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Author Topic: Balun Question?  (Read 653 times)
KA0GKT
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Posts: 555




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« on: January 19, 2006, 07:03:49 PM »

The VSWR for the 200 to 300-Ohm mis match will be 1.5:1 assuming that is actually what will be presented at the end of your ladder line.

In actuality, depending upon the specific conditions and the frequency of operation, just about any impedance can appear at the feedpoint of the ladderline.  While a low impedance may be present, it is more likely that the impedance will be greater than 50-Ohms, perhaps as much as a several K.  

Odds are that the 4:1 balun will provide the closest match.  While a greater ratio balun could be used, as the ratios get higher keeping the output balanced becomes far more difficult.

73 DE KAØGKT/7

--Steve
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N3JBH
Member

Posts: 2358




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« Reply #1 on: January 18, 2006, 02:18:58 PM »

Balun Question?

I often see guys saying you need a 4:1 balun when you change from 50 ohm coax to ladder line or twin lead. Some how this seems wrong. If twin lead is 300 ohm should the balun be 6:1?  Secondly if you were doing the same coax to 450 ohm ladder line shouldn't the balun be 9:1?

So maybe some the kind and nice folks out there cab explain to me why every one says use a 4:1 balun.
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WB6BYU
Member

Posts: 17050




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« Reply #2 on: January 18, 2006, 02:38:58 PM »

If the 300 ohm twinlead really presented a 300 ohm
impedance at the junction point, then a 6 : 1 balun could
be used.  But that happens only when the antenna impedance
at the other end of the feedline is 300 ohms, and that
isn't very often with the antennas most hams use.

Generally, we'll take a doublet or a loop antenna of some
sort - without concern for the impedance at the operating
frequency - and connect it to a balanced feedline.  As
a result, the impedance at the bottom of the feedline is
anyone's guess.  (You can calculate it if you know the
impedance of the antenna and the length of the feedline.)

So the question is, why do folks recommend using a 4 : 1
balun with random impedances.  The higher the impedance
ratio a balun has, the more difficult it is to get it to
actually work like a balun.  Especially when the load impedance
is something other than the design impedance.  So the
choice really comes down to a 4 : 1 or a 1 : 1 balun in
this application.  Because of the range of impedances
to be expected in a wire doublet antenna and how those
impedances would get transformed in the feedline, the
load impedances are generally above 50 ohms, so the
4 : 1 balun is more likely to be a closer match for
a majority of the loads encountered.  (For lower impedance
loads, a 1 : 1 balun would be better.)


The key piece is that the impedance seen by the balun
is NOT the same as the characteristic impedance of the
feedline in most ham antennas using twinlead or ladder
line.
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AD6JN
Member

Posts: 172




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« Reply #3 on: January 26, 2006, 06:17:51 AM »

I wondered about this myself some time ago.  A lot of the science is lost in common usage.

The 4:1 balun is the 2nd easiest to wind on a single toroid.  The easiest to wind is a 1:1 balun.  Being easy to wind they are lower cost and etc.  The 9:1 needs to be on a rather large diameter toroid or on three toroids.  The 4:1 balun is usually a series wound toroid using 2 individual wraps of bifilar wire with Teflon insulation which is about 100 ohms for each bifilar set.  In series it gives 200 ohms and in parallel it gives 50 ohms (output and input of the balun) hence 4:1.  Remember the voltage ratio is only 2:1.  See various ARRL books on baluns.

The 4:1 balun makes a good middle of the road matching device and will allow for random lengths of ladder line.

A ladder line at 450 ohms feeding a 75 ohm resonant dipole will reflect the impedance of the antenna if the length of the ladder is in ½ wave length increments (remember to use the velocity coefficient).  This implies the use of a 1:1 balun at the TX end of the feed line.  The VSWR should be 1.5 if the frequency is at or near resonance.  A 1.56:1 balun will give close to VSWR=1.

Bob, AD6JN
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