RadioBanter - View Single Post - E/M radiation from a short vertical aerial

Brian Reay[_5_] · March 8th 15, 11:21 PM posted to rec.radio.amateur.antenna

On 08/03/15 20:06, wrote:
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,

With respect, the difference in local ground is rather over stated.

Taking the US as an example, the conductivity ranges from 0.5mS to 30mS,
which sounds a lot. However, compared to sea water, 5000mS, it is all
rather poor.

I noticed this some years back when reading a paper, as I recall written
by the US Navy, which played down the importance of ground conductivity,
other when either at sea or in close proximity to the shore.

There is a world atlas of conductivity which is on the web, I can't
recall the URL, but it is worth looking out. The various seas and oceans
do vary, I recall the Baltic being less conductive for example.
Likewise, some of the patterns in the various countries are rather
intriguing. Some areas you would expect to be conductive are not. I
assume due to local rock formations etc.