SWR Measurement for End‑Feds Using Coax as Counterpoise

When using end‑fed antennas that rely on the coaxial feedline as counterpoise, feedline length and measurement point critically affect your SWR readings and real on‑air behavior. This guide shows where to measure, how to place chokes, and how to adjust the system for reliable results.

1) Understanding the Feedpoint Configuration

a) End‑Fed Feedpoint

• End‑feds present an unbalanced high‑Z feed into a transformer (4:1, 9:1, 49:1, 56:1, 68–75:1 UNUN).
• If the coax shield serves as the counterpoise, the effective “feedpoint” extends down the feedline, not just at the transformer.

b) Implications

• Coax length determines its electrical role as part of the radiator/return.
• Measurement point (transformer vs radio end) changes what you see.
• Frequency matters: the shield’s electrical length varies with band.

2) Where to Measure SWR

a) At the Transformer Output (near the antenna)

Pros: Direct view of wire + transformer, minimal feedline transformation.
Cons: If the coax acts as counterpoise, this may not reflect the operating system in‑situ.

Use it to: Trim wire length, find resonance (EFHW), confirm non‑resonance (long wire), and verify transformer behavior.

b) At the Transmitter (end of the coax)

Pros: Reflects real‑world operation including the shield’s counterpoise effects.
Cons: Standing waves on coax transform impedance; common‑mode currents distort readings.

Tip to stabilize: Fit a common‑mode choke on the coax (often ≈ ¼λ from the transformer for the lowest band) and another at the shack entry. This suppresses shield currents so the meter “sees” the antenna rather than the feedline.

3) Adjusting for Coax as Counterpoise

a) Coax Length

• The shield is part of the system; length shifts impedance at your meter.
• Avoid feedline lengths at integer multiples of λ/4 on any operating band (exaggerates transformation).

b) Choke Placement

• Place a choke about ¼λ (lowest band) from the UNUN to limit the shield’s radiating length.
• Add a choke near the transmitter to keep RF out of the shack and stabilize readings.

c) Counterpoise Length Tuning

• Move the choke position to change the effective shield length.
• As needed, extend or shorten the coax to avoid problematic electrical lengths.

4) Practical SWR Measurement Steps

1. Measure at the transformer: Disconnect coax, connect analyzer at the UNUN, sweep, and trim wire as needed.
2. Measure at the transmitter: Reconnect coax and measure again; compare to see feedline effects.
3. Add chokes: Install a common‑mode choke (¼λ from UNUN and/or at shack) and re‑measure to validate improvement.

5) Common Issues (and Fixes)

a) Inconsistent SWR Across Bands

Coax length may be resonant on some bands → unexpected peaks. Fix: change length to avoid λ/4 multiples.

b) High SWR on Low Bands

Counterpoise too short at low frequencies. Fix: extend the coax or add a dedicated counterpoise wire + choke.

c) Unstable Readings

CMC on the shield. Fix: add chokes near UNUN and/or radio; tidy cable routing.

6) Tips for Multiband End‑Feds

• Expect compromises: Aim for your priority bands; perfect SWR everywhere is rare.
• Fine‑trim wire: Small changes (±10–20 cm) can clean up key bands.
• Respect the feedline’s role: The coax is part of the antenna system — length and choke positions matter.

7) Conclusion

For end‑feds using the coax as a counterpoise, measure both at the transformer and at the radio, avoid critical coax lengths, and deploy common‑mode chokes strategically. You’ll get truer SWR readings and more reliable multiband performance.

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