OT: Ground Loss (long)
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I don't know why the server always does that my posts. Sorry. Last try. Then I give up.
If you have an antenna modeling program like EZNEC you can perform a simple experiment to illustrate a strange behavior of ground loss. The EZNEC example "Elevrad1" is a 1/4-wave vertical with 4 radials about 2 cm above ground. I re-scaled the antenna to 40 meters while keeping the radial height the same. . The "common" or "average" ground that everybody talks about is conductivity = 5 milliSiemens per meter (mS/m) and relative permittivity = 13. With this ground EZNEC gives an antenna gain of -2.46 dB at a takeoff angle of 26 degrees. If conductivity = 30 mS ("very good ground") the gain improves to -0.47 dB while the elevation angle lowers to 22 degrees. This is what we expect. Now, we want to see what happens as the conductivity gets *worse*. Here are the EZNEC results, where the 1st column is conductivity in mS and 2nd column antenna gain in dB. 30 -0.47 15 -1.68 10 -2.25 8 -2.44 5 -2.46 4 -2.30 2 -1.52 1 -0.78 0.1 +0.12 0.01 +0.20 See what's happening? There's a point where the loss hits maximum, but then the system gain actually improves as the ground worsens more. In my experience poorer ground also has lower permittivity. Setting permittivity = 4, the losses do increase (so that there are no positive values) *but the trend remains*. It's also true that the elevation angle increases as the ground gets worse, but it stays in the 20 - 30 degree range and doesn't get crazy or anything. This suggests that ground loss is sort of an impedance-matching problem. Clearly, as your ground gets really bad your antenna can still work just fine, even better in some cases! Totally unintuitive. But upon further reflection it makes sense that an "insulator" as ground is okay, otherwise an antenna in free space wouldn't work. I never meant to suggest something crazy like that one not use radials; I only wanted to point out a peculiar characteristic of verticals above real ground. I will re-state my original thesis this way: If you have perfectly conducting ground, there is no ground loss. If you have perfectly insulating ground, there is no ground loss. There's always some ground conductivity in between those extremes at which the loss is maximum. This value depends on the frequency. Al W6LX ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
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On Wed,7/22/2015 6:15 PM, Al Lorona wrote:
> This suggests that ground loss is sort of an impedance-matching problem. Clearly, as your ground gets really bad your antenna can still work just fine, even better in some cases! Hi Al, Several observations. First, soil influences vertically polarized antennas in two ways. First, loss in the soil near the antenna as the field from the antenna causes current flow in the ground. The result is simple I squared R loss, and that power does not get radiated. Second is the reflection from the earth in the far field where the radiated field hits it. That reflection adds (algebraically) to the direct signal to form the vertical pattern. The strength of the combined direct signal plus reflected signal depends on the relative phase angle between them, which in turn is a factor of distance, the elevation angle, and the soil. The second point is that conductivity is not the only soil parameter. There is also the dielectric constant. In EZNEC, if you open the Ground Description tab and right click on either Cond or Diel Const, you'll get a window showing a continuum of soil types from very good to very poor. BOTH Cond and Diel Const vary with those soil types, and both quantities affect how the antenna behaves. A year or two ago, I did an extensive study of the interaction of vertically polarized antennas with soil of different types and at different mounting heights. I presented it to the Pacificon Antenna Forum using these slides. http://k9yc.com/VerticalHeight.pdf 73, Jim K9YC ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
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Near field soil conditions become less of a factor with the use of
elevated radials. Consider a "ground plane antenna" with the base mounted at the rooftop of a 1 or 2 story building - it is reasonably independent of near field ground conditions because it is an antenna within itself - much the same as a vertical dipole. A vertical with buried radials is quite dependent on ground conductivity unless the number of radials is large. The far field ground conditions still do play a part and what Jim has pointed out below is valid. IMHO, the main advantage of a vertical is that it has a null at the top of the elevation plots which will reduce the response to high angle reception (nearby stations). If you look at the elevation plot for a horizontal dipole along with a vertical, you will see that the dipole has almost as much gain at low angles as the vertical - so from a pure gain standpoint, the vertical offers no advantage. BUT that is comparing a vertical with a high dipole (at least 1/2 wavelength high). For the lower frequency bands, the height of a dipole with those characteristics is not practical for most hams and the vertical wins "hands down" for DX - provided a good radial field or elevated radials are used. 73, Don W3FPR On 7/23/2015 11:10 AM, Jim Brown wrote: > On Wed,7/22/2015 6:15 PM, Al Lorona wrote: >> This suggests that ground loss is sort of an impedance-matching >> problem. Clearly, as your ground gets really bad your antenna can >> still work just fine, even better in some cases! > > Hi Al, > > Several observations. First, soil influences vertically polarized > antennas in two ways. > > First, loss in the soil near the antenna as the field from the antenna > causes current flow