Para que toda la señal generada por un transmisor sea irradiada, es preciso que haya una correcta adaptación de impedancia entre la línea de transmisión y la antena. Las características físicas de la antena, aunque estén bien calculadas, difieren siempre ligeramente de las ideales, lo que provoca una falta de adecuación responsable por ondas estacionarias y, en consecuencia, por pérdidas. Para adaptar la antena con la línea de manera ideal, aumentando el rendimiento del sistema, la manera más simple consiste en el "gamma-match".

La finalidad del "gamma-match", es adecuar las características del cable de transmisión con la antena, de modo de reducir al mínimo la relación de ondas estacionarias (ROE).

Por su simplicidad, el gamma-match es el más usado, tanto por los radioaficionados de las bandas de 7,14 ó 28 MHz, como también por los operadores de la banda de los 11 metros (27 MHz). En términos técnicos, la finalidad del "gamma-match" es permitir el pasaje de las señales que vienen de forma desbalanceada por el cable coaxial hacia un sistema irradiante simétrico como la antena.

En la figura 1 tenemos el circuito equivalente a la transmisión de señal para la antena, en la que observamos un capacitor, en serie con el conductor central del cable coaxial.

 


 

 

La adaptación de características se resume en la práctica a adecuar el valor de este capacitor, a un punto en que la impedancia presentada por la antena se adapte a la que corresponda al cable. El uso de un componente real, un capacitor variable, por ejemplo, en esta función no es interesante, pudiendo ser sustituido por un componente ficticio, como muestra la figura 2.

 


 

 

El largo del cable determinará entonces la capacitancia presentada, pudiendo hallarse con facilidad un valor que adapte la impedancia de la antena con la del cable.

Para orientación general, la mayoría de los cables presentan una capacidad por metro de alrededor de 100 pF.

El largo (a) y la separación (b) de la figura dependerá de la banda de operación del transmisor, y en consecuencia, de las dimensiones de la antena.

La regulación para el máximo rendimiento de la antena se hace modificando la distancia (b) del elemento irradiante de la antena (figura 3).

 


 

 

La fijación con aisladores plásticos debe hacerse bien para evitar problemas con el viento y la lluvia.

Damos la siguiente tabla para la elaboración del "gamma-match":

20 metros (14MHz):

a = 0,9m

b = 0,05 rn

 

15 metros (21MHz):

a = 0,7m

b = 0,04m

 

10 metros (28MHz):

a = 0,45m

b : 0,025m

 

 

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