Feedline Loss and Antenna Tuner Placement on 80 m
An operator has a 40 m long dipole fed with 45 m of RG-213 coax (loss 1.2 dB/30 m at 3.5 MHz when matched). The antenna is cut for 3.75 MHz but the operator wants to work 3.5–3.8 MHz. At 3.5 MHz the feedpoint impedance is 18 − j210 Ω; at 3.8 MHz it is 90 + j120 Ω. An antenna tuner is available, but the operator is unsure whether to place it in the shack at the transmitter end or at the antenna feedpoint. The transmitter produces 100 W and requires a load VSWR below 2:1. A 1:1 balun at the feedpoint is rated 200 W and has 0.3 dB loss.
1. Calculate the VSWR at the transmitter end at 3.5 MHz with the tuner in the shack and no tuner at the antenna.
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Feedpoint impedance: 18 − j210 Ω. Reflection coefficient: Γ = (18 − j210 − 50)/(18 − j210 + 50) = (−32 − j210)/(68 − j210). |num| = √(1024 + 44100) = 212.4; |den| = √(4624 + 44100) = 220.7. |Γ| = 0.962. VSWR = (1 + 0.962)/(1 − 0.962) = 51.6:1 at the feedpoint.
Effect of the line: The 45 m line has 1.2 dB/30 m × 1.5 = 1.8 dB matched loss. With a 51.6:1 VSWR, the additional loss (mismatch loss on the line) is significant: the line loss increases roughly by the factor (1 + |Γ|²)/(1 − |Γ|²) ≈ (1 + 0.925)/(1 − 0.925) ≈ 25. So total line loss ≈ 1.8 × 25 ≈ 45 dB - but this is an overestimate because the line is not long enough to fully attenuate the reflection. More accurately, the input impedance is transformed by the line, and the VSWR at the transmitter is reduced by the line loss. The VSWR at the transmitter will be lower than 51.6:1 but still far above 2:1, and the power delivered to the antenna will be a small fraction of 100 W. The key point: with the tuner in the shack, the tuner can match the transmitter, but the line still carries a high VSWR and the loss is high - the tuner does not reduce line loss.
2. Compare tuner placement options and recommend one, with numbers.
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Option A - tuner in shack: The transmitter sees 50 Ω after tuning (good), but the line carries a high VSWR. At 3.5 MHz the line loss with a 51.6:1 VSWR is roughly 1.8 dB × (1 + 0.925)/(1 − 0.925) ≈ 45 dB in the worst case, but in practice a 45 m line at 3.5 MHz with 51.6:1 VSWR will lose perhaps 6–10 dB. Either way, a large fraction of the 100 W is dissipated in the coax. The balun sees the full mismatch and may saturate.
Option B - tuner at the feedpoint: The transmitter sees 50 Ω through the line (matched), so line loss is only 1.8 dB. The tuner matches 18 − j210 Ω to 50 Ω at the antenna, and the balun sees a matched load. Power at the antenna ≈ 100 W × 10^(−1.8/10) ≈ 66 W, minus tuner loss (say 0.5 dB) and balun loss (0.3 dB) ≈ 55 W. This is clearly superior.
Recommendation: Put the tuner at the feedpoint (or use a remote autotuner), with a 1:1 balun rated for the power. If the tuner must be in the shack, use low-loss line (e.g., ladder line or heliax) and accept the loss. At 3.8 MHz the feedpoint impedance is 90 + j120 Ω, |Γ| = |(40 + j120)/(140 + j120)| = 126.5/184.4 = 0.686, VSWR ≈ 5.4:1 - still high, so the same reasoning applies.
3. What additional practical concerns arise from the high VSWR on the line?
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- Voltage stress: At 51.6:1 VSWR with 100 W, the peak voltage on the line can exceed several kV, risking coax breakdown and balun arcing. The balun's 200 W rating is for a matched load; at high VSWR the core can saturate and overheat.
- Line loss and heating: The coax dissipates significant power; the cable may become warm, and the dielectric may degrade over time.
- Common-mode current: A high VSWR often accompanies feedline radiation; a 1:1 balun helps but a choke (ferrite beads or a coiled coax choke) at the shack end is advisable.
- Tuner losses: The tuner itself must handle high voltages and currents; a small autotuner may arc. Verify the tuner's ratings at the operating frequency.
- Receiver noise: High VSWR can increase noise pickup on receive; a matched system is quieter.
- Regulatory: Ensure spurious emissions and harmonic levels remain compliant; a mismatched system can radiate harmonics more readily.