Coax plug insulator

A coaxial connector is provided, of a type that has a front portion (12) where the inner conductor (14) forms a socket for receiving a pin (M) of predetermined diameter (A) (1 mm) and the outer conductor has a front end of predetermined outside diameter (B) (3.7 mm), which enables an increase in characteristic impedance at the front portion of the connector to more closely match the impedance of the middle portion (16) of the connector. A dielectric support (30) that positions the inner conductor within the over one, includes primarily air at the front portion, to increase the characteristic impedance of the front portion without changing its diameter. The insulative support preferably includes a middle portion (72) with ring-shaped locating portions (62, 64) that are closely received in the outer conductor, that closely surround the inner conductor, and that abut shoulders on the inner conductor to fix the position of the inner conductor in radial and axial directions. The support also includes a front portion (66) with a ring-shaped locating portion (56) that is closely received within the front end (60) of the outer conductor but which does not engage the inner conductor and that forms a lead-in (58), and with posts (66) extending axially from the front ring to the middle portion of the support.

BACKGROUND OF THE INVENTION 
Coaxial connectors include inner and outer conductors and an insulator, or 
dielectric, lying between them. Such connectors typically are designated 
to have a predetermined characteristic impedance, which is usually 50 
ohms, and sometimes 75 ohms to match the impedance of a cable and mating 
connector so as to minimize the standing wave ratio and consequent losses. 
A widely used and largely standard miniature plug coaxial connector has a 
front end where the inner conductor forms a socket for receiving a pin of 
1 mm diameters and where the outer conductor has an outer diameter of 3.7 
mm to engage the outer conductor of the mating connector. A dielectric 
material such as Teflon occupies substantially all of the space between 
the inner and outer conductors. The connector middle portion has a larger 
outer conductor diameter, and can have virtually any inner conductor 
diameter (since that inner conductor portion does not have to receive a 
pin) to achieve the desired impedance. 
At thee front portion of the above prior standard plug connector, the inner 
conductor has a diameter of 1.4 mm, and the outer conductor has an inside 
diameter of about 3 mm, with the space between them filled with Teflon 
which has a dielectric constant of 2.55. The result is that the front 
portion of the connector has a characteristic impedance of 28 ohms. With 
the connector front portion having a characteristic impedance of 28 ohms, 
there is a serious mismatch with the characteristic impedance of the 
connector middle portion which has an impedance of 50 or 75 ohms. As a 
result, the prior connector gave rise to a considerable VSWR (voltage 
standing wave ratio) of about 1.13 to about 1.15, resulting in 
considerable losses. Although this mismatch and the resulting losses were 
known, no steps were taken to reduce the mismatch of characteristic 
impedances. 
It is noted that a variety of dielectric materials are available for use in 
coaxial connectors, with Teflon (dielectric constant of 2.55) being the 
most common because of its relatively low losses especially at higher 
frequencies (on the order 1 GHz and higher). For example, U.S. Pat. No. 
5,100,344 by Truong shows a coaxial connector plug where the front portion 
has an even larger inside diameter than the rear portion so mismatch would 
not be a problem with only a solid dielectric, although the patent 
describes using primarily air as the dielectric. U.S. Pat. No. 4,981,445 
by Bacher et al describes a coaxial plug where the rear portion has about 
50% air and 50% of a solid dielectric and the front portion is not 
surrounded by an outer conductor. Neither of these patents show a plug 
coaxial connector where there is a reduced diameter front end that results 
in a lower impedance than the rear portion or describes how to correct 
this problem. 
SUMMARY OF THE INVENTION 
In accordance with one embodiment of the present invention, a coaxial 
connector is provided, of a type which has a reduced diameter front 
portion, and especially where the front portion has a socket inner 
conductor for receiving a pin of a 1 mm diameter and the outer conductor 
outer diameter is about 3.7 mm, which enables an increase in the 
characteristic impedance of the front portion to more closely match the 
characteristic impedance of the middle (rear) portion. The space between 
the inner and outer conductors at the reduced diameter front portion, is 
filled primarily with air, to increase the characteristic impedance of the 
front portion without reducing the diameter of the inner conductor thereat 
or increasing the diameter of the outer conductor thereat. 
A support molded of solid dielectric material lies in the space between the 
inner and outer conductors and positions the inner conductor so it lies on 
the axis of the connector and is prevented from moving axially. A pair of 
ring-shaped location parts includes a mid location part that closely 
surrounds the inner conductor immediately rearward of its socket and which 
is closely surrounded by the outer conductor, and a rear location part 
that closely surrounds the inner conductor and is closely surrounded by 
the outer conductor. Rods extending parallel to the connector axis connect 
the mid and rear location parts. The rods preferably do not closely 
surround the inner conductor and are not closely surrounded by the outer 
conductor, so they do not radially locate the inner conductor but merely 
space the ring-shaped location part. A ring-shaped front location part 
which forms a lead-in lies closely within the front end of the outer 
conductor but is spaced from the inner conductor. The front location part 
is connected by rods to the mid location part. 
