RF Fuse

An RF connector has an overrated fuse located therein which is electrically connected in series with the center conductor. An overrated fuse is utilized because a fuse rated to give the desired protection is undesirable due to the resistance caused by its small diameter. To obtain the desired protection with the overrated fuse, current shunting means, such as diodes, are mounted within the RF connector and are electrically connected between the outer and inner conductors. The current shunting means are positioned within the RF connector to present minimum inductance to the RF circuit. The operation of the fused RF connector is such that when the power applied to the connector exceeds a predetermined amount, the current shunting means conduct the excess current to ground to provide protection to the associated equipment until the input power is sufficient to blow the overrated fuse.

BACKGROUND OF THE INVENTION 
This invention relates to RF (radio frequency) fuses and more particularly 
to RF fuses that are contained within an RF connector. 
It is known to protect various RF equipment, such as, RF instrumentation 
apparatus, from excessive RF power by the use of fuses. It is also known 
to locate or mount the fuseable element into RF connectors that are 
utilized in conjunction with the RF apparatus to be protected. A problem 
is encountered, however, when the RF apparatus is to be protected from 
relative small amounts of RF power. For example, as the power level which 
blows the fuse is decreased, the diameter of the rated fuseable element 
also decreases. The small diameter fuseable element, however, presents a 
high resistance to radio frequencies thereby creating undesirable voltage 
standing wave ratios. In accordance with one feature of the present 
invention, the desired protection is obtained by using an overrated fuse 
in conjunction with current shunting means that shunt excess currents to 
ground until the applied power causes the fuse to blow. 
Accordingly, one object of this invention is to provide an improved RF 
fuse. 
Another object of this invention is to provide a fused RF connector which 
utilizes an overrated fusing element to eliminate the undesirable 
characteristics associated with small diameter fuseable elements. 
A further object of this invention is to provide a fused RF connector which 
utilizes current shunting means to enable an overrated fuseable element to 
be incorporated therein. 
Still another object of this invention is to provide a fused RF connector 
which utilizes current shunting diodes to enable an overrated fuseable 
element to be incorporated therein. 
An additional object of this invention is to provide a fused RF connector 
having current shunting diodes mounted within the dielectric material in 
such a manner as to present minimum impedance to excess currents resulting 
from RF overload. 
BRIEF SUMMARY OF THE INVENTION 
These and other objects, features and advantages of the present invention 
are obtained in an RF connector that comprises a center conductor portion 
and an outer conductor portion surrounding the center conductor portion 
and having its longitudinal axis substantially parallel to the 
longitudinal axis of the center conductor. A dielectric material is 
interposed between the center and outer conductor portions and includes 
four openings extending therethrough from one end thereof to the other end 
thereof. The openings have a longitudinal axis that is substantially 
parallel to the inner conductor. A first diode is located within one 
opening and has its anode lead extending from the one end and its cathode 
extending from the other end. A second diode is located in another opening 
and has its anode lead extending from the one end. The cathode lead of the 
first diode is electrically coupled to the center conductor of the RF 
connector while the anode lead of the second diode is electrically coupled 
to the outer conductor of the RF connector. The cathode lead of the second 
diode and the anode lead of the first diode are electrically connected to 
one another. A third diode is located within a remaining opening and has 
its anode lead extending from the other end and its cathode lead extending 
from the one end. A fourth diode is located within the one remaining 
opening and has its cathode lead extending from the other end and its 
anode lead extending from the one end. The anode lead of the third diode 
is electrically coupled to the center conductor while the cathode lead of 
