Buck wrote:
On Thu, 08 Mar 2007 08:03:56 -0800, Richard Clark
wrote:

On Thu, 08 Mar 2007 05:24:52 -0500, Buck
wrote:

If an OCF is shorter than 1/2 wave for the frequency of operation, it
will not perform as well as the 1/2 wave dipole.
Hi Buck,

Modeling would suggest otherwise.

The difference between a 1/4 wave center fed and off-center fed (at
10% from the end) is about a 12% increase in real resistance, and
.25dB gain improvement, both favoring the off-center fed.

Are you comparing short antennas to long antennas? If so, it wouldn't
be a surprise, would it?

Coming in late, did I miss someone's extravagant claim that a OCF
could do better?

73's
Richard Clark, KB7QHC


Sorry, I haven't been able to find replies until now- musta been a news
server burp or something..


The original comment was that a multiband OCF antenna is a
'compromise' antenna and wouldn't be as good as dedicated 1/2 wave
dipole cut to frequency (or so I understood it.)

If a 135 foot OCF were compared to a 1/2 wave 160 meter dipole, the
OCF would lose, but if it were compared to a 20 meter dipole, it would
have gain in the direction of various lobes.

And I'm not sure I would define those lobes as something other than a
compromise. If the lobe is in a good place for you, fine. If not, not so
fine.


Yes, the comparison is between different length antennas, and you are
right, generally speaking, the longer, the better. (no doubt someone
can find an exception to the rule, but that isn't the point of this
discussion.)

I'm a little dense here. 8^) Is a antenna cut for a half wavelength at
80 meters a better antenna at 10 meters than an antenna specifically cut
for 10 meters?

I'd also have to go back and look, but isn't the SWR on some bands on
the Capacitive reactance end, even though it may be 50 ohms?

I know my old Icom did not like capacitive reactance very much.

The idea that an OCF is superior to a dipole, and certainly Richard's
statements would indicate that; makes me wonder why everyone isn't using
them!

- 73 de Mike KB3EIA -

On Thu, 15 Mar 2007 15:57:50 -0400, Michael Coslo
wrote:

On Thu, 08 Mar 2007 08:03:56 -0800, Richard Clark
wrote:
Coming in late, did I miss someone's extravagant claim that a OCF
could do better?

The idea that an OCF is superior to a dipole, and certainly Richard's
statements would indicate that; makes me wonder why everyone isn't using
them!

Hi Mike,

With all the intervening comments removed, want to reconsider that
again?

An OCF is simply resonant at exactly (by modeling) the same points as
the dipole of the same length. Resonance resides in the wire, not the
drive point. Of Course, F***! (expletive deleted to explain the
meaning of OCF) as the drivepoint is moved through the length, its
fundamental Z (still real) varies from low (at midpoint) to high (near
endpoint) in much the same manner as we would expect for the
difference between a conventional halfwave dipole and conventional
halfwave end-fed. The harmonic drivepoint Zs follow their own
sinusoidal roller coaster through the shift in feed point.

73's
Richard Clark, KB7QHC

On Mar 15, 10:40 pm, Richard Clark wrote:
On Thu, 15 Mar 2007 15:57:50 -0400, Michael Coslo
wrote:

On Thu, 08 Mar 2007 08:03:56 -0800, Richard Clark
wrote:
Coming in late, did I miss someone's extravagant claim that a OCF
could do better?
The idea that an OCF is superior to a dipole, and certainly Richard's
statements would indicate that; makes me wonder why everyone isn't using
them!

Hi Mike,

With all the intervening comments removed, want to reconsider that
again?

An OCF is simply resonant at exactly (by modeling) the same points as
the dipole of the same length. Resonance resides in the wire, not the
drive point. Of Course, F***! (expletive deleted to explain the
meaning of OCF) as the drivepoint is moved through the length, its
fundamental Z (still real) varies from low (at midpoint) to high (near
endpoint) in much the same manner as we would expect for the
difference between a conventional halfwave dipole and conventional
halfwave end-fed. The harmonic drivepoint Zs follow their own
sinusoidal roller coaster through the shift in feed point.

73's
Richard Clark, KB7QHC

Fine Guys except for one point WHY does a caroliner Windom have 2
Balums? I suggest for one reason to heat the garden I have known
several catch fire. also the performance is not that great.
mike M0DMD

wrote:
Fine Guys except for one point WHY does a caroliner Windom have 2
Balums?

