Overview
In the microwave transmission lines overview we started from the parallel-wire line as a basic electromagnetic-wave transmission medium. However, standard microwave engineering texts often lack a dedicated chapter on this line, and many RF designers have not encountered this type of line in modern practice.
Many microwave theoretical calculations nevertheless begin with the parallel-wire line. The open-ended parallel-wire configuration can be used to explain the basic principles of antenna radiation, and the parallel-wire model is also useful for deriving transmission-line circuit models.
Historical usage
This transmission line, now rarely seen, was once both a pedagogical starting point for electromagnetic-wave transmission and a common practical cable. For many users it was the white pair of wires that connected the rooftop antenna to the household television. The antenna that required rotation to adjust TV signal was the well-known Yagi-Uda antenna.
A common scene was one person on the roof adjusting the antenna while another watched the television for signal clarity. With technological development, these older components have become increasingly rare.
As a foundational transmission-line concept, the parallel-wire form is still useful for RF engineers. This article reviews key features of the parallel-wire line.
Construction
A parallel-wire line consists of two parallel conductors, typically stranded or solid copper. Sometimes copper-clad steel is used to reduce cost, and plastic spacers are used to maintain precise conductor spacing.
The structural layout is illustrated in figure b. The characteristic impedance depends on the spacing between the parallel conductors and the conductor diameter.
Uniform spacing is a critical dimension for a parallel-wire line; any change in spacing produces abrupt impedance variation.
Typical characteristic impedances include 600 ohm, 450 ohm, 300 ohm and 75 ohm. The 300 ohm parallel-wire line was most common and was widely used to connect televisions and FM radios to their receiving antennas.
Transmission performance
For low-frequency transmission, the parallel-wire line exhibited much lower loss than small flexible coaxial cable. For example, at 30 MHz an RG-58 coax loses about 6.6 dB per 100 m, while a 300 ohm parallel-wire line loses only about 0.55 dB per 100 m. Parallel-wire line was also much cheaper than flexible coax, which explains its historical prevalence in consumer television connections.
Balanced operation and immunity to interference
A parallel-wire line is a balanced transmission line. The spacing between the two conductors is much smaller than the wavelength of the transmitted electromagnetic wave. The RF current on one conductor equals in magnitude and is opposite in direction to the RF current on the other conductor. Therefore, in the far field the waves radiated by the two conductors are equal and opposite and cancel, so the line radiates almost no electromagnetic energy.
Similarly, an external interfering radio wave induces equal in-phase RF currents in the two conductors. Because the load at the transmission end is connected across the two conductors, only the differential opposite currents produce current in the load. Common-mode interference currents are therefore canceled, and the parallel-wire line does not introduce interference noise under ideal balanced conditions.
If a nearby metallic object is at unequal distances from the two conductors, the induced currents will differ and will not cancel, causing power loss in the line. Therefore the parallel-wire line must be kept a certain distance from metallic objects.
Shielding and replacement by coax
As frequency increases, both radiation loss and dielectric loss of the parallel-wire line increase significantly, and higher frequencies are more susceptible to external interference. Engineers therefore sometimes added a shield around the parallel-wire line, similar to the shield on flexible coax, to reduce radiation loss and improve immunity to interference.
The introduction of the coaxial transmission line addressed these issues. As RF operating frequencies rose, the parallel-wire line was gradually replaced by coaxial cable and fell out of common use.
