Sleeve Baluns
also see
common mode
isolation
Sleeve baluns are normally used at VHF and higher. As a
general rule, they are not practical at HF.
The balun can be
inverted and used as
a skirt to form part
of the antenna
element. Here is an
example:
Note
at figure 22-5(b),
the middle antenna,
a skirt does not
fully decouple the
coax shield.
Many
collinear antennas
depend on this
effect to excite the
collinear elements!
Stopping common mode
requires both a
skirt and radials,
or multiple large
inside diameter
skirts.
Decoupling Impedance
Depending on which layout is mechanically easier to build
and if you want the sleeve to radiate, the sleeve can be connected like this
or
reversed. In the layout below the sleeve is non-radiating. In a collinear
antenna we might want to reverse the sleeve so it forms half of a dipole
element.
The left hand side of the drawing, once past the
sleeve connection point, is UNBALANCED. The
right hand side is balanced. The choking impedance of the balun is
the impedance between the outer sleeve and the outside of the shield of the
coaxial cable inside at the open end of the sleeve.
The choking impedance represents the same type of impedance
we would have from any common-mode choke, such as a coil of cable or a string of
ferrite beads would produce. A sleeve balun only works where it is ¼ wl
electrical length, or odd multiple thereof.
Obviously we want the highest possible choking impedance,
because the balanced load terminal voltage on the shield side to unbalanced
(shorted end side) voltage and impedance sets the current though this balun (as
it does with any choke balun).
We can use TLA (free with the purchase of ARRL Antenna
Books) to calculate balun’s choking impedance.
Below is a calculation of sleeve balun choking impedance
with 100-ohm Zo 1dB loss sleeve. I used a .5vF inside the sleeve.
Balun choking impedance 50-ohm Zo sleeve and .351dB
internal sleeve loss (sleeve 35 feet long and matched loss 1dB per hundred
feet).
Impedance is now 1237 ohms compared to 2474 ohms for
100-ohm sleeve Zo.
As we double sleeve Zo, all other things equal, choking
impedance doubles. What happens if we only change loss in the 100-ohm line? We
can do this either by doubling the loss per hundred feet or doubling Vf at the
same loss, making the sleeve twice as long (and twice the loss).
Now we have 1240 ohms with a 100-ohm Zo sleeve. The reason
we have less Zo is the attenuation or loss INSIDE the sleeve is twice as high. If you look at
Matched Line Attenuation above, it is now .703dB versus .351dB in the 0.5 Vf
case.
From
the above, we observe the following characteristics in a sleeve balun:
1.)
The highest possible choke sleeve impedance (largest ratio of balun
sleeve diameter to outside of transmission line) is desired. We won’t have a
good balun if the choking Zo (ratio of sleeve inner diameter to coaxial shield
outer diameter) is small.
2.)
The balun requires the lowest possible loss over the length of the
sleeve. It forms a transmission line from the inside of the sleeve to the
outside of the coax. The coax jacket is a dielectric, so we need to keep a lot
of air inside of the choking sleeve or the coax jacket will increase loss and
reduce impedance, both being very undesirable.
3.)
The velocity factor of the sleeve, based on the dielectric between the
sleeve and the shield of the coaxial cable we are trying to balance or choke, is
very important to length of the sleeve.
The
following construction guidelines apply:
The cable should have a good low-loss jacket or a very
large air or low loss dielectric gap between the shield and the sleeve. Since
energy is normally confined to the inside of a coaxial cable manufacturers are
not concerned about jacket losses. They use outer materials with long life, not
low RF loss. It is advisable to use a filler material with a high volume of air
to maximize sleeve impedance and minimize sleeve losses.
It is also advisable to use the largest practical diameter
sleeve with the smallest diameter coaxial cable inside to maximize choking
impedance.
The sleeve length has to account for velocity factor of the
sleeve, since the sleeve forms a coaxial transmission line with the outer
conductor of the coaxial cable it is intended to choke or decouple.
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