End fed antennas are especially popular among ham radio operators—and for good reason. They work on multiple bands, are affordable, easy to build yourself, and quite forgiving when not perfectly installed. However, many amateurs fail to get the most out of their EndFed. The reason? It all comes down to the counterpoise.
What Is an EndFed Antenna and Why Does It Work on Multiple Bands?
In this article, we’re specifically talking about half wave end fed antennas (EFHW). In essence, an EFHW is a dipole antenna that’s fed at the end instead of the center. One of its most attractive features is multiband operation. For example, a half wave on 40 meters is also two half waves on 20 meters, three on 15 meters, and four on 10 meters.
49:1 Transformer for Extremely High Impedance
An EFHW antenna presents a very high impedance, typically around 2500 Ohms—far beyond the 50 Ohms your transceiver expects. To bridge this gap, a 49:1 impedance transformer is used. This is often—though incorrectly—referred to as a ’49:1 balun or unun.’ These transformers come in various forms, which we won’t discuss in detail here.
Every Antenna—Including an EndFed—Needs a Counterpoise
To radiate effectively, any antenna needs HF current to flow out and return. The returning current is known as the return current. In a dipole, it flows back through the leg connected to the coaxial shield. In a ground plane antenna, it flows through the radials and possibly a ground rod. Similarly, a half wave EndFed also needs a counterpoise. If you don’t deliberately add one, things can go wrong.
Debunking Myths About EndFed Antennas
“You don’t need a counterpoise with a half wave EndFed and a 49:1 transformer‘ someone confidently claimed in a Facebook group. That statement is false. As we’ve explained, every antenna requires a counterpoise. If you don’t provide one, the antenna will eventually take one on its own. In the case of an EFHW, it typically uses the outer shield of the coax cable. That may seem convenient, but without precautions, it can lead to serious problems:
- Interference on your equipment while transmitting
- Increased noise floor or reception issues
- Poor and inefficient radiation
More Than Just the Coax Shield
Let’s examine what happens when the coax shield becomes your only counterpoise. The coaxial shield is electrically connected to the metal chassis of your transceiver. Through the power supply’s negative lead, that chassis connects to the power supply’s metal casing, which—like most—is grounded to your home’s safety earth.
The Number One Cause of RFI with EndFed Antennas
Many appliances in your home are also connected to that same ground: your washing machine, refrigerator, dishwasher, computer, boiler, central heating system, and so on. All these metal housings can unintentionally become part of your antenna system, acting as counterpoise elements.
All HF return currents then flow through every component electrically bonded to your coax shield. That means your entire home may start radiating RF energy. This is the number one cause of RFI (radio frequency interference) when using EndFed antennas.
Even worse, the coax shield—and everything connected to it—can act like an antenna for unwanted signals inside your home. This can lead to all kinds of reception problems, from a high noise floor to severe crackling and S9++ interference.
The Solution: Stop Unwanted HF Return Currents with a Common Mode Choke
The fix? You need to stop those HF return currents where you don’t want them. That’s exactly what a common mode choke is for.
Isolating with a Common Mode Choke
A common mode choke suppresses HF currents that travel on the outside of the coaxial shield. By installing one, you electrically isolate all components that shouldn’t act as counterpoise from your antenna system at HF.
This type of choke is also known as a line isolator. In practice, a 1:1 balun or 1:1 unun serves the same purpose as a common mode choke.
There are two main ways to implement a choke in your antenna setup. But first, have a look at the ideal length of a counterpoise.
The Ideal Length for an EFHW Counterpoise
Let’s begin by looking at the ideal length for a counterpoise in an EFHW setup. This should be a non-resonant length—long enough to carry HF return currents, but short enough not to radiate.
The rule of thumb is:
0.05 × the longest wavelength you plan to transmit on.
For example, if your EFHW covers 10 to 40 meters, the longest wavelength is 40 meters.
So: 0.05 × 40 m = 2 meters — your ideal counterpoise length. For 80 meters, that would be 0.05 × 80 m = 4 meters
Using a Section of Coax as a Counterpoise
One option is to use a section of the coaxial shield as a counterpoise. In this case, insert a length of coax between the 49:1 transformer and the common mode choke equal to 0.05 × the longest wavelength you’re transmitting on.
The type of coax doesn’t matter much, as long as it’s 50 Ohms. However, you should route this section in the opposite direction of the EFHW antenna wire to ensure optimal radiation. This segment of coax should hang freely and must not run alongside or touch any metal objects like a mast.
Important: This method is not recommended if that coax segment continues into your shack, as it can still cause RFI issues.
Using a Separate Counterpoise Wire
A better alternative is to add a dedicated counterpoise wire. Again, use the same rule of thumb for its length:
0.05 × the longest wavelength you intend to use.
Most EFHW enclosures provide a terminal for this purpose—usually a screw with a wing nut. Simply connect your wire here. Stretch this wire as far as possible in the opposite direction of the EFHW antenna wire. This helps achieve better radiation.
Make sure the wire hangs freely, does not touch other metal parts, and isn’t routed too close to metal structures to avoid interference and unwanted coupling.
Where to Place the Common Mode Choke
When using a separate counterpoise, it’s crucial to prevent the coax cable from acting as an unintended counterpoise. To do this, install the common mode choke as close as possible to the 49:1 transformer. A short jumper cable is acceptable, but keep it under 30 centimeters (1 foot) in length.
More about common mode chokes
Want to learn more about common mode chokes? Or build one yourself? You might want to read this article about the broadband common mode choke.