in the ground. The result is simple I squared R > loss, and that power does not get radiated. > > Second is the reflection from the earth in the far field where the > radiated field hits it. That reflection adds (algebraically) to the > direct signal to form the vertical pattern. The strength of the > combined direct signal plus reflected signal depends on the relative > phase angle between them, which in turn is a factor of distance, the > elevation angle, and the soil. > > The second point is that conductivity is not the only soil parameter. > There is also the dielectric constant. In EZNEC, if you open the > Ground Description tab and right click on either Cond or Diel Const, > you'll get a window showing a continuum of soil types from very good > to very poor. BOTH Cond and Diel Const vary with those soil types, and > both quantities affect how the antenna behaves. > > A year or two ago, I did an extensive study of the interaction of > vertically polarized antennas with soil of different types and at > different mounting heights. I presented it to the Pacificon Antenna > Forum using these slides. http://k9yc.com/VerticalHeight.pdf > > 73, Jim K9YC > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[hidden email] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > Message delivered to [hidden email] > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
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True, at least from my experience. If you're on a pointy part of the
Earth [aka summit such as in SOTA], the far field effects can and often do lower the max elevation angle to or below the apparent horizon. Most antennas on summits exhibit a host of other inefficiencies as well, but the effect is there. I used a vertical GP with my Buddipole on summits only because the horizontal OCF loaded dipole configuration was worse. One thing that hasn't been mentioned is noise on RX. It seems like the preponderance of man-made noise is vertically [more or less] polarized. Until last Sunday, I had a Gap Titan on the roof. Good antenna, very low SWR on all its bands. Nearly all the time, noise was 1-3 S-units higher than on my tribander or dipoles. I did keep checking because occasionally, mainly on 12 and 10, it was lower than the TB. Man-made noise may not be a problem in a remote, back-packing environment of course. 73, Fred K6DGW - Northern California Contest Club - CU in the 50th Running of the Cal QSO Party 3-4 Oct 2015 - www.cqp.org On 7/23/2015 4:30 PM, Don Wilhelm wrote: > For the lower frequency bands, the height of a dipole > with those characteristics is not practical for most hams and the > vertical wins "hands down" for DX - provided a good radial field or > elevated radials are used. > > 73, > Don W3FPR ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
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In reply to this post by alorona
Hi All,
I cannot improve, or add much to, the excellent commentary provided by Jim Brown and others. However, I can say that my "experience" tends to confirm a lot these findings. First, I originally had an R7 vertical ground mounted. Performance was "O.K.", but certainly not very exciting. Later I elevated that antenna to about 15 ft. above ground, and the improvement was very noticeable. Subsequently I have replaced the R7 with an F8 (acquired in an estate sale), and the results are very much the same--the antennas are not all that much different anyway, but the R8 is a bit taller, and does cover 6 meters (after a fashion!). If you look at the study Ward Silver, N0AX, did many years ago on various vertical antenna systems, the R8 is rated one of the very best. In any event, I've worked well over 100 countries QRP on 40 meters using this arrangement. After hearing Rudy Severns' presentation about ground systems at Pacificon several years ago, I began elevating radials when using my portable antenna systems while camping. Most of this was done using either an MP-1 or a Biddipole configured as a Buddistick. Again, the results were very rewarding. As Rudy indicates, elevating your radials even just a small amount can result in significant benefit. Usually I try to deploy 4 elevated radials. Sometimes, though, only 1 or 2 are practical under the circumstances. Still, it helps! I also acquired a used Sigma 40XK, and now use it as my primary 40 meter antenna at home. That antenna is raised about 6 feet off the ground, so the overall height is about 20 feet or so. I wish I could go higher, but neighborhood restrictions prevent that. I get a 1 to 3 Db improvement with this antenna over the R8, measured by comparisons using the RBN system. I've become a big fan of vertical dipoles! When I go QRO (500 watts for me), I get some reports from DX stations that I consider to be exceptional. I also monitor myself on the RBN, so I have some idea as to who might hear me and who can't. It's almost better than trying to interpret some of the propagation software. I also have one of N6BT's Bravo 7 antennas, which he calls vertical dipoles also. By the way, N6BT also designed the Sigma 40KX when he owned Force 12. Anyway, the Bravo 7 works pretty well as a portable system, but I'm not quite as happy with it as a 40 meter system. Still, it is very portable, and easy to put up and take down. N6BT makes some pretty aggressive claims about the performance of this system, but I am pretty sure most of his "testing" was done over salt water. In any event, the higher up I deploy that system, the better it seems to work. I think Jim Brown's suggestion of adding a better radial system to vertical dipoles may be exactly what I need to try and do with the Bravo 7, particularly for 40 meter operation. Soil conditions here in Arizona are pretty awful. It's not very often that I go camping anywhere that offers anything much better. Deploying elevated radials can be problematic, but it certainly seems worth the effort most of the time. Dave W7AQK ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
