WINDOM OCF DIPOLE

[KB3TKQ]

I am researching the construction of a 6 banded windom off center fed dipole antenna. Obviously, the overall construction is very straight forward. The only issue I am having is what size balun to use. Some sites say 4:1, and others say 9:1. Is having a balun to compensate for lets say a 400 ohm impedance ok on 200 ohms?

KB3TKQ

[W5DXP]

The only issue I am having is what size balun to use.

Please know that you are going to need a good tuner.

The "theory" is that there exists a 300 ohm resistive feedpoint on harmonic bands so a 6:1 balun would be perfect. The "theory" is an old wives' tale. (The original Windom was not an OCF dipole as it had no insulator and was fed by a single vertical wire, Marconi style, against ground.)

I used an OCF dipole with 300 ohm twinlead and a 6:1 Heathkit air-core balun when I was at Texas A&M in 1957. It worked well with my DX-40 which had a pi-network on the output.

What I would do with a multi-band OCF dipole is feed it with ladder-line and have both a 1:1 balun and 4:1 balun available to use at the tuner. Then to overcome common-mode problems, I would change it to a center-fed dipole. :)

[G3TXQ]

As Cecil pointed out there are a lot of "myths" surrounding the OCF dipole; another one is that a wire which is half-wave resonant at 3.5MHz will also be resonant at 7.0MHz, 14MHz etc - it wont be!

So, across the various bands you'll see quite a spread of impedances at the feedpoint. In the modelling excersises I've tried, I've found a 4:1 ratio a reasonable compromise; 9:1 is too high.

73,
Steve G3TXQ

[W0HC]

I've had good luck with my OCF dipole using a 4:1 balun at the feedpoint.
Check my website to see the antenna sweep results I have with mine.
http://www.w0hc.com/ocf-dipole/

[G3TXQ]

The link from your web page to the sweep results is broken!

Steve G3TXQ

[AE5JU]

Since it is seldom possible to mount antennas as high as theortical, a Windom is going to be lower impedance than that optimum height. And that 300 ohm feedpoint is really more like 200 ohms. So a 4:1 balun is the better choice.

Here's some reading:

http://www.packetradio.com/windom.htm

[ZL3GSL]

I wish people wouldn't call off-centre fed dipoles, fed with coax or twin lead, "Windoms". :(

A (luckily!) well adjusted 80m OCF can be usable without a tuner on most bands (except 10m). An auto tuner can make it even nicer. The one I know uses legs of 13.5m and 27m long with a 4:1 current balun.

The feedpoint impedance varies in each band, of course. 4:1 seems to be a reasonable compromise to use with 50 ohm coax.

[W5DXP]

I wish people wouldn't call off-centre fed dipoles, fed with coax or twin lead, "Windoms".

Thank the "Carolina Windom" for spreading that semantic error. My 1957 ARRL Handbook says a Windom is a single horizontal wire fed with a single vertical wire against ground (no insulators).


A (luckily!) well adjusted 80m OCF can be usable without a tuner on most bands (except 10m). An auto tuner can make it even nicer. The one I know uses legs of 13.5m and 27m long with a 4:1 current balun.

My 1957 ARRL Handbook gives the same dimensions (44'4", 136') for an OCF. EZNEC gives the following feedpoint impedances with the 50 ohm SWR after being transformed by an ideal 4:1 balun for such an OCF at 40 feet.

3.8 MHz, (120+j240 ohms)/4, 4.4:1 50 ohm SWR
7.2 MHz, (148+j64 ohms)/4, 1.6:1 50 ohm SWR
10.125 MHz, (3163-j1660 ohms)/4, 20:1 50 ohm SWR
14.2 MHz, (136-j99 ohms)/4, 2:1 50 ohm SWR
18.14 MHz, (171+j100 ohms)/4, 1.7:1 50 ohm SWR
21.3 MHz, (3450-j221 ohms)/4, 17:1 50 ohm SWR
24.95 MHz, (157-j137 ohms)/4, 2.2:1 50 ohm SWR
28.4 MHz, (178-j228 ohms)/4, 3.2:1 50 ohm SWR

Looks reasonable except for 30m and 15m. It might need a tuner on 75m, 20m, 12m, and 10m.

[K4SAV]

Have you wondered why few people are responding to the poll?

Answer: The question did not include the intended purpose of the antenna, nor any details of the antennas. Most people realize there is no one correct answer.

Besides, an inverted L is not a dipole.

Jerry, K4SAV

[WA7OET]

Besides, an inverted L is not a dipole.

Jerry, K4SAV

It would be if it was a half wave.

[K4SAV]

It would be if it was a half wave.

Aha. You got me. Yep, a half wave inverted L fed in the center is a dipole.

[W5DXP]

Aha. You got me. Yep, a half wave inverted L fed in the center is a dipole.

The IEEE Dictionary says any antenna with the radiation pattern of a dipole is a dipole, by definition. :eek:

[K4SAV]

The IEEE Dictionary says any antenna with the radiation pattern of a dipole is a dipole, by definition. :eek:

A duck by any other name?

There seems to be several definitions of a dipole floating around. Here is the ARRL definition:

Dipole—An antenna that is split at the exact center for connection to a feed line, usually a half wavelength long. Also called a “doublet.”

I also saw another one (but I forget where) that said a dipole is a class of antennas consisting of antennas having two poles.

[KC8VWM]

A "dipole" doesn't have to be a "half wave" antenna to be considered a dipole antenna, but this is not to be confused with a half wave monopole antenna design. Feedpoint location for a dipole can be center, end fed, or off center fed but actual feedpoint location doesn't determine if it is a dipole antenna or not. A "common dipole" is considered a half wave antenna that is center fed, but it is not the only type of dipole.

