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Off Center Fed Dipoles

Feb 4th, 19:28

AG6CX

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Various sources state the OCF Dipole is resonant at Fo, 2xFo, 4xFo, 8xFo. Others sources say it is resonant at even harmonics. Sources also say the CF Dipole is resonant at Fo, 3xFo, 7xFo.

We know that a CF dipole cut for 7.0MHz will resonate at 21.0MHz.

But for the OCF Dipole with Fo = 3.55MHz, the literature says that resonance will be at 3.55. 7.10, 14.2, and 28.4 MHz. If the OCF Dipole is supposed to be resonant at even harmonics. why don't these antennas work at 6xFo, which for Fo=3.55 MHz would be 21.3 MHz. Isn't a sixth harmonic an even harmonic?

What am I missing?
Feb 5th, 18:08

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0

Typically, the resonances are only approximate in relation to the harmonic frequencies--I just ran an EZNEC model that predicted that a 7.0MHz dipole will resonate at 21.6MHz.

A common OCFD design is to place the feedpoint 1/3 the way from one end. If the lowest band is 80 meters, this means that on 15 meters, the feedpoint will be 1 wavelength from one end and 2 wavelengths from the other end--which is actually resonant, but high impedance, and not useful unless you add an impedance transformer to get back to a low impedance.

Zack Lau W1VT
ARRL Senior Lab Engineer
Feb 5th, 23:16

AG6CX

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Thanks for your observations Zack.

I guess I have to read your Mar 2001 article QEX (Pg. 55)
Making Off-Center Fed Dipoles Work (RF). I remain perplexed that the article download is not available to members, but would appreciate a copy if you can spare one, or request TIS to scan and send to edwmccann@aol.com. Happy to pay whatever is needed.

In any event, my basic question remains unanswered and I can find no answer: If the OCF antenna is supposed to be resonant on even harmonics, why isn't such an OCF antenna cut for Fo = 3.55 MHz resonant on 6xFo = 21.3 MHz ? (We can worry about the impedance with a balun).

Odd harmonics are 3,5,7,9, etc. So the 40m dipole works on 3x7.0=21.6MHz, as has been known.

Even harmonics would be 2,4,6,8,10, 12, wouldn't they? So wouldn't one expect the OCF Dipole to be resonant at the 6th, 8th. 10th, and 12th harmonics? Or are the 6th, 10th, 14th harmonics not as"even" as we think, being twice the 3rd, 5th, and 7th harmonics?

I look forward to your article to shed some more light on the matter.

73
AG6CX


Feb 5th, 23:16

AG6CX

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Thanks for your observations Zack.

I guess I have to read your Mar 2001 article QEX (Pg. 55)
Making Off-Center Fed Dipoles Work (RF). I remain perplexed that the article download is not available to members, but would appreciate a copy if you can spare one, or request TIS to scan and send to edwmccann@aol.com. Happy to pay whatever is needed.

In any event, my basic question remains unanswered and I can find no answer: If the OCF antenna is supposed to be resonant on even harmonics, why isn't such an OCF antenna cut for Fo = 3.55 MHz resonant on 6xFo = 21.3 MHz ? (We can worry about the impedance with a balun).

Odd harmonics are 3,5,7,9, etc. So the 40m dipole works on 3x7.0=21.6MHz, as has been known.

Even harmonics would be 2,4,6,8,10, 12, wouldn't they? So wouldn't one expect the OCF Dipole to be resonant at the 6th, 8th. 10th, and 12th harmonics? Or are the 6th, 10th, 14th harmonics not as"even" as we think, being twice the 3rd, 5th, and 7th harmonics?

I look forward to your article to shed some more light on the matter.

73
AG6CX


Feb 6th, 12:03

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
According to EZNEC, a 136' long dipole made out of #12AWG wire at 40 ft height has resonances at 3.5MHz and 21.7MHz.

But, the impedance at "15M" is 2827 ohms!

No ham I know has been clever enough to design a 6:1 balun, 300 to 50 ohms, that will handle that sort of load at the 100 watt power level, that also works well from 80 through 10 meters. Most wideband baluns have a choking impedance less than that, which means that the balun effectively acts as a lossy resistor.

You have to be very careful about using a transmatch in this situation--if you see SWR drift, this is a warning sign that you are burning up your balun--if you persist, it won't work anymore as a 6:1 balun. You might be able to get away with it if your feedline and transmatch is lossy enough to get rid of enough power to reduce ferrite core heating to a safe level.

One solution to covering 15 meters with an OCFD is to start at 40 meters, allowing coverage on the 40, 20, 15 and 10 meter bands.

Richard Feynman, the Noble Prize winner who helped design the atomic bomb, said that you really need to do some experiments with real stuff in order to learn physics--I think this is true with antennas as well. I'd suggest getting an antenna analyzer and a roll of #14 THHN house wire to experiment. Build another OCFD and measure the harmonic resonances. You can measure high impedance resonances with a quarter wave impedance transformer--10.5 feet of 380 ohm open wire will bring 2900 ohms down to 50 ohms at 21.7 MHz.

Zack W1VT
ARRLSenior Lab Engineer

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