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#1
April 2nd 14, 06:55 PM
posted to rec.radio.amateur.antenna
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Radials
Ian Jackson wrote:
In message , writes The ideal radial length for ANY ground plane antenna is slightly longer than 1/4 wavelength, no matter for what frequencey. Why is this? I would have thought that a 1/4 wave would be best, as it offers the lowest impedance. First you have to define what "best" means. All antennas are a trade off for impedance, bandwidth, gain and in most cases physical ability to build the structure. Changing the radial length will have a small effect on impdedance and resonant point but changing the radial angle will have a bigger effect on impedance and a very small effect on resonant point. I would suggest downloading the demo version of EZNEC and modeling a GP to see what small changes in various parameters do. -- Jim Pennino |
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#3
April 2nd 14, 08:25 PM
posted to rec.radio.amateur.antenna
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Radials
El 02-04-14 20:25, Ian Jackson escribió:
In message , writes Ian Jackson wrote: In message , writes The ideal radial length for ANY ground plane antenna is slightly longer than 1/4 wavelength, no matter for what frequencey. Why is this? I would have thought that a 1/4 wave would be best, as it offers the lowest impedance. First you have to define what "best" means. Yebbut ........ You've just said "the ideal radial length for ANY ground plane antenna is slightly longer than 1/4 wavelength, no matter for what frequency". I assumed that "ideal" = "best". . All antennas are a trade off for impedance, bandwidth, gain and in most cases physical ability to build the structure. Changing the radial length will have a small effect on impdedance and resonant point but changing the radial angle will have a bigger effect on impedance and a very small effect on resonant point. True - but what's the angle of the radials got to do with their length? I would suggest downloading the demo version of EZNEC and modeling a GP to see what small changes in various parameters do. I had presumed you had already do this (or something similar) in order to say that slightly longer than a 1/4 wavelength was ideal. However, I have always assumed that the steeper the angle of the radials, the more the groundplane becomes like a vertical halfwave dipole - and the lower becomes the angle of radiation. You are right, very steep radials become the lower half of a half wave dipole as the currents do not cancel eachother and contribute to the field of the quarter wave monopole. The "ultimate" version is the sleeve dipole. When they are in the horizontal plane, the contribution to the total radiation pattern is very small, and the contribution from the radials is even zero for the vertically polarized component at zero elevation. The "somewhat longer then 1/4 wavelength" I also noticed with radials connected to a coaxial braid to form a narrow band common mode choke. the choking effect (common mode insertion loss) is better when they are somewhat longer then 0.25lambda (depending in thickness). The effect of sloping angle on zero elevation gain is small, and you get hardly measurable more gain when they are almost vertical. Sloping radials have some other advantage: less birds. -- Wim PA3DJS Please remove abc first in case of PM |
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#4
April 2nd 14, 09:32 PM
posted to rec.radio.amateur.antenna
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Radials
Wimpie wrote:
El 02-04-14 20:25, Ian Jackson escribió: In message , writes Ian Jackson wrote: In message , writes The ideal radial length for ANY ground plane antenna is slightly longer than 1/4 wavelength, no matter for what frequencey. Why is this? I would have thought that a 1/4 wave would be best, as it offers the lowest impedance. First you have to define what "best" means. Yebbut ........ You've just said "the ideal radial length for ANY ground plane antenna is slightly longer than 1/4 wavelength, no matter for what frequency". I assumed that "ideal" = "best". . All antennas are a trade off for impedance, bandwidth, gain and in most cases physical ability to build the structure. Changing the radial length will have a small effect on impdedance and resonant point but changing the radial angle will have a bigger effect on impedance and a very small effect on resonant point. True - but what's the angle of the radials got to do with their length? I would suggest downloading the demo version of EZNEC and modeling a GP to see what small changes in various parameters do. I had presumed you had already do this (or something similar) in order to say that slightly longer than a 1/4 wavelength was ideal. However, I have always assumed that the steeper the angle of the radials, the more the groundplane becomes like a vertical halfwave dipole - and the lower becomes the angle of radiation. You are right, very steep radials become the lower half of a half wave dipole as the currents do not cancel eachother and contribute to the field of the quarter wave monopole. The "ultimate" version is the sleeve dipole. Not really. When they are in the horizontal plane, the contribution to the total radiation pattern is very small, and the contribution from the radials is even zero for the vertically polarized component at zero elevation. The theoretical gain of a GP with horizontal radials, radials drooping 45 degrees and and drooping 85 degrees is 1.42, 2.22, and 3.67 dbi. The "somewhat longer then 1/4 wavelength" I also noticed with radials connected to a coaxial braid to form a narrow band common mode choke. the choking effect (common mode insertion loss) is better when they are somewhat longer then 0.25lambda (depending in thickness). The effect of sloping angle on zero elevation gain is small, and you get hardly measurable more gain when they are almost vertical. Sloping radials have some other advantage: less birds. Changing the angle of the radials has little to no effect on elevation gain unless the radial ends are a very tiny fraction of a wavelength above ground. Elevation radiation angle is almost totally determined by the antenna height above ground. -- Jim Pennino |
