Contents: Basics; Coax; The PL259; Soldering Tools; Preparing the Coax; Soldering; Weather Sealing; Odds and Ends;
Poor quality coax, and cheap PL259 connectors, are failures waiting to happen.
With a few exceptions, most coax is well made, and will provide years of service if you follow a few basic rules. What you read here is pointed toward the mobile operator, but the same basic information may be used for base station use too. There are a few special considerations when coax is used in a mobile installation, and I cover those below.
The coax prep tools you see listed here can be ordered from Sunset Enterprises (for all sizes of coax), and DX Engineering. These tools are some of the best money can buy, and make proper stripping of coax a very simple job. However, they will not strip some kinds of coax. I'll cover this later on too.
The lowly PL259 connector isn't given a second thought either. If they were, those $1 hamfest specials wouldn't sell like hot cakes. By the time you finish reading this article, you'll know why good quality connectors are so important to long-lasting, trouble-free, installations.
Coax comes in many configurations. Center conductors can be solid or stranded, the dielectric can be solid, foam, and air with the latter not just air. Shields can be solid, corrugated, copper or braided copper, silver or braided silver, aluminum in many different configurations, and even steel. The outer covering can be made of a myriad of materials exotic and otherwise, but usually consists of polyvinyl chloride or polyethylene. The nominal Z can be from 35 ohms to as high as 125 with the basic standards being 50 and 75 ohms If you want to know why, read this.
Amateurs typically lump the various types into three “standard” configurations; RG8, RG8X, and RG58. However, these three different configurations are far from standardized. Since there isn't a mil-spec on any coax cable, relying on that specific designation is all but moot.
The ARRL Handbook lists eleven types of RG8, six types of RG8X, and seven types of RG58. Since our focus here is mobile operation, we can narrow down the list rather quickly by eliminating RG8 sized coax, and here's why. First, power levels are seldom over 500 watts. Standard RG8X can easily handle that much power, even at an elevated SWR. RG8 is also rather stiff, and in the tight confines of a mobile, this fact makes installation more difficult. Its lower loss characteristics doesn't mean much in a mobile environment either, as the difference in loss between 10 feet of RG8, and 10 feet of RG8x (an average mobile installation length), is a fraction of a dB.
Tech Talk: Here is a web page RF Transmission Line Loss Calculator written by Owen Duffy, VK1OD. If you calculate the loss difference between RG8X, and RG8 using the lengths typically used in a mobile scenario, you'll see why RG8 isn't worth the effort or cost.
We can also eliminate most foam insulated varieties too, as their cores tend to migrate in warm weather, especially in tight radiuses. For this reason, RG8X shouldn't be in the running, but it is with a caveat. Recently-manufactured examples, are far superior to the older RG8X types. However, you still need to use care when installing connectors.
There's at least one other reason RG8X is a better choice. Even the best of mobile installations will have some common mode currents flowing on the coaxial feed line. This fact is exacerbated by poor mounting locations and/or methodology, when using short, stubby antennas, and when bonding is inadequate or nonexistent. Common mode currents can play havoc with automatic controllers, and can cause some radios to shut down especially when they're remoted. The cure is simply a chunk of ferrite, but not just any chunk of ferrite.
The right photo depicts a 3/4 inch ID, mix 31 split bead, with 6 turns of RG8X through wound through it. Note the minimum 3 inch diameter turns which prevents the coax core from migrating. The resulting choke exhibits an impedance of about 2,500 ohms at 10 MHz. Since the amount of impedance is to the square of the turns, an equivalent choke for RG8 sized coax will require 36 beads! That's over $100 worth!
If you use RG8X, remember it's easily crushed, and the minimum bend radius is twice that of RG58. I might add, some brands of RG8X aren't worth the time it takes to solder it. Coleman® comes to mind. It ages so badly, that after just a few weeks in the outdoors (no sun needed), the core cracks like popcorn. When it does, an RF arc can occur as shown in the left photo. I've never had this happen with Belden® 9258.
For most installations, RG58 is as good as any. Some might argue about that fact, so let’s look at some real world numbers. One hundred feet of RG58 has a loss of 2.5 dB at 30 MHz. In a mobile installation, coax runs are seldom longer than 10 feet, so our loss would be .25 dB. If we add an SWR of 3:1, the total loss is still less than .5 dB. In short, nothing to worry about as long as you don't run high power. If you do, remember this; 500 watts CW is very close to the limit for RG58A. Keep in mind the center conductor is 20 AWG, and with a 2:1 SWR (25 ohms) its current carrying capacity will be exceeded (its 1,900 volt rating notwithstanding). By the way, 500 watts is well past the limit for most other types of RG58, especially at VHF.
