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#1
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Richard Fry wrote:
. . . Using your statements above the line, one might think that it rather pointless to use anything longer than a 1/4-wave vertical. But going from a 1/4-wave to a 1/2-wave vertical in fact will add ~1.6dB of gain at the peak of the pattern envelope, and a 5/8-wave vertical will add almost 3dB. These are worthwhile improvements in system performance. Broadcast engineering consultants have recognized this, and used it to advantage for decades. . . . It's important to realize that the graphs you posted are for surface wave field strengths. This is equivalent to far field strengths at zero elevation angle over perfect ground. Amateurs seldom communicate by surface wave, except for local contacts. When the vertical is surrounded by real ground, attenuation of the sky wave at lower angles occurs. One of the results of this is that the antennas which concentrate energy more at lower angles end up losing a greater fraction of the total radiated energy. This tends to decrease the gain difference between a 5/8 and 1/4 wave vertical, for example, over a typical sky wave path. In the case of VHF/UHF mobile operations, which are essentially line of sight, the finite size of most ground planes (e.g. a car top) can affect the pattern considerably, again altering the gain difference between various heights of verticals. While there's an extensive body of well established and proven knowledge in the broadcast industry, we have to be careful in applying it to typical amteur communications. Often, the conditions are different (as in this discussion, of surface vs sky wave propagation; or fixed vs variable frequency operation), and the important criteria are different (a few percent difference in coverage area is important to a broadcaster because of its impact on advertising revenue, but a fraction of a dB is seldom important to an amateur; a broadcast phased array can take a long time to design and adjust, but amateurs want to switch or change directions). So we can't just assume that an antenna or method that's best for a broadcaster is best for us. Roy Lewallen, W7EL |
#2
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"Roy Lewallen" wrote:
Amateurs seldom communicate by surface wave, except for local contacts. When the vertical is surrounded by real ground, attenuation of the sky wave at lower angles occurs. One of the results of this is that the antennas which concentrate energy more at lower angles end up losing a greater fraction of the total radiated energy. This tends to decrease the gain difference between a 5/8 and 1/4 wave vertical, for example, over a typical sky wave path. __________________ I investigated your concept statements using NEC-2 models of 1/4-wave and 5/8-wave verticals in the 40m band (7.3MHz), working against the same infinite ground plane of "Average" parameters. * The 5/8-wave vertical has a peak gain of 0.2dBi, 16 degrees above the horizon. * The 1/4-wave vertical has a peak gain of -6.4dBi, 26 degrees above the horizon, and its entire radiation envelope is always within that of the 5/8-wave. I don't know which range of elevation angles is considered most useful for skywave paths on 40m, but it would appear that with equal tx power, a 5/8-wave vertical always will have a usefully better skywave than a 1/4-wave vertical over a typical ground plane -- and probably by more than 3dB. If you could check my conclusions on this I'd be grateful. RF |
#3
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It sounds like you might have made the mistake of connecting a wire
directly to Sommerfeld or reflection coefficient ground. Doing this with NEC-2 (or EZNEC) produces a resistance of unpredictable and meaningless value at the connection point, lowering the indicated field strength by an unpredictable amount. (EZNEC gives you a warning message when you try to do this.) EZNEC provides an option not available in NEC-2, a "MININEC-type" ground. This functions as a perfect ground when calculating impedances and currents, but uses the user-specified ground constants (conductivity and dielectric constant) when calculating the pattern. It simulates an antenna with a lossless ground system, allowing you to separately see the effect of ground conductivity on the pattern without the magnitude of the field being affected by changes in the ground system loss. The best you can do with either EZNEC or NEC-2 if you want to include ground system loss is to include radial wires just above the ground in the model and connect the vertical to them. Then, however, any differences you see will hold only for that particular ground system -- and, the above-ground approximation isn't