Tom et al,'this a hobby, pse dont take it that seriously
73
Len
SM7BIC
-----Ursprungligt meddelande-----
Från: Topband [mailto:topband-bounces@contesting.com] För Tom W8JI
Skickat: den 17 december 2012 18:55
Till: topband@contesting.com
Ämne: Re: Topband: GAP Vertical Question
To work at its maximum efficiency a vertical needs a real ground
system and the image is its fictitious counterpart to isotropic. Im
oversimplyfing here so no need to pick nits.
Besides being untrue, that is confusing or misleading.
1.) Some verticals need no ground. What would also be true is that end-fed
antennas always require a counterpoise of some sort, because there always
has to be a second terminal of some type for the feedline to "push
against".
2.) The image is a shortcut tool used to allow longhand pattern
calculations. It is not used for efficiency, antenna descripition, or
actual
operation.
3.) dBi, on the other hand, is a reference condition for a field strength
ratio.
According to Kraus that image, mirror, or whatever you care to call it
occurs at a distance from the base and at a mathematical relationship
to where the current peak is on the vertical radiator.
In the case of this half wave discussion the reflection occurs around
.35 wave out unless you, or others, want to try and discredit Kraus.
Thus radials do work with a 1/2 wave and "system" efficiency is
dependent upon the actual ground and how well the radials perform.
Since this forum is predominantly DX oriented I prefer to qualify the
"system" efficiency by how well the main lobe extends between its peak
and the ground. IOW those low angles needed for DX.
There is not wrong with what Kraus teaches. The problem comes when we
misunderstand or misapply what he teaches.
Using modeling it is easy to realize that significant degradation of
the radiated field at the lower angles is very real.
I'm not sure models we use are all that meaningful at low angles on low
bands. They are OK on extended groundwave, and probably OK on upper HF.
They are, however, all we have.
Various verticals (mostly VHF/UHF) on tall buildings or towers are not
subjected to those ground losses and place a strong signal at the
horizon.
I'm not going to touch that one, other than to say ground losses for a
given
soil and condition are dependent of intensity of the electric, magnetic,
and
electromagnetic fields in a given volume of lossy media.
This is why we can have moonbounce, even though losses in the moon's
surface
are horrible, and why moving an antenna up away from earth or distributing
the fields over a wider area by using more radials reduces loss.
Where we create a misunderstanding or problem is when we ignore how it
works, and pretend all field intensities in a given volume of lossy media
are equal at all distances with all antennas. When we do that, we get
false
ideas................such as half wave verticals have high loss without
large radial fields. If that was true, our horizontal half-wave dipoles 1/4
wave or more high would have poor efficiency without large counterpoise
fields below the dipole.
73 Tom
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Topband reflector - topband@contesting.com
_______________________________________________
Topband reflector - topband@contesting.com