David,
The other day you asked a pair of questions that I have not seen
directly andwered for you so far. Both of these questions
deserve an answer because they reveal the persistence of a widely
held myth about SWR. So, I'll take a stab at putting the "SWR
causes feedline radiation" myth to rest.
>From: "DavidC" <davidc@bit-net.com>
To: <towertalk@contesting.com>
>Date: Fri, 21 Aug 1998 23:47:56 -0400
Snip...
>When the feedline, or anything other than the antenna radiates,
>is that considered loss?
No.
If it happens, it simply means that the feedline, etc. has
_BECOME_ part of the antenna. Whether this is desirable or not
depends on a number of things. But mostly it depends on whether
it helps or hurts the ability to accomplish the intended task.
Either result is possible. Radiation is considered radiation.
Loss generally converts a desirable form of energy (RF) into an
unwanted form (heat). This conversion makes it useless for its
intended purpose. Only one outcome is possible from this change.
The intentionally radiated signal strength will be decreased.
>
>Is it not common that when a tuner compensates for a high vswr at
>the antenna that it turns much of an open line feedline (or other
>feedline) into part of the radiating antenna?
>
No.
A correctly designed and applied tuner will not cause the
transmission line to radiate.
A high SWR will not cause a correctly installed and lead dressed
balanced transmission line which is connected to a balanced
antenna to radiate.
A high SWR on a correctly applied and installed coaxial
transmission line will not cause the line to radiate.
What usually causes a transmission line to radiate is:
1. Using the wrong type of line for the load (unbalanced line,
balanced load, etc.). Note that this is the typical case for
a center fed dipole or Vee antenna fed with coaxial feedline.
2. Feedline not properly decoupled from the fields around the
antenna. This is normally due to improper routing of the
line away from the feedpoint or failure to use a balancing
device when connecting coax to a balanced antenna.
3. Intentionally using the feedline as top loaded radiator.
73, Eric N7CL
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