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It's kind of interesting that hams see buried/on ground
On Saturday, July 25, 2015, dyarnes <[hidden email]> wrote: > Hi All, > > I cannot improve, or add much to, the excellent commentary provided by Jim > Brown and others. However, I can say that my "experience" tends to confirm > a lot these findings. > > First, I originally had an R7 vertical ground mounted. Performance was > "O.K.", but certainly not very exciting. Later I elevated that antenna to > about 15 ft. above ground, and the improvement was very noticeable. > Subsequently I have replaced the R7 with an F8 (acquired in an estate > sale), and the results are very much the same--the antennas are not all > that much different anyway, but the R8 is a bit taller, and does cover 6 > meters (after a fashion!). If you look at the study Ward Silver, N0AX, did > many years ago on various vertical antenna systems, the R8 is rated one of > the very best. In any event, I've worked well over 100 countries QRP on 40 > meters using this arrangement. > > After hearing Rudy Severns' presentation about ground systems at Pacificon > several years ago, I began elevating radials when using my portable antenna > systems while camping. Most of this was done using either an MP-1 or a > Biddipole configured as a Buddistick. Again, the results were very > rewarding. As Rudy indicates, elevating your radials even just a small > amount can result in significant benefit. Usually I try to deploy 4 > elevated radials. Sometimes, though, only 1 or 2 are practical under the > circumstances. Still, it helps! > > I also acquired a used Sigma 40XK, and now use it as my primary 40 meter > antenna at home. That antenna is raised about 6 feet off the ground, so > the overall height is about 20 feet or so. I wish I could go higher, but > neighborhood restrictions prevent that. I get a 1 to 3 Db improvement with > this antenna over the R8, measured by comparisons using the RBN system. > I've become a big fan of vertical dipoles! When I go QRO (500 watts for > me), I get some reports from DX stations that I consider to be exceptional. > I also monitor myself on the RBN, so I have some idea as to who might hear > me and who can't. It's almost better than trying to interpret some of the > propagation software. > > I also have one of N6BT's Bravo 7 antennas, which he calls vertical > dipoles also. By the way, N6BT also designed the Sigma 40KX when he owned > Force 12. Anyway, the Bravo 7 works pretty well as a portable system, but > I'm not quite as happy with it as a 40 meter system. Still, it is very > portable, and easy to put up and take down. N6BT makes some pretty > aggressive claims about the performance of this system, but I am pretty > sure most of his "testing" was done over salt water. In any event, the > higher up I deploy that system, the better it seems to work. I think Jim > Brown's suggestion of adding a better radial system to vertical dipoles may > be exactly what I need to try and do with the Bravo 7, particularly for 40 > meter operation. > > Soil conditions here in Arizona are pretty awful. It's not very often > that I go camping anywhere that offers anything much better. Deploying > elevated radials can be problematic, but it certainly seems worth the > effort most of the time. > > Dave W7AQK > > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[hidden email] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > Message delivered to [hidden email] > -- Sent via Gmail Mobile on my iPhone ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
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Please ignore. Unintended "send".
On Sat, Jul 25, 2015 at 2:20 PM, Guy Olinger K2AV <[hidden email]> wrote: > It's kind of interesting that hams see buried/on ground > > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
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In reply to this post by w7aqk
On Sat,7/25/2015 10:16 AM, dyarnes wrote:
> I also have one of N6BT's Bravo 7 antennas, which he calls vertical > dipoles also. By the way, N6BT also designed the Sigma 40KX when he > owned Force 12. Anyway, the Bravo 7 works pretty well as a portable > system, but I'm not quite as happy with it as a 40 meter system. > Still, it is very portable, and easy to put up and take down. N6BT's verticals ARE vertical dipoles, loaded in some quite innovative ways. W6GJB and I are building an 80M vertical dipole for FD and CQP based on some of Tom's ideas. > N6BT makes some pretty aggressive claims about the performance of this > system, but I am pretty sure most of his "testing" was done over salt > water. I've not known Tom to "puff" his antennas with exaggerated claims. He's a very good designer, and my experience with him is nothing but honesty. It is, however, well known that verticals work awfully well over salt water, and he's written extensively about that. > In any event, the higher up I deploy that system, the better it seems > to work. I think Jim Brown's suggestion of adding a better radial > system to vertical dipoles may be exactly what I need to try and do > with the Bravo 7, particularly for 40 meter operation. After having done some more modeling, I'm backpedaling on that suggestion. You'd need a LOT of radials over pretty awful ground to see a dB or so. Note also that the horizontal elements at the bottom of Tom's vertical dipoles are NOT radials, they are capacitive loading for the bottom half of the dipole. Don't try to add to them -- you'll detune the antenna. :) 73, Jim K9YC ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
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