Practical_Antenna_Handbook_0071639586

C h a p t e r 1 2 : T h e Y a g i - U d a B e a m A n t e n n a 303
L
D
Center
Shorting clamp
L = K
F MHz
Feet
C
Insulator
L/70
D/3
L/10
C =
8 pF
Meter()
Figure 12.6 Gamma feed provides impedance matching.
The gamma match shown in Fig. 12.6 is one popular example of a shunt-feed system
that attaches the feedline in parallel across a portion of the element that remains electrically
continuous across its centerline. This approach is easy to describe: We are simply
connecting the transmission line at a point (or tap) on the element where the impedance
of the transmission line is comparable to the impedance of the standing wave of RF
energy on the driven element (or dipole). We can best understand this by visualizing
each half-element as a tappable resistor that is l/4 in length. Thus, the role of the
gamma match is to transform the impedance at some tap point along the element half to
a resistive part that matches the Z 0 of the line and to simultaneously cancel out any
leftover reactance.
To repeat for emphasis and clarity: The dipole or driven element of the gammamatched
Yagi is not broken in the center, as it is in the case of the simple series fed dipole.
The outer conductor, or shield, of the coaxial cable is connected to the precise
center point of the driven element, which is often directly grounded to the boom. The
center conductor of the feedline is connected to one end of a capacitor that is in series
with a component known as the gamma rod. If you are comfortable analyzing electronic
circuits, one way to view the gamma rod is to think of it as link coupling to a zero-turn
coil (the element). Early link coupling circuits used a moveable link to vary the “tightness”
of coupling between the link and the primary resonating inductance in antenna
tuners and plate tank circuits. Later antenna tuners used a fixed link but varied a capacitor
in series with the link to ground—a configuration quite analogous to the gamma
configuration of Fig. 12.5. Moving the shorting bar is equivalent to changing the number
of turns on the link.
Here is one set of “cookbook” dimensions for gamma matching a Yagi to 50-Ω line:
• Distance from centerline of element to gamma rod shorting bracket: L/10
• Gamma rod diameter: D/3
• Spacing of gamma rod from driven element: L/70
where L = length of entire driven element, roughly equal to l/2
D = diameter of driven element