Transmission Line 'mask effect or 'masking effect' on VSWR
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 "mask 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:
1) Results with Transmission Line and Antenna connected in series:
2) Results with Transmission Line masking removed, revealing true antenna efficiency:
3) Additional details about this location based on input values:
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