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#7
March 8th 15, 08:06 PM
posted to rec.radio.amateur.antenna
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E/M radiation from a short vertical aerial
On Sunday, March 8, 2015 at 3:40:21 AM UTC-5, Spike wrote:
On 08/03/15 00:17, wrote: True ground wave, which to me, is the same as the surface wave, actually can follow the curvature of the earth, which a space wave cannot do. But true ground or surface waves are generally only taken advantage of on the lower frequencies such as MW, or LW. That's true, which is a shame as useful ground-wave/surface wave can be had on 28 MHz; a maximum range figure for a path over ground of average conductivity might be 25 miles, and considerably more if the path is over water (especially sea-water). That's space wave on 10m. Even seeing a surface wave on 40m is a bit of a stretch from the norm. As I mentioned in my 2nd post, the reason I saw farther than expected from space wave operation on 40m, could well have been due to refraction of the space wave, and due to the fact that the radio horizon is farther than the visual horizon. I used to work local 10m all the time back in the 80's, early 90's.. 25 miles is fairly easy with any decent antenna, at a decent height above ground. I used to work a good bit farther than that fairly often, when using an antenna at 35-45 feet up. The reason I think so, is because the distances I could work with it were a good bit farther than what I would expect with the space wave alone. Maximum surface wave over ground with average conductivity might be 40 to 45 miles on 40m; if you were getting ranges over this, then your ground conductivity might have been enhanced, or due to the height of your ground-plane, you could have experienced refraction of the space wave. If your location was on a hill-top or other high ground, this could have helped the space wave refraction as well. The ground is good here, and the ground plane was full size at 36 ft at the base of the antenna. But it may well have been an enhanced space wave. I was often working well over 100 miles away in such a case. I believe that the availability 24/7/365 of the space-wave and surface-wave is one of Amateur Radio's undervalued assets. On 160m the surface wave might reach over 100 miles, including behind hills and into valleys, which here in the UK would enable a station to reach a significant proportion of the UK Amateur population. Unfortunately, people dismiss verticals in favour of horizontals of one form or another, the usefulness of which drops to zero when the sky wave disappears (apart from any vertically-polarized radiation from a mismatched feeder or unbalanced elements). Well, not everyone does. I know many on 160m who favor verticals. Not only for ground wave, but better DX. The ground wave is pretty good on 160m if using a vertical. Nearly as good as on the MW AM broadcast band, being the two bands are right next door to each other, so to speak. |
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