One more comment about RG58. At least two, HF mobile antenna manufacturers, ship installation kits with their products with contain RG58U, with a solid, copper-plated, steel center conductor. It's usually supplied along with a pair of #18 control wires, inside a vinyl outer cover. This center conductor is difficult to solder, and the steel wire easily breaks. Add a little mobile vibration, and breakage is a given. Use it at your own risk!
The one thing you can't do with coax is pinch it. I've seen literally hundreds on installations where the coax is routed through a door or trunk seal. This is not an ideal situation, and is yet another reason to properly mount antennas by drilling the necessary holes. This is doubly true with trunk lip mounts where the coax is not only pinched but sharply bent around the mounting bracket.
If you just can't help yourself, and you use RG8 type coax, remember the minimum bend radius is about 6 to 8 inches so care must be taken when installing it. Installing connectors is less problematic than RG58, which is the only good attribute.
Tech Talk: Common mode current is almost a given in a mobile installation, especially one which uses abbreviated mounting schemes. The usually way to deal with common mode is to wind a choke using a mix 31 split bead. They costs just under $5 each. As mentioned in the article, you can get about 6 turns of RG8X through a 3/4 inch ID split bead. You can only get one turn of RG8 through the same bead. To duplicate the reactance of 6 turns, you'd have to use 36 individual beads. That's about $160 worth! Just one more reason not to use RG8-sized coax.
Besides avoiding foam insulated coax, I suggest staying away from those with a "rubbery" outer cover. Not only will coax prep tools not cut them properly, it's nearly impossible to screw on a PL259 without distorting the cover, and the shield under it. Unfortunately, this is also true of a lot of cheaply made coax, and especially some of the imported junk. I recently measured a 10 foot piece of RG8 purchased from Radio Shack. The OD varied from .398 to .428! This makes installing PL259s very problematic. If there is one thought to take away from this article, it is this; don't buy cheap coax!
In my humble opinion, the single most prevalent problem amateurs face is caused by the ubiquitous PL259. They typically are poorly or incorrectly soldered (if at all), and the coax preparation is almost never done properly. Adding insult to an already terse situation, the material making up cheap PL259s seemingly can't be soldered. Some of them easily corrode or rust, and their sleeve threads are pressed, not cut. As a result, far too many amateurs end up blaming all matter of station equipment, and antennas for their problems, rather than the real culprit; a cheaply made, and incorrectly installed PL259. Therefore, this is an attempt to address the situation by making a few pertinent suggestions.
So called teflon (note lower case T) insulated ones can be easily damaged by too much soldering heat. The reason is, most of them are polypropylene, not real, honest-to-john, Teflon®! So when you try to solder them, they melt. Depending on the quantity purchased, real Teflon® (note the capital T) insulated PL259s, cost about $9 to $12 each (the latter with silver plated shells). Few amateurs will pay this much for a PL259, even though they should!
While a lot of different companies make PL259s, I prefer to buy genuine Amphenols®. The part number for the silver plated, Teflon® insulated ones, is 83-822 (Mouser number 523-83-822), and cost about $5 each in quantity. They come in both silver plated and nickel plated shells (that's the ferrule you screw on to the SO239 chassis connector). The silver plated ones are preferred if you can find them. Mouser does not carry them, but they will order them.
A word of caution. Part number 83-1SP-1050 has an Astroplate® body, and shell. Astroplate® is extremely difficult to solder, and it takes an iron with a lot of latent heat (no gun here folks!). Current prices are around $3.50 each.
Another word of caution. The 83-1SP-RFX has what is commonly referred to as a Japanned finish, but it's actually a nickel-based alloy. They're all but impossible to solder! You can file the finish off down to the brass, but that doesn't help much as the inside is also Japanned. This brings up an important point.
In order to make a good RF connection, it is necessary to get the solder to flow into the shield of the coax, and provide a well soldered joint. The surface the shield will be soldered to, is the inside of the PL259, not the outside! In other words, applying solder just to the braid in, and around the holes in the body, does not assure a robust connection. Remember, the solder must flow into the coax shield! This is especially important when using reducing sleeves.
Reducing sleeves are a requirement for both RG58 (.192" OD), and RG8X (.242" OD) coax. For RG58, the correct Amphenol® part number is 83-185 (Mouser part number 523-83-185). You can also buy 83-185-RFX sleeves for RG8X. Unfortunately, they have a Japanned finish, and are very difficult to solder! There are two solutions.
You can buff the reducing sleeve with a wire brush, and pre-tin them. Or, you can do what I do if you have a drill press and/or vice. I buy the 83-185, silver-plated sleeves. I chuck them in my drill press vice, and drill them out to .25 inches. Care must be taken, as brass tends to snatch badly. The silver plating makes soldering a snap!
