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Balanced-Twin Transmission Lines Used as Tuned Circuits

Author: R.J.Edwards G4FGQ © 27th April 2005

At VHF and above with a short circuit at one end, a 1/4-wave balanced-twin line can be used as a tuned circuit with a high Q. With a variable capacitor across the open end an adjustable parallel tuned circuit results. The capacitor can be a pair of spaced disks, a few picofarads, at the ends of threaded rods. Precision balanced lines also form parts of RF impedance measuring equipment.

For given line dimensions, this program calculates the value of the tuning capacitor needed to resonate the line to a particular frequency. Tuned circuit Q, input impedance, parallel R and X, and other properties of interest of the line itself are also calculated.

A common application is a tank circuit in a push-pull RF power amplifier. Coupling to the antenna is via a rectangular loop of thick wire running in parallel with the pair of tuned line conductors which also form a rectangle. The amplifier's DC supply is connected to the short circuit at the end of the line.

Q increases with conductor diameter and frequency. Conductors are often silver plated to marginally decrease transmission loss and increase Q.

Computed line input resistance at resonance is the same as that of an ordinary parallel LC tuned circuit. Computed line input reactance is that tuned out by the parallel capacitor simultaneously with the input reactance of a transistor or tube used as an RF amplifier of which the tuned circuit forms a part.

Note that as line length nears 1/4-wave, tuning capacitor approaches zero pF. In practice the line will be made somewhat shorter than 1/4-wave to permit the capacitor to be adjusted precisely to resonance. Line length, by itself, is too imprecise for accurate adjustment of resonant frequency.

Dielectric loss in the insulant, especially with air spacing, is very small and is neglected. But it has the effect of reducing Q and input R above 2 or 3 GHz.

Computed negative capacitor values and Q are incorrect at line lengths greater than 1/4 wavelengths. Other computed data is correct at all line lengths.

Note that as the short-circuited line length and attenuation increases above 13 or 14dB, resonant line input resistance oscillates-about and eventually converges on the line impedance Zo. In practice Zo is independent of frequency above VLF. For exact Zo and attenuation down to power frequencies see other programs. This program is useful for calculating Zo and loss in ordinary HF feedlines.

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