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EZNEC Vertical interpretation
Showing my ignorance (or perhaps deteriorating intellect?) again:
I am attempting to compare the gain of a full size (1/4 wave) vertical to various shorter verticals at 160 meters. I am looking at the gain in dbi on the 2D plot. I see 1.39 dbi for the full 1/4 wave and I see 1.52 dbi for the 28 foot version. I don't believe the shorter antenna has less gain. Where am I going wrong? de W8CCW John Ferrell W8CCW |
EZNEC Vertical interpretation
On Thu, 20 Apr 2006 17:51:02 GMT, John Ferrell
wrote: I am looking at the gain in dbi on the 2D plot. I see 1.39 dbi for the full 1/4 wave and I see 1.52 dbi for the 28 foot version. I don't believe the shorter antenna has less gain. Where am I going wrong? Hi John, Well, at a minumum, your statements conflict from one paragraph to the next. That is one indication of something wrong even before we consider the numbers. Unravelling that knot, and taking only the first paragraph, yes that looks wrong (in comparison) too. On the third hand, even if it were true 0.13dB is something you will never measure accurately in your lifetime. Even more, you would never perceive it. Your question should be more of the nature, "how did I misread the results?" to which we could respond to the tenor of your opening statement: Showing my ignorance (or perhaps deteriorating intellect?) again Perhaps if you were to explain how you got here instead (the problem) of what you found (the symptom). 73's Richard Clark, KB7QHC |
EZNEC Vertical interpretation
"John Ferrell" wrote:
I am attempting to compare the gain of a full size (1/4 wave) vertical to various shorter verticals at 160 meters. I am looking at the gain in dbi on the 2D plot. I see 1.39 dbi for the full 1/4 wave and I see 1.52 dbi for the 28 foot version. I don't believe the shorter antenna has less gain. Where am I going wrong? Take a look at the phase angle between the feedpoint voltage and the feedpoint current for the 28 foot version and try to figure out an efficient matching network. That phase angle is ~89.95 degrees. The feedpoint voltage is 1400 volts for only 1.2 watts input. The feedpoint impedance is about 1.2-j1400 ohms. Try using the helix feature to install a loading coil in the center of the 28 foot antenna. The gain will drop as expected but the feedpoint impedance will be easier to match. -- 73, Cecil, W5DXP |
EZNEC Vertical interpretation
Oops! Thanks for the quick response!
That should have read "I can't believe the shorter antenna has MORE gain". I am not normally troubled by such small differences in db's but for that small of a gain difference and that large of a physical difference (104 feet!) I am certain I have missed more than a decimal point. The tone of the message was intended as apologetic. I really hate to bring dumb questions here to the experts but other than my books and the Internet I have no where else to go with them. I also hate to take the dumb questions to Roy directly for fear I will wear out my welcome with him. On Thu, 20 Apr 2006 10:59:22 -0700, Richard Clark wrote: On Thu, 20 Apr 2006 17:51:02 GMT, John Ferrell wrote: I am looking at the gain in dbi on the 2D plot. I see 1.39 dbi for the full 1/4 wave and I see 1.52 dbi for the 28 foot version. I don't believe the shorter antenna has less gain. Where am I going wrong? Hi John, Well, at a minumum, your statements conflict from one paragraph to the next. That is one indication of something wrong even before we consider the numbers. Unravelling that knot, and taking only the first paragraph, yes that looks wrong (in comparison) too. On the third hand, even if it were true 0.13dB is something you will never measure accurately in your lifetime. Even more, you would never perceive it. Your question should be more of the nature, "how did I misread the results?" to which we could respond to the tenor of your opening statement: Showing my ignorance (or perhaps deteriorating intellect?) again Perhaps if you were to explain how you got here instead (the problem) of what you found (the symptom). 73's Richard Clark, KB7QHC John Ferrell W8CCW |
EZNEC Vertical interpretation
I don't find the Helix feature in my copy of EZNEC+ V4.
