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Jerry Martes wrote:
I'd submit that, in a practical situation, the mismatch of an antenna for receiving can be as high as 2:1 or even 3:1 without degrading the strength of the received signal when the transmission line losses are low. I thought that, if the receiver is tuneable, the actual impedance the transmission line presents to the receiver can be 'accounted for. I thought that, for a given antenna and transmission line, the effects of VSWR are less important that for delivering power by a transmitter. Am I wrong when I consider VSWR to be less important for receivers than for transmitters? Jerry It sounds like you (and some other posters) might be confusing the transmission line SWR with the impedance seen by the transmitter/receiver, which is often indicated with an SWR meter. The two aren't the same. Consider, for example, a 50 ohm antenna and 50 ohm tx/rx, with a 300 ohm half wavelength transmission line connecting the two. The antenna sees a perfect match (50 ohms) when receiving, and the transmitter sees a perfect match (50 ohms) when transmitting. The transmission line SWR is 6:1 when transmitting and receiving. On the other hand, if the antenna and transmission line are both 300 ohms (+ j0), the line SWR will be 1:1 when transmitting, 6:1 when receiving. And so forth. The effects of impedance mismatch seen by the transmitter when transmitting, the impedance mismatch seen by the antenna when receiving, and the transmission line SWR are three separate issues. Each has its own effect on system performance, and each needs to be treated separately. The importance of one or the other depends on the individual situation. Roy Lewallen, W7EL |
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