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#5
April 2nd 14, 10:43 PM
posted to rec.radio.amateur.antenna
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Radials
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#6
April 3rd 14, 12:04 AM
posted to rec.radio.amateur.antenna
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Radials
Ian Jackson wrote:
In message , writes The theoretical gain of a GP with horizontal radials, radials drooping 45 degrees and and drooping 85 degrees is 1.42, 2.22, and 3.67 dbi. I would have thought that the 'ultimate' would be when the droop IS 90 degrees (ie essentially a sleeve dipole). With a droop of 90 degrees, is the gain slightly more than 3.67? You can't physically have a 90 degree droop. The radials would have to extend horizontally for some distance, then drop to 90 degrees. That is a different antenna. How come that you can have a 1/4 wave radiator groundplane type of antenna with a gain that is more than a halfwave dipole (2.15 dBi) -even if it is more-or-less a sleeve dipole? When the radial droop approaches 90 degrees it really isn't a GP antenna anymore, it is something else. -- Jim Pennino |
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#7
April 3rd 14, 01:23 PM
posted to rec.radio.amateur.antenna
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Radials
El 03-04-14 1:04, escribió:
Ian wrote: In , writes The theoretical gain of a GP with horizontal radials, radials drooping 45 degrees and and drooping 85 degrees is 1.42, 2.22, and 3.67 dbi. I would have thought that the 'ultimate' would be when the droop IS 90 degrees (ie essentially a sleeve dipole). With a droop of 90 degrees, is the gain slightly more than 3.67? You can't physically have a 90 degree droop. The radials would have to extend horizontally for some distance, then drop to 90 degrees. This is the same as saying, you can't have a 90 degree radiator, as due to wind it will bend. You know that going horizontally a few inch and then 90 degrees down doesn't make big difference compared to 85 degrees sloping. You only may experience some length difference to get lowest common mode current in the mast or feeder. Both option will not give you more gain compared to a half wave dipole (free space). That is a different antenna. How come that you can have a 1/4 wave radiator groundplane type of antenna with a gain that is more than a halfwave dipole (2.15 dBi) -even if it is more-or-less a sleeve dipole? When the radial droop approaches 90 degrees it really isn't a GP antenna anymore, it is something else. Is this because of electrical operation (I doubt), or naming convention? -- Wim PA3DJS Please remove abc first in case of PM |
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#8
April 3rd 14, 01:11 PM
posted to rec.radio.amateur.antenna
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Radials
El 02-04-14 22:32, escribió:
wrote: El 02-04-14 20:25, Ian Jackson escribió: In , writes Ian wrote: In , writes The ideal radial length for ANY ground plane antenna is slightly longer than 1/4 wavelength, no matter for what frequencey. Why is this? I would have thought that a 1/4 wave would be best, as it offers the lowest impedance. First you have to define what "best" means. Yebbut ........ You've just said "the ideal radial length for ANY ground plane antenna is slightly longer than 1/4 wavelength, no matter for what frequency". I assumed that "ideal" = "best". . All antennas are a trade off for impedance, bandwidth, gain and in most cases physical ability to build the structure. Changing the radial length will have a small effect on impdedance and resonant point but changing the radial angle will have a bigger effect on impedance and a very small effect on resonant point. True - but what's the angle of the radials got to do with their length? I would suggest downloading the demo version of EZNEC and modeling a GP to see what small changes in various parameters do. I had presumed you had already do this (or something similar) in order to say that slightly longer than a 1/4 wavelength was ideal. However, I have always assumed that the steeper the angle of the radials, the more the groundplane becomes like a vertical halfwave dipole - and the lower becomes the angle of radiation. You are right, very steep radials become the lower half of a half wave dipole as the currents do not cancel eachother and contribute to the field of the quarter wave monopole. The "ultimate" version is the sleeve dipole. Not really. When they are in the horizontal plane, the contribution to the total radiation pattern is very small, and the contribution from the radials is even zero for the vertically polarized component at zero elevation. The theoretical gain of a GP with horizontal radials, radials drooping 45 degrees and and drooping 85 degrees is 1.42, 2.22, and 3.67 dbi. You may check your simulations, as in free space you will not exceed the half wave dipole gain with near vertical radials (for the quarter wave version). A quarter wave monopole with near vertical radials has same current distribution as a vertical half wave dipole (use sum of current in all radials). Of course provided that you don't have significant common mode current in the mast or coaxial cable, as this may increase or decrease the