Smith-Kettlewell Soldering Series Part 4

Table of Contents

Soldering I
Soldering II
Soldering III: Tinning Stranded Wire
 Soldering IV: Popular RF Connectors
Soldering V: RCA and Motorola Plugs
Soldering VI: Resistance Soldering
Soldering VII
JA3TBW Solder Guide
Soldering Kinks
Vinther Fingertip Soldering Iron

SOLDERING Part IV: Popular RF Connectors

Having been a ham for two decades, and having worked in a lab full of test equipment on which many of these connectors are used, the editor has accumulated a bag of tricks for installing RF connectors. Besides making the job of installation easier, these tricks are intended to address two objectives; one is to prevent damage to the cable in preparing it, and the other is to give the blind technician positive assurance that the connector is properly fitted and soldered to the cable.

RCA phone plugs shall not be called RF connectors (in my presence), and are taken up in part V, as is their cousin the Motorola plug.

Cable-hyphen mounted male connectors are the ones addressed in this discussion. Cable-mounted female units are used less frequently, and the principles used in working with them are exemplified in the discussion of their male counterparts. Chassis-mounted females often have conventional solder terminals and warrant no special mention.

The Standard UHF Connector (PL259)

The so-called "UHF" connector (whose name goes back to a time when F's were not very H and were not at all U) has been widely accepted by radio amateurs and CB equipment manufacturers. In a way, this is unfortunate, since installation is time consuming, and proper soldering of the shield braid is difficult. What's worse is that this connector is poorly understood even by veterans of radio work who often invent short-cuts to proper installation, making it imperative that knowledgeable people (such as we) oversee the preparation of cables at antenna parties.

The male center-conductor pin is tubular so that the cable's center conductor can be passed through it. The overall length of this tube is 5/8 in. from its tip to the bottom of the well inside the connector shell.

In the connector shell directly behind the center-conductor pin is a chamber whose inner diameter is just large enough to accept the braided portion of the cable. This chamber is about 1/2 in. deep and has an inner diameter of .350 in. Four holes around the perimeter of this chamber allow access to the braid through the shell of the connector; it is intended that the braid be soldered to the shell through these holes.

Further back, the inner diameter of the connector is large enough to accept the plastic-covered portion of large coaxial cable such as RG8/U. Internal threads at the rear of the connector are provided to accommodate "reducing adaptors" to be used with small-sized cable.

Preparation of Large Cable (RG8/U, RG11/U)

About 3/4 in. from the end, cut through the outer sheath, the shield braid, and the diaelectric insulator, and remove these pieces to expose the center conductor. Check with a fingernail to see that none of the strands of the center conductor were badly damaged by the knife. If obvious damage has been done, it will save you future grief to start over.

Using the procedures outlined in "Soldering Part III", smooth out and rebundle the strands of the center conductor and generously wrap them in solder. Using a heavy-duty soldering iron or gun, tin this conductor with the end of the cable pointing slightly downward to prevent a bead of solder from collecting at the base of the lead; such a solder bead can prevent insertion of the wire all the way into its tubular pin. Before continuing, insert the lead into the tubular pin from the front end of the connector to make sure that it still fits after being tinned.

Cut through the outer sheath 5/8 in. back from its end (5/8 in. back from the previous cut). Make no sawing motions with the knife that might cut into the braid; gently press the knife through the sheath on all sides, being careful not to knick the braid. Carefully remove this short length of outer covering and assess the damage. If whiskers around the knife cut indicate that a half dozen braid wires have been severed, it would be wise to start over.

Essential to installation of the connector is that the braid be completely tinned so as to make soldering inside the connector shell possible. Furthermore, the braid must be smoothed down so as to make it small enough to fit into its chamber. What makes tinning the braid difficult is that the dielectric insulator melts at a comparatively low temperature. Bring the braid to soldering temperature is often very injurious to the center insulation, and melting of the plastic can even contaminate portions of the braid, making it unsolderable.

I find that a thin strip of paper wrapped around the center insulator under the braid can protect the plastic from the heat. The braid can be flared out temporarily while the paper is wrapped around the plastic insulator beneath, then the braid can be closed over the paper. You may wish to "train" the paper by first wrapping it around a pencil to make it more manageable. The strip should be at least 1/2 in. wide and about an inch long. One-half of the slip from a Chinese fortune cookie works very well; not only is it precut to the right width, but this paper is very thin and supple.

It is nearly impossible to keep the braid smoothed down enough to fit into its intended space, and you will more than likely have to file the tinned braid down to size before the connector can be installed. As long as filing will have to be done anyhow, a piece of thin bare wire can be used to tie the braid into place. (Without the extra paper insulator, this trick can get you into trouble, since the wire will assure firm contact between the hot braid and the dielectric insulator.) Loop the wire around the end of the braid, and twist its ends together, tightening it just enough to keep it in place around the braid.

To keep the tip of the iron from contacting the plastic outer sheath, you may wish to wrap the cable in gummed paper or masking tape just behind the exposed braid. Whatever you use, it should be of a material that will not readily contaminate the tip of the iron.