Here's yet another item that shouldn't be scrimped on; a good soldering iron. To properly solder PL259s, you have to have a soldering iron with a lot of latent heat. At a minimum, this is a 75 watt iron, and a 100 watt works even better. Notice I said iron! A soldering gun is not the tool to use. The reason is, they have very little thermal mass and as soon as you touch the tip to a cold connector what little latent heat exists is drawn away. While you're waiting for the material to get hot enough to melt the solder, the coax core is slowly but surely being damaged beyond use.
If you just can't find a decent iron locally, here's a good source. This is Hobby Lobby's on-line ordering company, but if you have a store near you, then that's the place to go. It sells for $50, and is as good as you can buy even if you spend twice the amount. It comes with two tips, and the smaller one perfectly fits the groove in a PL259. It takes less than 2 minutes to solder a PL259 with this iron.
Digressing again. Hobby Lobby also sells what they call an embossing gun. It actually is a small heat gun, just right for those little bench jobs that don't require a lot of heat. If you use polyolefin, dual-walled, adhesive heat shrink to seal your connectors (as outlined below), this $20 tool is just the ticket! By the way, it comes in blue too!
Proper soldering of PL259s requires two soldering irons. The aforementioned iron, and a smaller one to solder the center conductors. I use a Weller pencil type, but others will work just as good (except a gun). I clamp the iron in the vise and rotate the work, not the other way around, as this makes soldering easier.
When I wrote the original article for eham.net (these words are an expansion to that article), several folks commented that I didn't know what I was talking about with respect to using a soldering gun. One even suggested removing the tip and using the current to heat the PL259 body directly. Here is my answer to those critics. I mentioned above, that in order to make a good RF connection, it was necessary to flow solder into the shield of the coax, and wet it to the inside surfaces of the PL259. This takes a source of latent heat to get the job done quickly before too much heat is transferred to the insulating material, and outer cover of the coax. It doesn't make much difference whether you heat the connector with the tip, or use current flow from the gun, it takes a lot longer to reach soldering temperatures. In the mean time, the coax is being damaged. The same goes for butane powered soldering irons, no matter how handy they may be for tower work.
Never cut coax with wire cutters! Doing so distorts the core and the center conductor making installation of the PL259 body rather difficult. If you don't have a proper cable cutter, a heavy-duty box cutter with a new blade is your best bet. Lay the coax on a scrap chunk of lumber and tap the box cutter through the coax with a small hammer. The cut needs to be clean and even.
The Cablematic® Division of Ripley Tools Corporation makes the best coax prep tool I have ever used. The UT8000 (right photo) is specifically for RG8U and RG213, and the UT5800 for RG58. There is also a model for RG8X (RG59 OD). Their US distributor is Sunset Enterprises. They sell the tool for $50 plus shipping. The "First Cut" end cuts the coax outer jacket, shield, and core, but not the center conductor. The center conductor extends the exact length needed; 5/8". The "Second Cut" end of the tool removes just the outer jacket to the correct length, 5/8". The photo below left below shows each cut the tool makes. Incidentally, the center conductor should not stick out of the end of the PL259, but just shy of the end as seen in the photo below right. This is the correct length!
Tech Talk: The aforementioned tools are for 83-1SP style PL259s. If you're using PL259s based around the 83-1SP-15RFX style, the outer cover cut will be tool long (1.25 inches verses 1.13 inches). Since no one makes a tool for the 83-1SP-15RFX style PL259s, this is yet another good reason to purchase the correct 83-1SP units.
As I pointed out above, these tools will not cut coax with a rubbery outer cover, or RG8 larger than .405 inches in outside diameter. In fact, the tougher the outer cover the better it cuts, so it works exceptionally well for direct bury coax.
Always use good quality, rosin core solder. Never use acid core! I prefer Kesters 60/40 because I know of its quality. That cheap stuff you buy from Radio Shack or Wal-Mart is not the solder of champions. By the way, I don't recommend lead-less solder for coax connections, as it doesn't wet nearly as well as lead-based ones. So, here's a good suggestion. Buy yourself a small roll of 1/8 inch, 60/40, rosin core solder. It's too big for circuit board use, but it fits the bill connector-wise.
I digress. All too often I have seen amateurs cut the jacket away to the point the shield shows after assembly. This is not the proper way. Conversely, if you follow the cutting table listed in the ARRL Handbook, the jacket stripping length is too short, and on some PL259s, the jacket bottoms out before the core is snug against the back of the tip. Further, the jacket could cause contamination of the solder thus impeding its flow. Both good reasons for purchasing a UT8000.