the fourth diode is electrically coupled to the outer conductor. The 
cathode lead of the third diode and the anode lead of the fourth diode are 
electrically coupled to one another. An overrated current sensitive 
interrupting means is located in the RF connector and is connected 
electrically in series with the inner conductor.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
A fused RF connector in accordance with the present invention is 
schematically illustrated in FIG. 1 and includes an input portion 11 and 
an output portion 13. An outer conductor 17 surrounds an inner conductor 
15 which has a fuseable link 19 connected in series therewith. In 
accordance with one embodiment of the present invention which was 
constructed the fuseable element 19 constituted a 0.125 amp pico fuse 
manufactured by Littelfuse Incorporated and was utilized with a 50 ohm RF 
connector. The 0.125 amp fuse 19 will blow at approximately 0.8 watts or 
6.25 volts. These power and voltage levels, however, are too high for many 
RF applications. However, the use of a properly rated fuseable element 19 
introduces resistance into the RF circuit due to the small diameter of the 
fuseable element 19. Such resistance produces undesirable electrical 
characteristics such as an undesirable voltage standing wave ratio. In 
order to obtain the desired protection, current shunting means 12, 14, 16 
and 18 are connected between the inner 15 and outer 17 conductors. In the 
specific embodiment described herein, protection was obtained from RF 
power levels in excess of about 0.1 watts. As shown in FIG. 1, a first 12 
and second 14 diode are connected between the center 15 and outer 17 
conductors while a third 16 and fourth 18 diode are connected between the 
center 15 and outer 17 conductors. The cathode of the first diode 12 is 
connected to the center conductor 15 while its anode is connected to the 
cathode of the second diode 14 which has its anode connected to the outer 
conductor 17. The anode of the third diode 16 is connected to the center 
conductor 15 while its cathode is connected to the anode of the fourth 
diode 18 which has its cathode connected to the outer conductor 17. In 
accordance with one embodiment of the present invention which was 
constructed the diodes 12, 14, 16 and 18 constituted 1 N 4149 silicon type 
diodes. As is well known, silicon diodes will normally conduct when the 
forward voltage thereacross is approximately 0.6 volts. By placing two 
such diodes in series, the doides will not conduct until the voltage 
thereacross is about 1.2 volts. Accordingly, a DC voltage of 1.2 volts or 
greater of positive potential on the center conductor 15 will cause diodes 
16 and 18 to conduct whereas the same voltage of opposite polarity on the 
center conductor 15 will cause diodes 12 and 14 to conduct. In like manner 
an instantaneous radio frequency potential of 1.2 volts peak or greater 
which is positive will cause doides 16 and 18 to conduct while a similar 
instantaneous RF potential of opposite polarity will cause diodes 12 and 
14 to conduct. 
Briefly described, the operation of the fused RF connector shown in FIG. 1 
is such that at low power levels the power output obtained therefrom is 
substantially equal to the power applied thereto. As the input power 
increases, however, the diodes 12, 14, 16 and 18 will conduct to shunt 
excess currents to ground to protect the associated equipment (not shown). 
This continues until the input power applied to the RF connector causes 
the overrated fuse 19 to blow. 
The RF connector in accordance with the present invention will be readily 
understood by a perusal of FIGS. 1 and 2. In FIG. 2 the horizontal 
coordinate is input power, the left hand vertical coordinate is output 
power and the right hand vertical coordinate is time in milli seconds. The 
curves shown in FIG. 2 represent the operational characteristics when the 
specific components described hereinabove are utilized. The dotted line 21 
shows the relationship between power obtained from the RF connector with 
respect to power applied to the RF connector in the absence of the fuse 19 
and diodes 12, 14, 16 and 18. The solid curve 22 shows the relationship 
between power applied to the RF connector and the power obtained from the 
RF connector when the fuse 19 and diodes 12, 14, 16 and 18 are utilized in 
accordance with the present invention. The solid curve 23 indicates the 
time required for the fuse 19 to create an open circuit once sufficient 
power has been applied to the fuse 19. 
