Why Coax Length Matters Less at VHF and Above (Except for Loss)
On HF, feedline length can radically change what your tuner sees due to impedance transformations. But as you move up into VHF, UHF, and microwave, coax length (beyond loss) usually matters much less. Here’s why.
At VHF and Above, Antennas Are Usually Resonant
Unlike HF multiband wires with wild impedance swings, most VHF and UHF antennas (like Yagis, verticals, or dipoles) are designed for narrowband resonance near 50 Ω. That means the feedpoint impedance is already close to what the rig expects — no heavy transformation needed.
Low-Loss Matching Beats Transformation
- Short runs dominate: At 145 MHz, a quarter wave is only ~50 cm of coax. You won’t use such short runs except in lab setups, so practical feedlines are always multiple wavelengths long.
- Stable impedance: Since the antenna is resonant, the impedance seen at the coax input remains close to 50 Ω regardless of coax length — transformation effects are minimal.
- Loss hides mismatch: Even if a mismatch exists, the higher attenuation of coax at VHF/UHF damps standing waves, further reducing line transformation effects.
Why Loss Is the Real Enemy
At VHF and above, the real factor to watch is coaxial attenuation:
- At 30 MHz, 30 m of RG-213 loses ~1 dB.
- At 144 MHz, the same run loses ~3.5 dB — half your power is gone before reaching the antenna.
- At 432 MHz, losses can exceed 7 dB — nearly 80% wasted in heat.
(Figures indicative — exact values depend on coax type and installation.)
When Length Still Matters
- Phased arrays: In VHF contesting, EME, or repeater work, precise coax lengths are used to set element phase. Here, length is critical by design.
- Duplexers and filters: Some VHF systems rely on exact quarter-wave stubs. These are intentional line lengths, not random feedline runs.
Bottom Line
On HF, coax length is part of the matching game. On VHF and above, it’s not — provided your antenna is resonant. The only thing that grows more important is using low-loss coax like Messi & Paoloni Hyperflex or Ultraflex instead of legacy RG-58 or RG-213.
Mini-FAQ
- Do I need a tuner at VHF? — Rarely. Most VHF/UHF antennas are designed to present ~50 Ω directly.
- Can I ignore coax length completely? — For single-antenna, resonant setups: yes. For phased arrays or stubs: no.
- What’s the best upgrade for VHF? — Swap long RG-213 runs for low-loss coax like Messi Hyperflex or Ecoflex equivalents.
Interested in more technical content? Subscribe to our updates for deep-dive RF articles and lab notes.
Questions or experiences to share? Feel free to contact RF.Guru.