Greetings

A gentleman on another group.. is telling me that he wants to put up an
offset dipole antenna and feed it with a balum.. He is a new amateur and I
can tell he does't have much antenna knowledge...

He is being advise to use an offset antenna wich in my mind isn't the best
answer to his problem....

Does anyone know of a website that describes the offset antenna better than
I can....

Larry ve3fxq

Greetings

A gentleman on another group.. is telling me that he wants to put up an
offset dipole antenna and feed it with a balum.. He is a new amateur and I
can tell he does't have much antenna knowledge...

He is being advise to use an offset antenna wich in my mind isn't the best
answer to his problem....

That may depend to some extent on what his problem is grin

Does anyone know of a website that describes the offset antenna better than
I can....

http://www.cebik.com/gup/gup9.html seems a good place to start.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

"Dave Platt" wrote in message
...
Greetings

A gentleman on another group.. is telling me that he wants to put up an
offset dipole antenna and feed it with a balum.. He is a new amateur and I
can tell he does't have much antenna knowledge...

He is being advise to use an offset antenna wich in my mind isn't the best
answer to his problem....

That may depend to some extent on what his problem is grin

Does anyone know of a website that describes the offset antenna better than
I can....

http://www.cebik.com/gup/gup9.html seems a good place to start.

--
Dave Platt AE6EO

Larry, what do you mean by 'offset'? Do you really mean 'off-center fed'? If
that's the case the only reason one would want to feed an antenna off center
would be if it permitted a better chance for a 90° angle between the antenna and
the feedline from where the feedline enters the house.

A dipole is a dipole, no matter how it's fed, center, off center, or end. The
only difference in the radiation pattern between the various methods of feeding
is the re-radiation effect of the feeder if it's not in a neutral portion of the
radiation from the dipole, causing current to flow on the feeder and thus
radiating in addition to that of the dipole.

The dipole fed at the center with the feedline coming away from the dipole at
90°, the feedline will not be a part of the radiator, unless the feedline is
coax with no balun. The balun prevents antenna currents from flowing on the
outside of the coax shield, thus preventing radiation fromthe feedline.

Incidentally, the spelling is baluN, not baluM, the term coming from the
combination of two words, BALance to UNbalance.

Walt, W2DU

In article , says...

Larry, what do you mean by 'offset'? Do you really mean 'off-center fed'? If
that's the case the only reason one would want to feed an antenna off center
would be if it permitted a better chance for a 90° angle between the antenna
and
the feedline from where the feedline enters the house.

I can think of a couple more. One reason would be
to achieve a reasonably good match to the antenna
on more than one band using coax and no transmatch.
Another reason is that an off-center-fed dipole tends
to have a broader SWR curve than a center-fed dipole
(coax feed and single-band operation assumed).

73,
Bob

--
+----------------------------------------------+
| Bob Schreibmaier K3PH | E-mail: |
| Kresgeville, PA 18333 | http://www.dxis.org |
+----------------------------------------------+

Thank you to all three of you...
I saw the webpage.. I must admit I did have too much faith in this 'offset'
kind of antenna.... I am going to send this webpage to the other group...
Personally, I don't believe in baluns even thought they seem to have their
benefits.. I still believe that matching circuts should be variable but I am
always open to more eduction...
Thanks for your input... much appreciated...
Larry ve3fxq


"Dave Platt" wrote in message
...
Greetings

A gentleman on another group.. is telling me that he wants to put up an
offset dipole antenna and feed it with a balum.. He is a new amateur and
I
can tell he does't have much antenna knowledge...

He is being advise to use an offset antenna wich in my mind isn't the
best
answer to his problem....

That may depend to some extent on what his problem is grin

Does anyone know of a website that describes the offset antenna better
than
I can....

http://www.cebik.com/gup/gup9.html seems a good place to start.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

"larry" wrote in message
...
Greetings

A gentleman on another group.. is telling me that he wants to put up an
offset dipole antenna and feed it with a balum.. He is a new amateur and I
can tell he does't have much antenna knowledge...

He is being advise to use an offset antenna wich in my mind isn't the best
answer to his problem....

Does anyone know of a website that describes the offset antenna better
than
I can....

Larry ve3fxq

Hi, Larry,

An off-center fed antenna doesn't necessarily produce unbalanced feeder
currents - - just a different feeding impedance level, which he will no
doubt take care of with a tuner or a 4:1 ratio in the balun if single band
use is all he wants and he can find the right spot for the feeder. Of course
the balun takes care of any unbalance that might be due to the feeder
not making a 90 degree angle with the antenna. There's nothing wrong
with his idea.

I just typed it balum, too, so don't be concerned - - we all make typos.

73, Chuck W6PKP

Chuck Olson wrote:

Hi, Larry,

An off-center fed antenna doesn't necessarily produce unbalanced feeder
currents - -

I'm sorry to say, it does. There's coupling from the feedline to each
side of the antenna, and unlike a symmetrical dipole with feedline at
right angles, the coupling from each side is unequal. This results in
unbalanced feeder currents. If the feedline is twinlead or open wire
line, this shows up as unequal currents on the two conductors. On coax
it manifests itself as current on the outside of the shield. In both
cases, the result is the same -- feedline radiation.

