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#1
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NVIS Dipoles Directional?
My experience seems to be that the performance of NVIS dipoles (at or under around 30 feet high for 80 meters) doesn't noticeably change with orientation. In other words ... and despite what modeling programs say .... BVIS dipoles are pretty much omnidirectional. I haven't scientifically tested that but it does seem to me that since most NVIS radiation goes approximately straight up (definition of NVIS), it really shouldn't matter much how the antenna is oriented. Have you all found that to be true, or false? If I do want near-omnidirectional performance, in the real world (versus in a software program's modeling), am I better off with an inverted vee that's up 30 feet in the center, vs a dipole that's 30 feet up along its full length? How about an inverted vee that's up 50 feet in the center and 10 feet at the ends, versus a dipole that's 30 feet up along its full length? |
#2
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NVIS Dipoles Directional?
Rick wrote:
I haven't scientifically tested that but it does seem to me that since most NVIS radiation goes approximately straight up (definition of NVIS), it really shouldn't matter much how the antenna is oriented. Have you all found that to be true, or false? With a 130 ft dipole at 30 feet, the azimuthal radiation pattern at 60 degrees is 3 dB down off the ends of the dipole. I have never seen the minimum NVIS angle defined anywhere. -- 73, Cecil http://www.w5dxp.com |
#3
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NVIS Dipoles Directional?
Rick wrote in newsan.2007.04.09.16.49.43.321645
@reply.in.gp: My experience seems to be that the performance of NVIS dipoles (at or under around 30 feet high for 80 meters) doesn't noticeably change with orientation. In other words ... and despite what modeling programs say ... BVIS dipoles are pretty much omnidirectional. I don't know that modelling programs are in conflict with the the observation that dipoles low over real ground are nearly omni directional at high elevations. I haven't scientifically tested that but it does seem to me that since most NVIS radiation goes approximately straight up (definition of NVIS), it really shouldn't matter much how the antenna is oriented. I wrote an article on the design of an antenna for local contacts on 40m, it is at http://www.vk1od.net/7MDipole/7MDipole.htm . It may be of interest. Have you all found that to be true, or false? If I do want near-omnidirectional performance, in the real world (versus in a software program's modeling), am I better off with an inverted vee that's up 30 feet in the center, vs a dipole that's 30 feet up along its full length? I think you are on the wrong tram about the modelled performance. How about an inverted vee that's up 50 feet in the center and 10 feet at the ends, versus a dipole that's 30 feet up along its full length? Mounting a half wave dipole low over real ground modifies the pattern from the free space pattern, the the whole pattern is deformed by the presence of the ground and nulls previously off the end are not nearly as pronounced. When you rig the dipole as an inverted V, the pattern is further deformed and the nulls are even less pronounced. Ask yourself the question "is a dipole horizontal over ground (at any height) omni-directional at the zenith?". If you found a modelling program that, used properly, says otherwise, tell us about it. NVIS isn't strictly about the zenith, but an antenna that is omni at the zenit, and doesn't have deep nulls (dipole low over real ground), is close to omni at high elevations. The exact variation will depend on frequency, mounting height, ground parameters, and leg angles if an inverted V. If you could rotate such a dipole, I would defy you to reliably measure the out-of-omni above 50 deg elevation with an S meter. Owen |
#4
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NVIS Dipoles Directional?
Rick wrote in newsan.2007.04.09.16.49.43.321645
@reply.in.gp: My experience seems to be that the performance of NVIS dipoles (at or under around 30 feet high for 80 meters) doesn't noticeably change with orientation. In other words ... and despite what modeling programs say ... BVIS dipoles are pretty much omnidirectional. NVIS propagation is pretty high angle stuff. If you look at the three dimensional patterns for NVIS antennas you will see that they have a large lobe at high angles and an almost circular omnidirectional pattern at those angles. We're looking at 80 degrees and up mostly here, maybe 70 at the low end....so that antennas are mainly designed to illuminate the patch of ionosphere directly above the antenna. -- Dave Oldridge+ ICQ 1800667 |
#5
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NVIS Dipoles Directional?
