[CQ-Contest] Feedline length

[Marijan Miletic, S56A]

In personal corespodence with Marco, IK2XSL I claimed that certain length of
coaxial feedlinea have some desirable features.

Let us assume dipole high in the air directly fed with perfectly matched 75
Ohms coax and a radio with a perfect grounding.
Using (2n+1)*L/4 would isolate dipole half  connected to coax shield from
the ground making balun redundant.

Second scenario might involve perfectly grounded vertical antenna. n*L/2
feedline is recommended in order to preserve good earthing.

Real life situations might fall between although shorter is better :-)

73 de Mario, S56A, N1YU
  ----- Original Message -----
  From: Tom Rauch
  To: cq-contest at contesting.com ; Marco
  Sent: Wednesday, October 10, 2001 12:20 AM
  Subject: Re: [CQ-Contest] Feedline length



  > Hi guys, I've heard several times that the length of a feedline must
  > be = cut to 1/2 wavelength or multiple (1, 1.5, 2, ...). Someone else
  > say it's better odd multiples of a 1/4 wavelength (1/4, = 3/4, 5/4,
  > ...) to minimize interaction between feedlines running the = same
  > path, for example when feeding monoband beams. Is there any particular
  > reason to do in a way or another

  If a feedline is matched, it has the same impedance along the
  entire length...assuming reasonable loss.

  If it is not matched, the impedance will vary with length but not the
  SWR...assuming minimal loss.

  Coupling between antennas can absolutely NOT be defined by
  anything like line electrical length. Coupling between lines is a
  mostly a function of lack of proper feed methods or use of baluns
  when required.

  Make the lines whatever length you need to reach the rig, unless
  you have a real reason to make them a special length (like you
  intentionally have SWR, or are phasing antennas).
  73, Tom W8JI
  W8JI at contesting.com


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