I believe I can get the same results by inserting an appropriate load. It will just be a little more cumbersome. I was not aware that the feeding of the radiator was affected by the driving source. My "lab models" require a base inductance to get the radiator into a range I can feed. I have not been including that in the models. I added a base inductor of 100uh(r=3) and got the results you predicted. The 28 foot radiator is now showing -3.75 dbi gain! Lesson learned! The matter of the phase angle and the feed point voltage is currently over my head, I will study it further. I am using the TLW program from the Antenna Handbook to determine an appropriate match. I have not tried to verify those calculations with a lab project YET! Once again, thank you for the assistance. I was getting pretty frustrated... de W8CCW John On Thu, 20 Apr 2006 18:23:34 GMT, "Cecil Moore" wrote: Take a look at the phase angle between the feedpoint voltage and the feedpoint current for the 28 foot version and try to figure out an efficient matching network. That phase angle is ~89.95 degrees. The feedpoint voltage is 1400 volts for only 1.2 watts input. The feedpoint impedance is about 1.2-j1400 ohms. Try using the helix feature to install a loading coil in the center of the 28 foot antenna. The gain will drop as expected but the feedpoint impedance will be easier to match. John Ferrell W8CCW |
EZNEC Vertical interpretation
Correction: the base inductor was 75uh
On Thu, 20 Apr 2006 19:06:17 GMT, John Ferrell wrote: I don't find the Helix feature in my copy of EZNEC+ V4. I believe I can get the same results by inserting an appropriate load. It will just be a little more cumbersome. I was not aware that the feeding of the radiator was affected by the driving source. My "lab models" require a base inductance to get the radiator into a range I can feed. I have not been including that in the models. I added a base inductor of 100uh(r=3) and got the results you predicted. The 28 foot radiator is now showing -3.75 dbi gain! Lesson learned! The matter of the phase angle and the feed point voltage is currently over my head, I will study it further. I am using the TLW program from the Antenna Handbook to determine an appropriate match. I have not tried to verify those calculations with a lab project YET! Once again, thank you for the assistance. I was getting pretty frustrated... de W8CCW John On Thu, 20 Apr 2006 18:23:34 GMT, "Cecil Moore" wrote: Take a look at the phase angle between the feedpoint voltage and the feedpoint current for the 28 foot version and try to figure out an efficient matching network. That phase angle is ~89.95 degrees. The feedpoint voltage is 1400 volts for only 1.2 watts input. The feedpoint impedance is about 1.2-j1400 ohms. Try using the helix feature to install a loading coil in the center of the 28 foot antenna. The gain will drop as expected but the feedpoint impedance will be easier to match. John Ferrell W8CCW John Ferrell W8CCW |
EZNEC Vertical interpretation
"John Ferrell" wrote:
I don't find the Helix feature in my copy of EZNEC+ V4. I believe I can get the same results by inserting an appropriate load. It will just be a little more cumbersome. The helix feature is found under the Wires window under the create menu. But it is somewhat complicated. A lumped inductance load will get you started. The problem with feeding an impedance of 1.2-j1400 ohms is getting power into the antenna without dissipating most of it in the matching network. -- 73, Cecil, W5DXP |
EZNEC Vertical interpretation
On Thu, 20 Apr 2006 18:28:33 GMT, John Ferrell
wrote: Oops! Thanks for the quick response! That should have read "I can't believe the shorter antenna has MORE gain". I am not normally troubled by such small differences in db's but for that small of a gain difference and that large of a physical difference (104 feet!) I am certain I have missed more than a decimal point. Hi John, Well, this still does not tell us how you got here, only that when you did you immediately stumbled. Could it be that you inverted the results too? You know, it would make more sense as an explanation than anything else. Besides, if you skip the losses, the two will have such similar gain figures that no one would be surprised by the razor thin margin of difference. 73's Richard Clark, KB7QHC |
EZNEC Vertical interpretation
On Thu, 20 Apr 2006 19:22:17 GMT, "Cecil Moore"
wrote: "John Ferrell" wrote: I don't find the Helix feature in my copy of EZNEC+ V4. I believe I can get the same results by inserting an appropriate load. It will just be a little more cumbersome. The helix feature is found under the Wires window under the create menu. But it is somewhat complicated. A lumped inductance load will get you started. The problem with feeding an impedance of 1.2-j1400 ohms is getting power into the antenna without dissipating most of it in the matching network. Not to worry. Ground loss will help with that problem. |