As shown by the curve 22 in FIG. 2, the output power from the fused RF 
connector equals the input power until the point X is reached which is 
above a power level of 0.01 watts and below a power level of 0.1 watts. At 
this point the peak voltage on the center conductor 15 is such that the 
diodes 12 and 14 and the diodes 16 and 18 become conducting on alternate 
half cycles of RF energy. Beyond the point X the output power will 
continue to increase as the input power is increased but at a much slower 
rate due to the conduction of the diodes 12, 14, 16 and 18. The space 
between the dotted line 21 and the solid curve 22 illustrates the energy 
that is shunted to ground by the diodes 12, 14, 16 and 18 to protect the 
equipment associated with the fused RF connector. As shown by the vertical 
dotted lines 24 and 26, the fuse 19 will blow when the power applied to 
the RF connector is between 0.8 and 3 watts. Once the power required to 
blow the fuse 19 has been applied, a finite time is required for the 
fuseable element 19 to actually separate and create an open circuit. As 
shown by the solid line 23, the fusing time delay of the fuseable element 
19 is inversely proportional to the power applied to the RF connector. The 
characteristics illustrated in FIG. 2 are obtained for a RF connector 
constructed in accordance with the present invention for a frequency 
operating range of from zero to about one giga hertz. As will be apparent 
to those skilled in the art, the fused connector can provide protection 
for power applied at the output 13 by connecting another fuse (not shown) 
in series with the fuse 19 with the diodes 12, 14, 16 and 18 being 
connected between the junction of the two fuses and the outer conductor 
17. 
An inline RF connector in accordance with the present invention is 
illustrated in FIGS. 3A, 3B and 3C as including an outer conductor 25 
portion. A first annular opening 29 is located at one end of the outer 
conductor 25 while a second annular opening 31 is located at the other end 
of the outer conductor 25. The outer conductor 25 includes a reduced 
diameter portion intermediate the annular openings 29 and 31. A fuse 35 
having leads 37 and 39 extending from opposite ends thereof is surrounded 
by the reduced diameter area of the outer conductor 25. The exterior body 
of the fuse 35 typically consist of an insulating material (not shown) 
which electrically isolates the fuse 35 from the outer conductor 25. The 
internal wall 27 of the annular opening 29 is threaded to threadably 
receive a standard type BNC coaxial connector 41 which includes a outer 
conductor 43 electrically connected to the outer conductor portion 25 and 
a center conductor 47 which is insulated from the outer conductor 43 by a 
dielectric material 45. One end of the center conductor 47 slidably 
receives the lead 39 of the fuse 35. The exterior surface at the other end 
of the outer conductor 25 adjacent to the annular opening 31 is threaded 
to threadably receive another standard type BNC coaxial connector 49 which 
includes an outer conductor 53 which is electrically connected to the 
outer conductor portion 25. The connector 49 also includes a center 
conductor 55 which is insulated from the outer conductor 53 by a 
dielectric material 51. The lead 37 of the fuse 35 is serially connected 
to the center conductor 55 of the coaxial connector 49 by a conductor 60. 
The lead 37 of the fuse 35 slidably engages the end of the center 
conductor 60 adjacent thereto. As will now be apparent, the outer 
conductor portion 25 and the outer conductor 53 of the coaxial connector 
49 are electrically connected together to form the outer conductor for the 
complete connector assembly. In a like manner the center conductor 47 of 
the coaxial connector 41, the fuse element 35, the conductor 60, and the 
center conductor 55 of the coaxial connector 49 constitute the center 
conductor portion for the complete connector assembly and is electrically 
insulated from the outer conductor portion of the complete connector 
assembly. 
