5. Networks and Transmission Lines

Two port networks and transmission lines connect between two wire elements.

We have in the previous chapters already mentioned a special use of a two port network in NEC-2 to simulate a current source. In the case of the current source, NC creates an additional small wire far away from the antenna structure, and then connects a special two-port network between the new wire and the wire for which the current source is needed. In this chapter, we will show how you can create arbitrary two-port networks (and also transmission lines) between the centers of any two wire elements.

A NEC 2-port network is defined by a 2 by 2 complex admittance matrix. NEC-2 only allows passive 2-port networks, thus the y12 and y21 terms of the matrix are assumed to be identical.

To connect between the center of two wire elements, you use the network function call.

network( wire1, wire2, y11r, y11i, y12r, y12i, y22r, y22i ) ;

wire1 and wire2 are two element variables. The arguments y11r, y11i, y12r, y12i, y22r and y22i are the real and imaginary parts of the admittance matrix (with y21 = y12). For example, the following NC code will connect a network between wire1 and wire2 .


element wire1, wire2 ;

wire1 = wire( 0, -4.0, height, 0, 4.0, height, #14, 21 ) ;
wire2 = wire( 1.25, -4.0, height, 1.25, 4.0, height, #14, 21 ) ;

network( wire1, wire2, 0.0, 0.0, 0, -1.0, 0.0, 0.0 ) ;



In the above example, the network function will force the current at the center of wire2 to be the same as the voltage at the center of wire1, together with a 90 degree phase shift.

Please note that NEC-2 will not physically connect wires between wire1 and wire2, so the locations of wire1 and wire2 themselves will have no effect how the network function works.


Transmission Lines

NC has functions to connect a transmission line between the center of two wire elements.

The simplest ones are the transmissionLine and crossedTransmissionLine functions. The transmissionLine function connects a transmission line directly between the center of the two elements, using the distance between the centers as the electrical length of the transmission line.

Please note that NEC-2 will not physically create wires between the two elements. It merely sets up the proper current conditions.

The crossedTransmittionLine function is identical to the transmissionLine function, except that it reverses the phase of the transmission line (e.g., swapping the two conductors at one end of the transmission line to affect a 180 degree phase reversal).

These two functions take the two wire elements as arguments, followed by the impedance of the transmission line. E.g.,

transmissionLine( element1, element2, impedance ) ;

The crossedTransmissionLine function is a convenient way to connect the elements of a Log Periodic antenna or to directly feed an "HB9CV beam," e.g.,


element hb9cv, reflector ;

hb9cv = wire( 0, -4.9, height, 0, 4.9, height, #14, 21 ) ;
reflector = wire( 2.66, -5.3, height, 2.66, 5.3, height, #14, 21 ) ;

crossedTransmissionLine( hb9cv, reflector, 1000.0 ) ;



For other transmission line lengths, use the longTransmissionLine and the crossedLongTransmissionLine functions. A "long" function has an additional real number which represents the electrical length of the transmission line.

E.g.,


element feed, reflector ;

feed = wire( 0, -4.9, height, 0, 4.9, height, #14, 21 ) ;
reflector = wire( 2.66, -5.3, height, 2.66, 5.3, height, #14, 21 ) ;

crossedLongTransmissionLine( feed, reflector, 300.0, 2.66/0.91 ) ;


Note that "long" does not mean that the electrical length of the transmission line has to be longer than the separation of the wire elements; the length that is passed to the function can be any positive, non-zero value. The transmission line can be physically shorter than the separation of the wire elements (but how you would actually build the antenna defined that way is an interesting topology question).

NEC-2 transmission lines can also be terminated by admittances. NC implements the general NEC-2 transmission line by adding four more arguments to the longTransmissionLine argument list.

The four additional arguments to the terminatedTransmissionLine and the crossedTerminatedTransmissionLine functions are the real and imaginary components of the admittance at the first wire element, followed by the real and imaginary components of the admittance at the second wire element.

E.g.,


element feed, reflector ;

feed = wire( 0, -4.9, height, 0, 4.9, height, #14, 21 ) ;
reflector = wire( 2.66, -5.3, height, 2.66, 5.3, height, #14, 21 ) ;

terminatedTransmissionLine( feed, reflector, 300.0, 2.66/0.91, y1r, y1i, y2r, y2i ) ;

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