EZNEC Vertical interpretation
If there's no conductor loss in a model, and no ground loss from a
ground system (which can be simulated with a MININEC type ground), the gain of an infinitesimally short vertical should be about 0.45 dB less than for a quarter wave vertical. The gains for intermediate lengths should fall between. The gain difference is caused by the change in current distribution as the antenna length changes, which in turn causes a slight change in the pattern shape. If you're seeing a report of higher gain from the shorter antenna, something's wrong. Do an Average Gain test to make sure there are no numerical problems, and also check the segmentation to make sure it's reasonable. And, of course, make sure that the length is the only difference between the models. Roy Lewallen, W7EL John Ferrell wrote: Showing my ignorance (or perhaps deteriorating intellect?) again: I am attempting to compare the gain of a full size (1/4 wave) vertical to various shorter verticals at 160 meters. I am looking at the gain in dbi on the 2D plot. I see 1.39 dbi for the full 1/4 wave and I see 1.52 dbi for the 28 foot version. I don't believe the shorter antenna has less gain. Where am I going wrong? de W8CCW John Ferrell W8CCW |
EZNEC Vertical interpretation
John Ferrell wrote:
I don't find the Helix feature in my copy of EZNEC+ V4. You'll find it in the EZNEC manual index under "Helix Creation". (There's lots of other good information in the manual, too.) I believe I can get the same results by inserting an appropriate load. It will just be a little more cumbersome. It'll be less cumbersome, but you probably won't get the same results. The amount difference will depend on the geometry of the helix. However, a lumped load is adequate to illustrate the phenomena you're investigating. I was not aware that the feeding of the radiator was affected by the driving source. My "lab models" require a base inductance to get the radiator into a range I can feed. I have not been including that in the models. I added a base inductor of 100uh(r=3) and got the results you predicted. The 28 foot radiator is now showing -3.75 dbi gain! Lesson learned! The loss is due solely to the inductor's resistance -- you'll get the same result by replacing the inductor with a 3 ohm resistor. The amount of gain reduction due to the inductor will get worse and worse as the antenna gets shorter and shorter, for two reasons. The first is that the antenna's radiation resistance drops as it gets shorter, so the inductor loss becomes a greater fraction of the total feedpoint resistance. The second is that you need a larger inductor for a shorter antenna and, assuming a constant Q, that means more inductor resistance. . . . Roy Lewallen, W7EL |
EZNEC Vertical interpretation
"John Ferrell" wrote
I am attempting to compare the gain of a full size (1/4 wave) vertical to various shorter verticals at 160 meters. I am looking at the gain in dbi on the 2D plot. I see 1.39 dbi for the full 1/4 wave and I see 1.52 dbi for the 28 foot version. I don't believe the shorter antenna has less gain. Where am I going wrong? _____________ Assuming that your vertical is ground-mounted, getting the "right answer" for this question depends on the quality of your wire models, and especially your assumptions about the ground plane, and the way your vertical connects to it. A classic 1/4-wave vertical monopole working against a very good radial ground system such as used in AM broadcast should produce a peak gain of around 5 dBi. The theoretical max for such a system with perfect conditions is 5.15 dBi. Electrically short(er), base-insulated vertical monopoles, for a given frequency, have lower radiation resistance at the feedpoint near the tower base. So, given the same r-f resistance in the path to the ground plane, the radiation efficiency of the shorter system will drop. RF |
EZNEC Vertical interpretation
Richard Fry wrote:
. . . A classic 1/4-wave vertical monopole working against a very good radial ground system such as used in AM broadcast should produce a peak gain of around 5 dBi. The theoretical max for such a system with perfect conditions is 5.15 dBi. . . . Unfortunately, "perfect conditions" includes a perfectly conducting ground far beyond the radial field, a luxury neither we nor the broadcasters have. The maximum far field (sky wave) gain of a ground mounted quarter wave vertical over average ground, with a completely lossless ground system, is on the order of 0 dBi, and this occurs at roughly 25 degrees above the horizon (both depending on frequency as well as ground characteristics). See the EZNEC example file Vert1.ez as an illustration. Its MININEC type ground simulates a lossless ground system. Roy Lewallen, W7EL |
EZNEC Vertical interpretation
Thank you, I get that!
