On Mar 5, 9:18 am, John Passaneau wrote:

We have used OFC dipoles here for a few years at field day. They fill a
very specific set of needs for us. 1) They are simple to put up
2) The feed line coming from one end of the dipole is shorter than from
center feed dipoles in our setup. 3) OFC dipoles offer an impedance at
the end of the coax that is within the range of the tuners built into
our radios on the bands that are important to field day. This simplifies
our setup and operation. 4) They work as well as an antenna of that
physical length on any one frequency would no mater how it's feed.

The radiation pattern from an OFC is set by the length of the wire not
where RF is feed in/out of the antenna. In our setup open wire line and
tuners would be a pain in the butt, and an operational inconvenience
that gains us nothing. Fan dipoles or separate dipoles are hard to setup
and or tune and would perform no better for us. The antennas we use were
built by myself and use a 4:1 current balun which minimize feed line
radiation. On 80/40/20m we can easily match the antenna with the built
in tuners so the SWR must be under 3:1. OFC dipoles don't work well on
15m but with the current sun spot cycle not a problem. We see no
indication of common mode current problems, so we don't worry about it,
we just operate and have fun.

John W3JXP



Sounds fine... Just as long as I don't have to use it.. Myself, I
prefer either
separate, or fan dipoles on the low bands.. 20-10, a tribander.. "A4S"
I never use a tuner. All coax fed too... To each his own I say...
MK

wrote in
ps.com:

On Mar 5, 9:18 am, John Passaneau wrote:

We have used OFC dipoles here for a few years at field day. They fill
a very specific set of needs for us. 1) They are simple to put up
2) The feed line coming from one end of the dipole is shorter than
from center feed dipoles in our setup. 3) OFC dipoles offer an
impedance at the end of the coax that is within the range of the
tuners built into our radios on the bands that are important to field
day. This simplifies our setup and operation. 4) They work as well as
an antenna of that physical length on any one frequency would no
mater how it's feed.

The radiation pattern from an OFC is set by the length of the wire
not where RF is feed in/out of the antenna. In our setup open wire
line and tuners would be a pain in the butt, and an operational
inconvenience that gains us nothing. Fan dipoles or separate dipoles
are hard to setup and or tune and would perform no better for us. The
antennas we use were built by myself and use a 4:1 current balun
which minimize feed line radiation. On 80/40/20m we can easily match
the antenna with the built in tuners so the SWR must be under 3:1.
OFC dipoles don't work well on 15m but with the current sun spot
cycle not a problem. We see no indication of common mode current
problems, so we don't worry about it, we just operate and have fun.

John W3JXP



Sounds fine... Just as long as I don't have to use it.. Myself, I
prefer either
separate, or fan dipoles on the low bands.. 20-10, a tribander.. "A4S"
I never use a tuner. All coax fed too... To each his own I say...
MK

I have worked with one of John's OCF dipoles on the FD night shift,
and hold and run frequencies on 75 meters all night long - with 100
watts. Can't say the exact performance of the antenna, but if I'm
holding a frequency along with the big guns on a busy day, it isn't
too bad at all.

OCF dipoles are obviously a compromise. As a multiband antenna it
better not outperform a specific band dipole.

If you have found such a *drastic* difference however, perhaps
there was something wrong with your particular antenna or setup?

- 73 de Mike KB3EIA -

On Wed, 07 Mar 2007 19:16:57 -0600, Mike Coslo
wrote:

OCF dipoles are obviously a compromise.

Hi Mike,

What happens to be the compromise?

As a multiband antenna it
better not outperform a specific band dipole.

Seems unlikely to perform any different than any equal length of wire
(the length of the fundamental band, that is). Only the feed Z
changes is all (or so modeling would lead me to believe).

73's
Richard Clark, KB7QHC

On Wed, 07 Mar 2007 19:16:57 -0600, in rec.radio.amateur.antenna you
wrote:

OCF dipoles are obviously a compromise. As a multiband antenna it
better not outperform a specific band dipole.

That's a general statement, but for a correctly fed OCF, your
statement is generally false.