a terrifically accurate representation of a buried system. Using the MININEC-type ground with EZNEC (and only 10 segments, so this can easily be done with the demo program) and starting with the Vert1.ez example file, the gain of a resonant (~0.24 wavelength) high vertical at 7 MHz with "average" ground is -0.0 dBi at an elevation angle of 26 degrees. Changing the height to 0.625 wavelengths (easily done by first changing Units to Wavelengths) produces a maximum gain of 1.19 dBi at 15 degrees elevation angle. The 1/4 wave trace protrudes outside the 5/8 wave trace only from about 25 to 41 degree elevation. But more interesting is the gain difference at various low elevation angles. The comparison is easily done with EZNEC v. 4.0 by saving the trace from one antenna, then superimposing that pattern on the pattern of the second antenna. By clicking the name of the superimposed pattern in the 2D plot window, a new entry appears in the data box showing the difference between the two at the angle of the cursor. It turns out that the 5/8 wave really shines at really low angles when the ground is poor, but isn't so impressive when the ground is very good -- at least at 7 MHz. Over average ground, the gain difference is at or just above 3 dB up to about 10 degrees. (My explanation of the reason for the difference over real ground was overly simplistic. I apologize.) Above 10 degrees, the difference decreases. Over poor ground, the gain difference is about 4.5 dB up to 5 degrees, and over 4 at 10. So if you have poor ground, you can really benefit from a higher radiator. Over very good ground, though, the difference is about 2 dB up to 5 degrees elevation, only 1.2 at 10 degrees, and less than a dB at 12 degrees and above. So it might or might not be worthwhile to extend the height of a tower for that amount of benefit. Those figures depend on frequency, too, and the pattern shape varies considerably with frequency and ground characteristics. So modeling the particular situation would be a good idea before doing any expensive and extensive tower lengthening. In all the cases I looked at, however, the 5/8 wave vertical did show some gain over a quarter wave vertical up to at least 14 degrees. Whether the difference is worth the added height is up to the individual. Roy Lewallen, W7EL Richard Fry wrote: __________________ I investigated your concept statements using NEC-2 models of 1/4-wave and 5/8-wave verticals in the 40m band (7.3MHz), working against the same infinite ground plane of "Average" parameters. * The 5/8-wave vertical has a peak gain of 0.2dBi, 16 degrees above the horizon. * The 1/4-wave vertical has a peak gain of -6.4dBi, 26 degrees above the horizon, and its entire radiation envelope is always within that of the 5/8-wave. I don't know which range of elevation angles is considered most useful for skywave paths on 40m, but it would appear that with equal tx power, a 5/8-wave vertical always will have a usefully better skywave than a 1/4-wave vertical over a typical ground plane -- and probably by more than 3dB. If you could check my conclusions on this I'd be grateful. RF |
#4
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Roy Lewallen wrote:
[...] modeling the particular situation would be a good idea before doing any expensive and extensive tower lengthening. In all the cases I looked at, however, the 5/8 wave vertical did show some gain over a quarter wave vertical up to at least 14 degrees. Whether the difference is worth the added height is up to the individual. Another height-related factor that may be worth considering is the effect of surrounding the lower part of the antenna by nearby buildings. Inside a typical 2-floor home are 3-D grounded meshes of electrical wiring and (in many countries) central heating pipes. The wiring mesh typically extends up to 6m/20ft above ground, which may be a significant fraction of the antenna height. For example, in the suburban situation here, the lower part of my vertical for 40m was almost completely surrounded by these "scattering objects" at distances ranging from 0.5 to 2 wavelengths. I never got around to modeling the effects of these objects... though someone could easily try it. -- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
#5
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"Roy Lewallen" wrote:
... the gain of a resonant (~0.24 wavelength) high vertical at 7 MHz with "average" ground is -0.0 dBi at an elevation angle of 26 degrees. Changing the height to 0.625 wavelengths produces a maximum gain of 1.19 dBi at 15 degrees elevation angle... Thanks for your comprehensive, civil analysis. It appears either that my incarnation of NEC-2 doesn't deal with this situation properly, or I didn't use it right (the latter is more likely). I'll have a look into it. In all the cases I looked at, however, the 5/8 wave vertical did show some gain over a quarter wave vertical up to at least 14 degrees. ...Over average ground, the gain difference is at or just above 3 dB up to about 10 degrees. .... which supports my contention earlier in this thread: The peak gain increase between a 1/4-wave and a 1/2-wave or 5/8-wave vertical is 3dB above the gain differences of those antennas as dipoles of _twice_ that length in free space. Repeating the reasons for this... * the electrical length of the vertical is doubled by its image below the ground plane (a 1/4-wave vertical monopole becomes an electrical 1/2-wave dipole) * the peak "free space" gain of the monopole and its image is increas- ed 3dB, because all radiation from it is confined to one hemisphere (above the ground). RF |
#6
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Richard Fry wrote:
* the peak "free space" gain of the monopole and its image is increas- ed 3dB, because all radiation from it is confined to one hemisphere (above the ground). Remember, that's for perfect ground only (and maybe salt water ground). If one buries half of a dipole in earth ground, one loses most of that 3 dB to the ground. For instance, EZNEC reports: The max gain of a 40m 1/4WL vertical with 8 horizontal radials one foot above average ground is -0.29 dBi. Raising the radials to one wavelength above ground increases the max gain to +3.23 dBi. (Of course, the 3D radiation patterns are not exactly the same but the correlation to that 3 dB of image power is in there because of decreased ground losses at increased height.) Problem: Most everyone with a 1/4WL vertical and four buried radials is throwing away about half of his/her source power. Solution: Put up a horizontal dipole. :-) -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
#7
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"Cecil Moore" wrote
Richard Fry wrote: * the peak "free space" gain of the monopole and its image is increas- ed 3dB, because all radiation from it is confined to one hemisphere (above the ground). Remember, that's for perfect ground only (and maybe salt water ground). If one buries half of a dipole in earth ground, one loses most of that 3 dB to the ground. ____________ According to the empirical results of AM broadcast radiators, and also Roy Lewallen's EZNEC numbers in his last post in this thread, the ground plane itself doesn't need to be perfect, or maybe salt water to realize the gain improvement. It's just that a very low resistance connection to it must exist for the vertical to work against. RF |
#8
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Richard Fry wrote:
According to the empirical results of AM broadcast radiators, and also Roy Lewallen's EZNEC numbers in his last post in this thread, the ground plane itself doesn't need to be perfect, or maybe salt water to realize the gain improvement. Point is, it has to be near perfect. 4 buried radials just won't hack it. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
#9
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A good fraction of your power is lost to the ground even if you have a
perfectly lossless ground system. To find the amount, use EZNEC (the demo program is perfectly adequate), select MININEC-type ground and appropriate ground constants. Set the plot type to 3D and run a pattern calculation. The "Average Gain" reported at the bottom of the main window is the loss due to ground reflection. Short of making a ground screen several wavelengths in diameter, all you can do to improve this is to move to a location where the ground is more conductive. For example, the example file Vert1.ez shows a loss of over 5 dB in the total radiated signal. Ground reflection loss applys to horizontal antennas, too. But with horizontal antennas it primarily attenuates the very high angle radiation, while with verticals it gets the low angle radiation. You can easily see its effect by superimposing a plot calculated with perfect ground over a plot calculated with MININEC-type (if there's a direct ground connection) or Real, High Accuracy(*) (if there isn't) ground. (*) For NEC-2 users, this is EZNEC-speak for Sommerfeld ground. Reflection coefficient ground doesn't provide any real advantage with modern computers, so it's no longer an option in EZNEC. In v. 3.0 and earlier versions it was called "Real, Fast Analysis" ground. Roy Lewallen, W7EL Cecil Moore wrote: Richard Fry wrote: * the peak "free space" gain of the monopole and its image is increas- ed 3dB, because all radiation from it is confined to one hemisphere (above the ground). Remember, that's for perfect ground only (and maybe salt water ground). If one buries half of a dipole in earth ground, one loses most of that 3 dB to the ground. For instance, EZNEC reports: The max gain of a 40m 1/4WL vertical with 8 horizontal radials one foot above average ground is -0.29 dBi. Raising the radials to one wavelength above ground increases the max gain to +3.23 dBi. (Of course, the 3D radiation patterns are not exactly the same but the correlation to that 3 dB of image power is in there because of decreased ground losses at increased height.) Problem: Most everyone with a 1/4WL vertical and four buried radials is throwing away about half of his/her source power. Solution: Put up a horizontal dipole. :-) |
