On Sat, Jan 1, 2022 at 08:00 PM, Jeff Green wrote:
MAXIMUM loss that you can have due to this mismatch (an SWR of 1.5:1) is 0.18 dB for ANY length of cable! These equations are also displayed in the Handbook as graphs."
I'm not sure that I entirely agree with that figure, but it does highlight a difference between a transmission system, where the transmitter is the signal source and the antenna is the load, and a reception system where the antenna is the signal source and the receiver is the load.
As the receiver presents a constant impedance load at the far end of the transmission line, and if the characteristic impedance of the transmission line is similar to that of the load, even though the antenna source impedance may vary considerably, the SWR and line losses are constant and the only variable is the mismatch loss between the antenna and it's connection to the transmission line.
However in a transmission system, the transmitter is a constant impedance source and the antenna becomes the load at the far end of the transmission line. As the antenna load impedance is likely to vary with frequency, this is often not of the same value as the characteristic impedance of the transmission line, so the SWR varies considerably and there are two lots of mismatch losses, one at the interface between the transmitter and transmission line and another at the other end of the transmission line where it connects to the antenna. Plus there are additional lines losses due to the increase in SWR, but these can also partially offset the source to line mismatch loss and reduce the losses across this interface, so things are not quite as bad as they could be.
Regards,
Martin
