Practical_Antenna_Handbook_0071639586

C h a p t e r 1 0 : W i r e A r r a y s 261
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Resultant pattern
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Figure 10.9 Nonresonant vee beam antenna.
The layout of the rhombic antenna is characterized by two angles. One-half of the
included angle of the two legs of one wire is the tilt angle (f), while the angle between
the two wires is the apex angle (q). But the two are not independent; from Fig. 10.10 it
should be obvious that q/2 = 90 - f. A common rhombic design uses a tilt angle of 70
degrees, a length of 6l for each leg (two legs per side), and a height above the ground
of 1.1l. q for this antenna must necessarily be 40 degrees.
The termination resistance for the nonresonant rhombic is 600 to 800 Ω, and it must
be noninductive across the entire range of operating frequencies. For transmitting
rhombics, the resistor should be capable of dissipating at least one-third of the average
power of the transmitter. For receive-only rhombics, the termination resistor can be a
2W carbon composition or metal-film type. Such an antenna works nicely over an octave
(2:1) frequency range.