Transmission Line masking effect (or Cable Loss mask effect) on VSWR
Coax cable mask effect, or transmission line masking effect, is defined as the misleading and more favorable efficiency measurement taken at the base of a tower of an installed antenna that is usually located at the top of a tower and at the end of a length of coaxial cable transmission line, due to the attenuation loss inherent in all coaxial cables.
Refer to the attached image. If a technician becomes aware of a high reflected power situation at a broadcast location, the first action taken is to test the antenna atop the tower by reading the Return Loss (or VSWR) value present in the telecom shelter, or location A in the image. Doing so does provide valuable information, however, it could be very misleading as to the severity of the fault when the coaxial cable transmission line is a part of the test, particularly if the length of transmission line going up the tower feeding the antenna array has high loss. This is because 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 from B to A as the incident signal in the forward path going up from A to B. Thus, the forward-to-reflected ratio of the antenna array is always lower (worse VSWR) when measured at point B because the reflected signal has not yet been attenuated through the coaxial transmission line on its way back down the tower, as would be the case if metered at point A.
It is difficult to accurately measure the VSWR of an antenna system at point A that is already installed on a tower without physically climbing the tower and sweeping the antenna on its own at point B, bypassing the long length of transmission cable altogether. More favorable Return Loss and VSWR readings can occur when metering at point A 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 (point B) will always be higher than when measured with the transmission line and antenna connected in series (point A), due to the loss inherent in all transmission line. 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. If you don't see your particular transmission line listed, choose a different cable listed and adjust the length value until you are able to mimic the same dB loss readings to those you have. Transmission line characteristics are shown in 3) below.
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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