#10
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Richard Fry wrote:
... which supports my contention earlier in this thread: The peak gain increase between a 1/4-wave and a 1/2-wave or 5/8-wave vertical is 3dB above the gain differences of those antennas as dipoles of _twice_ that length in free space. Things seem to be getting a little confused here. When you replace a free space environment with a perfect ground plane, the *average* field strength of *all antennas* increases by 3 dB for a given power input because of the reduced volume. This shows up as a 3 dB gain increase when the gain is referenced to a free-space antenna such as an isotropic source. No antenna is given any additional advantage over any other - they all get the same amount of increase. So if I read the above statement correctly, it's not true. The gain increase between a 1/4 and 1/2 or 5/8 wave antenna over a perfect ground is the *same* as the gain increase between a 1/2 and 1 or 5/4 wave dipole in free space. Not 3 dB greater. If you'll look at the patterns of the antennas, you'll find that the pattern of a 1/4 wave vertical over perfect ground is identical in shape to half the pattern of a 1/2 wave free space dipole, but 3 dB stronger. Likewise for any other vertical and its twice-as-long free space dipole counterpart. When the perfect ground is replaced by real ground, an attenuation factor is introduced which actually changes the pattern shape. This pattern shape change is different for each height of vertical because it depends on the angle at which the radiation from each part of the antenna strikes the ground. The different antenna heights have different current distributions and so different fractions of the total radiation hits the ground at different angles. The effect of the attenuation at each elevation angle depends on the ground constants and the frequency. You're probably more used to looking at surface wave attenuation, where this ground reflection effect doesn't exist. Instead, there's a single frequency and ground dependent attenuation that's essentially the same for all antenna heights. What I'm talking about here is sky wave radiation which consists of both a directly radiated "ray" (which undergoes no attenuation other than that caused by its expanding volume with distance) and a "ray" reflected from the ground. It's the attenuation and phase shift of this second "ray", which depends on the elevation angle, ground constants, and frequency, which causes the pattern shape modification and attenuation of low angle signals. If you look into the way NEC-2 operates you'll see that it does just this calculation. The relationship of the reflected ray before and after striking the ground is described by a fairly simple reflection coefficient, which is quite different for horizontally and vertically polarized waves. If you assume a current distribution, it's not difficult to calculate the pattern manually. The reflection coefficients can be found in Kraus. Repeating the reasons for this... * the electrical length of the vertical is doubled by its image below the ground plane (a 1/4-wave vertical monopole becomes an electrical 1/2-wave dipole) I don't think that's a good use of the term "electrical length". It is true that the radiation pattern of a 1/4 wave vertical over perfect ground (but not imperfectly conducting ground) is the same as that of a half wave dipole in free space. Also, its feedpoint impedance assuming no loss is exactly 1/2 that of a 1/2 wave dipole in free space. * the peak "free space" gain of the monopole and its image is increas- ed 3dB, because all radiation from it is confined to one hemisphere (above the ground). Yes, but this is altered if the ground isn't perfect. When the ground isn't perfect, the shape of the pattern of the monople is no longer the same as half a free space dipole, so the gain difference is no longer a constant 3 dB at all angles. Some of the radiated energy is lost in the ground reflection, and the fraction which is, depends on the angle at which it strikes the ground. Roy Lewallen, W7EL |
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