Feeder loss related to swr measured at load end and transmitter end
Transmission line loss can be so considerable that it should be included as it effects the entire system gain.
In the graph below the SWR is measured at the load on the y-axis and SWR measured at the input of the transmission line on the x-axis.By taking readings on the graph the loss measured in dB can be found. The curve with 0 dB feeder loss is a straight line, as assumed, although the SWR will generally be higher at the load.
SWR Feeder Loss in relation to measured SWR at load end
and SWR at the transmitter end
The graph can also be used to find SWR at the load as long as it is possible to find the SWR at the transmitter and the feeder loss of the transmission line is known.
When transmission line losses are negligible, the VSWR may be measured at any convenient point on the line.
However, when measurements are taken at the transmit end of the system, a lossy feeder will tend to mask the effects of poor VSWR at the antenna end and the VSWR will look better than it actually is.
The graph shows how measured VSWR (at the antenna) will be greater than the VSWR measured at the transmit end and the ratio will vary with line loss.
Normally, it is power that is stated and not SWR. Below is an example of a calculation in which both power, SWR and feeder loss are used. The formular for SWR is used, however an SWR Nomograph could have been used as well.
The transmit power is 100 W and feeder loss is 3 dB. As 3 dB loss diminishes the power by one half only 50 watt arrives at the load. An SWR meter has been placed on the transmitter part giving an SWR of 1.85:1. To be able to find the reflected power the SWR formular can be used. When the reflected power is isolated the formula is:
Reflected power = 9 W. The reflected power at the load is 18 W as there is also 3 dB loss in the transmission line on the reflected power. SWR at the load can now be calculated as 4:1. The curve above can be used to make a much faster calculation because here only feeder loss and SWR at the transmitter part is needed to reveal that the SWR is 4 at the load.
The transmit power is 100 W, feeder losses are 3 dB. Only 50 W arrives at the load (which has a termination VSWR of 4:1). So, 36% of the 50 W (18 W) is reflected back (of which a further 9 W (50%)) is lost before reaching the measurement point.
The forward and reflected (measured) powers give an apparent VSWR of 1.85:1, and not the actual value of 4:1.
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