The 160 m Inverted-L and the Tuner That Won't Tune
You erect a 160 m inverted-L: 60 ft vertical section, 70 ft horizontal top section, fed at the base against a ground rod and a few radials. You feed it with 100 ft of RG-8X and an antenna tuner rated 300 W. The transmitter is 100 W. On 1.8 MHz the tuner will not find a match; SWR stays above 5:1. On 3.5 MHz the same antenna tunes easily. You measure the feedpoint impedance with an analyzer at the shack end of the coax: it reads about 15 ohms resistive with a large capacitive reactance on 1.8 MHz. You have a 1:1 current balun, a 4:1 voltage balun, and additional radial wire.
1. Explain why the tuner fails on 1.8 MHz but succeeds on 3.5 MHz, using the measured impedance.
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The analyzer reading of ~15 ohms resistive with a large capacitive reactance on 1.8 MHz is the key. A short vertical against a poor ground system is electrically short at 1.8 MHz: radiation resistance is low (a few ohms) and the ground/radial loss plus the feedline transform the impedance. The large capacitive reactance means the tuner must supply a large inductive reactance to cancel it. A tuner rated 300 W may have limited reactance range and limited matching range; at 5:1+ with a big reactive component, the required L/C values and the voltages/currents inside the tuner exceed what it can produce or safely handle, so it cannot find a match. On 3.5 MHz the antenna is electrically longer, the reactance is smaller and the resistive part closer to a matchable value, so the tuner succeeds. The fix is not 'more tuner' - it is to make the antenna's impedance more favorable: improve the ground/radial system (lower loss resistance), and/or lengthen the antenna to bring the feedpoint impedance closer to resonance and to 50 ohms.
2. Which of the available components would you add, in what order, and what result would you expect?
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First: add radials. The ground system is the dominant loss and the cause of the low, highly reactive impedance. Add as many radials as practical (the additional radial wire), each at least 1/8 wavelength on 160 m (~66 ft) if possible, laid on or just below the surface and bonded to the ground rod. More radials raise the effective ground conductivity, lower loss resistance, and move the feedpoint impedance toward a more favorable value. Second: consider the 1:1 current balun at the feedpoint to choke common-mode current on the coax shield - on an inverted-L with a poor counterpoise, shield current is likely and distorts the pattern and the impedance. A 1:1 current balun is appropriate; a 4:1 voltage balun is generally not the right choice here because it can worsen imbalance and is unnecessary if the impedance is being brought near 50 ohms. Expected result: after adding radials, re-measure; the resistive part should rise and the capacitive reactance should fall, bringing the impedance within the tuner's range on 1.8 MHz. If it still will not tune, lengthen the horizontal section to bring the antenna closer to resonance. Verify with the analyzer before transmitting at full power.