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 -

"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.