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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? |
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 |
NVIS Dipoles Directional?
Rick wrote in news:pan.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 |
NVIS Dipoles Directional?
Rick wrote in news:pan.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 |
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? |
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 |
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 |
NVIS Dipoles Directional?
Rick wrote in
: 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. |
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 |
NVIS Dipoles Directional?
"J. Mc Laughlin" writes:
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. Or, in the case of 80 m, between 16 and 24 m up (for the metric challenged: between 50 and 75 ft). Excellent for those who can, but most can't. 73 de LA4RT Jon |
NVIS Dipoles Directional?
On 10 abr, 06:34, 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? Hello Rick, At 100 miles distance, the TOA is almost vertical (maybe 75 degr), so with respect to signal strength, it is practically impossible to detect whether the antennas are broad side. Because of the distance, ground wave propagation loss is far higher with respect to NVIS propagation loss under these circumstances. One would mention polarization of the waves going up and down. I would not matter about this. These low frequencies are strongly affected by faraday rotation. On the way up and down, the polarization rotates several times and several wave fronts do exist. While you transmit (nearly vertical) with linear polarization, the down coming wave may have a strong circular component. With respect to the radiation pattern, you are right, the differences in pattern are minimal below 30 feet. However the overall efficiency is strongly depended on antenna height and soil properties. I did some simulation and practice. I made a short document of it (for JOTA porpuse), however the document is in Dutch Language (http:// www.tetech.nl/divers/NVISantenneNL1.pdf). Maybe you can get some useful info out of it. At low heights, much power is dissipated into the ground (resulting in a useful bandwidth). Best Regards, Wim PA3DJS |
NVIS Dipoles Directional?
Rick wrote in
: 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? Absolutely true. Any difference would be insignificant. The path elevation is about 79 degrees for that path. -- Dave Oldridge+ ICQ 1800667 |
NVIS Dipoles Directional?
LA4RT Jon ?Q?K=C3=A5re?= Hellan wrote in
: "J. Mc Laughlin" writes: 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. Or, in the case of 80 m, between 16 and 24 m up (for the metric challenged: between 50 and 75 ft). Excellent for those who can, but most can't. A horizontal quad loop at 50 feet is almost ideal for NVIS. -- Dave Oldridge+ ICQ 1800667 |
NVIS Dipoles Directional?
Dear Wim PA3DJS:
How I wish that I could read Dutch. However, in looking over all of your work, it continually occurred to me that much of the base of English comes from Northern Nederland. I looked at the first table on page 20 of your work. The information so intrigued me that I ran almost the same information through NEC4, which does well with antennas close to the ground. I assumed: a center frequency of 3.6 MHz, a horizontal half wave dipole slightly adjusted to be resonant at 3.6 MHz at each height, the use of Cu wire 2 mm in diameter, and earth with a sigma of 0.01 and a relative dielectric constant of 15. This is what I found without too obsessive an amount of tweaking: The data is in this order: height of the dipole in meters the real part of the dipole's impedance in ohms (the phase angle of the impedance was kept under 2 degrees) the approximate SWR=2 BW in a 50 ohm system in kHz the total apparent loss of the system in dB (this is found by integrating gain in all directions) the peak antenna gain in dBi 4.2 38.1 125 6.25 2.0 5.8 37.7 120 4.31 4.1 8.3 42.1 130 2.59 5.8 10.4 49.0 140 1.81 6.5 15 66.5 145 1.04 6.9 20.8 85.7 125 0.7 6.5 Most of my results are close to your results. It is interesting to know if I used the same assumptions. Please let me know. I encourage you to consider translating your work, or at least parts of it, into English. 73, Mac N8TT -- J. Mc Laughlin; Michigan U.S.A. Home: "Wimpie" wrote in message Hello Rick, At 100 miles distance, the TOA is almost vertical (maybe 75 degr), so with respect to signal strength, it is practically impossible to detect whether the antennas are broad side. Because of the distance, ground wave propagation loss is far higher with respect to NVIS propagation loss under these circumstances. One would mention polarization of the waves going up and down. I would not matter about this. These low frequencies are strongly affected by faraday rotation. On the way up and down, the polarization rotates several times and several wave fronts do exist. While you transmit (nearly vertical) with linear polarization, the down coming wave may have a strong circular component. With respect to the radiation pattern, you are right, the differences in pattern are minimal below 30 feet. However the overall efficiency is strongly depended on antenna height and soil properties. I did some simulation and practice. I made a short document of it (for JOTA porpuse), however the document is in Dutch Language (http:// www.tetech.nl/divers/NVISantenneNL1.pdf). Maybe you can get some useful info out of it. At low heights, much power is dissipated into the ground (resulting in a useful bandwidth). Best Regards, Wim PA3DJS |
NVIS Dipoles Directional?
