HORIZONTAL RHOMBIC ANTENNAS 



153 



The measurement consists in replacing the antenna by a value of 

 resistance R which gives the same thermocouple deflection. The 

 reactive components involved are tuned out in each case by adjusting 

 the condenser C^. The change in the dial setting of this condenser, as 

 the resistance is substituted for the antenna, permits the determination 

 of the reactive component of the antenna impedance. 



Curve 3, in Fig. 18, shows the terminal resistance versus frequency of 

 a properly terminated experimental antenna constructed by the authors. 

 Curve 1, of that figure, results from a termination resistance which is 

 too large while that in curve 2 is too small. Incidentally, it is inter- 

 esting to note that the value of the measured terminal resistance of this 

 antenna is lower than the resistance required, at the other end of the 

 antenna, for the correct termination. This behavior is probably an 

 effect of the radiation resistance of the system. 



10 2 



zZ700 



7 8 9 10 II 12 13 14 15 16 



FREQUENCY IN MEGACYCLES PER SECOND 



Fig. 18 — Rhombic antenna resistance versus frequency for three values of termination 



resistance. 



When a lumped termination resistance is employed, measurements 

 reveal that it operates as if paralleled by a small effective capacitance, a 

 capacitance probably due to the proximity of the adjacent converging 

 antenna wires. This capacitance prevents the resistance from acting 

 as an entirely satisfactory termination. Its effect can be minimized 

 through a proper distribution of the termination resistance over a short 

 length of the converging part of the antenna. This distribution tends 

 to isolate the harmful capacitances by means of the resistances and also 

 introduces a small effective series inductance which is helpful in 

 neutralizing the residual capacitive reactance. Other circuits have 

 been devised, but this one is often employed because of its simplicity 

 and ruggedness. 



Antenna-to-Pipe Line Coupling Circuits 



Concentric pipe transmission lines are usually favored for receiving 



antenna installations as they can be buried in the ground to give a 



substantial and weather-proof construction, free from reactions on 



adjacent antennas. This pipe-line runs up one pole of a rhombic 



