When using end-fed antennas that rely on the coaxial feedline as a counterpoise, the feedline length and measurement point play a crucial role in obtaining accurate SWR readings and optimizing performance. This guide explains how to measure SWR effectively and adjust the system for the best results in such setups.

1. Understanding the Feedpoint Configuration

a. End-Fed Antenna Feedpoint

• The feedpoint of an end-fed antenna is typically an unbalanced, high-impedance point connected to a transformer, such as a 4:1, 9:1, 49:1, 56:1, 68:1, 70:1 or 75:1 UNUN.

• When the coaxial shield acts as a counterpoise, the actual “effective feedpoint” extends along the length of the coaxial cable itself, rather than being confined to the transformer.

b. Implications of Coax as a Counterpoise

The SWR readings and overall system behavior are influenced by:

1. Coaxial Cable Length: The length of the coaxial shield determines its electrical properties as part of the counterpoise.

2. Measurement Point: Where you measure SWR (at the transformer or the transmitter end) significantly affects the results.

3. Operating Frequency: The counterpoise behaves differently across frequencies due to its varying electrical length.

2. Where to Measure SWR

a. At the Transformer Output (Near the Antenna Feedpoint)

Advantages:

• Measuring close to the transformer provides a direct view of the antenna’s performance, unaffected by feedline transformations.

• This is ideal for assessing the antenna wire and transformer behavior.

Challenges:

• In setups where the coaxial cable acts as a counterpoise, this measurement may not represent the system’s true performance during operation.

Tip:

Use this measurement to:

• Fine-tune the antenna wire length for optimal performance across desired bands.

• Achieve near-resonance (for EFHW), non-resonance (for long wires), or resonance (for EFHW designed for a specific frequency).

b. At the End of the Coaxial Feedline (Near the Transmitter)

Advantages:

• Measuring at the transmitter end reflects the system’s real-world behavior, including the effects of the coax acting as a counterpoise.

Challenges:

• Standing waves on the feedline cause impedance transformation, which can affect SWR readings.

• Common-mode currents on the coax shield can distort measurements.

Tip:

To minimize the feedline’s influence:

1. Place a common-mode choke on the coaxial cable (e.g., 1/4 wavelength from the transformer for the lowest operating frequency).

2. This suppresses unwanted currents on the shield, providing a more accurate representation of the antenna’s performance.

3. Adjusting for Coax as a Counterpoise

a. Coax Length

• The coaxial shield acts as part of the radiating system, so its length directly impacts the impedance seen at the measurement point.

• Avoid feedline lengths that are exact multiples of λ/4 for any operating frequency, as this can exacerbate impedance transformation effects.

b. Choke Placement

Strategically placing a common-mode choke helps isolate the coax shield from acting as part of the antenna. Consider:

• 1/4 wavelength from the transformer (at the lowest frequency of operation).

• Near the transmitter to suppress common-mode currents and prevent RF interference in the shack.

c. Counterpoise Length Adjustment

If the coaxial shield is the counterpoise, you can modify its effective length by:

• Adjusting the choke’s position along the coax.

• Extending or shortening the coaxial feedline as needed.

4. Practical Steps for Measuring SWR

1. Measure at the Transformer:

• Disconnect the coaxial feedline and connect an antenna analyzer directly to the transformer output.

• Perform a frequency sweep to identify resonance points and adjust the antenna wire length as needed.

2. Measure at the Transmitter:

• Reconnect the coaxial feedline and measure SWR at the transmitter end.

• Compare these results with transformer-only measurements to assess the feedline’s effects.

3. Add a Choke for Isolation:

• Install a common-mode choke at the desired location to isolate the antenna system from the coaxial feedline.

• Re-measure SWR to evaluate the impact of suppressing common-mode currents.

5. Common Issues with Coax as a Counterpoise

a. Inconsistent SWR Across Bands

• The coax length may resonate at certain frequencies, causing unexpected SWR peaks.

• Solution: Adjust the coax length to avoid resonant multiples.

b. High SWR on Lower Bands

• The counterpoise may be too short for lower frequencies.

• Solution: Extend the coax length to provide sufficient electrical length for low-band operation.

c. Unstable SWR Readings

• Common-mode currents on the coax shield can lead to erratic SWR behavior.

• Solution: Install a common-mode choke near the transformer or the transmitter.

6. Tips for Multiband End-Fed Antennas

1. Expect Compromises:

Multiband end-fed antennas often involve trade-offs, so achieving a perfect SWR across all bands is unlikely. Focus on optimizing performance for your most-used bands.

2. Fine-Tune Wire Lengths:

Small adjustments to the antenna wire (e.g., ±10–20 cm) can significantly improve resonance or near-resonance across key bands.

3. Understand the Role of the Coax:

The coaxial feedline is an active part of the system. Its length and configuration directly affect the antenna’s performance. (See also: “Does Feedline Length Matter?”)

7. Conclusion

For end-fed antennas that rely on the coaxial feedline as a counterpoise, accurate SWR measurements require a clear understanding of how the coax interacts with the system. By:

• Measuring at both the transformer and transmitter ends,

• Adjusting coax length to avoid critical multiples,

• Using common-mode chokes strategically,

you can effectively optimize the antenna for multiband operation. While some compromise is inherent in multiband systems, these steps will help you achieve reliable performance across your desired frequency range.

If you have questions or need assistance, don’t hesitate to contact us for expert guidance!

Article written by Joeri Van Dooren, ON6URE – RF engineer, antenna designer, and founder of RF.Guru. With extensive experience in active and passive antenna systems, high-power RF transformers, and custom RF solutions, Joeri shares insights into cutting-edge radio communication technologies.