"Dave Oldridge" wrote in message 9... Rick wrote in : 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? Absolutely true. Any difference would be insignificant. The path elevation is about 79 degrees for that path. -- Dave Oldridge+ ICQ 1800667 Absolutely??? So if you suspended another dipole above your NVIS dipole and oriented 90 degrees to each other, the difference would be insignificant? Then move it up into the "clouds", then move down to earth at the distance and you will see "insignificant" difference in signal levels? Seems that direction finders should not work according to this "verdict", Eh? One thing is the direction of the signals (maximum) another one is the polarization. Based on the orientation of antennas, one can orient the antenna to find the minimum signal. Yuri, K3BU.us |
NVIS Dipoles Directional?
So if you suspended another dipole above your NVIS dipole and oriented 90 degrees to each other, the difference would be insignificant? Then move it up into the "clouds", then move down to earth at the distance and you will see "insignificant" difference in signal levels? Seems that direction finders should not work according to this "verdict", Eh? One thing is the direction of the signals (maximum) another one is the polarization. Based on the orientation of antennas, one can orient the antenna to find the minimum signal. Yuri, K3BU.us Hi Yuri, As the wave pass through the ionosphere, strange things happen. You could google on Faraday rotation, ordinary and extraordinary waves to find out that at low frequency, the change in polarization is significant. Based on the down coming wave, you cannot determine the orientation of the transmitting antenna, neither the position with reasonable accuracy (for NVIS propagation). When you place the receiving antenna just above the transmitting antenna you are right, but we were discussing NVIS propagation. Best Regards, Wim PA3DJS |
NVIS Dipoles Directional?
"Yuri Blanarovich" wrote in
: "Dave Oldridge" wrote in message 9... Rick wrote in : 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? Absolutely true. Any difference would be insignificant. The path elevation is about 79 degrees for that path. -- Dave Oldridge+ ICQ 1800667 Absolutely??? So if you suspended another dipole above your NVIS dipole and oriented 90 degrees to each other, the difference would be insignificant? Then move it up into the "clouds", then move down to earth at the distance and you will see "insignificant" difference in signal levels? Seems that direction finders should not work according to this "verdict", Eh? And they don't actually work on NVIS signals unless you use them to measure elevation, in which case they point fairly high up. One thing is the direction of the signals (maximum) another one is the polarization. Based on the orientation of antennas, one can orient the antenna to find the minimum signal. You're assuming the ionosphere doesn't rotate the signal (or even render its polarization elliptical). Whether that's happening or not will likely depend on conditions at the time. But if it doesn't, then it doesn't matter which way the antennas are oriented in azimuth so long as they are both oriented the same. -- Dave Oldridge+ ICQ 1800667 |
NVIS Dipoles Directional?
"Wimpie" wrote in message ps.com... So if you suspended another dipole above your NVIS dipole and oriented 90 degrees to each other, the difference would be insignificant? Then move it up into the "clouds", then move down to earth at the distance and you will see "insignificant" difference in signal levels? Seems that direction finders should not work according to this "verdict", Eh? One thing is the direction of the signals (maximum) another one is the polarization. Based on the orientation of antennas, one can orient the antenna to find the minimum signal. Yuri, K3BU.us Hi Yuri, As the wave pass through the ionosphere, strange things happen. You could google on Faraday rotation, ordinary and extraordinary waves to find out that at low frequency, the change in polarization is significant. Change does not mean that polarization dissapears. Based on the down coming wave, you cannot determine the orientation of the transmitting antenna, neither the position with reasonable accuracy (for NVIS propagation). When you place the receiving antenna just above the transmitting antenna you are right, but we were discussing NVIS propagation. Best Regards, Wim PA3DJS So when signal is reflected, the polarization disappears? 73 Yuri, K3BU |
NVIS Dipoles Directional?