On Thu, 20 Apr 2006 13:12:20 -0700, Roy Lewallen wrote: The loss is due solely to the inductor's resistance -- you'll get the same result by replacing the inductor with a 3 ohm resistor. The amount of gain reduction due to the inductor will get worse and worse as the antenna gets shorter and shorter, for two reasons. The first is that the antenna's radiation resistance drops as it gets shorter, so the inductor loss becomes a greater fraction of the total feedpoint resistance. The second is that you need a larger inductor for a shorter antenna and, assuming a constant Q, that means more inductor resistance. . . . Roy Lewallen, W7EL John Ferrell W8CCW |
EZNEC Vertical interpretation
The Vert1 example does illustrate the point well by varying the ground
options. On Thu, 20 Apr 2006 16:16:21 -0700, Roy Lewallen wrote: Richard Fry wrote: . . . A classic 1/4-wave vertical monopole working against a very good radial ground system such as used in AM broadcast should produce a peak gain of around 5 dBi. The theoretical max for such a system with perfect conditions is 5.15 dBi. . . . Unfortunately, "perfect conditions" includes a perfectly conducting ground far beyond the radial field, a luxury neither we nor the broadcasters have. The maximum far field (sky wave) gain of a ground mounted quarter wave vertical over average ground, with a completely lossless ground system, is on the order of 0 dBi, and this occurs at roughly 25 degrees above the horizon (both depending on frequency as well as ground characteristics). See the EZNEC example file Vert1.ez as an illustration. Its MININEC type ground simulates a lossless ground system. Roy Lewallen, W7EL John Ferrell W8CCW |
EZNEC Vertical interpretation
Found the helix. I will have to puzzle it out. I have a lot to digest.
I am glad I asked the question. The answers turned on a lot lights. On Thu, 20 Apr 2006 19:22:17 GMT, "Cecil Moore" wrote: "John Ferrell" wrote: I don't find the Helix feature in my copy of EZNEC+ V4. I believe I can get the same results by inserting an appropriate load. It will just be a little more cumbersome. The helix feature is found under the Wires window under the create menu. But it is somewhat complicated. A lumped inductance load will get you started. The problem with feeding an impedance of 1.2-j1400 ohms is getting power into the antenna without dissipating most of it in the matching network. John Ferrell W8CCW |
EZNEC Vertical interpretation
"Roy Lewallen" wrote:
The maximum far field (sky wave) gain of a ground mounted quarter wave vertical over average ground, with a completely lossless ground system, is on the order of 0 dBi, and this occurs at roughly 25 degrees above the horizon (both depending on frequency as well as ground characteristics). _____________ The above is an understandable conclusion using NEC analysis, however it is not supported empirically. If it was, AM broadcast stations would perform very much differently than they do. The measured data in Brown, Lewis & Epstein's 1937 benchmark paper "Ground Systems as a Factor in Antenna Efficiency" proved that the *radiated* groundwave field from a vertical monopole working against 113 buried radials each 0.41 lambda in length was within a few percent of its calculated peak value for a radiation pattern with maximum gain in the horizontal plane. The path length for the measurement was 0.3 miles, which was in the far field of the vertical monopole configurations measured. BL&E's measurements, and the results of thousands of measurements made of the groundwave fields of MW broadcast stations using such radial ground systems ever since demonstrate that their peak gain always lies in the horizontal plane. It is true that, as a groundwave propagation path becomes longer, the field measured at increasing elevations above the earth at distant ranges might be higher than measured at ground level at those ranges. But that is not because more field was launched by the original radiator toward those higher elevations -- it is because the the groundwave path has higher losses, which accumulate as that path lengthens. Therefore a NEC plot showing the conditions reported in the quote above do not accurately depict the elevation pattern as it is launched from the radiator, and the groundwave field it will generate. There are software programs designed for calculating MW groundwave field strength given the FCC "efficiency" of the radiator and the conductivity of the path. The radiator efficiency is the groundwave field developed by the radiator with a given applied power at a given distance (1 kW @ 1 km). These values must meet a certain minimum level for the class of station. I think in all cases, they must be within ~0.5 dB of the