[Elecraft] SWR, Transmission lines and Antenna Matching

[Larry East]

At 12:53 PM 10/15/02 -0500, Dave, N0IT wrote:
>True Larry, but I had already said that he had to measure true swr at the
>antenna. I was simply pointing out that if he wanted to lower the swr in his
>shack to get it down to his vaunted 1:1 so his rig wouldnt reduce output, he
>could mess around inside.

Well, I believe that you are perhaps confused - or at least confusing 
others - about SWR. If one uses a "reflectometer" to measure forward and 
reflected power on a relatively loss-less transmission line, it matters not 
where the measurement is done along the transmission line -- one will still 
get the correct SWR. It also matters not where one does the matching -- at 
the antenna, at the rig or someplace in between.

One only needs to make changes in the transmission line length if the 
matching device at the transmitter end (ATU, xmitr tank circuit, whatever) 
has a limited impedance matching range; then it might be necessary to 
fiddle around with the line length to get the input impedance of the line 
within the range that can be matched.

One does not get a "1:1 SWR" at the transmitter output using a matching 
device between the transmitter and the antenna, but rather a 1:1 impedance 
match. The SWR on the transmission line between the matching device and the 
antenna feed-point is not altered.

If you haven't read "Reflections II", you really should. It clears up a lot 
of mysteries and misconceptions about transmission lines, SWR and antenna 
matching.

73,
Larry, W1HUE/7