Located within the annular opening 31 and surrounding the center conductor 
60 is an annular piece of dielectric material 59, such as nylon. As best 
shown in FIGS. 3B and 3C, first, second, third and fourth substantially 
equally spaced openings 61, 63, 65 and 67, respectively, extend through 
the dielectric material 59. As shown, the longitudinal axis of these 
openings 61, 63, 65 and 67 are substantially parallel to the longitudinal 
axis of the outer 25 and inner 60 conductors. A metallic washer 57 is 
secured to one end of the outer conductor 25 and is electrically insulated 
from the center conductor 60 by the dielectric material 51 and functions 
to retain the dielectric material 59 within the annular opening 31. A 
diode 69 is located within the first opening 61 and has its cathode 
terminal 76 connected to the outer conductor 25 by being connected to the 
washer 57 by any suitable means such as by soldering. A diode 71 is 
located within the second opening 63 and has its anode terminal 77 
connected to the center conductor 60 by any suitable means such as by 
soldering. The anode lead 75 of the diode 69 and the cathode lead 79 of 
the diode 71 extend from the same end of the dielectric material 59 and 
are electrically connected together by any suitable means such as 
soldering. In like manner a diode 73 is located within the third opening 
65 and has its anode lead (not shown) connected to the outer conductor 25 
by being electrically connected to the washer 57. A diode 83 is located 
within the fourth opening 67 and has its cathode lead (not shown) 
connected to the center conductor 60. The cathode lead 81 of the diode 73 
and the anode lead 75 of the diode 83 extend from the same end of the 
dielectric material 59 and are electrically connected together. Input 
power is applied to the left side of the RF connector by way of the 
coaxial connector 41 and output power is obtained from the right side of 
the RF connector by way of the coaxial connector 49. 
As will be apparent from a perusal of FIGS. 3B and 3C the placement of the 
diodes 69, 71, 73 and 83 and the electrical connection of the diodes 69, 
71, 73 and 83 with the inner 60 and outer 25 conductors is such that the 
diode 69, 71, 73 and 83 leads are kept short thereby presenting minimum 
impedance to excess currents resulting from RF overload. A spent fuse 35 
is replaced merely by disengaging the coaxial connector 41 from the outer 
conductor portion 25, removing the spent fuse 35 and reinserting a new 
fuse 35, and then again threadably engaging the coaxial connector 41 with 
the outer conductor 25. The electrical circuit of the fused RF connector 
illustrated in FIGS. 3A, 3B and 3C is substantially identical to that 
shown in FIG. 1 and described hereinabove in conjunction with FIG. 2. 
A panel mounted fused RF connector in accordance with the present invention 
is illustrated in FIGS. 4A, 4B and 4C. As shown in FIG. 4A the exterior 
surface of the outer conductor portion 90 adjacent to the annular opening 
92 contains a flanged or shoulder portion 83 adjacent to a threaded area 
85. The shoulder portion 83 and threaded area 85 enable a hex nut 87 and 
washer 89 to be utilized to secure the fused RF connector to a panel 
member 91. Additionally, a portion of the internal walls of the annular 
opening 93 adjacent the other end of the outer conductor 90 are threaded 
to enable a standard type SMA coaxial connector 95 to be threadably 
secured to the right-hand portion of the outer conductor portion 90. In 
other respects, the panel mounted RF connector illustrated in FIGS. 4A and 
4B and 4C is substantially identical to the fused in line RF connector 
illustrated in FIGS. 3A, 3B and 3C. FIG. 4A shows a standard type SMA 
coaxial connector 95 threadably secured to the right-hand portion of the 
outer conductor 90. It is to be understood, however, that the present 
invention is not limited to this type of connector. For example, the semi 
rigid coaxial connector illustrated in FIG. 5 as well as the SMB type 
coaxial connector illustrated in FIG. 6 and the standard type SMC coaxial 
connector illustrated in FIG. 7 may be utilized in lieu of the SMA type 
coaxial connector. Additionally, the inline RF connector illustrated in 
FIGS. 3A, 3B and 3C and the panel mounted RF connector illustrated in 
FIGS. 4A, 4B and 4C can be fitted at one or both ends with a standard type 
N coaxial connector (now shown) or with any combination of the coaxial 
connectors described herein. As in the case of the inline RF connector, 
the input to the RF connector illustrated in FIGS. 4A, 4B and 4C is 
applied to the left-hand side by way of the coaxial connector 96 and the 
output is obtained from the coaxial connector 95 at the right-hand side.