free space gain. When extending both sloping radials and radiator you can get more gain, but you get significant increase in common mode current as the radial ground no longer act as a floating ground point, and the input impedance has a reactive part. The "somewhat longer then 1/4 wavelength" I also noticed with radials connected to a coaxial braid to form a narrow band common mode choke. the choking effect (common mode insertion loss) is better when they are somewhat longer then 0.25lambda (depending in thickness). The effect of sloping angle on zero elevation gain is small, and you get hardly measurable more gain when they are almost vertical. Sloping radials have some other advantage: less birds. Changing the angle of the radials has little to no effect on elevation gain unless the radial ends are a very tiny fraction of a wavelength above ground. I can't match this statement with your earlier gain figures, or did I misunderstand you. Elevation radiation angle is almost totally determined by the antenna height above ground. Agree, but now the number of variables increases as you need to take into account nearby ground conductivity and far away ground conductivity. -- Wim PA3DJS Please remove abc first in case of PM |
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#9
April 3rd 14, 05:41 PM
posted to rec.radio.amateur.antenna
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Radials
Wimpie wrote:
El 02-04-14 22:32, escribió: wrote: El 02-04-14 20:25, Ian Jackson escribió: In , writes Ian wrote: In , writes The ideal radial length for ANY ground plane antenna is slightly longer than 1/4 wavelength, no matter for what frequencey. Why is this? I would have thought that a 1/4 wave would be best, as it offers the lowest impedance. First you have to define what "best" means. Yebbut ........ You've just said "the ideal radial length for ANY ground plane antenna is slightly longer than 1/4 wavelength, no matter for what frequency". I assumed that "ideal" = "best". . All antennas are a trade off for impedance, bandwidth, gain and in most cases physical ability to build the structure. Changing the radial length will have a small effect on impdedance and resonant point but changing the radial angle will have a bigger effect on impedance and a very small effect on resonant point. True - but what's the angle of the radials got to do with their length? I would suggest downloading the demo version of EZNEC and modeling a GP to see what small changes in various parameters do. I had presumed you had already do this (or something similar) in order to say that slightly longer than a 1/4 wavelength was ideal. However, I have always assumed that the steeper the angle of the radials, the more the groundplane becomes like a vertical halfwave dipole - and the lower becomes the angle of radiation. You are right, very steep radials become the lower half of a half wave dipole as the currents do not cancel eachother and contribute to the field of the quarter wave monopole. The "ultimate" version is the sleeve dipole. Not really. When they are in the horizontal plane, the contribution to the total radiation pattern is very small, and the contribution from the radials is even zero for the vertically polarized component at zero elevation. The theoretical gain of a GP with horizontal radials, radials drooping 45 degrees and and drooping 85 degrees is 1.42, 2.22, and 3.67 dbi. You may check your simulations, as in free space you will not exceed the half wave dipole gain with near vertical radials (for the quarter wave version). Addressed in another post. A quarter wave monopole with near vertical radials has same current distribution as a vertical half wave dipole (use sum of current in all radials). Of course provided that you don't have significant common mode current in the mast or coaxial cable, as this may increase or decrease the free space gain. In the simulation there is no mast or cable. When extending both sloping radials and radiator you can get more gain, but you get significant increase in common mode current as the radial ground no longer act as a floating ground point, and the input impedance has a reactive part. There is always a frequency where the reactive part is zero. See my long post comparing configurations. As the simulations have no cable, you can not see any common mode current effects. The "somewhat longer then 1/4 wavelength" I also noticed with radials connected to a coaxial braid to form a narrow band common mode choke. the choking effect (common mode insertion loss) is better when they are somewhat longer then 0.25lambda (depending in thickness). The effect of sloping angle on zero elevation gain is small, and you get hardly measurable more gain when they are almost vertical. Sloping radials have some other advantage: less birds. Changing the angle of the radials has little to no effect on elevation gain unless the radial ends are a very tiny fraction of a wavelength above ground. I can't match this statement with your earlier gain figures, or did I misunderstand you. See my long post comparing configurations. Elevation radiation angle is almost totally determined by the antenna height above ground. Agree, but now the number of variables increases as you need to take into account nearby ground conductivity and far away ground conductivity. Only if you want to simulate some particular and specific place. For the vast majority of problems you just use average ground values. EZNEC does allow you to change the ground values if you want to see what happens in someplace like a desert with poor ground. -- Jim Pennino |
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#10
April 3rd 14, 06:31 PM
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Radials
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