The novel features of the invention are set forth with particularity in the 
appended claims. The invention will be best understood from the following 
description when read in conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 illustrates a type of miniature plug coaxial connector 10 that is in 
common use and sold in large numbers by applicant which is of a "standard" 
size. That is, the connector has a front portion 12 and has an inner 
coaxial conductor 14 with a front portion that is designed to receive a 
pin M of a diameter A of 1 mm. Also, the front portion is designed to mate 
with an outer conductor N of the mating connector, which is accomplished 
where the outer diameter B of the front portion is about 3.7 mm. The 
connector 10 has a middle or rear portion 16 whose outer diameter is not 
specified (it does not mate with the other connector). A coaxial cable is 
assembled to the connector by engaging a center conductor of the cable to 
the rear end 20 of the inner conductor and engaging the cable outer 
conductor to an extension 22 of the shell or outer conductor 24 of the 
connector. It is noted that after most of the connection is made, another 
extension 26 of the shell is bent downwardly against the extension 22. 
As shown in FIGS. 3 and 4, the inner conductor 14 lies concentric with the 
shell-shaped outer conductor 24 along a connector axis 26. A dielectric 
support 30 positions the coaxial conductors so they remain concentric, and 
prevents front or rear F, R movement of the inner conductor with respect 
to the outer one. The inner conductor 14 has forwardly and rearwardly 
facing shoulders 32, 34, and the support 30 has corresponding shoulders 
36, 38 to prevent axial movement of the inner conductor. The outer 
conductor has tabs 41 that prevent rearward movement of the insulator and 
has a shoulder 42 that abuts a surface 44 of the insulator to prevent its 
forward movement. The connector part 46 which lies rearward of the support 
(where it engages inner conductor shoulder 32) is considered to be the 
rear portion of the connector in the following discussion. 
Coaxial cables and connectors are usually designed to have a predetermined 
characteristic impedance, with the most common probably being 50 ohms and 
next most common being 75 ohms. To minimize the VSWR (voltage standing 
wave ratio) and consequent losses, it is desirable to construct the 
connector so it has a characteristic impedance as close as possible to 
that of the other components of the circuit; i.e. to make the connector so 
it has a characteristic impedance of 50 ohms or 75 ohms throughout. The 
middle portion 16 which generally has an outer diameter C of 4.6 mm (more 
than 4.1 mm), can be easily sized to create a desired impedance (as can 
the rear portion). Generally, the diameter D of the inner conductor middle 
portion is adjusted so that with the particular dielectric lying between 
the inner and outer conductors thereat, the desired characteristic 
impedance (50 or 75 ohms) is achieved. However, previously it has not been 
possible for the connector designer to construct the connector front 
portion 12 so it had an impedance close to the desired level, such as 50 
ohms. 
As discussed above, the front portion 40 of the inner conductor had to 
receive a pin M of 1 mm diameter. The inner conductor front portion 40 was 
divided into two tines 70, each having a thickness E such as 0.008 inch 
(0.2 mm), resulting in an outside diameter J of about 1.4 mm. Since the 
outer diameter B of the connector front portion was about 3.7 mm (less 
than 3.9 mm) and the thickness G of the shell walls was about 0.014 inch 
(0.35 mm) the inside diameter H of the shell was about 3 mm. When Teflon 
(dielectric constant of 2.55) filled the space between the inner and outer 
conductors at the front portion 12 of the connector, the characteristic 
impedance of the front portion was about 28 ohms. A characteristic 
impedance of 28 ohms for the connector front portion, when used with a 
circuit and connector middle portion of 50 ohms, resulted in a large VSWR 
and corresponding losses. Although connector designers were aware of this 
difference in characteristic impedance and the consequent losses, 
designers previously were not able to rectify the situation. 
In accordance with the present invention, applicant significantly raises 
the characteristic impedance of the plug connector front portion 12 from 
28 ohms to 45 ohms to achieve a much closer match to the characteristic 
impedance of the rear portion 16 of the connector and to the 
characteristic impedance of circuitry including the cable and mating 
connector) that is electrically connected to the plug connector. Applicant 
accomplishes this by constructing the dielectric support 30 so there is a 
minimum of solid dielectric material in the front portion 50 of the space 
52 between the inner and outer conductors. That is, solid material 
occupies less one-third of the front space portion 50. Although applicant 
could make the front space portion 50 completely devoid of insulation, 
applicant prefers to provide a front ring portion or front locator part 56 
which forms a lead-in at 58. The front locator part 56 is closely 
surrounded by the front 60 of the outer conductor, but preferably does not 
closely surround the deflectable tines 70 of the inner conductor. The 
dielectric support also includes mid and rear ring-shaped locating parts 
62, 64 that are each closely surrounded by the outer conductor 24, and 
that each closely surrounds the inner conductor 14. The mid locating part 
62 lies at the rear of the inner conductor front portion to avoid 
interference with the tines 70. Each of the locating parts comprises a 
ring that extends substantially 360.degree. (more than 320.degree.) around 
the axis. It is noted that the locating portions 62, 64 form the shoulders 
32, 34 that fix the axial position of the inner conductor. 