On 12 abr, 00:47, "Yuri Blanarovich" wrote:
"Wimpie" wrote in message ps.com... So if you suspended another dipole above your NVIS dipole and oriented 90 degrees to each other, the difference would be insignificant? Then move it up into the "clouds", then move down to earth at the distance and you will see "insignificant" difference in signal levels? Seems that direction finders should not work according to this "verdict", Eh? One thing is the direction of the signals (maximum) another one is the polarization. Based on the orientation of antennas, one can orient the antenna to find the minimum signal. Yuri, K3BU.us Hi Yuri, As the wave pass through the ionosphere, strange things happen. You could google on Faraday rotation, ordinary and extraordinary waves to find out that at low frequency, the change in polarization is significant. Change does not mean that polarization dissapears. Based on the down coming wave, you cannot determine the orientation of the transmitting antenna, neither the position with reasonable accuracy (for NVIS propagation). When you place the receiving antenna just above the transmitting antenna you are right, but we were discussing NVIS propagation. Best Regards, Wim PA3DJS So when signal is reflected, the polarization disappears? No, mostly the down comming wave has a circular component (eliptical polarization), therefore the orientation is not of significant importance. Wim, PA3DJS. 73 Yuri, K3BU |
NVIS Dipoles Directional?
On 11 abr, 06:14, "J. Mc Laughlin" wrote:
Dear Wim PA3DJS: How I wish that I could read Dutch. However, in looking over all of your work, it continually occurred to me that much of the base of English comes from Northern Nederland. I looked at the first table on page 20 of your work. The information so intrigued me that I ran almost the same information through NEC4, which does well with antennas close to the ground. I assumed: a center frequency of 3.6 MHz, a horizontal half wave dipole slightly adjusted to be resonant at 3.6 MHz at each height, the use of Cu wire 2 mm in diameter, and earth with a sigma of 0.01 and a relative dielectric constant of 15. This is what I found without too obsessive an amount of tweaking: The data is in this order: height of the dipole in meters the real part of the dipole's impedance in ohms (the phase angle of the impedance was kept under 2 degrees) the approximate SWR=2 BW in a 50 ohm system in kHz the total apparent loss of the system in dB (this is found by integrating gain in all directions) the peak antenna gain in dBi 4.2 38.1 125 6.25 2.0 5.8 37.7 120 4.31 4.1 8.3 42.1 130 2.59 5.8 10.4 49.0 140 1.81 6.5 15 66.5 145 1.04 6.9 20.8 85.7 125 0.7 6.5 Most of my results are close to your results. It is interesting to know if I used the same assumptions. Please let me know. I encourage you to consider translating your work, or at least parts of it, into English. 73, Mac N8TT -- J. Mc Laughlin; Michigan U.S.A. Home: [rest removed] Dear Mac, Thank you for sharing your results with me. You are fully right, the results are almost the same. The difference with my assumptions is wire diameter only. I used 10mm. I expected more difference in useful bandwidth, because of the factor 5 in wire diameter. The simulations were carried out on IE3D with a lossy dielectric as ground. The first table on page 17, shows the results for a perfect ground. I wasn't satisfied with the bandwidth, so the actual antenna consists of 2 wires (1m separation). This gave me sufficient bandwidth to use the antenna without a tuner. The results between the brackets are with 3 floating reflector wires under the antenna (about 1 m above ground). This gives an evident increase in gain. The gain increases somewhat by raising the 3 wires to about 1.5..2m above ground. At this moment there are no plans to translate the document into English language. As you have seen, the text in the images is in English language, so you never know what happens during a vacation..... The birdth of the Dutch version was in Spain. Best Regards, Wim PA3DJS. |
NVIS Dipoles Directional?
