Why an Asymmetric Inverted-V Is Not Really Efficient
The Problem with the Asymmetric Inverted-V (Coax-fed)
When you feed an asymmetric inverted-V (e.g. one leg 9 m, the other 4 m) using coax directly, the system suffers from:
1. Severe Current Imbalance
- The short leg carries much less current than the long leg.
- That unbalanced current forces the feedline shield to act as the “missing leg,” resulting in common-mode radiation.
- This leads to:
- RF in the shack
- Unpredictable pattern shifts
- Increased noise pickup
2. Unpredictable and Mismatched Impedance
- The feedpoint impedance becomes unstable and often far from 50 ohms.
- This causes high SWR unless tuned or lossy tuners are used.
- Coax losses increase, especially on lower bands and longer runs.
3. Wasted Energy
- Part of your transmitter power is lost as heat in mismatched components.
- Another portion radiates as noise from common-mode currents.
The Better Way: Use an UNUN with a Counterpoise
If the antenna is asymmetric anyway, don’t treat it like a balanced dipole. Treat it as what it really is: an end-fed or unbalanced radiator.
How to Do It
- Use a 4:1 UNUN at the junction.
- Connect the longer leg as the radiator.
- Use the shorter leg purely as a counterpoise, not a radiator.
- Add a 1:1 choke after the UNUN to block any remaining common-mode current.
Why It Works Better
- All the RF current flows into the longer wire, maximizing far-field radiation.
- The counterpoise offers a clean return path.
- Radiation pattern remains nearly the same for inverted-V geometries.
- Power is no longer wasted in the coax or in radiated noise.
Impedance Matching Is Simpler
- Impedance at the UNUN input is typically in the 200–600 ohm range, depending on radiator length.
- Tuners work more effectively, and losses are minimized.
Summary
| Approach | Pattern | Efficiency | Common Mode | Noise Pickup | Matching |
|---|---|---|---|---|---|
| Coax-fed asymmetric V | ~ OK | Poor | High | High | Difficult |
| UNUN + counterpoise | ~ OK | Much better | Low (with choke) | Lower | Easier |
If it’s not balanced, don’t feed it like it is. Use an UNUN, let the short leg be a counterpoise, and let the longer wire do the real work.
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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, he has also engineered telecom and broadcast hardware, including set-top boxes, transcoders, and E1/T1 switchboards. His expertise spans high-power RF, embedded systems, digital signal processing, and complex software platforms, driving innovation in both amateur and professional communications industries.