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Gamma Match Question

Oct 14th 2014, 01:45

AC8SW

Joined: Jul 28th 2014, 19:56
Total Topics: 0
Total Posts: 0
Question, please. For a gamma match that uses a coaxial capacitor formed by the interior surface of the hollow gamma rod and an insulated conductor running part way down the interior of the rod, should the far end of the hollow gamma rod (the end away from the coax cable connection) be electrically closed?

If the far end is electrically closed, it seems the RF current flowing radially through the coaxial capacitor will return on the interior surface to the open end (feedline end) of the gamma rod, then travel the entire length of the transmission line formed by the exterior surface of the rod and the antenna driven element before reaching the connection point between the two.

If both ends of the hollow gamma rod are open, it seems the capacitor current will flow out of both ends of the rod and travel to the element connection point by two parallel paths, only one of which is the intended gamma rod transmission line.
Oct 14th 2014, 13:36

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0

In most cases, it is impractical to close off the end of an aluminum tube electrically in a fashion that will hold up to the rigors an antenna must endure.

Zack W1VT
ARRL Senior Lab Engineer

Oct 31st 2014, 17:23

AC8SW

Joined: Jul 28th 2014, 19:56
Total Topics: 0
Total Posts: 0
After further consideration, I believe the far-end of the hollow gamma rod may be either electrically open or closed. The choice should have no impact on the gamma match performance.

Rationale: The inner surface of the hollow gamma rod and the conductor running along its axis form a coaxial structure often referred to as a “trombone capacitor”. But for RF signals, this structure is a coaxial transmission line with an open-circuit load. Because the line length is less than 1/4 wavelength (and neglecting any loss), the impedance at the input is entirely capacitive reactance. The reactance value is a function of the characteristic impedance of this line and its electrical length. Gamma match analysis can assume an equivalent capacitor having this reactance to be connected between the antenna feed line and the gamma match transmission line formed by the antenna driven element and the outer surface of the hollow gamma rod. (Please refer to the W3PG gamma match analysis article in the Jan 1969 issue of QST.)

The central point is that when the gamma rod inner surface and center conductor act as a transmission line, the RF signal is reflected back to the input at the open end of the conductor. Neglecting a small, local end-effect field, no electromagnetic field exists within the hollow gamma rod beyond the end of the center conductor. Consequently, the far end of the hollow gamma rod may be either electrically open or closed without affecting the gamma match performance.

73s

KD8ZIH
kd8zih@arrl.net

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