"Wimpie" wrote in message ups.com... On 12 abr, 00:47, "Yuri Blanarovich" wrote: "Wimpie" wrote in message ps.com... So if you suspended another dipole above your NVIS dipole and oriented 90 degrees to each other, the difference would be insignificant? Then move it up into the "clouds", then move down to earth at the distance and you will see "insignificant" difference in signal levels? Seems that direction finders should not work according to this "verdict", Eh? One thing is the direction of the signals (maximum) another one is the polarization. Based on the orientation of antennas, one can orient the antenna to find the minimum signal. Yuri, K3BU.us Hi Yuri, As the wave pass through the ionosphere, strange things happen. You could google on Faraday rotation, ordinary and extraordinary waves to find out that at low frequency, the change in polarization is significant. Change does not mean that polarization dissapears. Based on the down coming wave, you cannot determine the orientation of the transmitting antenna, neither the position with reasonable accuracy (for NVIS propagation). When you place the receiving antenna just above the transmitting antenna you are right, but we were discussing NVIS propagation. Best Regards, Wim PA3DJS So when signal is reflected, the polarization disappears? No, mostly the down comming wave has a circular component (eliptical polarization), therefore the orientation is not of significant importance. Wim, PA3DJS. 73 Yuri, K3BU So if there is ANY polarization left how can you claim that is not of significant importance. The proof in the pudding is that on any signal one can with any antenna, especially directional, find the spot where signal is minimal or nulled out. Nulls are sharp vs. broad pattern, but one has to be aware of them and understand the difference between the pattern and polarization. Yuri, K3BU.us |
NVIS Dipoles Directional?
Dear Wim:
Thank you for the additional information. The information fills in the rest of the story. Before I shut down the dedicated computer used for the calculations, I looked at the last entry again. The 20.8 m high dipole in a 50 ohm system seemed to have significantly smaller BW than what you reported. I changed the impedance to 85.7 ohms and then found (using the same data) that the SWR=2 BW is about 250 kHz. With a 1.3:1 turns ratio transformer, this is also the expected BW that would be seen in a 50 ohm system. In short, my reported BW numbers may need qualification. It is interesting to see how two different simulation techniques reach similar conclusions. I too expect that increasing the wire to 10 mm has little effect. Vacations can be very creative times. Thank you for your work. 73, Mac N8TT -- J. Mc Laughlin; Michigan U.S.A. Home: "Wimpie" wrote in message oups.com... On 11 abr, 06:14, "J. Mc Laughlin" wrote: Dear Wim PA3DJS: How I wish that I could read Dutch. However, in looking over all of your work, it continually occurred to me that much of the base of English comes from Northern Nederland. I looked at the first table on page 20 of your work. The information so intrigued me that I ran almost the same information through NEC4, which does well with antennas close to the ground. I assumed: a center frequency of 3.6 MHz, a horizontal half wave dipole slightly adjusted to be resonant at 3.6 MHz at each height, the use of Cu wire 2 mm in diameter, and earth with a sigma of 0.01 and a relative dielectric constant of 15. This is what I found without too obsessive an amount of tweaking: The data is in this order: height of the dipole in meters the real part of the dipole's impedance in ohms (the phase angle of the impedance was kept under 2 degrees) the approximate SWR=2 BW in a 50 ohm system in kHz the total apparent loss of the system in dB (this is found by integrating gain in all directions) the peak antenna gain in dBi 4.2 38.1 125 6.25 2.0 5.8 37.7 120 4.31 4.1 8.3 42.1 130 2.59 5.8 10.4 49.0 140 1.81 6.5 15 66.5 145 1.04 6.9 20.8 85.7 125 0.7 6.5 Most of my results are close to your results. It is interesting to know if I used the same assumptions. Please let me know. I encourage you to consider translating your work, or at least parts of it, into English. 73, Mac N8TT -- J. Mc Laughlin; Michigan U.S.A. Home: [rest removed] Dear Mac, Thank you for sharing your results with me. You are fully right, the results are almost the same. The difference with my assumptions is wire diameter only. I used 10mm. I expected more difference in useful bandwidth, because of the factor 5 in wire diameter. The simulations were carried out on IE3D with a lossy dielectric as ground. The first table on page 17, shows the results for a perfect ground. I wasn't satisfied with the bandwidth, so the actual antenna consists of 2 wires (1m separation). This gave me sufficient bandwidth to use the antenna without a tuner. The results between the brackets are with 3 floating reflector wires under the antenna (about 1 m above ground). This gives an evident increase in gain. The gain increases somewhat by raising the 3 wires to about 1.5..2m above ground. At this moment there are no plans to translate the document into English language. As you have seen, the text in the images is in English language, so you never know what happens during a vacation..... The birdth of the Dutch version was in Spain. Best Regards, Wim PA3DJS. |
NVIS Dipoles Directional?
