Hello all
While compiling an article about the german LW-BC transmitter
site "Donebach", I come acros some interesting facts about the
first antenna system which was in use there from 1967 through
1970 - a multiple tuned vertical, based on the design principle
of Ernst F.W. Alexanderson.
Four steel lattice masts, height 200m, cross section 2m by 2m
are aligned in a star figure. All mast are isolated from ground
at their bases. Each mast has its individual burried radial
system consisting of 100 radials 200m each. The radials are
made of zinc plated steel "tape" (30mm by 3mm approx.) burried
half a meter below ground level. The total length burried
calculates as 80km (nowadays system 2*60*360m=43.2 km).
The mast feeding the antenna system is placed in the center of
a triangle.
From center 330m to zero degrees is placed the first,
From center 330m to 120 degrees is placed the second,
From center 330m to 240 degrees is placed the third periferal mast.
A top load is spun from the central mast to each of the periferal
masts. Each of the three toploads comprise 10 "wires" spaced by 1m
making a rectangle 10m by 330m (there is no means to connect one
periferal mast to the other directly, only via central mast).
Operating frequency that time was 151.0 kHz (1987m WL).
The base of the central mast is fed by a T-Network
(base)series L1, shunt C, series L2(coax).
The bases of the three periferal mast are connected via
L3 to ground.
The approximate Reactances are:
X(L1)=j157 Ohms
X(L2)=j60 Ohms
X(L3)=j94 Ohms
X(C)=-j60 Ohms
The head designer of the first Donebach antenna system
Dr. Heinz Graziadei claimes the efficiency of the antenna
system to reach 95% (the two mast system in use nowadays is
said to have an efficiency of 88%).
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Hi there down under in ZL:
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It is said that in Paengatoa (New Zealand) we may find
(if still existing) a 100% copy of the Donebach-1 Antenna System.
Perhaps you can check that unconfirmed info and take pictures ??
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Using a very simplified model (top load modeled by one wire only
having 2m equivalent diameter, all masts modeled as wires 2m in
diameter) of the antenna, defining Ground as perfect, wire losses
as zero (to obtain the radiation resistance only) using a test
version of ELNEC I get the following numbers:
Rfeedpoint (Rrad) is about 118 Ohms
current in central mast (base/top) 1.00A/0.62A
current in all periferal masts (base/top) 1.02A/0.65A
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Historical add-on:
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From 1967-1970 the LW-BC transmitter site Donebach
(49N39, 09E11) was allowed to use 250 kW transmitter
(HF output power) and an omnidirectional radiation
pattern in azimuth on the assigned frequency of 151 kHz.
The Romanian LW-BC station in Brasov (45N43, 25E36) used the same
frequency with 2000 kW transmitter and omnidirectional
radiation pattern in azimuth. There are even more stations
sharing the same allocated frequency but their ground distances
to Donebach are at least twice as much as in between Donebach and
Brasov.
The ground distance in between latter sites is about 1300 km
and was assumed sufficient to provide protection of each stations
intended coverage area - a false assumption. Receiption of
Brasov Program in the intended coverage area of Brasov was
systematically "jammed" by the Donebach Program - WHY ???
The Donebach site is at 510m above sea level, an area of 2 sqkm
with level variations below 10m. The site, in the middle of the little
mountain area of "Odenwald", formerely was a marshland. During
WW2 there was an attempt to install an airfield there. The terrain
was drained and leveled to some extend. The site nowadays is still
marshy, i.e. you get wet shoes when walking around, except for
the hottest summer season. Ground conductivity approximates
three milliSiemens. There is a virtually unobstacled "free-shot" to
the horizon in any direction.
I do not have details of the Brasov site, all I heared, the ground
conductivity is "very bad", perhaps caused by the porous carbon
type of ground.
The Donebach site therefore has an natural benefit over the
Brasov site. Rating the excellent Donebach-1 Antenna System
it is easy to understand, why the 250kW classed Donebach site
could outperform the 2MW classed site in Brasov in it's
own coverage area !!
As a temporary countermeasure, Donebach reduced power to 120kW
and 60kW without much help for the Brasov coverage area. In 1970
the service in Donebach was stopped (Mainflingen took over temporarily)
in order to convert the omnidirectional Donebach-1 Antenna System into
a 2 mast directional antenna system (Donebach-2 Antenna System).
This did not help either, as two of the former masts were re-used at
their original location, the relative distance (330m) was insufficient to
provide the required cardioide azimuth pattern with a deep enough
Null in direction to Brasov.
The final solution was to completely redesign the antenna system
(went in operation Oct 1982) to what we know today as the
Donebach-3 Antenna System (see my recent "profile").
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73 de Gamal
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