UN7GUN / REFERENCE
Antennas
Dimensions, SWR, cable loss and adjustment.
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Antenna quick reference
Starting dimensions for resonant antennas and feed lines. Measure the finished installation and trim gradually.
| Band | MHz | ½-wave dipole | ¼-wave | Fine trim / 1 MHz | Vertical radiator | Full-wave loop | RG-58 × 0.66 | RG-213 × 0.82 |
|---|---|---|---|---|---|---|---|---|
| 160 m | 1.84 | 77.4 m | 38.7 m | 3.99 m | 38.7 m | 166.3 m | 26.9 m | 33.4 m |
| 80 m | 3.65 | 39.0 m | 19.5 m | 1.04 m | 19.5 m | 83.8 m | 13.6 m | 16.8 m |
| 40 m | 7.10 | 20.1 m | 10.0 m | 27.9 cm | 10.0 m | 43.1 m | 6.97 m | 8.66 m |
| 30 m | 10.12 | 14.1 m | 7.04 m | 13.8 cm | 7.04 m | 30.2 m | 4.89 m | 6.08 m |
| 20 m | 14.15 | 10.1 m | 5.04 m | 7.1 cm | 5.04 m | 21.6 m | 3.50 m | 4.35 m |
| 17 m | 18.12 | 7.86 m | 3.93 m | 4.3 cm | 3.93 m | 16.9 m | 2.73 m | 3.39 m |
| 15 m | 21.20 | 6.72 m | 3.36 m | 3.2 cm | 3.36 m | 14.4 m | 2.33 m | 2.90 m |
| 12 m | 24.94 | 5.71 m | 2.86 m | 2.3 cm | 2.86 m | 12.3 m | 1.98 m | 2.47 m |
| 10 m | 28.50 | 5.00 m | 2.50 m | 1.7 cm | 2.50 m | 10.7 m | 1.74 m | 2.16 m |
| 6 m | 50.15 | 2.84 m | 1.42 m | 0.6 cm | 1.42 m | 6.10 m | 98.7 cm | 1.23 m |
| 2 m | 145.0 | 98.3 cm | 49.1 cm | — | 49.1 cm | 2.11 m | 34.1 cm | 42.4 cm |
| 70 cm | 433.0 | 32.9 cm | 16.5 cm | — | 16.5 cm | 70.7 cm | 11.4 cm | 14.2 cm |
SWR and power
| SWR | Reflected power | Delivered power |
|---|---|---|
| 1.0 | 0.0% | 100.0% |
| 1.1 | 0.2% | 99.8% |
| 1.2 | 0.8% | 99.2% |
| 1.3 | 1.7% | 98.3% |
| 1.5 | 4.0% | 96.0% |
| 1.7 | 6.7% | 93.3% |
| 2.0 | 11% | 89% |
| 2.5 | 18% | 82% |
| 3.0 | 25% | 75% |
| 4.0 | 36% | 64% |
| 5.0 | 44% | 56% |
Typical cable loss, dB / 100 m
| Cable | 7 MHz | 14 MHz | 28 MHz | 50 MHz | 145 MHz | 435 MHz |
|---|---|---|---|---|---|---|
| RG-58 | 2.7 | 5.4 | 7.7 | 10.8 | 19 | 36 |
| RG-213 | 1.2 | 2.4 | 3.4 | 4.6 | 8.5 | 16 |
| LMR-400 | 0.8 | 1.5 | 2.2 | 2.9 | 4.9 | 8.8 |
Cable figures are approximate. Connector quality, age, moisture and the exact cable construction affect real loss.
Working formulas
- Half-wave dipole: 142.5 / f (MHz), total length.
- Quarter-wave element: 71.25 / f (MHz).
- Full-wave loop: 306 / f (MHz).
- Coax electrical length: free-space wavelength × velocity factor.
Adjustment
- Resonance too low: shorten both sides equally.
- Resonance too high: lengthen both sides equally.
- Start 2–3% longer and cut in small steps.
- For a 50 Ω transceiver, aim for the lowest practical SWR at the working frequency.