SWR Losses Are Manageable – But Know Your Coax
Even with premium coax such as Messi & Paoloni’s 7 mm EXTRAFLEX BURY, a mismatch at the antenna can turn a surprising fraction of your RF into heat before it even reaches the feedpoint. The table below shows what happens when the tuner sits in the shack and reflections keep bouncing down the line.
Setup Details
- Cable: Messi & Paoloni EXTRAFLEX BURY 7 mm
- Length: 25 m (≈ 82 ft)
-
Matched loss (20 °C):
- 3.5 MHz — 0.325 dB (7.2 %)
- 14 MHz — 0.550 dB (11.9 %)
- 28 MHz — 0.750 dB (15.9 %)
- Scenario: Lossless tuner at the rig (conjugate match). Reflected power reaching the shack is re-reflected, so SWR on the coax remains high; multiple passes increase I²R and dielectric heating according to Pant = a (1 − ρ²)/(1 − ρ² a²). With an absorbing source or lossy tuner, coax heating is lower — but the “saved” power is dissipated in the shack.
Feedline Loss vs SWR (ATU in the Shack)
(Power delivered to the antenna feedpoint for 25 m of EXTRAFLEX BURY 7 mm; “Additional loss” is over the 1:1 baseline.)
| Freq | SWR | Effective Loss (dB) | Additional Loss (dB) | Power Delivered (%) | Power @ Feedpoint (100 W in) | Power @ Feedpoint (500 W in) | S-points Down |
|---|---|---|---|---|---|---|---|
| 3.5 MHz | 1:1 | 0.325 | – | 92.8 % | 92.8 W | 464 W | 0.05 |
| 5:1 | 0.783 | 0.458 | 83.5 % | 83.5 W | 417 W | 0.13 | |
| 14 MHz | 1:1 | 0.550 | – | 88.1 % | 88.1 W | 441 W | 0.09 |
| 5:1 | 1.265 | 0.715 | 74.7 % | 74.7 W | 373 W | 0.21 | |
| 28 MHz | 1:1 | 0.750 | – | 84.1 % | 84.1 W | 421 W | 0.12 |
| 5:1 | 1.662 | 0.912 | 68.2 % | 68.2 W | 341 W | 0.24 |
Technical Tip — Estimating Delivered Power
To estimate power at the antenna from any coax loss in dB:
Every 3 dB loss halves your power — or drops your signal by half an S-point at the other end.
To estimate power at the antenna from any coax loss in dB:
Pant = Pin × 10(−Loss / 10)Example: 1.66 dB loss on a 100 W signal → 100 × 10(−1.66 / 10) ≈ 68 W reaching the antenna.
Every 3 dB loss halves your power — or drops your signal by half an S-point at the other end.
Interpreting the Numbers
- At 28 MHz and 5:1 SWR, only ≈ 68 % of your power reaches the antenna — 1.66 dB down, about a quarter S-point on the other end.
- Moving the tuner to the feedpoint could avoid the re-reflections, but even then the savings at 5:1 SWR aren’t spectacular. It’s often better to leave the ATU indoors and invest in a PA instead.
- Important: This loss is not caused by SWR itself — it’s the coax’s attenuation that turns reflected power into heat. If you use a low-loss feedline like 600 Ω open-wire line with only 0.05 dB loss over 25 m, even a 10:1 SWR shows virtually no extra loss compared to 1:1. The difference comes from the cable’s attenuation, not the SWR.
Coax vs Open-Wire Feedline (25 m @ 14 MHz, 10:1 SWR)
(Shows why open-wire feedline remains far more efficient under mismatch.)
| Feedline Type | Matched Loss (25 m) | Effective Loss @ 10:1 SWR | Power Delivered (%) | Relative Efficiency |
|---|---|---|---|---|
| EXTRAFLEX BURY 7 mm (50 Ω coax) | 0.55 dB | ≈ 2.17 dB | ≈ 60 % | Baseline |
| Open-Wire 600 Ω (typ. 450–600 Ω) | 0.05 dB | ≈ 0.06 dB | ≈ 99 % | ≈ 1.65× higher power delivered |
Key Insight — Why Open-Wire Line Hardly Cares About SWR
For very low-loss feedline (a ≈ 1), the formula η = a(1 − ρ²)/(1 − ρ² a²) → 1. Even with extreme SWR, there’s almost no extra loss because the line doesn’t dissipate much per pass. That’s why open-wire line can run 10:1 mismatch and still beat coax at 1:1.
For very low-loss feedline (a ≈ 1), the formula η = a(1 − ρ²)/(1 − ρ² a²) → 1. Even with extreme SWR, there’s almost no extra loss because the line doesn’t dissipate much per pass. That’s why open-wire line can run 10:1 mismatch and still beat coax at 1:1.
Notes
- Matched-loss data from Messi & Paoloni EXTRAFLEX BURY 7 mm datasheet (20 °C). Multiply the 100 m values by 0.25 for 25 m.
- Reflection coefficient ρ = (SWR − 1)/(SWR + 1).
- Delivered power Pant = 10−L/10(1 − ρ²)/(1 − ρ² 10−2L/10); the remainder is coax heating.
- 1 S-unit ≈ 6 dB; table values show approximate S-point drops vs matched line.
Mini-FAQ
- Does SWR alone cause loss? — No. SWR only changes voltage and current distribution; loss comes from the feedline’s resistance and dielectric loss.
- Why is loss worse with the ATU in the shack? — Because the tuner re-launches reflections until steady state, raising circulating currents and total heating.
- Would open-wire line fix this? — Yes. Open-wire feedline has so little loss that even at 10:1 SWR it’s still more efficient than coax at 1:1.
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