<LOL> The reason for the CM choke on the 75 meter sloping dipoles is
"because it is unbalanced" Without the choke the feedline is so hot the
200W rig could start lighting the LEDs up in the shack. With one choke
800 watts was the limit before the LEDs in the shack light up. It took
two chokes before I could run the legal limit. The tuner let the rig
see R=50 and an SWR of 1:1 and still the lights lit up.
With the choke not at the antenna, the feedline becomes part of the antenna.
As to the feedline and a center fed half wave sloping dipole. As close
to the feedpoint as is practical works for me.
Only a few feet of feedline are perpendicular to the antenna. From the
bottom of the catenary to the tower they are almost parallel (about a
third of the feedline length.) Even if the feedline were perpendicular
all the way to the ground, it would still need to be decoupled.
The chokes work and serve a very useful function.
73
Roger (K8RI)
On 7/5/2016 Tuesday 1:32 PM, jimlux wrote:
On 7/5/16 7:34 AM, Jim Thomson wrote:
Date: Tue, 5 Jul 2016 02:40:24 -0400
From: "Roger (K8RI) on TT" <K8RI-on-TowerTalk@tm.net>
It's my understanding that a choke of (theoretical) infinite Z does not
disconnect the antenna. It would reduce the current flowing on the
outside of the coax (common mode) to zero at that point. It has no
effect on the interior currents
The choke does nothing to the balance of the antenna. It just stops
(more correctly reduces) the CM current caused by a reasonable imbalance
such as a sloping dipole, or that caused by nearby objects. The greater
the imbalance the greater the power dissipated in the core material.
The difference between sloping, center fed dipoles on 80 Vs 40 is very
pronounced
Different core materials will likely have different dissipation values.
73
Roger (K8RI)
## IF your sloping half wave dipole is UN balanced...then why
insert a CMC at the feed point at all ??
Think of the choke as being equivalent to putting an insulator to
break up a guy wire. the idea is to prevent segments of the feedline
from coupling to the antenna and reradiating because of the current
induced in the shield.
In some cases, there's no real point in trying to break up the
feedline (a 100 foot piece of coax running alongside a 100ft tower...
the tower is the big conductor in the near field of the antenna)
If you've got a sloping dipole, and the feedline comes off at right
angles, in free space, the coupling is small, and, you're right,
probably don't need a choke at the feedpoint.
on a real antenna, where the feedline probably isn't exactly
perpendicular, and there's a big conductive supporting structure, and
there's a somewhat conductive ground plane underneath.. maybe there is
a significant current induced, maybe there isn't.
One could easily model this and *see* if there's significant current
on the feedline in a non-ideal scenario. If I ran the model, and I
saw that the current everywhere in the wire that represents the
feedline was <1/10th the current at the dipole feed, I'd call it done
and go home. Nobody is looking for 20dB nulls from a dipole, and
that's what that stray current would do.
It's sort of the same as the "is that rain gutter/downspout" going to
cause a problem modeling.
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73
Roger (K8RI)
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