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Off-center fed dipole, fed with ladder line?



 
 
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  #1  
Old July 31st 07, 05:07 PM posted to rec.radio.amateur.antenna
Rick (W-A-one-R-K-T)
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Posts: 150
Default Off-center fed dipole, fed with ladder line?


On page 7-10 of the ARRL Antenna Book 20th Edition, there is a discussion
of "Off-Center-Fed Dipoles". It shows a dipole 130 feet long, fed 45.3
feet (a bit over 1/3 wavelength) from one end with a 4:1 balun, then coax
to the shack, and used on 80 thru 20.

I'm wondering if I can feed that with ladder line instead of coax and
balun.

It's not a "balanced" antenna (if there even is such a thing in the real
world) so maybe ladder line isn't a good choice here, but it would make
the installation a bit easier...

  #2  
Old July 31st 07, 05:53 PM posted to rec.radio.amateur.antenna
Wimpie
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Posts: 106
Default Off-center fed dipole, fed with ladder line?

On 31 jul, 18:07, "Rick (W-A-one-R-K-T)"
wrote:
On page 7-10 of the ARRL Antenna Book 20th Edition, there is a discussion
of "Off-Center-Fed Dipoles". It shows a dipole 130 feet long, fed 45.3
feet (a bit over 1/3 wavelength) from one end with a 4:1 balun, then coax
to the shack, and used on 80 thru 20.

I'm wondering if I can feed that with ladder line instead of coax and
balun.

It's not a "balanced" antenna (if there even is such a thing in the real
world) so maybe ladder line isn't a good choice here, but it would make
the installation a bit easier...


Hi Rick,

With respect to radiated power and SWR, basically you can. There are
many "but","however", etc.

As you mentioned, the antenna is fed off-center, hence there will be
reasonable common mode voltage at the feed point. This results in
unequal current in both conductors of the ladder line (result is
radiation). You should evaluate whether this is desired in your
situation. When the low end of the ladder line radiates, it will also
pick up noise (from your PC display for example).

I think it will be difficult to make a small current mode choke for
your balanced line to suppress the common mode component (as is done
many times for the unbalanced situation). The reason is that because
of the separation of the ladder line conductors, the wanted fields
extend somewhat around the line. These will interact with the
ferrite. Maybe someone built a common mode filter (and have
experience with it) for twin lead and can advise you.

I don't know the type of tuner you have, but it is very likely that
you need to a make reasonable ground to connect to the tuner. With
"ground" I do not mean just the safety or lightning ground, but a
ground that is low impedant for the RF operating frequency (multiple
wires or wire mesh).

Another option is to add a differential coil (center tapped coil)
between the ladder line at some point where you have a good ground.
The center tap of the coil is connected to that ground. From there you
can go to your tuner (with ladder line) without the need for a good RF
ground. This doesn't solve the radiation problem, but the ladder line
between the differential coil and your tuner will not radiate
significantly.

Hope this will help you a bit.

Best regards,

Wim
PA3DJS
www.tetech.nl

  #3  
Old August 1st 07, 04:01 AM posted to rec.radio.amateur.antenna
Cecil Moore[_2_]
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Posts: 3,522
Default Off-center fed dipole, fed with ladder line?

Rick (W-A-one-R-K-T) wrote:
On page 7-10 of the ARRL Antenna Book 20th Edition, there is a discussion
of "Off-Center-Fed Dipoles". It shows a dipole 130 feet long, fed 45.3
feet (a bit over 1/3 wavelength) from one end with a 4:1 balun, then coax
to the shack, and used on 80 thru 20.

I'm wondering if I can feed that with ladder line instead of coax and
balun.


My 1957 ARRL Handbook shows the OCF fed with 300 ohm
twinlead using an air-core balun which was popular
at that time. The feedline will radiate some but
that may not be a bad thing. (The Carolina "Windom"
feedline is designed to radiate.) Link coupling,
as used in the Johnson Matchboxes, would minimize
the common-mode current at the tuner.
--
73, Cecil http://www.w5dxp.com
  #4  
Old August 2nd 07, 01:51 AM posted to rec.radio.amateur.antenna
afcsman
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Posts: 63
Default Off-center fed dipole, fed with ladder line?

Rick (W-A-one-R-K-T) wrote:
On page 7-10 of the ARRL Antenna Book 20th Edition, there is a discussion
of "Off-Center-Fed Dipoles". It shows a dipole 130 feet long, fed 45.3
feet (a bit over 1/3 wavelength) from one end with a 4:1 balun, then coax
to the shack, and used on 80 thru 20.

I'm wondering if I can feed that with ladder line instead of coax and
balun.

It's not a "balanced" antenna (if there even is such a thing in the real
world) so maybe ladder line isn't a good choice here, but it would make
the installation a bit easier...

My experience with this antenna (fed with 300-ohm TV twinlead, which was
all I could afford at age 16) was electrifying. Using a DX-20
transmitter, I was able to draw nice sparks on 20 mtrs. You may want to
do some pruning to find a length that will keep RF out of the shack. My
own preference would be to center-feed the 130-foot flat-top with
ladder-line and use a link-coupled transmatch to work 80 through 10,
including WARC, and not worry about coax losses. You'll be the proud
owner of a "Zepp!"

73---W9NPI
  #5  
Old August 3rd 07, 01:53 PM posted to rec.radio.amateur.antenna
hasan schiers
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Posts: 34
Default Off-center fed dipole, fed with ladder line?

The whole idea of the OCF is to moderate the wild excursions of impedance
that is so typical of a center fed antenna operated at even harmonics. I've
used both over the years (OCF and Center Fed). I prefer the OCF for general
use and the balanced fed dipole for best overall performance. I will say the
impedance excursions of the dipole were annoying (of course, one can always
employs Cecil's trick and avoid the problem completely). Cecil approach,
while effective, makes the entire project more and more complicated.

For a "throw it up and use it" approach, the coax fed OCF is hard to beat.
It works and presents impedances that are typically handled quite easily by
even the minimal tuners incorporated into many of today's radios. I use a
135' OCF and have no trouble matching it at all (it's a CW-80 Carolina
Windom). I have found its performance on 80/40/30 to be quite good.

....hasan, N0AN

"afcsman" wrote in message
...
Rick (W-A-one-R-K-T) wrote:
On page 7-10 of the ARRL Antenna Book 20th Edition, there is a discussion
of "Off-Center-Fed Dipoles". It shows a dipole 130 feet long, fed 45.3
feet (a bit over 1/3 wavelength) from one end with a 4:1 balun, then coax
to the shack, and used on 80 thru 20.

I'm wondering if I can feed that with ladder line instead of coax and
balun.

It's not a "balanced" antenna (if there even is such a thing in the real
world) so maybe ladder line isn't a good choice here, but it would make
the installation a bit easier...

My experience with this antenna (fed with 300-ohm TV twinlead, which was
all I could afford at age 16) was electrifying. Using a DX-20
transmitter, I was able to draw nice sparks on 20 mtrs. You may want to
do some pruning to find a length that will keep RF out of the shack. My
own preference would be to center-feed the 130-foot flat-top with
ladder-line and use a link-coupled transmatch to work 80 through 10,
including WARC, and not worry about coax losses. You'll be the proud owner
of a "Zepp!"

73---W9NPI



 




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