Make sure you put the threaded barrel over the coax and in the correct direction first! Then carefully slide the PL259 body over the end of the coax and screw it down over the jacket. A small pair of channel lock pliers works well for this purpose. Once resistance is felt stop threading. And speaking of resistance, once you get connectors screwed on both ends is a good time to use your DMM or VOM to check for shorts between the center and shield.
I often see folks pre-tin the coax shield before they screw on the connector. This is an unnecessary step, and one which can actually lead to a failure. Remember, the shield should be properly wetted to the inside of the connector. Pre-tinning might speed up that process, but what usually happens, once the solder flows into the hole, most folks stop. This leaves the shield only partly soldered to the connector.
So we've got our coax prepped, our irons hot, and it's time to solder. The tip should be soldered first. If you use good PL259s as I suggest, this operation takes about 10 to 15 seconds. Enough solder should be used to close the connector tip entirely to aid in keeping out moisture. Avoid slopping solder on the outside of the center pins. Let the tip cool before soldering the shield.
If the soldering iron tip is well tinned, about 6 to 10 seconds later the barrel will be hot enough to accept solder. The solder should be applied just at the edge of each hole so it flows into the shield. Enough solder should be applied to fully wet the shield and close all four holes, one at a time. Use too much and it could flow to the tip and cause a short. Use too little, and you won't have a good connection. This operation should take about 20 to 30 seconds and requires both hands; one to apply the solder and the other to rotate the coax. Now you know why I clamp the iron in a vise. Once soldering is complete, it's time to recheck for shorts and continuity.
If you follow the aforementioned installation and soldering techniques, and completely fill-in all the solder holes, PL259 connectors are rather weather tight. You can make them even better by sealing the coax entry into the connector with polyolefin, dual-walled, adhesive heat shrink. All Electronics is one source. Nonetheless, they're not made to be immersed, and you should protect them from direct water spray.
Here's an alternative. A recent QST article suggested using a hot melt glue gun, and standard heat shrink to effect the same type of seal. I've not tried this, but if the hot melt was applied in the correct places, it should work quite well. You're on your own here, as I don't scrimp when it comes to heat shrink. Here's why. Standard heat shrink tubing reduces about 50%. Polyolefin, dual-wall, adhesive tubing, reduces about 65%, and is much thicker than single wall. That's not to say the hot glue technique won't work, but I'd bet it'll take a few tries to get it right.
Rescue Tape® works well too if you apply it correctly. It comes in several colors so frequency marking coax is easy too. If it has a drawback, it is the fact is isn't very scuff resistant. If you break through the top layer, it has a tendency to unravel. In any case, it does seal out moisture better than any other type of tape. Ace Hardware now carries Rescue Tape® in stock.
It seems everyone uses vinyl electrical tape and/or coax seal on outdoor connections. Just remember this; even though you might get the connection well covered, water can still seep in, where it tends to stay. And, when you have to take it off, it always leaves a sticky mess, no matter how good the quality of the tape. If you just have to use it, use 3M's #88, all-weather style, and not #99! It has a tendency to corrode even silver plated connectors.
A few last comments about tapes. Forget friction tape, rubber self-vulcanizing tape, mylar mending tape, masking tape (seriously, I've seen it used), and duct tape. If that's not all, heaven help us, packaging tape!
Salvaging PL259s is a losing battle, but lots of amateurs do it. I've done it when I've had to, but it should be avoided. As I said, scrimping on coax and PL259s is a prescription trouble, and it always happens at the most inopportune times.
PL259s are available that crimp on. Personally I don't like them, but they offer a rather good moisture seal, so some amateurs use them. The crimping tool is not cheap (about $145), and they require a different prep tool as well. Sunset Enterprises sells the proper tools if you're into crimped connectors. However, almost all of the readily available crimp connectors are RFX style, and should be avoided. The exception to this are the various N connectors made by Amphenol. Good quality crimp style N connectors cost about $18 each.
There are several styles of screw on (to the coax cover) PL259s. I can't think of one application where they are apropos.
Over the last 4 years or so, copper pricing have increased several hundred percent. Coax connectors are made of brass, which is an alloy of copper and zinc. As a result, prices of coax connector has risen too. If the present trend continues, it won't be long before PL259s will sell for over $15 each. This fact exemplifies the need to properly install your coax connectors.
Once upon a time, the official designation for RG8 was not 50 ohms, but 52 ohms. Nowadays, it's difficult to find references to the latter, even though some brands actually measure closer to 52 ohms than they do 50 ohms. This brings up an interesting fact. Different runs of the same brand and type, may have different characteristic impedances. Nominally this isn't a worry unless you're using it for phasing lines, especially VHF ones. In this case, the coax should be measured to make sure the true impedance is known.