WINDOM OCF DIPOLE

Discussion in 'Survey Center' started by W5RJW, Feb 1, 2010.

?

best dipole antenna:

Poll closed Feb 2, 2010.
  1. Flat Top

    85.7%
  2. Inverted V

    0 vote(s)
    0.0%
  3. Invertel L

    14.3%
Multiple votes are allowed.
Thread Status:
Not open for further replies.
ad: L-HROutlet
ad: l-rl
ad: abrind-2
ad: Left-2
ad: Left-3
ad: l-BCInc
ad: Subscribe
ad: L-MFJ
  1. W5RJW

    W5RJW Ham Member QRZ Page

    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
     
  2. W5DXP

    W5DXP Ham Member QRZ Page

    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. :)
     
  3. G3TXQ

    G3TXQ Ham Member QRZ Page

    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
     
  4. W0HC

    W0HC XML Subscriber QRZ Page

    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/
     
  5. G3TXQ

    G3TXQ Ham Member QRZ Page

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

    Steve G3TXQ
     
  6. AE5JU

    AE5JU Ham Member QRZ Page

    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
     
  7. ZL3GSL

    ZL3GSL Ham Member QRZ Page

    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.
     
  8. W5DXP

    W5DXP Ham Member QRZ Page

    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).

    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.
     
  9. K4SAV

    K4SAV Ham Member QRZ Page

    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
     
  10. K7NNO

    K7NNO Moderator QRZ Page

    It would be if it was a half wave.
     
Thread Status:
Not open for further replies.

Share This Page