...The explination then rather ungracefully jumps to an explination that in unballanced (still no clear defnintion) on coax you may have current flow on the outside of the braid of the coax.
With a “balanced” antenna or feedline, the
RF currents are equal and out of phase in
both sides. In that case, radiation from the
feedline is minimum, and most of your
power is radiated from the antenna.
When that isn’t the case, you have “common
mode current” on your feedline. With a
balanced feeder like open wire line, some
component is in phase between the two
wires, and that radiates just like a single
wire would. With coax cable, the common
mode current is on the outside of the shield,
with the same effect. The feedline becomes
part of the antenna.
This has a number of side effects.
For example: a friend put a 40m dipole in
his attic. It took a lot of work to get it tuned,
but, in spite of the low height, he could make
contacts out to several thousand km. Then
one day he plugged another feedline into the
unused port on his antenna switch, and it
messed up his SWR.
The explanation, of course, is that the
outsideof his coax (and everything attached to it) was
an active part of his antenna, and he had, in
effect, added another piece of wire to it that
changed the tuning, just as adding a wire to a
dipole would.
After adding a balun and retuning the antenna,
adding cables to the switch no longer affected
the SWR. That’s a good sign. But he found he
was no longer making contacts as far away as
before. That was because his feedline radiation
was vertically polarized, which gave a lower
angle of radiation than the low dipole by itself.
That component as eliminated when he added
the balun to eliminate common mode current.
So there are several effects of common mode
current. It changes your radiation pattern:
sometimes this isn’t a bad thing, but if you
are using a beam to focus your power in one
direction and reduce interference from other
directions, it can spoil that effect.
Your SWR can change based on your feedline
configuration and how the station is grounded.
I remember trying to tune a portable antenna:
we kept taking it down, making adjustments,
and putting it back up, with inconsistent
results. Then we took it down and put it
back up without making any adjustments, and
the resonant frequency still changed. Turns
out that the way the extra coax was laying on
the ground made a difference. In another
case, plugging in my headphones changed the
SWR.
Having the feedline act as part of the antenna
can be a real problem in a modern home filled
with electronic noise generators, as it will
pick up a lot of noise from close proximity
on its way to the antenna. And if there is a
path from the coax shield to the mains wiring
(via the negative power supply lead, for
example) then it is even easier for all that
noise to reach the antenna, even when it
looks like the antenna is at some distance
from the house.
What would happen if I simply ignoe this and feed a ballanced antenna from unballanced coax?
My experience is that, around 80% of the time,
a ham putting up a dipole with no balun doesn’t
notice any problems that he associates with the
lack of a balun. I often use such antennas,
especially for portable operation.
Here’s the issue: at the antenna, the coax center
conductor is connected to one dipole wire. The
other side of the transmission line (that is, the
inside of the coax shield), connects to
two wires:
the dipole wire, and the
outside of the coax, which
acts as another antenna wire. The current will
divide between those two wires depending on their
relative impedances, just as it does with two resistors
in parallel.
The impedance looking into a 1/4 wave dipole wire
is relatively low, while the impedance of the coax
shield depends on the length and what is connected
to it. Often, most of the RF flows on the dipole wire,
and things seem to work OK. Sometimes not.
Of course, that ham may end up with a high noise
level, or other quirks, without realizing that they
are due to the lack of an effective balun.
Lack of a balun doesn’t guarantee problems, but
using one makes things much more predictable.
2. Impedence matching
Baluns can also transform impedence.
Why is it important to have "ballanced" (?) impedence. Ballance here probaly is better described as matched between the source and the antenna.
What happens if they are not ballanced/matched?
Why is it important?
Balance and matching are two different things.
You can have a balanced antenna with a poor
match, and a matched antenna with high
common mode current. (In fact, some small
commercial antennas rely on common mode
current to achieve performance and low SWR
that the antenna by itself is incapable of.)
“Balanced” means “no common mode current”.
“Matched” means that the impedance at the
antenna feedpoint (or wherever we are measuring
it) is reasonably close to the characteristic
impedance of the feedline.
Some baluns transform impedance due to the
way they balance the current: there are
several types of 4 : 1 baluns for this reason.
The simplest types of voltage baluns often
had that property. And you can design
impedance transformation into some baluns.
That may be convenient when matching an
antenna like a loop or folded dipole with an
impedance around 200 to 300 ohms to
standard coax.
But, while simple, the
voltage baluns don’t
do as good of a job of reducing common
mode current as
current baluns do,
so 1 : 1 baluns are more common these days.
3. Sleeve balluns
I read that sleeve act as band stop filters. So this means that if I fit a sleeve balun that will block all the signals in the relevant band and hence the antenna can be used for reception/transmission of all the frequencies it could be used for before fitting execpt the band that the sleeve balun blocks (which is as I understand 4x the length of the sleeve divided by the velicity factor of the resultant balun). This could be really useful for example to block for example FM broadcast?
Have I got this correct?
No. Well, not quite...
The sleeve balun blocks the design frequency
from the
outside of the coax, but not from
the inside (which is how the signal gets to
the receiver).
Way back up the page I said that there were two
wires connect to the shield side of the feedline,
and the current divides between them depending
on the relative impedance of the two paths.
The sleeve balun increases the impedance of
the outside of the coax shield at the design
frequency, so more of the RF flows on the
intentional antenna, thus acting as a
frequency-specific balun. At other frequencies
the antenna just works as though it doesn’t
have a balun.
Author
Topic: Newbie question on baluns and feaders ballanced and unballanced and impedence ma (Read 395 times)