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Author Topic: When to use a 4:1 or 1:1 balun?  (Read 5513 times)
WB6BYU
Member

Posts: 13171




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« Reply #15 on: February 23, 2013, 08:34:21 AM »

Probably not.

The balun at the antenna prevents the outside of the coax from being
part of the antenna, which serves to reduce the noise pickup along the
line on receive and reduces any pattern distortion on both transmit and
receive.

We demonstrated this on an OCFD at the County EOC where there was
a very high noise level on receive from the computer network (since the
feedline shared cable ducts with the network cables.)  Adding a feedline
choke (which is a 1 : 1 balun) at the rig end didn't help, but putting it
at the antenna reduced the noise floor by at least 2 S-units.  (We ended
up leaving the one in place at the rig end as well, but in practice it made
no difference.)
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KF5PGT
Member

Posts: 38




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« Reply #16 on: February 23, 2013, 04:07:30 PM »

Is there a way to calculate how many turns of coax is needed for a 1:1 balun at different frequencies?

I like building things. It's one of the things that drew me into ham radio.
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W5DXP
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Posts: 3562


WWW

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« Reply #17 on: February 23, 2013, 07:21:07 PM »

Is there a way to calculate how many turns of coax is needed for a 1:1 balun at different frequencies?

Here's some info: http://www.karinya.net/g3txq/chokes/

It looks like a crude rule-of-thumb for 4.25" diameter might be

number of turns = 1/2 the wavelength, i.e. 5 turns for 10m, 20 turns for 40m, ...
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
N4CR
Member

Posts: 1664




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« Reply #18 on: February 25, 2013, 06:05:26 AM »

thanks for such a wonderful article abut balun's.
still i always buy the best coax i can, as 90 percent of your signal will radiate from it!
jim, w4rs

The quality of the coax has little or no affect on common mode currents.
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
K7KBN
Member

Posts: 2788




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« Reply #19 on: February 25, 2013, 07:17:18 AM »

"When to use a 4:1 or 1:1 balun?"?

When the feed point impedance and the feed line itself so dictate.  A balun is about transforming impedances between input and output.  You have to know what that impedance is, and for a multiband antenna you have to know it for every band you plan to use for transmitting.  A balun that works fine for one band might be terrible on another.
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73
Pat K7KBN
CWO4 USNR Ret.
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