Anritsu 11410-00293 User Manual
Microwave
Connector Thread Gaging
Technical Note
2
When making a microwave connection with a coaxial
connector, a good ground connection must be achieved. If not,
poor return loss will result. If the ground connection is loose,
performance will be inconsistent and will cause intermittent
performance problems. Connector threads play a crucial role
in achieving a good ground connection – they must be made
properly in order to achieve good consistent results.
It has come to our attention that there may be confusion
regarding the measurement of connector threads to determine
if they are properly made. The intention of this note is to
clarify methods used to measure threads and to define some
terms used in thread measurement.
This simplified diagram shows the major features of threads
that are important for this discussion
Figure 1.Thread definition
For the threads of a female K connector, the pitch diameter
of a perfectly made thread is 0.2311 inches (5.870 mm)
while the minimum pitch diameter is 0.2280 inches
(5.791 mm). The correct flank angle is 60°. The major
diameter of the
1
/4-36 thread used on the K connector is
0.250 inches (6.35 mm) and the minor diameter is designed
to be 0.1885 inches (4.788 mm) while the correct pitch is
0.0278 inches (0.706 mm.)
If the female thread pitch diameter is too small, it may not be
possible to torque the nut to 8 in-oz or the connector may tilt
as the threads are tightened. Figure 2 shows what may occur.
Figure 2. Undersized Threads (exaggerated a bit)
Figure 3. Properly sized Threads
If the pitch diameter is too large, the specified torque specification may be reached before the center pin is properly
engaged, resulting in poor return loss due to excessively large
pin gap. Remember that housing threads can also cause
problems tightening to the proper torque.
Thread Measurements
There are two primary tools used to measure threads.
Go-No Go Gages
The first and easiest to use are Go-No Go gages. The type for
external threads look similar to die used to thread rods. They
come in pairs, the “Go gage” and the “No-Go gage.” The Go
gage should thread on easily while the No-Go gage, whose
threads are slightly smaller, should jam after 1
1
/2turns or so. If
the No-Go gage doesn’t jam, this indicates that the thread is
undersized. If the go-gage jams, this indicates that the threads
are over-sized. The gages should be lightly lubricated with
machine oil to minimize wear and prevent corrosion. Go NoGo gages comes in two types, adjustable and non-adjustable.
The adjustable type has a gap as shown in figure 4. Ascrew
sets the gap width and calibration lab personnel use the screw
to calibrate the gage when compensating for wear. The nonadjustable type must be discarded when worn. For measurements made by these gages to be meaningful, the customer
must have a process in place to regularly calibrate the gages
or to determine if a gage is worn. If a worn gage is not
discarded, a connector that is perfectly good may be rejected.
For measuring internal threads, the go-no go gage shown in
Figure 5 is used. The pass/fail indications for internal threads
are the same as for external threads – the No-Go end of the
gage should jam after 1
1
/2turns or so.
Major Diameter
Pitch Diameter
Minor
Diameter
Pitch
Flank
Angle 60˚
Loading...