like a Tupperware container).
Setting up my antenna for 20m the first time took about half an hour, and I was able to get very close to 1:1 SWR using the 9:1 input with a vertical wire supported by a tree. Over the next hour or so, operating from a park bench in Brooklyn, using 5 watts on sideband, I made contact with stations in the US, Europe, South America, the Caribbean, and Hawaii. The furthest station was about 5000 miles away, which means I was getting 1000 miles per watt out of this setup. I’m sure it was a big contest station and not another QRP operator sitting on a park bench with a wire antenna, but it was still fun.