theoretical value for a radiation pattern with its peak gain in the horizontal plane. In the case of directional MW antennas, this performance must be proven by field measurements. Finally, standard equations show a peak field of ~137.6 mV/m at 1 mile from a 1/2-wave dipole radiating 1 kW in free space. The calculated groundwave field at 1 mile radiated by 1 kW from a 1/4-wave vertical MW monopole over a perfect ground plane is ~195 mV/m. This is the same field as generated by the free space 1/2-wave dipole, when all radiation is confined to one hemisphere (137.6 x 1.414). The groundwave fields measured from thousands of installed MW broadcast antenna systems confirm that their intrinsic radiation patterns are within a fraction of a decibel of that perfect radiator over a perfect ground plane, no matter what is the conductivity at the antenna site (N.B. Reg). RF |
EZNEC Vertical interpretation
"Richard Fry" wrote The groundwave fields measured from thousands of installed MW broadcast antenna systems confirm that their intrinsic radiation patterns are within a fraction of a decibel of that perfect radiator over a perfect ground plane, no matter what is the conductivity at the antenna site (N.B. Reg). ======================================== So you cleverly avoid the problem of how to calculate the effects of ground conductivity by laying down more than enough radials of more than sufficient length to hide the effects. B,L & E sure had the right idea. But hardly a satisfactory economic engineering solution. ---- Reg. |
EZNEC Vertical interpretation
"Reg Edwards" wrote
So you cleverly avoid the problem of how to calculate the effects of ground conductivity by laying down more than enough radials of more than sufficient length to hide the effects. B,L & E sure had the right idea. But hardly a satisfactory economic engineering solution. _________________ So you think it is preferable to install a less than optimal radial ground system, as long as you can calculate how deficient it is? MW broadcast stations want/need maximum groundwave coverage. Installing a "BL&E" radial ground system will provide that for all conductivity conditions at the tx antenna site -- which equates to a very satisfactory and economic engineering solution for this application. RF |
EZNEC Vertical interpretation
Richard Fry wrote: "Roy Lewallen" wrote: The maximum far field (sky wave) gain of a ground mounted quarter wave vertical over average ground, with a completely lossless ground system, is on the order of 0 dBi, and this occurs at roughly 25 degrees above the horizon (both depending on frequency as well as ground characteristics). _____________ The above is an understandable conclusion using NEC analysis, however it is not supported empirically. If it was, AM broadcast stations would perform very much differently than they do. NEC analysis has been supported many times by measurement and observation. The measured data in Brown, Lewis & Epstein's 1937 benchmark paper "Ground Systems as a Factor in Antenna Efficiency" proved that the *radiated* groundwave field from a vertical monopole working against 113 buried radials each 0.41 lambda in length was within a few percent of its calculated peak value for a radiation pattern with maximum gain in the horizontal plane. The path length for the measurement was 0.3 miles, which was in the far field of the vertical monopole configurations measured. Yes. The question is what is the calculated value. B, L, and E normalized their measurements to the unattenuated field strength at one mile for 1000 watts radiated power. I couldn't find anywhere in their paper where they explained how they determined the ground attenuation between the antenna and their observation point. BL&E's measurements, and the results of thousands of measurements made of the groundwave fields of MW broadcast stations using such radial ground systems ever since demonstrate that their peak gain always lies in the horizontal plane. No, the field strength is strongest at low elevation angles only close to the antenna, as you further explain below. It is true that, as a groundwave propagation path becomes longer, the field measured at increasing elevations above the earth at distant ranges might be higher than measured at ground level at those ranges. But that is not because more field was launched by the original radiator toward those higher elevations -- it is because the the groundwave path has higher losses, which accumulate as that path lengthens. Therefore a NEC plot showing the conditions reported in the quote above do not accurately depict the elevation pattern as it is launched from the radiator, and the groundwave field it will generate. Of course the standard