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From: liz@poppyrecords.invalid.invalid (Liz Tuddenham)
Newsgroups: sci.electronics.design
Subject: Re: Pawsey stub velocity
Date: Mon, 17 Mar 2025 08:32:37 +0000
Organization: Poppy Records
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piglet <erichpwagner@hotmail.com> wrote:

> 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?

I agree, that seems like the correct explanation - mystery solved!


-- 
~ Liz Tuddenham ~
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www.poppyrecords.co.uk