Asymmetric Inverted-V Fix — UNUN + Counterpoise Guide
The Problem with the Asymmetric Inverted-V (Coax-Fed)
Feeding an asymmetric inverted-V (e.g., one leg 9 m, the other 4 m) directly with coax creates a fundamentally unbalanced system:
Severe Current Imbalance
- The short leg carries less current than the long leg.
- The coax shield becomes the “missing leg,” producing common-mode current and feedline radiation.
- Symptoms include RF in the shack, unpredictable pattern shifts, and higher receive noise.
Unstable, Mismatched Impedance
- Feedpoint impedance varies widely and is often far from 50 Ω.
- High SWR raises coax loss—worst on lower bands and with longer runs.
Wasted Energy
- Power is lost as heat in mismatched components and radiated as noise via common-mode currents.
The Better Way: Treat It as Unbalanced (UNUN + Counterpoise)
If the geometry is asymmetric, don’t force a balanced feed. Configure it as an unbalanced end-fed style radiator with a proper transformer and return path.
How to Configure It
- Install an impedance transformer (UNUN) at the feedpoint. For many cases a 4:1 UNUN is a good start (when feedpoint Z is a few hundred ohms). If Z is much higher across your bands, consider a 9:1; if near half-wave resonance, a 49:1 may be appropriate.
- Connect the longer leg as the radiator (“hot” side of the UNUN).
- Use the shorter leg as a counterpoise on the UNUN return side, not as a co-radiator.
- Add a 1:1 current choke immediately after the UNUN to block remaining common-mode current on the coax.
Tip: Measure the actual feedpoint impedance with an analyzer before choosing the ratio. Start with 4:1; if the tuner struggles or SWR plateaus, try 9:1. For near-half-wave cases, 49:1 often yields the cleanest match.
Why This Works Better
- RF current is directed into the long radiator for efficient far-field radiation.
- The counterpoise provides a clean return path, reducing feedline participation.
- The inverted-V geometry’s overall pattern remains similar, but with fewer distortions.
- Losses in coax and “RF in the shack” issues drop significantly.
Matching Is Simpler
- With an appropriate UNUN, input impedance is brought closer to tuner-friendly values.
- ATUs work more effectively; system loss is minimized when the UNUN ratio matches the measured Z and a 1:1 choke suppresses common-mode.
Summary
Approach | Pattern | Efficiency | Common-Mode | Noise Pickup | Matching |
---|---|---|---|---|---|
Coax-fed asymmetric V | ~ OK | Poor | High | High | Difficult |
UNUN + counterpoise + 1:1 choke | ~ OK | Much better | Low | Lower | Easier |
If it isn’t balanced, don’t feed it like it is. Use a UNUN, let the short leg act as a counterpoise, and let the long wire do the real work.
Mini-FAQ — Asymmetric Inverted-V
- Can I fix imbalance with a 1:1 balun? A 1:1 current choke is essential, but it won’t turn an asymmetric radiator into a balanced antenna. Use a UNUN + counterpoise.
- Which UNUN ratio should I start with? Start with 4:1. If measured Z is higher on your target bands, try 9:1. For near half-wave cases, 49:1 often works best.
- Do I still need a tuner? Usually yes. The UNUN brings Z into range; the ATU finishes the match.
- Where should the choke go? Install a 1:1 current choke immediately after the UNUN on the coax.
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Written by Joeri Van Dooren, ON6URE – RF, electronics, and software engineer, complex platform and antenna designer. Founder of RF.Guru. An expert in active and passive antennas, high-power RF transformers, and custom RF solutions.