One is to step down the feedpoint impedance and the
other is to choke common-mode current.
--
73, Cecil http://www.w5dxp.com

Richard Clark wrote:
On Thu, 15 Mar 2007 15:57:50 -0400, Michael Coslo
wrote:

On Thu, 08 Mar 2007 08:03:56 -0800, Richard Clark
wrote:
Coming in late, did I miss someone's extravagant claim that a OCF
could do better?

The idea that an OCF is superior to a dipole, and certainly Richard's
statements would indicate that; makes me wonder why everyone isn't using
them!

Hi Mike,

With all the intervening comments removed, want to reconsider that
again?

An OCF is simply resonant at exactly (by modeling) the same points as
the dipole of the same length. Resonance resides in the wire, not the
drive point. Of Course, F***! (expletive deleted to explain the
meaning of OCF) as the drivepoint is moved through the length, its
fundamental Z (still real) varies from low (at midpoint) to high (near
endpoint) in much the same manner as we would expect for the
difference between a conventional halfwave dipole and conventional
halfwave end-fed. The harmonic drivepoint Zs follow their own
sinusoidal roller coaster through the shift in feed point.


Sorry for the delay in response Richard, but as a confessed not-so-wise
guy, I've gone back to modeling to see exactly where I've erred and to
discover the source of my density.

And darned if I can't figure it out!

I've modeled Both OCF and frequency cut dipoles, and darned if the
frequency cut dipoles don't look better.

Your argument makes it sound as if the OCF has identical performance at
those same points as a frequency cut dipole. Resonance or not, there are
bands for which I still need a tuner, which makes the whole purpose a
little moot. The SWR curve of the OCF really doesn't look all that hot,
sometimes it is just passable at the frequencies of interest,and looks
better off frequency. and it looks like something a radio with a *good*
autotuner could take care of. That has been my experience with them.
Sure seems like a compromise to me. YMMV.

- 73 de Mike KB3EIA -

Mike Coslo wrote in
:

....
Sorry for the delay in response Richard, but as a confessed
not-so-wise
guy, I've gone back to modeling to see exactly where I've erred and to
discover the source of my density.

And darned if I can't figure it out!

I've modeled Both OCF and frequency cut dipoles, and darned if
the
frequency cut dipoles don't look better.

Your argument makes it sound as if the OCF has identical
performance at
those same points as a frequency cut dipole. Resonance or not, there
are bands for which I still need a tuner, which makes the whole
purpose a little moot. The SWR curve of the OCF really doesn't look
all that hot, sometimes it is just passable at the frequencies of
interest,and looks better off frequency. and it looks like something a
radio with a *good* autotuner could take care of. That has been my
experience with them. Sure seems like a compromise to me. YMMV.

Mike,

You seem to be considering just the flat-top of the OCF and that is not
the only conductor of an OCF dipole antenna system carrying current, the
other is the feedline.

If you offset the source in a halfwave dipole (zero length feedline), I
expect you will just see an increase in feed point R, and no significant
change in loss. So on that basis you could argue they are equivalent...
but you haven't compared an OCF dipole antenna system with a centre fed
dipole antenna system.

Then you talk about the SWR curve and ATU.

Aren't you trying to compare the entire system? Is there much point in
comparing the flat-top of an OCF with a centre fed, it is only part of
the picture.

Of course, the system performance will depend on assumptions that you
make about the ground, feedline route, length, type, ATU etc... but
having chosen a scenario, you can get to an overall performance figure
that properly deals with the complex interaction between components.

Owen

On Sat, 17 Mar 2007 21:17:00 -0500, Mike Coslo
wrote:

Your argument makes it sound as if the OCF has identical performance at
those same points as a frequency cut dipole.

Hi Mike,

Well, there are two "performances" to consider (and not just the
matinee and the evening show).

Resonance or not, there are
bands for which I still need a tuner, which makes the whole purpose a
little moot.

For that, the tuner will be called to perform different chores for
different bands for different offsets. However, the resonances will
fall principally at the same frequencies.

The SWR curve of the OCF really doesn't look all that hot,
sometimes it is just passable at the frequencies of interest,and looks
better off frequency.

Much the same could be said for any garden variety dipole.

and it looks like something a radio with a *good*
autotuner could take care of.


Much the same could be said for any garden variety dipole.

That has been my experience with them.
Sure seems like a compromise to me. YMMV.

Much the same could be said for any garden variety dipole.

The two performances would be tune-up and launch characteristics. If
modeling is any indication, the offset affects the magnitude of the R
at resonance (again, no different an experiance comparing a normally
fed half-wave dipole to an end-fed half-wave dipole). However, the
gain, number of nulls (or lobes) does vary at the higher frequencies
when offset is added to the variables. Higher gains for the near
end-fed (albeit 1dB).

73's
Richard Clark, KB7QHC

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