Re: Pawsey stub velocity

john larkin <jlArbor.com> wrote:

On Sun, 16 Mar 2025 12:57:20 -0400, ehsjr <ehsjr@verizon.net> wrote:
 

On 3/16/2025 6:39 AM, Liz Tuddenham 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).
 
The only possible explanation I can think of is that the current in the
braid of the shorted section of the feeder is cancelled by the current
in the stub, so that section of the feeder is not acting as co-ax and
the velocity factor doea not apply to it.  Nowhere can I find anything
which says that  - so it there another explanation?
 
 

>
It's an interesting question, but there is an inherent problem.
Published velocity factor may contain an error as great as 10%,
so what does that mean to the calculation of stub length?  Seems
to me that you are forced into empirical measurements either way
to determine the "proper" stub length - where "proper" is
whatever your design specs are. In other words, it sure would be
nice to be able to compute "the" answer, but I don't see how that
is possible without measuring the velocity factor - or measuring
the stub performance at the design frequency and over the design
frequency range.  :-(
>
Ed

 
Why not use a transformer?

I have one on order but I am looking at other solutions too.  Balancing
the centre point of the vertical diople is only part of the problem,
getting the co-ax to it from underneath is much more difficult if you
don't want standing waves on the braid.  A choke is a distinct
possibility and very cheap to make.


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