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. However, as the
length of the OCF increases over the 1/2 wave, it generates lobes of
gain in various directions. How many lobes and to where they point
changes according to the relative length to the band, but there is
actually gain.

You can find more information in the ARRL Antenna book or by modeling
the antenna on one of the antenna model ling programs.


--
73 for now
Buck, N4PGW
www.lumpuckeroo.com
N4PGW

On Mar 7, 7:16 pm, Mike Coslo wrote:


If you have found such a *drastic* difference however, perhaps
there was something wrong with your particular antenna or setup?

- 73 de Mike KB3EIA -

Wasn't mine..It was one they were using at a field day. As far as I
could
tell, it was a regular ole carolina windom, fed with their feedline. I
can't
remember if he had a tuner inline.
It was a dud though compared to a standard coax fed dipole.
A good 2 s units down on *everything*. "40m" Noise, desired signals,
the
whole ball of wax. Obvious feeder loss... Sure, you can make contacts
with such a device, but it's not for me.. Two S units difference is
about the equal of adding an average amplifier to a 100 watt radio.
I'm used to coax fed dipoles where the appx system efficiency is in
the
mid/upper 90's % range.. So almost any other compromise antenna is
going
to be inferior as far as total system loss. The main problem with the
carolina
windom I tried was the goofy feeder system with coax, choke, twin
lead,
etc... What a cluster%$#@ of engineering that is... :
If I'm going to use a compromise one wire/all band antenna, it's going
to be
fed with ladder line the whole way to a tuner which will be carefully
tuned using the least inductance possible. It will also be center fed.
Even that will be inferior to my usual coax fed... I've compared..
But usually my preferred multi band antenna will be paralled dipoles,
with the
legs spread apart as far as possible.. Fed with a single coax feed.
Thats what I use here at home. No loss in system efficiency compared
to many other multi band designs.
MK

"Owen Duffy" wrote in message
...

According to my ARRL Antenna Handbook, the Windom Antenna was described
by Loren Windom in QST in 1929.

The design is a horizontal half wave (ie single band) fed by a vertical
single wire feedline attached just off centre (~14%). Explanations go
that this approximately matches the feedline Zo (which is quite high)
with the horizontal wire. It is single wire (ie ultimately unbalanced)
feedline and therefore radiates. The Antenna is fed between the source
end of the feedline and ground, and the load impedance should be
somewhere in the many hundreds of ohms. The feedline carries an
appreciable net current.

More recently, the Off Centre Fed (OCF) Dipole design emerged,
principally as a multi-band antenna. The OCF Dipole is a horizontal wire
with a coaxial feed and coupling transformer (often called a balun)
attached offset from the centre of the dipole. The feedpoint excursions
at a half wave length and harmonic frequencies are much lower than centre
feeding, and may be operated as a multiband antenna with reasonable
efficiency, though it probably really needs an ATU at the tx end of the
coax. The OCF dipole feedline does have current flowing on the outer of
the outer conductor, at least as a result of the assymetric coupling to
the dipole legs, and to some extent because the ineffectiveness of
practical coupling transformers to isolate the feedline ends from the
differing voltages on each dipole leg. The feedline carries an
appreciable net current.

Then along came the Carolina Windom, which appears to be a OCF dipole
with a proprietary (ie secret, undescribed) coupling transformer, a
vertical coax section (feedline and radiator) and a proprietary (remember
the meaning) "isolator" located at a given distance along the coax to
prevent the current flowing on the outer of the outer of the coax from
flowing further towards the transmitter. The isolater would appear to be
a ferrite choke and it would introduce a series impedance (reactance and
resistance) to current on the coax, so influencing the establishment of
the standing wave pattern on the outer of the outer of the coax. You
might naively think that this isolator prevents current flowing into the
shack, but that is unlikely.

In all these cases, there is an expectation that the feedline carries a
net radiating current, and it seems to me, that if you don't want to
bring that into the shack, you need to design an appropriate solution.

In the case of the true Windom, it seems the easiest solution is to end
the single wire feedline outside the shack and place a matching unit
connecting to ground and the single wire feedline at that point, and
transforming the load to something suitable to coax or balanced feedline
to the shack.

