"ml" wrote in message
...
hi

Over the years, I've found it interesting and disturbing even
confusing, that there seems to be so much confusion regarding the
above.

Nothing gets a post responce, like the above topic and boy those
threads quickly grow to large numbers

I always read them trying to learn as well as books on the topics,
i don't pretend to have any formal engineering /scientific
knowledge just enjoy reading what i can

but i ponder why there seems to be so many opposing /contradicting
views on same subjects and topics and i am not referring to just
those here on the news groups

if i get x people in a room ask about baluns or chokes 1/2 say one
thing the other 1/2 say something else

dunno it's strange in all that confusion obviously we all pick
what we interpert and believe to be best but i hate when that happens

yeah, i just hate it when that happens... it all stems from many old myths
and misconceptions about what baluns do, or don't do... and confusing them
with coax cable chokes.

One important note... coax IS a balanced transmission line, its just that
the current that balances the one on the center conductor is confined
(normally) to the inside of the shield.

i look at it this way:

a 'true' balun is a transformer that converts from balanced to unbalanced
lines, it may or may not also transform impedances. this is the type of
thing you would put in between a coax and an open wire feeder, or what is
included in many tuners that are fed with coax and have balanced outputs.
in the degenerate case, which is actually now the most common use, the
balanced line is shortened to nothing and the balun is at the feedpoint of
the dipole or yagi driven element.

the coax choke is often confused with a balun because it is also used at a
balanced antenna feedpoint, but the actual physics of it is much different.
if you just connect a dipole to a piece of coax with no choke you really
have a 3 legged 'dipole'. current going out the center conductor wants to
return on the INSIDE of the coax shield... however there are 2 paths for
this current to take. it can come from the other side of the dipole, the
intended path, or it can come from the OUTSIDE of the shield up to the
feedpoint then back to the INSIDE of the shield... this is the one that
causes trouble. Not only is it usually vertically polarized instead of
horizontal like the intended current which distorts the pattern. But it is
also not just confined to the coax near the antenna, it can also exist on
the outside of the shield all the way back to the shack. Now, a choke on
the outside of the shield, either by winding coils of coax or adding ferrite
beads creates a high impedance bump between the shield below the choke and
the inside of the shield at the feed point. This high impedance reflects
any current on the outside of the shield back down, out of phase, thus
effectively canceling it out. This means the only route back for the
current to the inside of the coax shield is from the intended side of the
radiator... thus it forces a balance in the currents, but through a slightly
different mechanism than a 'true' balun transformer. Personally i prefer
the coil method because all it costs is a bit of coax. The ferrites that
are often used can have some unintended effects. first, they are lossy, and
some people assume this is their intended mode of operation and use even
lossier ones than they should. this results in heating at high power and
loss of transmitted power in the choke. also at high power if the cores are
not big enough they can saturate which causes harmonics and even more heat
plus a loss of effectiveness. The more correct way to look at it is as a
single turn choke, the ferrite core should present as high an impedance as
possible with as little loss as possible at the frequency its being used at.
This reduces losses and the associated bad effects. also note that the coax
choke can not transform impedances, the feedpoint must be matched to the
coax.