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Reflexion on ground and radials
Hi to all,
From the help file of EZNEC, I copy this sentence: ''The ground reflections which cause the low-angle pattern reflections take place farther from the antenna than most common radial ground systems extend, so ground radial systems have little or no effect on the ground reflection phenomenon.'' My question: Is this phenomenon dependent of the frequency? In other words, will the first reflexion point be farther away with lowering of the frequency or about at the same distance, independently of the wavelength**? **In that case, the ''long'' radials of AM broadcasting verticals 'may' also become reflectors... Thanks and 73 de Pierre VE2PID |
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Reflexion on ground and radials
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#3
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Reflexion on ground and radials
In daylight, with vertical LF and MF antennas, all radiation at an
angle above 0 dgrees and that reflected from the ground, whuch is most of it, is wasted and generally unwanted. Only the groundwave is useful. At night a virtue is sometimes made out of a vice. "Roy Lewallen" wrote in message ... wrote: Hi to all, From the help file of EZNEC, I copy this sentence: ''The ground reflections which cause the low-angle pattern reflections take place farther from the antenna than most common radial ground systems extend, so ground radial systems have little or no effect on the ground reflection phenomenon.'' My question: Is this phenomenon dependent of the frequency? In other words, will the first reflexion point be farther away with lowering of the frequency or about at the same distance, independently of the wavelength**? **In that case, the ''long'' radials of AM broadcasting verticals 'may' also become reflectors... For a given antenna height and elevation angle, it's the same distance from the antenna. Imagine yourself at some point on the antenna, say at the middle of a vertical antenna. Direct your gaze at an angle of 10 degrees below horizontal. Radiation traveling from the antenna at that angle will strike the ground at the point where your gaze does, and after reflection it will be going upward at an angle of 10 degrees above horizontal. The overall elevation pattern at 10 degrees elevation angle is formed by the sum of a direct ray going upward at 10 degrees and one going downward 10 degrees below horizontal, reflecting off the ground, and adding to it at a distant point. The lower the angle or the higher on the antenna you are, the farther the reflection point. Antenna programs repeat this process for every "segment", that is, points all along the antenna, to get the total field. So where does that reflection take place? Let's use your example of an AM broadcast antenna. Some broadcasters, I believe, use verticals which are around a half wavelength high. Suppose the frequency is 1 MHz, so the wavelength is 300 meters, and a half wave antenna would be about 150 meters high. Broadcast radials are, I think, around a half wavelength long. So the field from half way up the antenna would just strike the end of the radial field, using this simplified model, at a downward angle of arctan(75/150) ~ 26 degrees. Any radiation from the center of the vertical at an angle below 26 degrees would reflect beyond the radial field, so the elevation pattern for all angles below 26 degrees would be independent of the radial field. Likewise, radiation at 26 degrees from any portion of the antenna above half way up would strike the ground even farther away. Ironically, vertically polarized reflection at angles above this aren't too bad off plain dirt, so the radials don't help as much as you might think even at high angles. It's low angle reflections that really suffer when polarization is vertical, and those are the ones missing the radial field. This model is oversimplified, not taking into account the fact that the wavelength is large relative to some of the dimensions being considered or the considerable skin depth of the ground. The "NEC radial" option of NEC and EZNEC uses just this kind of analysis with the ground in the region of the radial field being assumed uniformly conductive, and I discourage its use. It's more accurate to model very slightly elevated radial wires where the fields from the wire currents make their true contribution. But when you do, you'll find that the radials still don't have much impact on the lower angle portion of the pattern. Roy Lewallen, W7EL |
#4
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Reflexion on ground and radials
Reg Edwards wrote: In daylight, with vertical LF and MF antennas, all radiation at an angle above 0 dgrees and that reflected from the ground, whuch is most of it, is wasted and generally unwanted. Only the groundwave is useful. At night a virtue is sometimes made out of a vice. While the above is generally true for broadcasting... I, like hundreds of others, can work 1000 miles or more during mid-day in winter on 160 meters. I, like many others, can work thousands of miles in mid morning or late afternoon. It isn't unusual to hear and work west coast stations that are 2000 miles away at 10 or 11 AM local time, and that sure isn't zero degree takeoff stuff. By the way the vast majority of AM BC stations use tower somewhere around 1/4 wl tall. There is nothing to be gained by going taller compared to the extra installation and upkeep expenses. AM BC stations used to use 5/8th wave verticals in early years of broadcasting, but they quickly learned that was actually a disadvantage because the earth around the antenna wasn't good enough to allow the full gain. All the extra height did was increase fading at modest distances. 73 Tom |
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