Wrap a coil of solder around the exposed braid, using perhaps as much as 6 inches of solder. Using a heavy duty iron or gun, proceed in tinning the braid on all sides of the cable. By starting on the under side, you will prevent large segments of the solder coil from dropping off as you go. Each time you contact the braid, linger just long enough to feel the squeakiness of clean solder wet metals, then draw the iron off the end of the braid. After you have done this on all sides, let the cable cool for a couple of minutes and then check your work for large globs or unmelted pieces of solder. Approach these soldered globs specifically with the iron and draw them off the end of the braid.

On the one hand, you must take long enough with this process to assure that the braid is thoroughly tinned. On the other hand, the intense heat inside the braid cannot help but damage the cable. Your objective should be to reach a good compromise.

The braid can now be filed to size. Wrap a piece of tape around the center conductor to protect it from being marred by the file. File down the jagged end of the braid until it is even with the end of the dielectric insulator. File around the outside of your tinning job until it is smooth and cylindrical. (For those of you who own a micrometer, the tinned braid should measure 349/1000 of an inch.) Concentrate your efforts on the 1/2 in. of braid nearest the end, since this is the portion which must fit into the soldering chamber.

Testing for Proper Fit

Unfortunately, the adaptor threads inside the back end of the connector make testing for a fit difficult. This threaded portion of the connector is small enough in diameter to bite into the outer sheath of large sized cable; screwing and unscrewing the connector during the trial-and-error process of fitting is laborious and damaging to sheath. A slip-on test jig can be built by cutting off the rear-most 1/2 in. of a connector, thus doing away with these internal threads. Slipping this test jig onto the cable will quickly indicate if the tinned braid is too large for the chamber, or if the tinned center conductor is too large for the tubular pin.

Two indications as to whether or not the cable can be fully inserted into a connector are:

  1. The center conductor can become a "measuring stick" if you cut it to the exact length of 5/8 in. When the braid and dielectric insulator hit bottom in the connector shell, the end of the center conductor will be even with the end of the tubular pin. (You will recall that advice was given to cut away 3/4 in. of material to expose this center conductor. Depending on the shape of the end of the cable initially, you may need this extra length to insure that a good clean 5/8 in. long piece of this lead is available.)
  2. By tapping through the soldering holes with the point of a braille stylus, you will be able to tell whether or not the braid is present underneath. The surface beneath the holes should feel and sound metallic when tapped, and the holes should be completely obstructed by the braid. If the holes are only half covered, either the soldering chamber or the tubular pin is unable to accept full insertion. If the surface beneath the holes feels rubbery, the braid has been forced back by the entrance of the chamber, and your stylus is contacting the center insulator.

Preparation of Small Sized Cable

Slide a reducing adaptor of appropriate size onto the cable. Carefully one inch of outer sheath from the end of the cable. Position the adaptor so that its front end is even with end of the sheath and wrap tape around the cable directly behind the adaptor to keep it from slipping backward. Fold the braid back over the small end of the adaptor and tie it into position with a wire loop. Wrap the braid in solder and tin it; this time, draw the iron in front to back motions as if to smooth out the braid. Solder droplets will appear to stick to the adaptor, but the fact that the metal of the adaptor never reaches soldering temperature makes picking these droplets off possible after the project has cooled.

About 1/16 in. ahead of the adaptor, and braid strip off the dielectric insulator to expose the center conductor. Tin this lead, even if it is solid wire, since this will make soldering easier.

File the braid to size and test its fit in the connector as before.

Installing and Soldering of the UHF Connector

The connector comes with a screw-on ring to be used in securing it to its mating socket. Be sure that this ring has been passed down the cable before installing the connector. Screw the connector onto the cable and repeat the above tests to ascertain that it is properly in place.

Especially with large sized cable, the center conductor should be soldered first to hold the connector in place. If this is not done, bringing the body of the connector up to soldering temperature will soften the outer sheath, thus allowing the connector to slip forward. Your subsequent unseemly remarks will have the neighbors concerned for you.

Clamp the connector in a vise with the tubular pin pointing upward. Some technicians heat the pin from the side and hope that the conductor inside reaches soldering temperature. For this to work, the lead must be wet with solder; it helps if there is room in the pin to slip thin solder in alongside the conductor. These connectors, however, have a spoon-shaped end on the tubular pin so that the iron can be brought into contact with both the pin and the lead.

Hold a straight piece of thick solder vertically and position it on top of the pin. You can either position the tip of a soldering gun at the opening of the spoon, or you can slide a hot iron up the pin until the tip finds the spoon. When the solder melts, feed a half inch of it straight down into the pin. Very often, you will lose the pin with the solder. Let the project cool off and try again. Solder spilled outside the pin can be wiped off with the iron later; do this with the pin pointing downward. Also, it is a good idea to check for spillage around the base of the pin with a stylus or other small probe. This spillage can be removed with a knife.

The braid must now be soldered through the holes in the shell. If the connector is to be used outdoors, soldering all four holes will help protect the cable from moisture.

