In many cases it is difficult to accurately measure the VSWR of an
antenna system that is already installed on a tower without physically climbing the tower
and sweeping the antenna on its own. This is due to the "masking effect"
that the long length of transmission line creates between the antenna at the top and the
transmitter below.VSWR of
the antenna system measured directly at the point of input to the antenna system will
always be higher than when measured with the transmission line and antenna connected in
series, due to the loss inherent in all transmission line. It is this loss that
"masks" the true VSWR of an antenna system once installed, as the reflected
signal in the transmission line is subject to the same loss characteristics on frequency
in the return path coming back down the tower as the main carrier in the forward path
going up. Thus, the forward-to-reflected ratio is always lower (worse VSWR) at the
antenna because the reflected signal has not yet been attenuated through the coaxial
transmission line, as is the case when metered at the input of the line and antenna
connected in series. The higher the transmission line loss, the higher the masking
effect the line produces.
Use the following to accurately find out
the efficiency of an antenna system already installed by removing the "masking
effect" of the transmission line:
