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You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND OF THE INVENTION 
     The invention relates to improvements in sonde housings used in horizontal direction drilling to carry a radio transmitter that indicates the location and orientation of a drill head. 
     Horizontal direction drilling in geological formations is widely used to place product such as pipe, conduit or cable underground. Typically, the location and orientation of the drill head is monitored as it progresses through the earth so that corrections can be made to keep the actual path as close as possible to the desired path. The location and orientation of the drill head is signaled to the surface by a radio transmitter carried in a so-called sonde housing that is interposed in the drill string just behind the drill head. The sonde housing includes passages for fluid that is used in the drilling process and that advantageously cools the sonde housing so that heat from the drilling operation does not overheat the electronics of the transmitter. 
     Conventional prior art sonde housings have been fabricated by machining steel bars or tubes to provide a chamber for the transmitter and axial passages for the fluid. That fluid creates a jet at the drill face or provides hydraulic power for a directional drill motor and, as mentioned, cools the transmitter. Typically, the prior art sonde housings are relatively expensive because of the special machining operations that are performed to create the chamber and various passages through the full length of the housing. This cost is significant to a drilling company because the typical sonde housing has a limited life. The fluid that passes through the sonde housing is continuously recycled. Although it is filtered, fine sand particles remain in the fluid causing it to be highly abrasive. The fluid, because of its abrasiveness, wears away at the passages in the housing eventually destroying it. Another problem frequently encountered with known types of sonde housings is related to slots or other apertures formed in the housing wall that allow transmission of radio waves out of the metal housing. The slots are frequently filled with epoxy or other non-metallic material to exclude fluid from the chamber in which the transmitter is received. This material is prone to leak internally after a period of use with the result that the transmitter and its associated battery can become cemented in the chamber by fluid borne solids making it very difficult to remove the transmitter without harm. 
     SUMMARY OF THE INVENTION 
     The invention provides an improved sonde housing that can be economically manufactured and that has improved performance both in resistance to wear and resistance to internal leakage. Various internal parts, while being made of relatively inexpensive materials, are capable of an extended service life meeting or exceeding that of more expensive traditional materials. Still further, internal parts that are susceptible to wear by abrasion from the fluid being conducted through the housing are replaceable at relatively low cost. As disclosed, the sonde housing comprises an outer metal cylindrical shell or main body having tool joints at each end. The shell wall is slotted at circumferentially spaced locations for transmission of radio signals from the transmitter carried within the shell body. A cartridge assembly is positioned in the shell body to provide a sealed chamber for the transmitter, an annulus for conducting fluid through the housing and a sleeve to seal the radio transmission slots in the shell wall and to protect the shell wall from abrasion from the circulating fluid. The main parts of the cartridge are formed of a suitable plastic so that they are extremely cost effective and, advantageously, are inherently transparent to the radio wave signals generated by the transmitter. 
     In the disclosed arrangement of the housing, the sleeve of the cartridge not only protects the shell body from abrasion, but also by sealing the radio signal emitting apertures in the shell wall, avoids the seal failure problems normally encountered in the prior art where the apertures are sealed with epoxy. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A and 1B are left and right-hand portions of a sonde housing, constructed in accordance with the invention taken in a longitudinal cross-sectional view; 
     FIG. 2 is a cross-sectional view of the sonde housing taken in a plane transverse to the longitudinal axis of the housing as indicated by the arrows  2 — 2  in FIG. 1A; and 
     FIG. 3 is a transverse cross-sectional view of a main shell body of the housing taken in the plane indicated at  3 — 3  in FIG.  1 A. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings, there is shown a sonde housing assembly  10 . FIGS. 1A and 1B are complimentary to one another; the housing assembly  10  is illustrated in two parts so that a larger drawing scale is obtained but it will be understood that the actual housing assembly is a single integrated assembly. The housing assembly  10  comprises a main shell body  11  having end pieces or tool joints  12 ,  13  at each end. The shell body  11  is preferably formed as a length of suitable round steel tubing which may be a high alloy hardened steel material for improved strength. The end pieces  12 ,  13  are also fabricated of a suitable steel and are in the form of hollow heavy wall sleeves having through bores  16 ,  17 . In the illustrated case, the end piece or tool joint  12  on the right has an internal thread form designated by the American Petroleum Institute as an API IF thread which is commonly used in drill pipe. The end piece or tool joint  13  at the left is an internal thread designated by the American Petroleum Institute as an API REG thread which is used to couple with the drill head of horizontal directional drilling apparatus known in the art. 
     An extension  21  on an inward end of each of the end pieces  12 ,  13  is telescoped into a bore  22  of the shell body  11  to facilitate alignment and assembly of these parts. Axially and radially outward of the extension  21 , each end piece  12 ,  13  is chamfered to permit a circumferentially continuous fluid-tight weld bead  23  to be formed between the end piece and the shell body  11  to thereby join these parts together. The shell body  11  has a plurality of apertures in the form of axially extending slots  26  cut through its wall  27  to permit external transmission of radio waves from a transmitter carried in the housing assembly  10  as discussed below. 
     Positioned in the shell body  11  before one or both of the end pieces  12 ,  13  are welded on is a cartridge assembly  31 . The cartridge assembly  31  includes an outer sleeve  32  and an inner tube  33  within the sleeve. The sleeve  32  and tube  33  are held in concentric relation by a pair of annular adapters  34  and a ring  36 . Preferably, the adapters  34  are identical units having the general form of a short tube or ring with an internal cylindrical surface or bore  37  and a cylindrical outer surface  38 . As shown in FIG. 2, a wall  39  of the adapter  34  is drilled or otherwise formed with a plurality of axial bores or passages  41  angularly spaced about its circumference. At an inner end, the adapter  34  has a counter bore  42  for receiving a short portion of the length of the inner tube  33 . Similarly, the inner end of each adapter  34  has a reduced diameter outer surface  43  that fits into the inside diameter of the sleeve  32 . When the ends of the sleeve  32  and the tube  33  are respectively assembled in and on the adapter  34 , these parts are held concentric with one another. The ring  36  is similar in cross-section to the adapters  34 , but shorter in length, and is disposed around the tube  33  and in the sleeve  32 . The ring  36  is adhesively attached or otherwise fixed at the mid-lengths of the tube  33  and sleeve  32 . The ring  36  includes circumferentially spaced axial passages  46  to permit fluid passage through an annulus  35  between the tube  33  and sleeve  32 . In the illustrated example, the tube  33  and sleeve  32  are made of rigid polyvinylchloride such as the type conventionally used for plastic pipe. The surfaces of contact between the adapters  34  and ring  36  with the tube  33  and with the sleeve  32  are joined together with a suitable adhesive. Outer ends of the tubular adapters  34  have internal threads  47 . A retainer  51  at one end of the cartridge assembly  31  (FIG. 1A) has external threads complimentary to the adapter threads  47 . The retainer  51  has an outer portion  53  with a hexagonal or other acircular cross-section in end view enabling it to be tightened or untightened in the adapter threads  47 . A radial shoulder  54  of the retainer  51  is proportioned to abut an end face  56  of the adapter  34  when the retainer is fully threaded into the adapter. The contact between the shoulder  54  and end face  56  prevents the retainer  51  from being over-tightened. The retainer  51  has a central axial bore  57  in which is received an indexer  58 . The indexer  58  has a cylindrical central portion  59  sized to rotate in the retainer bore  57 . The indexer  58  is captured on the retainer  51  with a metal snap ring  61  at one end and a radially extending flange  62  at the other end. An elastomeric O-ring  63  disposed in a peripheral groove on the central cylindrical portion  59  of the retainer seals with the bore  57 . An elastomeric O-ring  64  located in a groove in the flange  62  seals against a radial inner face of the retainer  51 . At an outer end  66 , the indexer  58  has a hexagonal profile, in end view, to permit the indexer to be selectively rotated with a wrench. On an inner radial face, the indexer  58  has an integral key  69  that enables it to be rotationally interlocked with a radio transmitter  67  disposed in a chamber  68  circumferentially bounded by the inner surface of the tube  33 . The transmitter  67  is manually rotated or “clocked” in the chamber  68 , as is known in the art, by rotating the indexer  58 . 