Orient the cable horizontally and clamp it in a vise about an inch behind the connector. (Clamping the connector in a vise directly will make heating it impossible.) Before securing the cable in the vise, turn it so that a solder hole is facing upward. It is easy to lose track of which holes you have soldered. You may wish to lay a strip of tape adjacent to the first hole to serve as a reminder.

I use a gun for soldering the holes, since positioning the tip of a hot iron can be tricky. In any case, hold the solder at an angle and insert it into the hole. Place the tip of the iron on the hole, or if possible, orient it so that a corner of the tip seats in the hole. The solder which melts immediately (disconnecting the solder in your hand from the connector) will serve a to create a column of molten metal under the iron in order to improve the efficiency of heat transfer.

Keep trying to find the edge of the hole with the solder. Spilling solder elsewhere on the connector will not hurt anything; just try to see that some of it spills into the hole. Do not leave the connector with the iron until rocking of the iron feels squeaky; this is your only true indication that wetting of the connector has occurred.

After the connector has cooled, clear some of the larger droplets out of the way with a knife blade and feel in the hole with a stylus. The hole should either not be there, or should feel more like a "dent" than a hole.

As a final test, try unscrewing the connector from the cable. Even with the center conductor soldered, the connector will rotate an eighth of a turn if no adhesion to the braid has occurred.

With knives, picks, files, grinders, hatchets, and soldering irons, remove enough spillage from the connector so that the outer ring can be attached as intended.

Salvaging UHF Connectors?

Don't! Even if you are successful in getting the cable free of the connector, it is impossible to assure that little whiskers of braid are not lurking in the shelter of the soldering chamber ready to arc to the center pin when power is applied.

I always buy more units than I need in case something goes wrong in soldering them, or in case I forget that pesky outer ring. Always start over with new connectors.

BNC and N Connectors

These connectors are much different from the "UHF" units discussed above. Their connection to the shield braid is solderless. The only soldering to be done is in attaching a tiny male connecting pin to the center conductor of the cable. Without effective holding clamps, attaching this pin can be difficult.

These connectors come with several small parts whose dimensions are specific to a given size of cable. A connector which has been purchased for one size of cable cannot effectively be used on another. (A list of connector numbers at the end of this article should aid in ordering appropriate units for standard sized cables.) Care must be taken so as not to lose any of the small parts, since all of them are necessary for proper installation. A list of these parts is given below:

  1. A very tiny pin which tapers to a point is intended for soldering to the cable's center conductor. A hole drilled axially into the rear of this pin accepts the center lead of the cable; the diameter of this hole closely fits the lead of its intended cable. Soldering is done through a hole in the side of the pin.
  1. A tapered washer is intended to fit into a matching counterbore inside the shell of the connector. The braid, which is folded back against the tapered washer, is sandwiched between the washer and the bottom of the counterbore.
  2. A rubber ring fits around the cable behind the tapered washer and is held in compression; it clamps the braid firmly between the washer and the connector shell, and it grips the outer sheath of the cable to act as a strain relief device.
  3. A thin metal washer behind the rubber ring prevents damage to the rubber as it is being compressed.
  4. A threaded insert screws into the shell of the connector behind all of the above pieces and is firmly tightened with a wrench to compress the rubber ring.

The threaded insert, the thin flat washer, and the rubber ring are slid onto the cable in that specific order. Next, strip off some of the outer sheath to expose the braid. (The amount of braid to be exposed will depend on the connector and the size of cable. Basically, the braid should flair out and fold back to cover the front surface of the tapered washer.) After the sheath is removed to expose the braid, slip the tapered washer over the braid (with the tapered side toward the end of the cable; press this washer back against the sheath and fold the braid back over it.

About 1/8 in. ahead of this braid assembly, remove the dielectric insulator to expose the center conductor. Generously tin this lead and cut it off to a length of 1/8 in. (beyond the end of its insulator).

The tiny center-conductor pin can be held in locking forceps, with one of the forceps' handles being held in a vise. You may wish to line the jaws of the forceps with bits of braille paper so as not to mar the pin with their serrations, and to keep the forceps from acting as a heat sink. Orient the pin horizontally with the soldering hole facing upward.

In order to keep the cable in position, it can be held in another vise located a few inches away from the pin. The center conductor should comfortably rest in the pin, i.e., there should be no sideways force on this lead which will cause it to cut through the softening dielectric insulator as the connection is heated.

Hitting the tiny pin with a soldering iron, without touching the dielectric insulator, is no easy task. You may wish to protect the insulator by poking the center lead through a small bit of braille paper, thus creating a barrier between the iron and the plastic. Hold a very straight piece of solder vertically and insert its end into the soldering hole in the side of the pin. Carefully follow the forceps over to the pin with the tip of the iron and position the iron on the under side the pin. Since the pin is machined to fit its center conductor, you do not need to apply very much solder to the connection. This is fortunate for us, since feeding the solder straight down into the small hole is often a matter of luck. You will often lose the pin with the solder, whereupon you should let it cool off and try again. Perhaps 1/8 in. of solder should be fed to the connector. Spillage onto the sides of the pin can be filed away after it has been secured to the center lead.

List of Connectors