[This message is repeated because it didn't make through few days ago.]
There are several point one should consider.
1. One doesn't have to prove that an antenna in question NEEDS
a balun. On the contrary, one should always assume that a balun IS
needed, unless it is possible to prove the opposite, which actually
happens infrequently. Literature (especially advertising literature :-)
is full of false claims that "no balun is needed", "works very well
without a balun", etc., which only serve to prove the incompetence
or negligence of authors.
2. Not all baluns are made equal, nor all the balun applications
impose equal requirements. Compare this to resistors: one does
not simply use a "resistor". There are 1 Ohm resistors, 1 kOhm
resistors, 1 MOhm resistors, etc., each of them serving some
specific purpose the best. It is exactly the same with baluns:
they must be carefully chosen for the application.
3. Most critical balun parameters are:
* common mode impedance vs. frequency;
* common mode voltage rating vs. frequency.
Balun vendors should be ashamed of not providing this information.
The "Power rating" generally provided applies only to the
transmission line within a balun, and gives no clue
whatsoever to actual choking capabilities of the product.
Even the W2FMI book shows only transmission line performance,
which is easy to achieve, disregarding common mode
performance, which is difficult.
4. Common mode impedance requirements vary widely with
application. A symmetric horizontal dipole or beam can
do with ~500 Ohms. An asymmetric antenna, such as an
"off-center fed dipole" or a "ground plane", may need
10 kOhms or more. Avoid asymmetric antennas as far as possible.
And do not "assume" that your balun has enough impedance,
I've seen some "factory made" ones showing a mere 1 Ohm
on some frequencies!
5. Common mode voltage requirements depend on three factors:
* antenna design, including feeding point/method;
* feedpoint impedance;
* TX power.
With a symmetric antenna the c.m. voltage is generally
less than a half of the feedpoint voltage. But the feedpoint
voltage (for a given TX power) increases rapidly off resonance,
to the point that most baluns can easily be destroyed with only
100 W from TX, if used on a wrong frequency. Adjust your
antennas to resonance and be careful when switching bands.
6. A choke (including choke baluns) is a transmission line,
not a "lumped" circuit element. Therefore, it's influence
on system operation is not always "obvious". An example
of this is a series connection of several chokes which
can actually have a smaller impedance than an individual choke.
For this reason it is always best to strive for the
shortest possible wire [coax] length.
7. For a given wire length and ferrite material, the highest
inductance is achieved by using as many turns as possible,
as opposed to using beads which effectively provide [almost]
a single turn.
8. A good 160 m balun can be wound on two stacked FT240-61 cores.
For high power I use a line made of two tightly spaced AWG 14 wires,
one of them inside a teflon tubing. Such a line has Zc ~ 70 Ohms,
but inserted SWR is very low due to short line length. With
22 turns I got 1620 Ohms on HP4193A, which is an excellent value
for ordinary symmetric antennas. The same balun can be used
on 80 m providing a healthy 5 kOhm isolation. For 14-28 MHz
I use FT240-67 due to much less loss.
73,
Sinisa YT1NT, VA3TTN
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