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End-Fed Half-Wave (EFHW) antenna is a popular choice among amateur radio operators, especially those who enjoy portable operations like QRP, POTA, or SOTA. It’s lightweight, easy to set up, and versatile across multiple bands. But how does it work, and what about the common question: “Do I need a counterpoise?”
How the EFHW Works
An EFHW antenna is essentially a half-wavelength of wire fed from one end, rather than the center. This end-fed design makes it simple to deploy—just throw one end over a tree or other support, and you’re ready to go. Unlike center-fed dipoles that require a feed point in the middle of the wire, the EFHW allows for more flexibility in placement and length.
The key to making an EFHW work is the 49:1 transformer. This transformer matches the high impedance of the antenna (around 2,500 ohms) down to something much more manageable for your radio (typically around 50 ohms). With this impedance match, the EFHW can efficiently radiate your signal.
The Question of the Counterpoise
A counterpoise is essentially a "ground plane" for the antenna, providing a return path for current. It can sometimes improve the performance of an antenna by stabilizing the feed point impedance, especially in end-fed designs. The question is, does an EFHW antenna always need a counterpoise?
In most cases, an EFHW can work effectively without a dedicated counterpoise. The coax feed line often acts as a counterpoise of sorts, especially when you're using shorter lengths of wire. However, some operators find that adding a counterpoise can improve the antenna's performance, particularly when operating on lower bands or in certain environments.
No Connector? No Problem
With my smaller version of the EFHW, there’s no dedicated connector for a counterpoise. But that doesn’t mean you’re out of options. If you find that your setup benefits from a counterpoise, you can simply connect a piece of wire to an alligator clip and attach it to the shield of the BNC connector. This provides a quick and easy way to add a counterpoise when needed without modifying the antenna.
The length of the counterpoise wire depends on the band you’re operating on. For example:
- On 20 meters (HF), a quarter wavelength counterpoise would be about 16.5 feet long.
While these lengths provide a good starting point, you may want to experiment with slightly different lengths depending on your environment and operating conditions.
When to Use a Counterpoise
Here are a few situations where adding a counterpoise might be beneficial:
- Low Bands (80m and 40m): If you're operating on lower frequencies, a counterpoise may help stabilize the feed point impedance and improve performance.
- Poor Ground Conditions: If you're setting up in an area with poor ground conductivity (like rocky or dry soil), a counterpoise might help.
- Interference or SWR Issues: If you're noticing a high SWR or unexpected interference, try adding a counterpoise to see if it stabilizes the signal.
Something else to note, if you use a common-mode choke to protect your equipment, place it as close to your radio as possible. This allows the coax from the choke to the transformer to act as the counterpoise. If your run of coax is too short, you may need to add a wire counterpoise as described above.
