Optimizing SWR Measurements for End-Fed Antennas

With end-fed antennas that use the coaxial feedline as a counterpoise, the measurement point and feedline configuration play a significant role in accurately measuring and interpreting SWR. Here’s an adjusted explanation for this specific scenario:

Using an Antenna Analyzer for End-Fed Antennas with Coax as a Counterpoise

Understanding the Feedpoint Configuration

End-Fed Antenna Feedpoint:

  • In an end-fed antenna, the feedpoint is typically an unbalanced, high-impedance point connected to a transformer (e.g., a 4:1, 9:1 or 49:1 (or more) UNUN).
  • When the coaxial shield acts as a counterpoise, the actual “effective feedpoint” is no longer at the transformer but distributed along the coaxial cable itself.

Implications:

  • SWR measurements are influenced by:
    • The length of the coaxial cable acting as the counterpoise.
    • The placement of the measurement device along the coax.
    • The operating frequency, which affects how the counterpoise behaves electrically.

Where to Measure SWR

a. At the Transformer Output (Near the Antenna Feedpoint after the choke on the coaxial cable ( refer to the antenna’s documentation for guidance))
Advantages:
  • Measuring as close as possible to the transformer provides a clear view of the antenna’s performance, independent of the feedline effects.
  • This is ideal for assessing the raw antenna and transformer behavior.
Challenges:
  • In setups where the coax is part of the counterpoise, this measurement might not reflect the system’s actual performance during regular operation.
Tip:
  • Use this measurement to tune the antenna wire length for best resonance across the desired bands.
b. At the End of the Coaxial Feedline (Near the Transmitter)
Advantages:
  • This reflects the system as it is used during operation, including the effects of the coax acting as a counterpoise.
Challenges:
  • The impedance transformation caused by standing waves on the feedline may affect SWR readings.
  • If common-mode currents are present, they can distort the results.
    Tip:
    • To isolate the feedline’s impact, place a common-mode choke ( refer to the antenna’s documentation for guidance) on the coax at a strategic point (e.g., 1/4 wavelength from the transformer) before measuring. This minimizes current on the shield and gives a more accurate representation of the antenna’s performance. (Line Isolators / Common-mode choke why ?)

    Adjusting for Coax as a Counterpoise

    1. Coax Length:

    • The coaxial shield acts as part of the radiating system, so its length affects the impedance seen at the measurement point.
    • For multiband operation, avoid feedline lengths that are exact multiples of for any of the operating frequencies, as this can exacerbate impedance transformation effects.

    2. Choke Placement:

      • Place a common-mode choke along the feedline at a point where you want to isolate the coax shield from acting as a counterpoise. (refer to the antenna’s documentation for guidance)
      • Common placement points include: (Line Isolators / Common-mode choke why ?)
        • 1/4 wavelength from the transformer for the lowest operating frequency.
        • Near the transmitter to prevent common-mode currents from affecting SWR readings and causing RF interference in the shack.

    3. Counterpoise Length Adjustment:

    • If the coaxial shield serves as the counterpoise, you can adjust its effective length by repositioning the choke closer to or farther from the feedpoint (refer to the antenna’s documentation for guidance).

    Practical Measurement Steps

    1. Measure at the Transformer:
    • Disconnect the coaxial cable and connect the 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 with the Coax Connected:
    • Reconnect the coaxial feedline and measure at the transmitter end.
    • Compare these results with the transformer-only measurements to assess the feedline’s effect.
    3. Utilize a choke to isolate the antenna system from the feedline:
    • Install a choke at the point where you want to isolate the coax from acting as a counterpoise (refer to the antenna’s documentation for guidance). ( Line Isolators / Common-mode choke why ?)
    • Re-measure to see the impact of suppressing common-mode currents on SWR and overall system behavior.

    Common Issues with Coax as Counterpoise

    Inconsistent SWR Across Bands:

    • The coax length may resonate at certain frequencies, causing unexpected SWR peaks.
      • Solution: Adjust the coax length.

    High SWR on Lower Bands:

    • The counterpoise may not be electrically long enough for lower frequencies.
      • Solution: Extend the coax length to accommodate these bands.

    Unstable SWR Readings:

    Tips for Multiband End-Fed Antennas

    Expect Some Compromise:
      • Multiband end-fed antennas inherently involve trade-offs, and perfect SWR across all bands is unlikely.
      • Use an antenna analyzer to identify the bands where performance is best and make adjustments as needed.
    Fine-Tune Wire Lengths:
      • Small adjustments to the antenna wire (±10–20 cm) can significantly improve (near) resonance across key bands.
    Understand the Role of the Coax:

    Conclusion

    When using an end-fed antenna where the coaxial feedline serves as a counterpoise, accurate SWR measurement requires understanding how the coax interacts with the system. By strategically placing a choke, carefully choosing the coax length, and using the antenna analyzer both at the transformer, the prolonged feed point (for end-fed's with a coaxial counterpoise) and transmitter ends, you can effectively optimize the antenna for multiband operation.