Re: Pawsey stub velocity

Dave Platt <dplatt@coop.radagast.org> wrote:

In article <1r9aa77.1vp4ggx1eqrulmN%liz@poppyrecords.invalid.invalid>,
Liz Tuddenham <liz@poppyrecords.invalid.invalid> wrote:
 

I've been mucking about with a design which includes a Pawsey stub.
Some sources say the velocity factor of the feeder co-ax and the
quarter-wave shorting stub, which is made of co-ax with the inner
disconnected, must be taken into account.  Other sources say that the
velocity factor is that of an open wire, not co-ax, because the stub is
only the braid acting as a piece of wire.
 
I can see that the stub does not need to be treated as co-ax, because it
is just acting as wire (and the fact that it is made from the braiding
of co-ax is irrelevant).  I can also see that the feeder co-ax
apparently *is* being  used as co-ax which means its velocity factor
should be taken into account.  This leads to the logical conclusion that
the length of feeder co-ax shorted by the stub needs to be a different
length from the length of the stub itself - which none of the
descriptions mentions or illustrates (the kinks would be obvious).

 
As I understand it:
 
-  The current flow on the _inside_ of the feeder coax is subject
   to the cable's velocity factor, because the electrical fields
   are applied across the cable dielectric.
 
-  The current flow back down the _outside_ of the feeder coax
   (which is what you want to choke off, in order to force
   balance in the antenna) is not subject to the cable's
   velocity factor, because the electrical field on the
   outside isn't going through the cable dielectric.  It's
   going only through the outer insulation and then out
   into space.
 
Again, if I understand it correctly, the presence of the outer
insulation (on both the feeder, and the choke section) does cause
current flow here to have a velocity factor of somewhat less than 1.0
(as you would see in a bare wire).  However, the velocity change is
much less than what occurs inside the cable (the VF here might be .98
rather than .67 as it might be inside the coax), and most opinions
I've read say that it can generally be neglected when figuring out the
length of the choke section (and thus the point at which the bottom of
the choke is soldered to the feeder).
 
I don't believe it matters significantly whether you remove
the center conductor from the choke section, or simply
trim it off flush at both ends and don't connect it.

Yes I think that’s right. The quarter wave transmission line we want is
formed between the stub shield and the feedline shield. These fields are in
the thin outer jacket insulation and air so velocity factor will be high
and near one. Spacing between the two shields should be minimised?