In the case of the OCF Dipole and the Carolina Windom, shunting the
current on the outer of the outer to ground outside the shack is a
potential solution. Series chokes might help, but the magnitude of the
choking impedance is limited, and their effectiveness could be improved
greatly by a low impedance shunt to ground.

Comments?

Owen

PS: In todays paranoid world where rules in many jurisdictions restrict
the maximum permitted exposure to electromagnetic radiation, antennas
such as these with radiating elements that are close to areas accessible
by people are a safety challenge.

Back in 1974 I used to visit an amateur radio repair shop in Jacksonville
FL, it was located on Pearl, ave or st, cant remeber which near 8th street.
I remember that quite a few hams would come into the place and often
conversations about different aspects of ham radio would get started. One
day they were discussing the Windom antenna with both praise and damnation.
At the end(3hrs or so) at least I had deciced the the antennas greatest
claim to fame was that it would load up on all the HF bands with at least
mediocre performance, Probably not a bad antenna if you could only have one
antenna. I do remember that someone working out current distribution in the
antenna and claimed that on some bands the current was distrubted primarily
in the vertical and the short section of the horizontal and in othe current
was in the vertical and the long section of the horizontal section for other
freqencies. I look back on this with a certain amount of aw(assuming he was
reasoably correct in his computations) that this was performed with pencil
and paper bag and not with aid of a computer.

Jimmie

"Owen Duffy" wrote in message
...

According to my ARRL Antenna Handbook, the Windom Antenna was described
by Loren Windom in QST in 1929.

The design is a horizontal half wave (ie single band) fed by a vertical
single wire feedline attached just off centre (~14%). Explanations go
that this approximately matches the feedline Zo (which is quite high)
with the horizontal wire. It is single wire (ie ultimately unbalanced)
feedline and therefore radiates. The Antenna is fed between the source
end of the feedline and ground, and the load impedance should be
somewhere in the many hundreds of ohms. The feedline carries an
appreciable net current.

More recently, the Off Centre Fed (OCF) Dipole design emerged,
principally as a multi-band antenna. The OCF Dipole is a horizontal wire
with a coaxial feed and coupling transformer (often called a balun)
attached offset from the centre of the dipole. The feedpoint excursions
at a half wave length and harmonic frequencies are much lower than centre
feeding, and may be operated as a multiband antenna with reasonable
efficiency, though it probably really needs an ATU at the tx end of the
coax. The OCF dipole feedline does have current flowing on the outer of
the outer conductor, at least as a result of the assymetric coupling to
the dipole legs, and to some extent because the ineffectiveness of
practical coupling transformers to isolate the feedline ends from the
differing voltages on each dipole leg. The feedline carries an
appreciable net current.

Then along came the Carolina Windom, which appears to be a OCF dipole
with a proprietary (ie secret, undescribed) coupling transformer, a
vertical coax section (feedline and radiator) and a proprietary (remember
the meaning) "isolator" located at a given distance along the coax to
prevent the current flowing on the outer of the outer of the coax from
flowing further towards the transmitter. The isolater would appear to be
a ferrite choke and it would introduce a series impedance (reactance and
resistance) to current on the coax, so influencing the establishment of
the standing wave pattern on the outer of the outer of the coax. You
might naively think that this isolator prevents current flowing into the
shack, but that is unlikely.

In all these cases, there is an expectation that the feedline carries a
net radiating current, and it seems to me, that if you don't want to
bring that into the shack, you need to design an appropriate solution.

In the case of the true Windom, it seems the easiest solution is to end
the single wire feedline outside the shack and place a matching unit
connecting to ground and the single wire feedline at that point, and
transforming the load to something suitable to coax or balanced feedline
to the shack.

In the case of the OCF Dipole and the Carolina Windom, shunting the
current on the outer of the outer to ground outside the shack is a
potential solution. Series chokes might help, but the magnitude of the
choking impedance is limited, and their effectiveness could be improved
greatly by a low impedance shunt to ground.

Comments?

Owen

PS: In todays paranoid world where rules in many jurisdictions restrict
the maximum permitted exposure to electromagnetic radiation, antennas
such as these with radiating elements that are close to areas accessible
by people are a safety challenge.