In article ,
Richard Clark wrote:

The harmonic drivepoint Zs follow their own
sinusoidal roller coaster through the shift in feed point.

Richard-

Someone asked that if the OCF Dipole was so good, why didn't everyone
use one? When I got my start back in the 50s, everyone did use one. I
used my "Full Windom" for several years on 80/75/40/10 CW and AM. In
today's world, the G5RV antenna appears to have taken over as the
popular antenna of choice, and is probably equally as bad as the OCF
Dipole.

As a teenager I knew little about SWR. I used a balanced tuner to match
the 300 Ohm feed-line, tuning for maximum brightness of a pilot lamp
connected to a loop of wire taped to the feed-line. I understood that
the feed-point was chosen so impedance was reasonably close to 300 Ohms
on all bands except 15 Meters. Your reference to a roller coaster
suggests that it might not be reasonably close.

Using the modeling software, is there a feed-point where impedance is
close to an available balanced feed-line on multiple bands? As close, I
would accept a 2:1 SWR.

Fred
K4DII

On Sun, 18 Mar 2007 14:31:38 -0400, Fred McKenzie
wrote:

Using the modeling software, is there a feed-point where impedance is
close to an available balanced feed-line on multiple bands? As close, I
would accept a 2:1 SWR.

Hi Fred,

The usual designs include a BalUn that transforms from a higher drive
Z to the 50 Ohms of a line. In that sense, the Off Center Dipole
introduces accessible resonances at every harmonic instead of at odd
harmonics. Depending upon the offset, some come into play, some go
out and for a variety of transformations. Some suggest 2:1, others
4:1, and yet others higher.

And you would still need to decouple the line (if the BalUn design
doesn't already answer that). Given the field imbalance, it may
require an aggressive decoupling (a second choke, or a distributed
choking). I have a large document available to those whose mail box
can stand the load.

73's
Richard Clark, KB7QHC

Fred McKenzie wrote in news:fmmck-9C2AC4.14313718032007
@nntp.aioe.org:

In article ,
Richard Clark wrote:

....
Someone asked that if the OCF Dipole was so good, why didn't everyone
use one? When I got my start back in the 50s, everyone did use one. I
used my "Full Windom" for several years on 80/75/40/10 CW and AM. In

Fred, I think the term "OCF Dipole" is usually used today to mean a
dipole fed with coax and balun (often 4:1, usually not 1:1) fed offset
from the centre and often operated at half wave resonance or harmonic
multiples.

....
Using the modeling software, is there a feed-point where impedance is
close to an available balanced feed-line on multiple bands? As close,
I
would accept a 2:1 SWR.

If you are going to use an ATU and open wire line (as distinct from
balanced line) why are you restricting the max VSWR to 2. Practical open
wire lines can operate at much higher VSWR with acceptable losses.

Once you have addressed that question, then ask yourself why you wouldn't
just feed such a dipole in the centre and reduce the common mode current
problem caused by the asymmetric feed.

A dipole of more than about 35% wavelength at its lowest operating
frequency, centre fed with practical open wire line and a good ATU will
allow multiband operation with efficiency should be acceptable as part of
the multiband compromise. For an example, look at Fig 10 in the article
http://www.vk1od.net/G5RV/index.htm . Although the article is about the
G5RV, Fig 10 is just a 100' dipole, centre fed with classic tuned feeder
and ATU.

Owen

In article ,
Owen Duffy wrote:

Fred, I think the term "OCF Dipole" is usually used today to mean a
dipole fed with coax and balun (often 4:1, usually not 1:1) fed offset
from the centre and often operated at half wave resonance or harmonic
multiples.

...
Using the modeling software, is there a feed-point where impedance is
close to an available balanced feed-line on multiple bands? As close,
I
would accept a 2:1 SWR.

If you are going to use an ATU and open wire line (as distinct from
balanced line) why are you restricting the max VSWR to 2. Practical open
wire lines can operate at much higher VSWR with acceptable losses.

Once you have addressed that question, then ask yourself why you wouldn't
just feed such a dipole in the centre and reduce the common mode current
problem caused by the asymmetric feed.

Owen-

My friends with money used a 4-to-1 BalUn coil with their Windoms and
drove them with rigs such as the DX-100 and Viking II. I think their
Pi-network output stages matched a wider range of impedances than the
modern solid state rigs can match, but I didn't know about that at the
time. I thought the 300 Ohm TV feed-line was a close match to the
antenna, and the BalUn transformed it to a nearly perfect 75 Ohms.

My current interest is two-fold. First, I was curious to know just how
good the match might have been on the old antenna. Second, it would be
handy to have a multi-band antenna that could be fed off-center so the
feed-line didn't have to run parallel to the antenna wire before
entering the shack.

I can afford a BalUn now. A little vertical radiation from the
feed-line would be OK unless there was a problem with RF burns!

Fred
K4DII

Owen Duffy wrote:
Fred McKenzie wrote in news:fmmck-9C2AC4.14313718032007
@nntp.aioe.org:

In article ,
Richard Clark wrote:

...
Someone asked that if the OCF Dipole was so good, why didn't everyone
use one? When I got my start back in the 50s, everyone did use one. I
used my "Full Windom" for several years on 80/75/40/10 CW and AM. In

Fred, I think the term "OCF Dipole" is usually used today to mean a
dipole fed with coax and balun (often 4:1, usually not 1:1) fed offset
from the centre and often operated at half wave resonance or harmonic
multiples.

That would be the type that I'm talking about. I've used them and like
them. For some reason I see them as a nice compromise antenna, and
Richard doesn't. Or maybe I'm talking about this type, and Richard is
speaking of the other, "classic" Windom. But then again, I'm way out of
me league here!


...
Using the modeling software, is there a feed-point where impedance is
close to an available balanced feed-line on multiple bands? As close,
I would accept a 2:1 SWR.

If you are going to use an ATU and open wire line (as distinct from
balanced line) why are you restricting the max VSWR to 2. Practical open
wire lines can operate at much higher VSWR with acceptable losses.

Once you have addressed that question, then ask yourself why you wouldn't
just feed such a dipole in the centre and reduce the common mode current
problem caused by the asymmetric feed.

A dipole of more than about 35% wavelength at its lowest operating
frequency, centre fed with practical open wire line and a good ATU will
allow multiband operation with efficiency should be acceptable as part of
the multiband compromise. For an example, look at Fig 10 in the article
http://www.vk1od.net/G5RV/index.htm . Although the article is about the
G5RV, Fig 10 is just a 100' dipole, centre fed with classic tuned feeder
and ATU.

That is pretty much exactly what I am using now, and it works a charm.
It's a fine antenna, if a little tight to match on 75/80 meters.

- 73 de Mike KB3EIA -