Part of our Building Your First Ham Radio Station guide.
The end-fed half-wave, or EFHW, is one of the most popular portable antennas in ham radio, and for good reason. It is a single wire, half a wavelength long on your lowest band, fed at one end through a matching transformer. You can throw the far end up into a tree and be on the air in minutes. But you will often see a short wire hanging off the feedpoint end called a counterpoise, and a lot of people are not sure why it is there or whether they really need it. Here is what it does.
Why an EFHW seems like it should not need one
A half-wave antenna has high voltage and low current at its ends, and low voltage with high current in the middle. Because you are feeding it right at that high-voltage, low-current end, the antenna presents a very high impedance, often in the thousands of ohms. That is why an EFHW uses a large step-up transformer (commonly around 49 to 1) to match that high impedance down to the 50 ohms your radio wants.
Since the current at the feed point is low, the antenna does not demand a big return path the way a quarter-wave vertical does. This is exactly why people say an EFHW does not need radials or a ground. And strictly speaking, that is mostly true. The antenna will work without a counterpoise. But work and work well are two different things.
The real problem: current on the coax shield
Even though the current at the feed point is low, it is not zero. That small amount of current still needs somewhere to go. If you do not give it a deliberate path, it will find one anyway, and the most convenient path is the outside of your coax shield.
When current flows on the outside of the coax, a few unpleasant things happen:
- Your feedline becomes part of the antenna and radiates, which distorts your radiation pattern in ways you cannot predict.
- That stray RF can come back into the shack and cause common-mode problems: RF in the shack, a hot microphone or chassis, computer and USB interference, and erratic behavior from your radio or accessories.
- The match can become touchy and change depending on how your coax is routed, which is a maddening symptom to chase.
What the counterpoise actually does
The counterpoise gives that small return current a defined place to flow instead of leaving it to wander onto your coax. A short wire, often somewhere around 0.05 wavelength on the lowest band (roughly a tenth of the antenna length is a common starting point), gives the transformer a bit of counterpoise to work against. This does two useful things.
First, it stabilizes the match. With a proper counterpoise, the transformer sees a more consistent load, and your SWR becomes less sensitive to how the coax is dressed. Second, it keeps RF off the feedline, which cuts down on the common-mode problems above and cleans up your pattern.
Many operators use both a short counterpoise wire and a common-mode choke a little way down the coax. The counterpoise gives the return current a home, and the choke blocks whatever is left from riding back down the shield. Together they make an EFHW behave much more predictably.
So do you have to use one?
You can get an EFHW on the air without a counterpoise, and plenty of people do. But if you are fighting an unstable SWR, getting RF bites in the shack, or seeing interference you cannot explain, adding a counterpoise wire is one of the first and cheapest things to try. It is a scrap of wire that turns a temperamental antenna into a reliable one.
Think of it this way. The counterpoise is not there to make the antenna radiate, it is there to keep everything else in your station from radiating when it should not. That is a small piece of wire doing a big job.