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