I just installed a Off Center Fed dipole where one leg is 45 feet in
length and the other is 90 feet long. When I checked
the balun feedpoint (SO-239) with my DC ohm meter, there was a circuit.
The manufacter states its a 6:1 ratio balun and after my
test, I assume its a trifilar type. The antenna truly is "no tune" on
the 80/75, 40, 20, 17, 12 and 10 meter bands.
60, 10 and meters are all within the range of the internal tuner of my
radio.

But I was wondering what impact using a voltage balun (assuming I am
correct) has? What would be the effect of changing to a
current balun in this design? Does the OCF dipole require feeding with
balanced voltage as opposed to balanced current?
Also, I've read things about voltage baluns indicating they can't
handle a mismatch well - is that true?

Thanks,
Scott, WU2X

wrote:
But I was wondering what impact using a voltage balun (assuming I am
correct) has? What would be the effect of changing to a
current balun in this design? Does the OCF dipole require feeding with
balanced voltage as opposed to balanced current?

An OCF antenna is designed to be unbalanced. If you balance the
voltages, the currents will be unbalanced. If you balance the
currents, the voltages will be unbalanced. Unbalanced antennas
are unbalanced antennas. As Aristotle and Ayn Rand said: "A is A".
--
73, Cecil http://www.qsl.net/w5dxp

If the current is unbalanced, this would mean current is likely to flow
on the coax then? So perhaps the reason a voltage balun is used is to
merely transform the impedance 6:1 and purposely allow the feedline to
radiate? The maker of the antenna (Buckmaster) doesn't indicate any
particular length of 50 ohm coax be used, but I saw a huge change in
SWR and bandwidth on the upper bands when I lowered the antenna by 5
feet. Maybe this explains the behavior I saw.

-Scott

Cecil Moore wrote:
wrote:
But I was wondering what impact using a voltage balun (assuming I am
correct) has? What would be the effect of changing to a
current balun in this design? Does the OCF dipole require feeding with
balanced voltage as opposed to balanced current?

An OCF antenna is designed to be unbalanced. If you balance the
voltages, the currents will be unbalanced. If you balance the
currents, the voltages will be unbalanced. Unbalanced antennas
are unbalanced antennas. As Aristotle and Ayn Rand said: "A is A".
--
73, Cecil http://www.qsl.net/w5dxp

On 19 Jun 2006 07:29:25 -0700, wrote:

If the current is unbalanced, this would mean current is likely to flow
on the coax then? So perhaps the reason a voltage balun is used is to
merely transform the impedance 6:1 and purposely allow the feedline to
radiate? The maker of the antenna (Buckmaster) doesn't indicate any
particular length of 50 ohm coax be used, but I saw a huge change in
SWR and bandwidth on the upper bands when I lowered the antenna by 5
feet. Maybe this explains the behavior I saw.

-Scott

Scott,

Here's some information you may find helpful:

http://w2du.com/r2ch21.pdf

and

http://www.eznec.com/Amateur/Articles/Baluns.pdf

73,

Danny, K6MHE

In my many years I have come to a conclusion that one
useless man is a shame, two is a law firm, and three
or more is a congress. - John Adams

email: k6mheatarrldotnet
http://www.k6mhe.com/

wrote:
If the current is unbalanced, this would mean current is likely to flow
on the coax then? So perhaps the reason a voltage balun is used is to
merely transform the impedance 6:1 and purposely allow the feedline to
radiate? The maker of the antenna (Buckmaster) doesn't indicate any
particular length of 50 ohm coax be used, but I saw a huge change in
SWR and bandwidth on the upper bands when I lowered the antenna by 5
feet. Maybe this explains the behavior I saw.

The Carolina Windom is designed to radiate from the vertical
section of the feedline. You could duplicate that function
by installing a husky choke a few feet above ground. The MFJ-
853 is an inexpensive current meter that allows one to
measure the current on the coax braid. Mine arrived today.
It needs a little modification to allow the core to open
without needing the strength of Sampson.
--
73, Cecil http://www.qsl.net/w5dxp

Cecil -

I was starting to think that this antenna is acting like a Carolina
Windom - and I was going to ask how to determine if there are common
mode current on the coax (thanks for the tip on the MFJ) . Is the 22'
length of the coax of the Windom critical or is that just a minimum
length? Also, if the coax is burried right below the balun at ground
level, does this at all act like a choke? Since I have a barrel
connector there I could also ground it at the point - any point to
that (besides lightning protection)? I guess if the choke
is working then grounding at that point will have no effect.

-Scott


Cecil Moore wrote:
wrote:
If the current is unbalanced, this would mean current is likely to flow
on the coax then? So perhaps the reason a voltage balun is used is to
merely transform the impedance 6:1 and purposely allow the feedline to
radiate? The maker of the antenna (Buckmaster) doesn't indicate any
particular length of 50 ohm coax be used, but I saw a huge change in
SWR and bandwidth on the upper bands when I lowered the antenna by 5
feet. Maybe this explains the behavior I saw.

The Carolina Windom is designed to radiate from the vertical
section of the feedline. You could duplicate that function
by installing a husky choke a few feet above ground. The MFJ-
853 is an inexpensive current meter that allows one to
measure the current on the coax braid. Mine arrived today.
It needs a little modification to allow the core to open
without needing the strength of Sampson.
--
73, Cecil http://www.qsl.net/w5dxp

wrote:
Is the 22'
length of the coax of the Windom critical or is that just a minimum
length?

I suspect it comes from the fact that 22 feet is very close
to 5/8WL on 28 MHz. If the vertical section were made any
longer, it might result in high angle radiation on 10m.
They probably also figure the antenna will be mounted at
least 25 feet in the air.

Also, if the coax is burried right below the balun at ground
level, does this at all act like a choke?

Burying probably dissipates common-mode power but if it's
dissipated, it's not radiated. Best to force that energy
into the antenna where it belongs.
--
73, Cecil http://www.qsl.net/w5dxp