On 12 abr, 23:40, "Yuri Blanarovich" wrote:
"Wimpie" wrote in message ups.com... On 12 abr, 00:47, "Yuri Blanarovich" wrote: "Wimpie" wrote in message oups.com... So if you suspended another dipole above your NVIS dipole and oriented 90 degrees to each other, the difference would be insignificant? Then move it up into the "clouds", then move down to earth at the distance and you will see "insignificant" difference in signal levels? Seems that direction finders should not work according to this "verdict", Eh? One thing is the direction of the signals (maximum) another one is the polarization. Based on the orientation of antennas, one can orient the antenna to find the minimum signal. Yuri, K3BU.us Hi Yuri, As the wave pass through the ionosphere, strange things happen. You could google on Faraday rotation, ordinary and extraordinary waves to find out that at low frequency, the change in polarization is significant. Change does not mean that polarization dissapears. Based on the down coming wave, you cannot determine the orientation of the transmitting antenna, neither the position with reasonable accuracy (for NVIS propagation). When you place the receiving antenna just above the transmitting antenna you are right, but we were discussing NVIS propagation. Best Regards, Wim PA3DJS So when signal is reflected, the polarization disappears? No, mostly the down comming wave has a circular component (eliptical polarization), therefore the orientation is not of significant importance. Wim, PA3DJS. 73 Yuri, K3BU So if there is ANY polarization left how can you claim that is not of significant importance. The proof in the pudding is that on any signal one can with any antenna, especially directional, find the spot where signal is minimal or nulled out. Nulls are sharp vs. broad pattern, but one has to be aware of them and understand the difference between the pattern and polarization. Yuri, K3BU.us High Yuri, I try again. Ratiation pattern As NVIS radiation comes from near 90 degrees elevation, the antenna receiving the incoming wave will operate at near maximum gain, no matter the orientation (in horizontal plane) of the dipole. The gain of a dipole under 75 degrees is less then 1 dB below the maximum. To orient your antenna to get a nul, you have to use a vertical dipole (or vertical monopole). So radiation pattern is not of importance. Polarization, If the ionosphere would act as a pure mirror reflector, and there was no free charge, you are right, orientation of both antennas would matter. As you do not know the amount of polarization change, you cannot predict the optimum orientation. Prove it yourself by using a hand held antenna (for example small vertical loop, or horizontal small dipole). Listen to an NVIS station (at about 50..100 mile from you) and rotate the antenna around the Z as. You should do this fast enough to distinguish between fading. You will find certain orientations with less reception, but the orientation (with minimum reception) will vary (that can be within a minute because of the change in overall faraday rotation). You will seldom find a real nul. This is because of the existence of a more or less circular component. If you want to be sure and also want to cover situations in which the refracted wave is almost linear polarized, you may consider two (small) antennas (perpendicular to each other) and switch between the two. During last year JOTA I had plenty of space to use two half wave dipoles, but it wasn't worth to do. I checked it with a simple hand held vertical tuned loop antenna by rotating the antenna round the Z- axis. Best regards, Wim |
NVIS Dipoles Directional?
Rick wrote in
: 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? True. Given the vagaries of propagation and the small differences in the pattern at that angle, he wouldn't notice. -- Dave Oldridge+ ICQ 1800667 |
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