Al wrote:
Hm, why is that? How does an open stub prevent coupling from the antenna
to the feedline?
Roy Lewallen, W7EL
I was hoping we would not have to go through this AGAIN.
The typical J-Pole (Closed Stub) was designed to feed with open feed
line. Because Hams insist on feeding it with coax, a Band-Aid is
needed to choke off feed line radiation & common mode currents.
The Open Stub J-Pole was designed to feed with coax. So it don't need
the Band Aid.
The difference between the two is discussed in several antenna books.
Just ask anyone the owns one.
(http://www.eham.net/reviews/detail/1613)
Besides your the antenna expert, tell us why it would.
73 Al Lowe N0IMW
Sorry, I didn't realize we'd been through it before. If you'll tell me
the date, I'll look up the postings and review them.
There are two things which can cause current on the feedline. One is by
conduction. That mechanism is illustrated in Fig 21-1 at
http://www.w2du/r2ch21.pdf, and elaborated on in
http://eznec.com/Amateur/Articles/Baluns.pdf. Some types of feed systems
can minimize this, and it looks like both the open and shorted stub feed
systems are in this category.
But it's the second mechanism I was talking about -- mutual coupling, or
induction. Even if you have a perfect current balun (common mode choke)
at the feedpoint or any sort of feed system you want, current can still
be coupled to the feedline. In a center fed dipole, it won't occur if
you first make sure there's no conducted current, and place the feedline
symmetrically relative to the dipole to make the coupling from the two
dipole halves cancel. But you can't do this with a ground plane, J-Pole,
or similar antenna -- there will always be coupling from the antenna to
the feedline. Fiddling with the feedpoint won't prevent it, although it
might change the current distribution on the feedline. Changing the
feedline length and/or orientation and/or using a couple of current
baluns spaced about a quarter wavelength apart are two ways it can be
minimized.
I'd be interested in seeing any book which explains how a feedpoint
modification can prevent current being coupled to the feedline (as
opposed to being conducted).
Roy Lewallen, W7EL