[W5DXP]

A "common dipole" is considered a half wave antenna that is center fed, but it is not the only type of dipole.

I think I remember the origin of the word "dipole" as being an antenna with two voltage maximum points, i.e. two electric poles, on the standing wave antenna conductor. That results in the familiar figure-8 radiation pattern and seems to fit with the IEEE definition of a "dipole".

[WA7OET]

I think I remember the origin of the word "dipole" as being an antenna with two voltage maximum points, i.e. two electric poles, on the standing wave antenna conductor. That results in the familiar figure-8 radiation pattern and seems to fit with the IEEE definition of a "dipole".

Yes, a dipole is defined by current distribution (IEEE) not it's physical construction. An end fed half wave is a dipole just as a center fed half wave is.

[K8JD]

I am researching the construction of a 6 banded windom off center fed dipole antenna. Obviously, the overall construction is very straight forward. The only issue I am having is what size balun to use. Some sites say 4:1, and others say 9:1. Is having a balun to compensate for lets say a 400 ohm impedance ok on 200 ohms?

KB3TKQ

The balun transformation ratio depends on what freq you want to use. That's why I stick to single band , coax fed dipoles and loops that require no tuner.:rolleyes:

[N4UM]

Why is everyone antisymantic? By definition dipoles are center fed, balanced antennas. How can a dipole be "off center fed?"

[K1LLR]

there is no such thing as a off center fed dipole..the definition of a dipole is two elements of equal length..Off center fed (Wire)..there is no advantage unless your realestate is longer or shorter on one end..>???I dont get it..

[W5DXP]

By definition dipoles are center fed, balanced antennas.


...the definition of a dipole is two elements of equal length.

Nope, those are the amateur definition. Remember that the 'A' in ARRL stands for "Amateur". The IEEE Dictionary contains the professional level definition and says:

"dipole antenna - Any one of a class of antennas having a radiation pattern approximating that of an elementary electric dipole... having a current node at each end of the antenna." For a standing-wave antenna, a current (magnetic) node is the same thing as a voltage (electric) POLE.

The "definition" of "dipole antenna", based on physical characteristics, is essentially common usage colloquial lingo. You see, the words "dipole antenna" have nothing to do with the physical characteristics - it's not the same thing as a fishin' pole. A dipole is any single element antenna with two and only two electric poles, i.e. standing-wave voltage maximum points. So any conductor, center-fed, off-center-fed, end-fed, or gamma-fed, that has two electric poles, is a dipole, by definition. Note that the "dipole antenna" is professionally defined by its radiation pattern, not by its physical characteristics.

[KI4TWB]

The original Windom antenna was fed with a single wire and was worked against good ground. It appeared similar to the OCF dipole because they are both fed at the 1/3 point. http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=15689 The two are NOT the same. Windom =/= OCF Dipole.

The BALUN you'll need for the OCF will depend on the height you can hang it. Up to about 40 feet or so, a 4:1 will work fine. At 50-60 feet, you'll need a 6:1. In any case, you WILL need a 1:1 BALUN immediately below the other BALUN, or you'll have more RF in the shack than you'll be able to shake a buddistik at.:)

[W5LDA]

The original Windom antenna was fed with a single wire and was worked against good ground. It appeared similar to the OCF dipole because they are both fed at the 1/3 point. http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=15689 The two are NOT the same. Windom =/= OCF Dipole.

The BALUN you'll need for the OCF will depend on the height you can hang it. Up to about 40 feet or so, a 4:1 will work fine. At 50-60 feet, you'll need a 6:1. In any case, you WILL need a 1:1 BALUN immediately below the other BALUN, or you'll have more RF in the shack than you'll be able to shake a buddistik at.:)

I dont use a 1:1 under my 4:1 and I have no RF. I think it depends on how you erect it whether you need the 1:1 or not.

[W5DXP]

I dont use a 1:1 under my 4:1 and I have no RF.

What is the magnitude of the common-mode current at the input of your tuner when you are outputting 100 watts?

[W6ECE]

Isn't any piece of wire a dipole at some frequency?

[WB2WIK]

Isn't any piece of wire a dipole at some frequency?

Yep!

And if my grandmother had wheels, she'd be a bus.

[AC0FP]

If I had a OCF dipole, I would feel like Don Quixote but instead of fighting windmills I would be fighting to keep RFI out of my shack! :eek:

fp

[PA5COR]

No problems with R.F. from my Fritzel OCF AMA 83 FD-4 antenna, lengthened it with coil and 30 feet for 160 meters flat top at 45 feet.
Not even using 1 Kw in the antenna.
The balun has 1:6 and is followed by a 1:1 unun.

Except 15/30 all other bands are lower as 1:3, no tuner needed.

And yes, my space is limited 2/3 and 1/3 fits the space here even when i have 165 feet of wire in the air.

Though for 160 meters and 10/12/15/17 i also have vertical antenna's up.

[KB0OU]

Howdy,
I don't care what ya want to call it.
My antenna is fed at about the 1/3 length and is cut for 160 metres.
It loads up good and people hear me when I speak, especially when I hit it with about 1200 watts at the antenna.
I use a big 4:1 balun rated for 3KW. I do not use a 1:1 and have not had RF problems in the shack even running full legal limit power.

I call it my Off Center Fed Dipole or "that there wire antenna up there on the tower" depending on who I am talking to and how smart they are.
Sometimes I have to point at it because it is not the only antenna around.

73
John kb0ou

[KB3QJA]

I use a "dipole" (two elements) fed not at the center which was cut for half wave 80 meters. I like the multiband capability which off center feeding facilitates. The original Windom was fed with a single wire which was part of the radiator. I feed mine with coax into homebrew air core 1:1 and then 4:1 current balans. I have been well satisfied with the DX performance of this simple wire antenna.
73
mike-KB3QJA