far field analysis doesn't accurately depict the field close to the antenna -- it's a plot of the field at points very distant from the antenna, as clearly explained in the manual. NEC allows you to include the surface wave if you want, and it accurately shows the total field including the surface wave at a distance of your choice. (Accurate, that is, up to a hundred km or so, beyond which the deviation of the flat ground model from the curved Earth begins affecting results.) Don't feel bad -- Reg has a lot of trouble understanding this, too. There are software programs designed for calculating MW groundwave field strength given the FCC "efficiency" of the radiator and the conductivity of the path. The radiator efficiency is the groundwave field developed by the radiator with a given applied power at a given distance (1 kW @ 1 km). These values must meet a certain minimum level for the class of station. I think in all cases, they must be within ~0.5 dB of the theoretical value for a radiation pattern with its peak gain in the horizontal plane. In the case of directional MW antennas, this performance must be proven by field measurements. Finally, standard equations show a peak field of ~137.6 mV/m at 1 mile from a 1/2-wave dipole radiating 1 kW in free space. The calculated groundwave field at 1 mile radiated by 1 kW from a 1/4-wave vertical MW monopole over a perfect ground plane is ~195 mV/m. This is the same field as generated by the free space 1/2-wave dipole, when all radiation is confined to one hemisphere (137.6 x 1.414). The groundwave fields measured from thousands of installed MW broadcast antenna systems confirm that their intrinsic radiation patterns are within a fraction of a decibel of that perfect radiator over a perfect ground plane, no matter what is the conductivity at the antenna site (N.B. Reg). No, the measured fields from quarter wave broadcast antennas are considerably less than 195 mV/m for 1 kW at one mile, unless perhaps there's only salt water between the antenna and measurement point. As you explained above, the surface wave is attenuated with distance. What you seem to be missing is that the attenuation is strongly dependent on ground conductivity (between antenna and measurement point, not just at the antenna site) and frequency, so the actual field strength at one mile for 1 kW radiated will always be considerably less than the perfect ground case. The 195 mV/m and associated values for various antenna heights is the "unattenuated" or "inverse" field, which doesn't include the surface wave attenuation beyond simple inverse distance field strength reduction. It's the field strength you'd get if the ground between antenna and measurement point were perfect, not what you get over real ground. I'm not very conversant with FCC antenna measurement methodology, but somewhere the measured field strength is normalized to the unattenuated field strength by fitting to a ground attenuation curve, which in turn depends on frequency and ground conductivity. (I've been told that this is the way broadcasters determine ground conductivity -- by seeing how far the measured field strength deviates from the unattenuated value.) I believe that the surface wave attenuation curves used by the FCC are from the 1937 I.R.E. paper by K.A. Norton. That paper is also the basis for NEC's surface wave calculations. Roy Lewallen, W7EL |
EZNEC Vertical interpretation
"Cecil Moore" wrote in news:JbR1g.62010$F_3.14225
@newssvr29.news.prodigy.net: "John Ferrell" wrote: I don't find the Helix feature in my copy of EZNEC+ V4. I believe I can get the same results by inserting an appropriate load. It will just be a little more cumbersome. The helix feature is found under the Wires window under the create menu. But it is somewhat complicated. A lumped inductance load will get you started. The problem with feeding an impedance of 1.2-j1400 ohms is getting power into the antenna without dissipating most of it in the matching network. A funny anecdote comes to mind in this connection. I was trying desperately to load a very short helical dipole on 40M and ended up hooking it to my old Dentron 1KW matchbox to see if that could do anything with it. Damn antenna loaded up perfectly. SWR at on the line to the matchbox 1.1 to 1, good power transfer at 25 watts from the exciter. So I went to listen with it. NOTHING! NADA! Not even a dit, even though the big vertical array just hummed with CW signals on the band. So I leaned over to check connections and darn near fried my hand on the case of the Dentron. THAT'S where all the watts were going. Damn tuner was loading its own case! -- Dave Oldridge+ ICQ 1800667 VA7CZ |
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