30 m of RG-58 on 40 m: is the ATU hiding a real loss?
Your 40 m inverted-V is resonant at 7.060 MHz. At the shack you measure SWR 1.2:1 at 7.060, 2.2:1 at 7.000 and 2.6:1 at 7.200 MHz, and the transceiver's internal ATU matches it everywhere in the band. The feeder is 30 m of RG-58C/U with a datasheet figure of 1.5 dB loss per 30 m at 7 MHz when properly terminated in 50 Ω; the maker notes that loss rises when the line is mismatched. The coax goes straight to the feedpoint with no balun or choke. You run 100 W, you are planning a 400 W amplifier, and you are deciding between a 30 m run of RG-213 (0.7 dB per 30 m at 7 MHz), relocating the ATU to the antenna, or leaving everything as it is.
1. Estimate how much of your 100 W reaches the antenna at 7.000 MHz with the ATU in the shack, and explain what the ATU does and does not do.
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What the ATU does. It transforms the impedance presented by the line into 50 Ω at the transceiver socket, so the transmitter sees 1:1 and delivers its full output. It does not change the standing wave on the cable, does not reduce the current maxima in the coax, and does not recover the power lost there.
Matched loss. 1.5 dB for the 30 m run.
Mismatch penalty. At SWR 2.2, the reflection coefficient is Γ = (2.2−1)/(2.2+1) = 0.375. The mismatch loss is −10·log(1−Γ²) = −10·log(0.859) = 0.66 dB, and the increased conductor and dielectric loss in the standing wave adds a little more, so take roughly 0.8 dB total.
Tuner loss. A built-in ATU working into a mismatch typically dissipates 0.3–0.5 dB, and it sits before the cable, so it reduces power entering the line, not power leaving it.
Result. Total roughly 2.2–2.5 dB from transmitter to antenna: about 55–60 W radiated out of 100 W. That is a real, permanent loss on every contact, and it gets worse at 7.200 MHz where SWR is 2.6.
The important judgement is that the ATU is not cheating physics on your behalf: it protects the transmitter, and it lets you use the whole band, but the antenna current is set by the power that survives the cable.
2. Choose between RG-213 and moving the ATU to the feedpoint, with numbers, and name two measurements that would confirm the decision.
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Option A - RG-213. Matched loss 0.7 dB plus the same 0.8 dB mismatch penalty gives about 1.5 dB, so roughly 65–70 W reaches the antenna: an improvement of about 0.9 dB, or 20–25 % more radiated power. No weatherproofing, no control cable, no new failure points.
Option B - remote ATU. With the tuner at the feedpoint the cable runs at 50 Ω, so the line contributes its 1.5 dB matched loss: 100 W in gives about 70 W to the tuner, and after 0.3–0.5 dB of tuner loss about 63–66 W reaches the antenna. In other words, about the same as RG-213, in exchange for a weatherproof box, a control line and a new common-mode problem to solve with a choke.
Verdict. For a single-band resonant dipole, RG-213 plus a 1:1 current choke at the feedpoint is the pragmatic choice. A remote tuner only pays off if you later move to a multiband doublet with high SWR on several bands, where the mismatch term would grow.
Measurements that confirm it. (1) Terminate one end of the actual 30 m cable in a good 50 Ω load and measure input and output power at 7 MHz - datasheet loss figures assume a perfect cable and perfect connectors. (2) Put the power/SWR meter at the antenna end during a live test, or measure the current on the outside of the coax with a clip-on RF ammeter, to see both the true power delivered and whether the choke is doing its job. Do the 400 W upgrade only after these numbers are in hand.
3. You plan to add a 400 W amplifier. What changes in your line, tuner and connector checks?
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Voltage on the line. At 400 W into 50 Ω the matched rms voltage is √(400×50) = 141 V. At SWR 2.6 it rises by √2.6 ≈ 1.61, giving about 227 V rms - roughly 320 V peak. RG-58's dielectric will tolerate that, but every mated connector, the centre-pin solder joint and any PL-259 adaptor now matter far more than at 100 W.
The tuner becomes the weak point. A built-in ATU rated 100 W must not be used at 400 W, and even a 400 W tuner is specified at a limited mismatch. Check the rating at the SWR you actually have at 7.200 MHz, and check the balun or choke: a ferrite rated for 100 W will saturate and heat at four times the power.
Licence first. Before anything is connected, confirm that 400 W is within what your licence authorises and that the amplifier is CE-marked and unmodified. An amplifier that pushes you over the licensed figure is a regulatory problem, not just a technical one, and it changes your RF exposure assessment entirely.
Practical order. Replace the feeder with RG-213, fit a properly rated 1:1 choke, check the tuner's power and mismatch ratings, then measure forward and reflected power at the antenna rather than trusting the shack meter. If the SWR at 7.200 MHz stays at 2.6, either trim the antenna for a better match at the top end or accept that you must reduce power there.