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ATU efficiency – an example

Performance of ATUs can range from quite good to very poor.

Problems are usually most evident at lower frequencies, so lets take a G5RV on 80m as an example for exploration.

The example is from the article the article Feeding a G5RV, and is fed with 9.85m (half wave at 14.15MHz) of Wireman 554 ladder line directly from the ATU with integrated 4:1 voltage balun. The feedpoint impedance of the dipole is taken as 9.7-j338Ω at 3.6MHz.

Using TLLC, the impedance looking into the line is 7+j16.6Ω and the line loss is 1.3dB.

A VNA was used to measuring the characteristics of the internal balun in a well known mid range commercial ATU over the range 1 to 60MHz, and two port model saved. The manufacturer states that it is rated for 1.8 to 30MHz, up to 300W, and tunes any antenna, and has an internal heavy duty 4:1 balun.

949eBalunG5rv

Balun peformance depends on frequency and the complex load impedance. Above, a chart of the efficiency of the ATU balun feeding the antenna and feedline described above.

Solving the balun with load 7+j16.6Ω gives an input impedance of 2.3+j6.4Ω and loss of 1.1dB (or 78% efficiency).

Using W9CF’s T tuner emulator to estimate the likely loss in the T-tuner, using 200pF caps and a load of 2.3+j6.4Ω gives 3.6dB loss.

Total loss is 6dB, so only 25% of the transmitter output power reaches the feedpoint.

Importantly, 66% of the transmitter power is converted to heat inside the ATU.

So using that model, if the tx output power was 100W, then 56W is lost in the tuner section, mainly the coil, 10W in the balun, and 8W in the feedline. Most of the remaining 26W is radiated!


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