Topband: 160 Meter BALUNS

[Sinisa Hristov]

[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