The concept of using two conductors to carry equal and opposite currents long predates radio. In the mid-19th century telegraph and, later, telephone engineers recognized that single-wire transmission with ground return was susceptible to interference and signal distortion. By the 1880s, the metallic circuit — a balanced pair of wires — was widely adopted by telephone companies to combat crosstalk and noise from adjacent lines. This early work in wired communication laid the technical foundation for what would later become balanced radio feeders1. It was thought that these lines confined radiation and thereby delivered power to the intended antenna while reducing susceptibility to external noise2.
The characteristic impedance of balanced line had been derived from Maxwell’s quaternion equations and recast by Oliver Heaviside into the modern form using the ∇ operator. With Heaviside’s addition of the inductor this resulted in the Telegraphers Equations model3. This model indicated that for practical lines, impedance is that of a TEM transmission line which within the 80-600 ohm range4 can to a good approximation be represented by:
where p is conductor radius, a is spacing between conductors or slightly more accurately as
where s is conductor diameter, d is spacing between conductors
When radio developed at the turn of the 20th century, antennas were typically large wire structures, such as inverted-L and T-antennas mounted as high as possible. Spark-gap and early vacuum tube transmitters could not tolerate high mismatch losses so a single conductor feed was thought to be inefficient, radiating energy along itself rather than delivering it to the desired antenna. The adoption of balanced open-wire transmission lines consisting of two parallel conductors separated by insulators provided a practical solution.
By the 1920s, open-wire balanced lines were in common use at both commercial and amateur stations. These balanced “feeder” lines were particularly suited for the high-power AM broadcast stations of the interwar years, where transmitter buildings were often located hundreds of feet from antenna towers. Long, low-loss feeders were strung across fields on poles with ceramic insulators, efficiently delivering kilowatts of RF power with minimal attenuation5 or radiation.
Despite their advantages, open-wire feeders presented practical challenges. The lines required significant physical space and their performance could be disturbed by nearby objects, wind, or weather. As coaxial cable technology matured in the late 1930s and 1940s, engineers increasingly favored it for its shielding, ease of installation, and mechanical stability, even though coax exhibited higher attenuation at HF compared to open-wire lines. Nevertheless, balanced feeders never disappeared. Radio amateurs in particular prized them for multi-band operation and low loss at high standing-wave ratios, qualities that remain attractive in certain high-efficiency antenna systems6. In retrospect, the balanced transmission line represented one of the earliest systematic solutions to the problem of feeding antennas. It not only improved efficiency but also introduced generations of engineers and amateurs to the principles of impedance matching, standing waves, and noise suppression. Balanced feeders thus occupy an important place in the early history of radio engineering, bridging the worlds of telephone wire pairs and the later dominance of coaxial cable.
The ability of feeder lines to avoid radiation was recognized for both single wire, concentric line as well as for two conductor balanced line but with cautions regarding their application in order to avoid radiation 7.
“In the twin line the two wires carry “forward and return” currents and the field is concentrated in their vicinity. When the spacing between wires is a very small fraction of the wavelength, the radiation is negligible provided the line is balanced e.g. the current are equal and opposite in the two parallel wires. In the H.F. range, spacings of several inches may be employed, but in the v.h.f. range small spacing is important.”8
The American Radio Relay League’s publications have described feeders in detail, recommending their use while cautioning:
“It is necessary to treat the two sides of the line similarly, for otherwise the fields about the two wires will not cancel and we will run into radiation troubles.”9
and with losses estimated for 50MHz and 144 MHz operation but:
“not recommended to use ”, “Open-Wire TV line at 220 Mc and 440 Mc”10
Although there seem to be no reports of it ever having been verified by careful measurement, the long-standing opinion that radiation occurs when conductor spacing becomes a large portion of a half-wavelength has persisted to the present day.
Experiment 5 and Experiment 18 in particular addresses this topic by measuring approximately 10 wavelengths of line separated by one half wavelength. Total attenuation of the line in these experiments may be compared to the results from other experiments of several line types to demonstrate that there is not significant difference due to the wide spacing.
J.J. Carty, The Metallic Circuit as Applied to Long Distance Telephony, Bell Telephone Technical Papers, 1885.↩︎
Marconi, G., Wireless Telegraphy and Signal Transmission, Proceedings of the IEE, Vol. 34,
1902.↩︎
Electromagnetic Induction and its Propagation, Oliver Heaviside, ‘ The Electrician ’ between 1885 and 1887↩︎
Radio Antenna Engineering, Laport, E.A., McGraw-Hill, 1952 p 396↩︎
Balanced Transmission Lines for Broadcast Antennas, Proceedings of the Institute of Radio
Engineers (IRE), Vol. 17, No. 12, 1929, pp. 2201–2210↩︎
Laport, op. Cit., , pp. 205–210↩︎
Radio Communication Handbook, Radio Society of Great Britain (RSGB), Fourth Edition, First Printing, September 1968, p 13.7↩︎
op. Cit., RSGB p 13.8↩︎
Radio Amateur ’ s Handbook, The American Radio Relay League(ARRL), First edition 1926, pp 101↩︎
The Radio Amateur ’ s VHF Manual, eleventh edition, 1968 ARRL Newington Connecticut,Table 8-III, p 173↩︎