     On an opposite end of the cartridge  31  (FIG.  1 B), a plug  71  with male threads complimentary to the adapter threads  47  is removably threaded into the adapter  34 . The plug  71  has a peripheral groove that receives an elastomeric O-ring  73  which seals with the adapter counterbore  42 . An outward portion  74  of the plug  71  has a hexagonal shape when viewed axially to permit the plug to be tightened or untightened into the threads  47  of the adapter. A radial shoulder  76  on the plug  71  abuts the end face  56  of the adapter  34  to prevent the plug from being inadvertently over-tightened. 
     The transmitter  67  and a battery  78 , both known in the art, can be disposed in the chamber  68 . A compression spring  79  holds the transmitter  67  and battery  78  in place with the transmitter coupled with the key  69  on the indexer  58 . The transmitter  67  and battery  98  can be assembled and removed from the chamber  68  through the end piece  12  by installing or removing the plug  71  with a wrench. 
     The cartridge assembly  31  comprising the outer sleeve  32 , inner tube  33 , adapters  34 , retainer  51  and plug  71  is inserted in the shell body  11  before at least a last one of the two end pieces  12  or  13  is welded or otherwise joined to the shell body. The cartridge assembly  31  is fixed relative to the shell body  11  by tightly fitting spring pins  81  extending through holes drilled through the shell wall  27  and into the walls of the adapters  34 . The outer periphery of the adapters  34  is machined or otherwise formed with a pair of spaced circumferential grooves in which are received elastomeric O-rings  82 . The O-rings  82  provide a fluidtight seal between the cartridge assembly  31  and interior surface of the bore  22  of the shell body  11 . 
     In use, fluid typically primarily recycled water is received by the end piece  12  (FIG. 1B) from a drill pipe string to which the end piece or tool joint is coupled by threading it onto the same. The fluid diverges over the plug  71  and passes through the several peripheral openings or passages  41  in the associated adapter  34 . This fluid then passes through the annulus  35  between the inner tube  33  and outer sleeve  32 , the passages  46  in the ring  36  and through the openings or bores  41  in the other adapter  34  and ultimately passing out of the end piece  13 . It will be understood that substantially the full circumference of the tube  33  and, therefore, the transmitter  67  is surrounded by this fluid so that full cooling of the transmitter is obtained. 
     The fluid pumped through the sonde housing assembly  10 , despite filtering, can become abrasive by picking up fine sand or other particulate material from the geological formation through which it is recycled. In this circumstance, the surfaces of the cartridge assembly  31  can become worn away with extended use even though it has been found that plastic material such as polyvinylchloride is remarkably durable when compared with the typical steels used in similar applications. The cartridge assembly  31  can be replaced by cutting off one of the end pieces  12  or  13  from the shell body  11  at the weld bead  23 , removing the worn cartridge assembly and replacing it with a new one. Thereafter, the end piece can be rewelded onto the shell. It will be understood that the inner tube  33  and outer sleeve  32 , being formed of a non-metallic material such as polyvinylchloride or other material of suitable structural strength and transparent to radio waves, eliminate the need for separately sealing the apertures or slots  26  in the wall  27  of the shell body  11 . 
     It should be evident that this disclosure is by way of example and that various changes may be made by adding, modifying or eliminating details without departing from the fair scope of the teaching contained in this disclosure. The invention is therefore not limited to particular details of this disclosure except to the extent that the following claims are necessarily so limited.

Summary:
A sonde housing construction that is cost effective to manufacture, has a prolonged service life and has an internal cartridge that has replaceable parts and that can be entirely replaced. The cartridge which in service contains and protects a radio transmitter also serves to protect a main shell body from abrasion by drilling/cooling fluid while sealing radio wave apertures formed in the shell body. The cartridge creates an annular flow path for drilling/cooling fluid that ensures complete cooling protection of the transmitter.