Pete Smith wrote:
>At the risk of fanning the flames again, I think the original question was
>whether a change in line length could cause the apparent resonant frequency
>of an antenna to change. My 20M antenna has a feedpoint impedance of 35-j14
>at 14 mhz. If I mathematically attach a half-wavelength of RG-8 (using
>TLA.EXE), the impedance at the input end of the cable becomes 36-j 13.3,
>and the result, as you'd expect, is slightly lower SWR due to line loss.
>however, if the line is pruned to 1/4 wave, the resulting input-end
>impedance is 61.75+j23, and it appears that the antenna has magically moved
>from being short at 14 MHz to being long. The SWR hasn't changed,
>fundamentally, but the mix of reactance and resistance confronted by the
>AMP sure has, unless I misunderstand what the program is telling me.
Correct, Pete.
As you move along the transmission line (assuming a 50 Ohm line in a 50
Ohm system), the VSWR remains constant. But the reactances definitely
change and at some points, you do have zero reactances as well.
So yes, I would suspect that your match would be different depending on
the line length.
73,
Jon
KE9NA
-------------------------------------
Jon Ogden
KE9NA
http://www.qsl.net/ke9na <--- CHECK IT OUT! It's been updated!!!!!
"A life lived in fear is a life half lived."
--
FAQ on WWW: http://www.contesting.com/ampsfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|