[Antennas] Balun for Windom

[Niel - W0VLZ]

Don,

Thanks for the response.

Referring only to the balun at the antenna, not the choke 10' away.

I found in the ARRL Antenna Book, 19th edition, the same balun labeled 
as a balanced to unbalanced voltage balun (pg 26-25). I also found under 
the off-center fed dipole paragraph (pg 7-7) that the  "RF impedance 
paths of the two wires at the feed point are unequal. If a voltage 
step-up transformer is used...antenna current can flow on the outside of 
the coaxial line". It appears that with a balanced to unbalanced voltage 
balun there will be no current on the coax shield only if the two halves 
of the antenna are perfectly balanced, not so in a OCF dipole and most 
real world dipole installations.

Might a 4:1 un-un voltage balun force more current onto the the coax 
shield? It looks like I could convert the 4:1 balanced to unbalanced 
voltage balun to an unbalanced to unbalanced voltage balun by feeding 
the hot side of the coax into the balun center tap and connecting one 
side of the antenna and one end of the balun to the coax shield. But 
with a voltage balun  the amount of current on the coax shield is a 
function of the antenna, not the balun. Either an unbalanced-balanced 
type or a un-un type balun should work the same.

Thanks again. I understand now what's going on.

73,
Niel

Donald Sanders wrote:
> Neil, I believe you are confusing baluns and transformers. Baluns come 
> in voltage and current types. Also there are UN-UNs and Balanced 
> types. Also 1:1 and other ratios. Multi ratio types are usually 
> voltage baluns. Choke baluns are usually coils of coax or coax with 
> ferrite beads added to "choke" the RF on the shield of the coax.
> On the windom a voltage balun is used at the feed point and a choke 
> balun is used at the 10 foot point on 40 meter and 21 foot on 80 meter 
> windoms. The feed balun is an un-un type and does not "Balance out" 
> the line rather it helps transform the feed point impedance from an 
> unbalanced impedance of the antenna to an unbalanced coax feedline, 
> usually around 50 ohms. The choke balun in the feedline has the 
> ferrite beads to stop feed line radiation at that point.
> I hope this has been helpful. For really good information on baluns, 
> read Jerry Sevick book available from ARRL, I believe.
> Don W4BWS/HC4
> In preparation for spring  (and the snow melting) here in Minnesota I
> put together a "New Carolina Windom" described at
> http://www.hamuniverse.com/k4iwlnewwindom.html 
> <mhtml:%7BAE8C98D0-C276-474E-878E-BC8D383A8369%7Dmid://00000749/%21x-usc:http://www.hamuniverse.com/k4iwlnewwindom.html> 
> and
> http://www.w5fc.org/files/how-to/QRP%20Expressions_version_1.pdf 
> <mhtml:%7BAE8C98D0-C276-474E-878E-BC8D383A8369%7Dmid://00000749/%21x-usc:http://www.w5fc.org/files/how-to/QRP%20Expressions_version_1.pdf> 
> . I
> have a question.
>
> Why not use a 4:1 transformer rather than a 4:1 balun so that radiation
> from that 10' feedline is maximized rather than balanced out?
>
> The articles specify using a 4:1 balun at the off-center feedpoint. I
> understand that you want the 4:1 transformation to match the off-center
> feed point impedance but why a 4:1 balun rather than just a 4:1
> transformer? I thought that a balun is typically used to reduce
> radiation from the feedline but, in fact, for this antenna you want
> feedline radiation. The feedline serves as a radiation pattern enhancing
> vertical radiator. Why use a balun? Won't that reduce radiation from the
> feedline?
>
> Thanks,
> Niel - W0VLZ
> http://www.prismnet.com/~nielw/wa5vlz.htm 
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