Hi Bob,
Good point but...let see what John says on he's book.
By the way, I have the 4th edition and the statement you refere to is on page
9-17 at the beginning of chapter 2.1.7.1 "Local ground characteristics".
"It is impossible to make a direct measurement of ground characteristics. The
most reliable source of information about local ground characteristics may be
the engineer of your local AM broadcast station. The so-called "full proof
of-performance" record will document the average soil conductivity for each
azimuth out to about 30 km (20 miles). But unfortunately this is hardly what
you need to know. What you need is the ground characteristics in a circle with
a λ/4 radius around the base of your vertical!"
I think John refere as "impossible" to measure the whole ground characteristics
of the ground around the antenna (at least in the near field), "piece by piece"
centimeter by centimeter which is right (correct)! But, based on several
measurements on the given area, the ground characteristics could be determined
statistically which is enough for our purpose.
Such measurements (as he stated above) can be done...by the AM broadcast
engineer.
One mistake is that the near field of an antenna is about half wave length
around it (not quarter wave length!). I think John wants to refere to a radial
length which make difference in the near field from efficiency point of view.
Another thing to consider is the ground on far field which is beyond half wave
length (let say till 30km away - to be in accordance with John's statement).
This ground have much influence on the vertical radiation angle of an antenna
but not on its efficiency!
Now, to all:
When I start those measurements I was interested to find out what are the
limits of the ground conductivity (in my place) and what is the impact of this
variation on the far field of the antenna, not the near field!!!
As one can observe from my previous e-mail, I was not interested of the antenna
efficiency but of its vertical radiation angle.
As "we all know", the antenna radiation pattern is formed on far field not on
near field.
As John says, is practically (I should say "nearly") impossible to measure the
ground characteristics on near and especially far field but...one can measure
accurately its limit (worst/best case) as I did. This is close correlated with
ground humidity.
The real point of such measurements/discussions is the way a radiated wave will
reach the ionosphere (incidence angle) and how will propagate further (i.e.
DX!).
All propagation models have as a primary input this angle.
As measurement shows, in my case this angle has 37% variation. Is it much?
From Terman (as Richard Fry mentioned):
http://i62.photobucket.com/albums/h85/rfry-100/TermanFig55.jpg
First hop distance on E layer reflection for given angles is:
20 degrees = 300miles
30 degrees = 200miles
I'll leave you all to consider this.
73 de YO3FFF
Cristi
________________________________
From: Bob Kupps <n6bk@yahoo.com>
To: topband <topband@contesting.com>
Sent: Wednesday, October 3, 2012 10:35 AM
Subject: Topband: ground characteristics
Hi I have no idea if it is a fact but on page 9-18 in ON4UNs book it is stated
without any further explanation -
"It is impossible to make a direct measurement of ground characteristics".
_______________________________________________
UR RST IS ... ... ..9 QSB QSB - hw? BK
_______________________________________________
UR RST IS ... ... ..9 QSB QSB - hw? BK
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