On Mon, 09 Apr 2007 23:36:56 +0000, Dave Oldridge wrote:
NVIS propagation is pretty high angle stuff. If you look at the three dimensional patterns for NVIS antennas you will see that they have a large lobe at high angles and an almost circular omnidirectional pattern at those angles. We're looking at 80 degrees and up mostly here, maybe 70 at the low end....so that antennas are mainly designed to illuminate the patch of ionosphere directly above the antenna. Right. That's my point. So, what I'm claiming ... and trying to get someone who knows more about this stuff than I do (which is just about all of you) to confirm or deny ... is that with an NVIS dipole, someone 100 miles away from me would not be able to perceive the difference if my antenna was broadside to him or oriented in line with him. True, or false? |
#6
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NVIS Dipoles Directional?
Rick wrote:
... is that with an NVIS dipole, someone 100 miles away from me would not be able to perceive the difference if my antenna was broadside to him or oriented in line with him. True, or false? The broadside radiation is mostly horizontally polarized while the radiation off the ends is mostly vertically polarized. I wonder if that would make a measurable difference? -- 73, Cecil http://www.w5dxp.com |
#7
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NVIS Dipoles Directional?
"Cecil Moore" wrote in message t... Rick wrote: ... is that with an NVIS dipole, someone 100 miles away from me would not be able to perceive the difference if my antenna was broadside to him or oriented in line with him. True, or false? The broadside radiation is mostly horizontally polarized while the radiation off the ends is mostly vertically polarized. I wonder if that would make a measurable difference? -- 73, Cecil http://www.w5dxp.com I think polarization will play in the picture. Dealing with NVIS, we do not get much of the polarization being rolled around as with signals coming from refractions/reflections via ionosphere. The signals at the receiving end would be coming from "above", but I would suspect that with distinct polarization component that should be detectable with receiving antenna if rotated. So I would vote that there would be difference in NVIS signals most likely characterized by sharp minimum at the opposite polarization receiving antenna orientation. Any RF signals have a distinct polarization at any point and time. Add some constructive or destructive interference and one would get the picture. 73 Yuri, K3BU.us |
#8
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NVIS Dipoles Directional?
Rick wrote:
On Mon, 09 Apr 2007 23:36:56 +0000, Dave Oldridge wrote: NVIS propagation is pretty high angle stuff. If you look at the three dimensional patterns for NVIS antennas you will see that they have a large lobe at high angles and an almost circular omnidirectional pattern at those angles. We're looking at 80 degrees and up mostly here, maybe 70 at the low end....so that antennas are mainly designed to illuminate the patch of ionosphere directly above the antenna. Right. That's my point. So, what I'm claiming ... and trying to get someone who knows more about this stuff than I do (which is just about all of you) to confirm or deny ... is that with an NVIS dipole, someone 100 miles away from me would not be able to perceive the difference if my antenna was broadside to him or oriented in line with him. True, or false? Most likely TRUE. Other conditions may affect the signal but for the most part the signal goes almost straight up in all directions and almost straight down in all directions. Your coverage can be up to 300 to 500 miles depending on conditions. The MUF for NVIS is dependent on the position of the sun and drops drastically at and after sun set. Dave WD9BDZ |
#9
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NVIS Dipoles Directional?
Rick wrote in
news Right. That's my point. So, what I'm claiming ... and trying to get someone who knows more about this stuff than I do (which is just about all of you) to confirm or deny ... is that with an NVIS dipole, someone 100 miles away from me would not be able to perceive the difference if my antenna was broadside to him or oriented in line with him. True, or false? I thought that was the meaning of this para that I wrote for you befo NVIS isn't strictly about the zenith, but an antenna that is omni at the zenith, and doesn't have deep nulls (dipole low over real ground), is close to omni at high elevations. The exact variation will depend on frequency, mounting height, ground parameters, and leg angles if an inverted V. If you could rotate such a dipole, I would defy you to reliably measure the out-of-omni above 50 deg elevation with an S meter. |
#10
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NVIS Dipoles Directional?
Dear Rick (no call sign):
A 0.5 WL dipole almost on the ground will be inefficient. What the dipole does radiate will, at high take-off-angles, be close to circular (independent of azimuth) well past 100 miles. If both efficiency and circularity are important, an excellent solution is what the CCIR calls a TR2/2/.2. That is two, parallel one-wavelength dipoles both in a horizontal plane 0.2 WL above the ground that are 0.5 WL apart. With the dipoles fed in the middle with equal lengths of open line that meet at a point midway between the dipoles, one can achieve a reasonable input impedance. In any case, I suggest you avail yourself of the benefit of raising whatever you use to a height of something like 0.2 WL and less than 0.3 WL. 73, Mac N8TT -- J. Mc Laughlin; Michigan U.S.A. Home: |
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