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Author Topic: 9:1 BALUN CONSTRUCTION  (Read 3779 times)

Posts: 9

« on: January 13, 2002, 02:22:55 PM »

i would like to construct a 9:1 balun for an swl receiving antenna. i need to know the following:  
1. gauge of wires i can use.
2. turns ratio.
3. are the turns spaced or wound side by side.
4. what type & size ferrite core should i use.
5. after completion, what should the approximate  
   inductance be on the input & output.

   an assistance concerning the above would be most

Posts: 550


« Reply #1 on: January 14, 2002, 06:42:22 PM »

1. gauge of wires i can use.
20 gauge enamel works well. Its easy to work with...

2. turns ratio.
A 3:1 turns ratio is a 9:1 impedance change. If you use trifilar winding techniques, about 10-12 turns work well for a broadband balun. Twisting 3 wires together (about 3 turns per inch) using a hand drill works well. Make 2 of the wires into the secondary and the third is the primary. Observe proper phasing!! Do a search on for 9:1 balun and get info in trifilar wound transformers.

3. are the turns spaced or wound side by side.
Spread them out evenly over the entire core best you can.

4. what type & size ferrite core should i use.
That depends on the freq range and power level. T-130 (red) works well for several hundred watts on the HF bands. Use a yellow core if you want to shift the freqs to the high side of HF and low end of VHF. Use T-50 for higher power.

5. after completion, what should the approximate
inductance be on the input & output.
That depends on the core, number of turns... Not important to know as long as you use the right material for the freqs you want to cover. You will need an ohm meter to detect the right wire to work on and make sure you get the phasing right! Go here to see a picture of a 4:1 bifilar wound balun.


Posts: 13015

« Reply #2 on: January 16, 2002, 02:33:37 PM »

First, I would use a ferrite core, not powdered iron.  You get higher
inductance with fewer turns, and you aren't worried about core
saturation in receive applications.

The wire size can be anything that fits the core and is easy to
work with.  Use whatever you can find from #20 to #26 or smaller,
preferably enamelled wire.

The turns ratio is 3 : 1, so the 450 ohm side needs 3 times the
turns on the 50 ohm side.  If you make a tri-filiar (3-wire) winding,
this requires that you use all three wires for the secondary while
using one for the primary.  The problem with this is that there is no
center tap on the secondary, so the coax braid will always be
connected to an unbalanced point.  There are several ways to get
around this problem:

1)  use 6-filiar winding.  Connect the windings in series for the
secondary.  Connect the coax shield to the center point, and the
coax center conductor across two of the windings.  However, the
coupling among the 6 wires will not be ideal, and this can get
difficult to wind.

2)  use the tri-filiar method, with all three windings in series for
the secondary and the coax cable connected across the center
winding.  Then put a  1 : 1 current balun in the coax, so the off-
center braid connection won't unbalance the system.

3)  In most cases, exact balance probably isn't required.  One of
the simplest approaches is to simply make two separate windings
on the core:  some number of turns for the secondary (say, 18), and
a primary with one third the number of turns over the top (in this
case, 6 turns.)

4)  I'd make a guess that you could use a standard 4 : 1 balun for
receive applications and not notice much difference.

The required inductance of the windings:  the reactance of the
windings should be at least 5 (preferably more) times the impedance
across the winding, so for a 450 ohm winding you will want to
have at least 2500 ohms or so of inductive reactance.  The
corresponding inductance will depend on what frequency you are
using - do the calculations for the lowest frequency.  The Amidon
web site should have the information you need to calculate the
inductance vs. number of turns for both powdered iron and ferrite
cores (but they may be given in different units for the two types,
so be careful when comparing them.)

One problem as the impedance gets high is that the stray
capacitance across the winding may well have a lower reactance
than that of the ideal coil.  This is why too large of an inductance
is not a good idea.  For this reason, it is more difficult to make
transformers to match high impedances without resorting to tuned
circuits to correct for stray reactances.
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