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Title: Dipole Radiation Pattern Question Post by: W1QWT on September 22, 2007, 10:55:40 PM Hi,
My center fed horizontal dipole cut for the 75 meter band came down cause one of the support limbs broke. I put it back up today but had to use another limb on the tree for that support. It caused the dipole to have a 10 to 15 degree bend in the horizontal plane at the feed point where the two wires meet. So instead of 180 degrees between the two wires it is 170 to 165 degrees. I don't have eznec so I am wondering if the radiation pattern is altered now? If so how? Is it no longer a donut because of such a small variation from an ideal dipole? The antenna is 60 feet above the ground. It is fed with 450 ohm balance line. Regards Q, W1QWT Title: Re: Dipole Radiation Pattern Question Post by: Steve - WB3HUZ on September 22, 2007, 11:26:45 PM At 60 ft above the ground, it never was a donut shaped pattern. Don't worry about it.
Hi, My center fed horizontal dipole cut for the 75 meter band came down cause one of the support limbs broke. I put it back up today but had to use another limb on the tree for that support. It caused the dipole to have a 10 to 15 degree bend in the horizontal plane at the feed point where the two wires meet. So instead of 180 degrees between the two wires it is 170 to 165 degrees. I don't have eznec so I am wondering if the radiation pattern is altered now? If so how? Is it no longer a donut because of such a small variation from an ideal dipole? The antenna is 60 feet above the ground. It is fed with 450 ohm balance line. Regards Q, W1QWT Title: Re: Dipole Radiation Pattern Question Post by: W1QWT on September 22, 2007, 11:55:11 PM Quote At 60 ft above the ground, it never was a donut shaped pattern. Don't worry about it. How about on 40 or 20? Isn't that theoretically high enough to produce a so called 'donut' pattern with lobes? I use this antenna with a balanced tuner from 160 to 10 meters. In any case I think the slight aberration from ideal is of no concern. I think your right. I am not to worry. If I can hear em and work em then that's what matters. I am not an antenna expert so I had to ask cause I'm curious. Regards Q W1QWT Title: Re: Dipole Radiation Pattern Question Post by: KA1ZGC on September 23, 2007, 10:51:37 AM Q, AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
Timmy's Wire Array is exactly the same way. The north legs of the dipoles are horizontal, the south legs slope downwards at a 10-15 deg. angle. The droop doesn't seem to have a negative effect on performance or pattern. It performs pretty much exactly the way you would expect if it were two perfect flat-tops. In your case, I would guess that your main lobes would be pulled ever-so-slightly towards the flat leg, and you might gain a very slight amount of high-angle energy, but that's just my gut feel. I expect both of these changes would be very slight. At 60 feet, it's a pretty high-angle radiator on 75 to start with, and not heavily directional, so any difference would probably be negligible. The heaviest skew would probably be on 20, but even there it probably won't be enough to be noticed. The most signifigant electrical difference would be reduced capacitance to ground on the drooping leg. If that were really pronounced, you'd see a difference in the tuning of the antenna caused by the imbalance. If it tunes up pretty much the way it always did, chances are the pattern won't have changed much, either. Try it out on all bands. If the tuning is pretty much what you remember it being before the end came down, then you probably won't see a noticable change in pattern. In the worst-case scenario, the antenna still works! --Thom Kilimanjaro Africa One Zulu Goat Cheese |