Electrical characteristics of the mesh or screen electrodes used in electron tubes are improved by decreasing the shunt capacitance of the tube while retaining the close spacing needed for the required resolution. Vibrational characteristics are enhanced by raising the natural resonant frequency

Nelson, R. E.

English

NASA Technical Reports Server

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Mesh Su
Prior Method
August 1969	 Brief 69-10097 
.
	
NASA TECH BRIEF 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
Mounting Method Improves Electrical and Vibrational

Characteristics of Screen Electrodes 
Improved Method 
Target Supports	 Approx. Target Supports	 Aprox. 
.tpport 
Close Spacing	 Insula 
Weld R 
Mesh Su
tpport 
red Mesh at 
Spacing 
A method has been devised to improve the elec-
trical and vibrational characteristics of the mesh or 
screen electrodes used in electron tubes. The primary 
application is in TV camera tubes, particularly SEC 
camera tubes. The SEC target consists of a thin film 
composite of aluminum on a substrate of aluminum 
oxide. The composite is then coated with a very low 
density layer of KCI, which functions as a secondary 
emission conduction member. A fine mesh screen 
electrode is placed between the KC1 side of the target 
and the electron gun and is closely spaced (0.010") 
from the KC1 layer. The screen is planar and lies 
parallel to the plane of the target. The spacing and 
parallelism are required during tube operation to 
create a uniform field between the target and mesh 
and to limit the surface charge on the target.
The prior method of suspending the suppressor 
mesh in typical secondary electron image conversion 
tubes is to stretch the mesh between concentric hoops 
of appropriate metal and to position the resultant 
disk of mesh close to the target by appropriate 
mechanical means. The resultant configuration has 
a natural resonant frequency that depends on the 
diameter of the disk, the mesh mass, and tension. 
The resultant configuration is adversely affected if 
the tube is subjected to mechanical vibrations. These 
vibrations may be severe, ranging from those encoun-
tered on ground'vehicles and aircraft to those of space 
vehicle launching. The resultant vibration of the mesh 
may be destructive to the point of mesh breakage; 
or by coupling energy into the fragile thin film target, 
it can cause partial or total disruption of the secondary 
Am	 (continued overleal) 
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emission layer. In severe cases, the mesh can 
touch the target and cause a catastrophic failure. 
In addition, the relative motion between the target 
and mesh due to the vibration of either member 
results in a variation of capacitance of the mesh-
target assembly. The change of capacitance causes 
a change in signal current which, under operational 
conditions, can be severe enough to obscure all 
usable information. The juxtaposition of the target 
to the mesh assembly is also the controlling element 
in the shunt capacitance of the tube. It is desirable 
to keep this capacitance as low as possible to mini-
mize the inherent noise of the preamplifier. 
In the new design, the electrical characteristics 
are enhanced by decreasing the shunt capacitance 
of the tube by 20 percent while retaining the close 
spacing needed to obtain required resolution. This 
is accomplished by the mesh configuration which 
has a much smaller area of piano mesh membrane 
in juxtaposition to the piano portion of the target. 
The remainder of the mesh falls away from the 
piano area of the target, thereby increasing the dis-
tance and decreasing the assembly capacitance. A 
small decrease in capacitance also occurs, as it is 
possible to use a 12 percent wider target-to-mesh 
ceramic spacing. The resultant capacitance has been 
decreased to 80 percent of the prior method. A lower 
capacitance could also be gained by use of ceramic 
pillars and smaller sections of ceramic flanges. The 
vibrational characteristics of the mesh are enhanced 
by raising the natural resonant frequency above the 
range encountered in military and space usage of 
the tube. This is accomplished by stretching the 
mesh membrane over a ridged area of the mesh 
support in the same manner that a violin string is 
stretched over a bridge. The ridged surface provides 
the mesh with a definite, continuous contact area. 
The mesh is always under tension across the ridged 
area and between the ridged area and the weld ring. 
This provides for maximum flatness and reproduci-
bility of the mesh assembly and defines the mesh's 
maximum area, shape, and vibrational characteristics. 
Previous methods of rastering the mesh (not illus-
trated) do not provide for a firm contact along the 
periphery of the raster. The ridged area also allows 
the mesh to be only slightly larger than the usable 
area of the target. The ridge allows adequate space 
for the mesh weld ring and use of the minimum 
mesh opening in close proximity to the target. In 
the prior method, the mesh opening must be larger 
than the total target support surface, which sets into 
a well that is defined by the mesh and mesh sup-
port. Because of this, a larger mesh diagonal is 
required. The new design makes it possible to use 
a mesh diagonal of about 80 percent of that required 
with the prior method, thereby increasing the resonant 
frequency of the system. The resonant frequency is 
also increased by 30 percent by use of the rectan-
gular shape of the rastered mesh as opposed to 
the circular shape of the standard mesh. The net 
result is to increase the natural resonance frequency 
of the mesh from the 2200 Hz area of an older 
design to the 3800 Hz area of the new design. 
The amplitude of the subharmonics and beats of the 
target mesh assembly are thereby decreased. 
Note: 
No further documentation is available. Inquiries 
may be directed to: 
Technology Utilization Officer 
Marshall Space Flight Center 
Huntsville, Alabama 35812 
Reference: B69-10097 
Patent status: 
No patent action is contemplated by NASA. 
Source: R. E. Nelson of 
Westinghouse Electric Corp. 
under contract to 
Marshall Space Flight Center 
(MFS-20169)
. 
Brief 69-10097	 Category 01


Mounting method improves electrical and vibrational characteristics of screen electrodes

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