Radio Antenna Engineering is a free introductory textbook on radio antennas and their applications. See the editorial for more information....

Quasi-coaxial Feeder

Author: Edmund A. Laport

Quasi-coaxial Feeder with Tubular Inner Conductor of Large Diameter and Eight Grounded Wires Enclosing It, the Latter Equally Spaced on a Concentric Circle. For very low impedance lines of very high power transmission capacity, this type of line (Fig. 4.9) has certain mechanical advantages over a full-concentric line. It has the mechanical simplicity and reliability of an open-wire system while giving the electrical characteristics ordinarily desired when a concentric feeder is specified. Its cost of construction is lower than for a full-concentric type, as can be judged immediately by reference to Fig. 4.10.
Fig. 4.9

When h » a



When h >>>a, k ->1.0 and Z0 - > 138 log10(a/ ρ). With ρ1 and ρ2 in inches,


An idea of the electrical possibilities is shown by the following values:

h (inches) p1 (inches) p2 (inches) a (inches) k Z0 (ohms)
100 2.00 0.1625 6.00 -0.950 76

Fig. 4.10: COnstruction for type VIII feeder

In regions devoid of ice, the inner conductor may be made of a circle of parallel wires, using a sufficient number closely to approximate, electrically, a continuous tubular conductor. This may allow the cost of the line to be reduced further, both in respect to weight of copper and perhaps also in respect to fewer supports.

Last Update: 2011-03-19