This can be shown quite dramatically by simple modeling. I've also seen
it myself by direct measurement(*). A current balun (NOT voltage balun)
at the feedpoint will reduce the conducted common mode current just as
it will with a dipole, but you still have to deal with common mode
current due to coupling. Depending on the frequency and the feedline
length and orientation, you might get lucky and not have too much
feedline current. On the other hand, it can be bad enough to aggravate
RFI problems if the feedline is routed near power or telephone lines, or
cause trouble with your rig or shack accessories.

just a different feeding impedance level, which he will no
doubt take care of with a tuner or a 4:1 ratio in the balun if single band
use is all he wants and he can find the right spot for the feeder. Of course
the balun takes care of any unbalance that might be due to the feeder
not making a 90 degree angle with the antenna. There's nothing wrong
with his idea.

See above comments. Routing the feeder at right angles to the antenna
doesn't eliminate current due to mutual coupling, and neither does a
feedpoint balun.

. . .

(*) You'll find some measured values of feedline current and more
discussion about it in "The Off-Center-Fed Dipole Revisited: A
Broadband, Multiband Antenna" by John Belrose, VE2CV and Peter Bouliane,
VE3KLO, QST, Aug. 1990.

Roy Lewallen, W7EL

Roy Lewallen wrote:
Chuck Olson wrote:


Hi, Larry,

An off-center fed antenna doesn't necessarily produce unbalanced feeder
currents - -


I'm sorry to say, it does. There's coupling from the feedline to each
side of the antenna, and unlike a symmetrical dipole with feedline at
right angles, the coupling from each side is unequal. This results in
unbalanced feeder currents. If the feedline is twinlead or open wire
line, this shows up as unequal currents on the two conductors. On coax
it manifests itself as current on the outside of the shield. In both
cases, the result is the same -- feedline radiation.

This can be shown quite dramatically by simple modeling. I've also seen
it myself by direct measurement(*). A current balun (NOT voltage balun)
at the feedpoint will reduce the conducted common mode current just as
it will with a dipole, but you still have to deal with common mode
current due to coupling. Depending on the frequency and the feedline
length and orientation, you might get lucky and not have too much
feedline current. On the other hand, it can be bad enough to aggravate
RFI problems if the feedline is routed near power or telephone lines, or
cause trouble with your rig or shack accessories.

just a different feeding impedance level, which he will no

doubt take care of with a tuner or a 4:1 ratio in the balun if single
band
use is all he wants and he can find the right spot for the feeder. Of
course
the balun takes care of any unbalance that might be due to the feeder
not making a 90 degree angle with the antenna. There's nothing wrong
with his idea.


See above comments. Routing the feeder at right angles to the antenna
doesn't eliminate current due to mutual coupling, and neither does a
feedpoint balun.


Roy, doesn't this suggest that there is benefit in twisting an open wire
feedline to attempt to expose each conductor to similar coupling to the
external fields. Clearly the benefit will be better for a higher twist
rate. Whilst achieving sufficient twist rate with a wide air-spaced line
may be impractical, it is probably quite realisable with ladder line
(notwithstanding the downsides of ladder line).

I suspect there is not much one could do to minimise the effects of
current on a coax shield other than placement of ferrite suppression
sleeves or loops in the coax at several places on the feedline to spoil
resonances in much the same way as one would try to prevent parasitic
excitation of a guy wire by breaking into non-resonant lengths with
insulators.

Owen

Owen wrote:
Roy Lewallen wrote:
. . .
See above comments. Routing the feeder at right angles to the antenna
doesn't eliminate current due to mutual coupling, and neither does a
feedpoint balun.


Roy, doesn't this suggest that there is benefit in twisting an open wire
feedline to attempt to expose each conductor to similar coupling to the
external fields. Clearly the benefit will be better for a higher twist
rate. Whilst achieving sufficient twist rate with a wide air-spaced line
may be impractical, it is probably quite realisable with ladder line
(notwithstanding the downsides of ladder line).

No. The problem isn't that the coupling is different to each conductor
of the transmission line, it's that the coupling is different from the
transmission line to each side of the antenna. Twisting the line won't
change the coupling of this common mode current by any appreciable
amount. Unfortunately.

I suspect there is not much one could do to minimise the effects of
current on a coax shield other than placement of ferrite suppression
sleeves or loops in the coax at several places on the feedline to spoil
resonances in much the same way as one would try to prevent parasitic
excitation of a guy wire by breaking into non-resonant lengths with
insulators.

The problem is exactly the same for coax and for two-wire line, and the
solution is the same for both -- common mode chokes (current baluns)
spaced about a quarter wavelength apart to break up resonances. In
practice, implementation is usually easier with coax.

Roy Lewallen, W7EL

On Sat, 25 Jun 2005 15:47:30 -0700, Roy Lewallen
wrote:

Owen wrote:
Roy Lewallen wrote:
. . .
See above comments. Routing the feeder at right angles to the antenna
doesn't eliminate current due to mutual coupling, and neither does a
feedpoint balun.


Roy, doesn't this suggest that there is benefit in twisting an open wire
feedline to attempt to expose each conductor to similar coupling to the
external fields. Clearly the benefit will be better for a higher twist
rate. Whilst achieving sufficient twist rate with a wide air-spaced line
may be impractical, it is probably quite realisable with ladder line
(notwithstanding the downsides of ladder line).

No. The problem isn't that the coupling is different to each conductor
of the transmission line, it's that the coupling is different from the
transmission line to each side of the antenna. Twisting the line won't
change the coupling of this common mode current by any appreciable
amount. Unfortunately.

Yes, of course. Twisting helps differential coupling (which in most
cases of sound installationwill not be an issue by virtue of the
relatively large distance from any other conductors to the feedline),
but not common mode.

Owen
--