[Amps] swr

[Bill Fuqua]

         Many people just see the capacitance per unit length but don't 
realize it also has inductance per unit length. And the square root of the 
inductance divided by the capacitance per unit length is the characteristic 
impedance of the cable. So as long as the cable is terminated by a 
resistance equal to its characteristic impedance these inductances and 
capacitances cancel each other out.  So, you don't include the capacitance 
of the cable as long as you have an impedance match at the output  end of 
the coax.

73
Bill wa4lav


At 03:00 PM 1/4/2007 -0500, c. wrote:
>       I've been following this thread on input swr VS. cable length.
>I've seen this many times but what has apparently not been considered is
>the capacitance per foot of the cable, which is in the range of 25-30 pf
>with the RG8 series of cables. That means 20 ft. of cable has around 600
>pfs.All that is being added to the input side of the pi-net inputs used
>in most amps.. When I see that problem My approach is to go into the amp
>and change the capacitance values as needed , especially on the exciter
>side of the pi-net.!!! I know that in the past some mfrs. have specified
>a certain length of coax for their amps, which of course could have been
>due to a couple other reasons, but I don't like having 20 ft. of coax
>running loose in my setup. Six ft. of cable is plenty for me. Of course
>, if you sell your amp after modifying it then the next owner will be
>asking the same questions as have been asked here already.!!!
>        I also have noted that some folks have changed to a parallel
>tuned input ckt and have wondered what the swr must be like.!!!!If the
>ckt is resonant in band will the swr be low ??
>         just another thought.!!!
>            any comments??
>       carl / kz5ca
>
>_______________________________________________
>Amps mailing list
>Amps at contesting.com
>http://lists.contesting.com/mailman/listinfo/amps