SWR Losses Are Manageable – But Know Your Coax
When feeding antennas with coaxial cable, it's easy to underestimate the importance of impedance matching. Especially for high-quality cable like Messi & Paoloni's 7mm EXTRAFLEX BURY, mismatch can still introduce measurable loss. Here’s how that loss builds up when there is no match at the antenna.
Setup Details
- Cable: M&P EXTRAFLEX BURY 7mm
- Length: 25 meters
-
Matched Losses:
- 3.5 MHz: 0.07 dB
- 14 MHz: 0.21 dB
- 28 MHz: 0.41 dB
- Mismatch Loss Factor Formula:
This formula accounts for extra loss due to SWR-generated reflections.
Table: Feedline Losses Without Matching at the Antenna
When there is a mismatch at the antenna, the coaxial cable sees standing waves, and reflections increase total loss.
| Freq | SWR | Matched Loss (dB) | SWR Loss Factor | Effective Loss (dB) | Power Loss (%) |
|---|---|---|---|---|---|
| 3.5 MHz | 1:1 | 0.07 | 1.00 | 0.070 | 1.6% |
| 2:1 | 0.07 | 1.25 | 0.088 | 2.0% | |
| 3:1 | 0.07 | 1.58 | 0.111 | 2.5% | |
| 5:1 | 0.07 | 2.00 | 0.140 | 3.2% | |
| 10:1 | 0.07 | 2.25 | 0.158 | 3.6% | |
| 14 MHz | 1:1 | 0.21 | 1.00 | 0.210 | 4.8% |
| 2:1 | 0.21 | 1.25 | 0.263 | 6.0% | |
| 3:1 | 0.21 | 1.58 | 0.331 | 7.1% | |
| 5:1 | 0.21 | 2.00 | 0.420 | 9.1% | |
| 10:1 | 0.21 | 2.25 | 0.473 | 10.4% | |
| 28 MHz | 1:1 | 0.41 | 1.00 | 0.410 | 9.1% |
| 2:1 | 0.41 | 1.25 | 0.513 | 11.1% | |
| 3:1 | 0.41 | 1.58 | 0.648 | 13.7% | |
| 5:1 | 0.41 | 2.00 | 0.820 | 17.0% | |
| 10:1 | 0.41 | 2.25 | 0.923 | 19.3% |
Key Takeaways
- Even with low-loss coax like EXTRAFLEX BURY, SWR increases effective line loss significantly.
- At 28 MHz with 5:1 SWR, nearly 17% of power is lost in just 25 m.
- Mismatches at the antenna can quietly reduce efficiency — especially on the higher bands.
For DXing, contesting, or QRP—every dB counts. Don’t let an unseen mismatch eat your signal.
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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.