Applicant connects the front locating part 56 to the middle locating part 
62 by a front dielectric portion 65 largely formed by plurality of 
axially-extending rods 66. As shown in FIG. 5, there are three rods 66A, 
66B, and 66C that are circumferentially spaced about the connector axis 
26. The rods do not closely surround the inner conductor at its tines 70, 
and are not closely surrounded by the front portion 25 of the outer 
conductor 24. The purpose of the rods is to axially position the front 
locating portion rather than to radially position anything (with respect 
to axis 26). As shown in FIG. 5, the three rods occupy only about 20% of 
the cross-sectional area of the connector front portion. Air occupies the 
rest. As a result, the characteristic impedance of the front portion is 
close to the level that would be achieved by providing only air in the 
front portion space 50. As mentioned above, this construction results in 
the front portion having a characteristic impedance of 45 ohms, which is 
close to the level of 50 ohms of the rear portion and of the most common 
specified level for the connector. 
The characteristic impedance I of a coaxial connector section is equal to: 
EQU I=138/.sqroot.e.times.Log.sub.10 D/d 
where D is the inside diameter of the outer conductor, d is the outside 
diameter of the inner conductor, and e is the dielectric constant of the 
material between the conductors. For the connector front portion 12, it 
was not possible to change the characteristic impedance by changing the 
diameters of the conductors, since it is a fixed design for engaging 
mating connectors of a predetermined size. However, applicant's 
substitution of primarily air for a solid material such as Teflon 
(dielectric constant of 2.55) increases the characteristic impedance to 
more closely match the desired level. 
The dielectric support has a middle portion 72 which could be all solid 
dielectric material. However, applicant prefers to form even the middle 
portion 72 primarily of air, to enable an increase in the diameter D of 
the middle portion so it is closer to the diameter F of the front portion 
of the inner conductor. By reducing the differences in diameters D, F, 
applicant reduces reflections that can lead to increased losses at higher 
frequencies (above about 750 MHz), although the losses due to reflections 
is secondary compared to the losses due to the previously greatly 
unmatched impedances. Applicant's connector is now used primarily for 
frequencies of up to about 2 GHz where the later is true. As shown in FIG. 
6, the middle portion 72 of the dielectric support includes three rods 
74A, 74B, and 74C, which together occupy about 20% of the cross-sectional 
area between the middle and rear locating portions 62, 64 (FIG. 3), except 
for a center flange 76 which provides a shoulder for the outer conductor 
tab 40. 
It is noted that in FIGS. 5 and 6, there are only three rods spaced 
90.degree. apart, with a gap 80, 82 of about 180.degree. between two of 
the rods. This construction aids in constructing the support by forming it 
as a one-piece plastic molded part. The three rods such as 66A, 66B, and 
66C can be withdrawn from a mold more easily than if the gap was less than 
about 180.degree.. The vertical sides such as 84, 86 of the posts are 
parallel to also ease in removal from a mold. 
Applicant has constructed and tested a connector of the above design, and 
one of the previous design (front socket end to receive a 1 mm pin, with a 
front outer diameter of about 3.7 mm). For an external impedance of 68.8 
ohms and a frequency of 1000 MHz and no load, the previous design (space 
between conductors filled with solid dielectric) resulted in a VSWR of 
1.145 while the new design described above produced a VSWR of 1.087. When 
a load was connected, the previous design produced a VSWR of 1.132 while 
the new design produced a VSWR of 1.081. 
Thus, the invention provides a coaxial connector of the type wherein the 
front end of the inner conductor forms a socket for receiving a pin of 
predetermined size and the outer conductor has a smaller diameter at its 
front portion than at its middle, which enables an increase in the 
characteristic impedance of the front portion of the connector. This is 
accomplished by providing primarily air as the dielectric that lies 
between the inner and outer conductors at the front portion of the 
connector. The dielectric can be formed by a dielectric support which 
preferably has a front locating part that forms a lead-in to the socket 
and that is connected by axially-extending rods to a ring-shaped mid 
locating part at the front of connector middle portion. The support 
preferably includes a rear ring-shaped locating part that is connected to 
the mid locating part by a plurality of rods, so there is primarily air in 
the space at the middle portion of the connector to allow a larger 
diameter inner conductor at the middle of the connector for lower 
reflections. The connector design is especially useful for a particular 
connector design where the socket contact at the front of the inner 
conductor is designed to receive a pin of 1 mm diameter and the outside of 
the front portion has a diameter of about 3.7 mm. Although a nonporous 
solid dielectric is shown for the support, it would be possible to use a 
rigid foam that fills the entire space but with a gas such as air 
occupying most of the foam volume. 
Although particular embodiments of the invention have been described and 
illustrated herein, it is recognized that modifications and variations may 
readily occur to those skilled in the art, and consequently, it is 
intended that the claims be interpreted to cover such modifications and 
equivalents.