The UHF connector is a name for a fairly common, but old type of threaded RF connector. The connector design was invented in the 1930s for use in the radio industry. PL-259, SO-239, PL-258, and several other related labels used by military services refer to one specific mechanical design, now collectively called UHF connectors, continuing the use of the now-obsolete meaning of 'UHF'. The designations come from the Joint Electronics Type Designation System, its predecessor, the AN system, and the earlier SCR (Set, Complete, Radio) system.
The name "UHF" is a source of confusion, since the name of the connectors did not change when the frequency ranges were renamed. The design was named during an era when "UHF" meant frequencies over 30 MHz. Today, the meaning of the term ultra high frequency (UHF) is instead defined to be frequencies between 300 MHz and 3 GHz and the frequencies formerly called UHF are now called very high frequency (VHF).
It is a widely used standard connector for HF transmission lines on full-sized radio equipment, with BNC connectors predominating for smaller, hand-held equipment. UHF connectors are still widely used in amateur radio, citizens' band radio, and marine VHF radio applications.
By design, all connectors in the UHF connector family mate using the 5/8 inch 24 tpi threaded shell for the shield connection and an approximately 0.156 inch-diameter (4 mm) pin and socket for the inner conductor. The mating receptacle is designated SO-239, creating the complete PL-259/SO-239 connector system.
Impedance Characteristics and RF Performance
UHF connectors have a non-constant surge impedance. For this reason, UHF connectors are generally usable through HF and the lower portion of what is now known as the VHF frequency range. Virtually all of the impedance bump and consequent loss is in the UHF female connector, the SO-239. A typical SO-239 UHF female, properly hooded, has a difference in impedance from the standard 50 Ohm line impedance of about 35 Ohms. The length of the bump is typically 1/2 inch, where the female pin flares to fit over the male pin.
Also an internal impedance of 30 ohms maximizes the power handling capability of a connector and the PL-259 comes in at 35 ohms. This impedance variation creates some controversy in the amateur radio community, though actual measured performance often differs from theoretical concerns.
Selecting optimal PL-259 connectors requires evaluating technical specifications against application requirements. Standard impedance is 50 ohms, with frequency ranges typically capped at 300 MHz. UHF connectors are used for frequencies from DC to a maximum of 300 MHz due to non-constant impedance across the length of the cable. Poor quality connectors will support substantially less than 300 MHz.
Common Applications in Ham Radio Stations
These are widely used in amateur ham radio, citizens band and marine VHF radio. As has been mentioned above, almost all amateur base and mobile radios have mating SO-239 connectors on them, even recent models like the 2015 Icom IC-7300. This includes models that span the HF/6M/2M/440 frequency range. The relatively low power (10W) Icom IC-705 is bucking this trend however.
UHF connectors are standard on HF amateur amplifiers rated at 1500+ Watt output. This widespread adoption in amateur radio equipment ensures continued relevance despite the availability of more modern connector designs.
PL-259 Connector Construction and Design
Physical Dimensions and Materials
PL-259 connectors feature a distinctive construction that has remained largely unchanged since their introduction. By design, all connectors in the UHF connector family mate using the 5/8 inch 24 tpi threaded shell for the shield connection and an approximately 0.156 inch-diameter (4 mm) pin and socket for the inner conductor.
The connector body is typically machined from brass or bronze, with various plating options available. For outdoor installations, prioritize IP67-rated units with nickel or silver plating. High-quality connectors use silver-plated brass construction for optimal conductivity and corrosion resistance.
I remember seeing hamfest vendors with signs advertising their cheap PL-259 connectors. As I recall, they were usually priced at $1 each, or less, with discounts for buying in bulk. I remember looking at these connectors and thinking "these are pretty low quality". The connector bodies and shells were usually un-plated aluminum which seems like it would have long-term corrosion issues. The center pins might be gold or silver plated. The insulators appeared to be nylon or teflon.
Internal Structure and RF Path
The internal design of PL-259 connectors creates their characteristic impedance profile. The UHF connector system presents a varying impedance to the signal that is dependent on frequency; This is the primary problem with this connector. The disturbance to the signal should be proportional to the length of the impedance deviation.
This bump can be mitigated by using a honeycomb dielectric in the female pin area. Many VHF / UHF amateur operators use special UHF female connectors that maintain a 50 ohm surge impedance. The dielectric material choice significantly affects performance, with Teflon offering superior characteristics compared to phenolic materials.
Quality Variations Between Manufacturers
What struck me most was the poor quality of the machining. The solder holes drilled through the body often had burrs on them. The threads on the bodies and the shells were rough so the two parts would not screw together smoothly. Just not a quality product, in my opinion.
I don't think you can get any better than Amphenol connectors, so that would be the brand I would get if possible. I've seen what you're seeing with the ones you have. Only thing that worked for me was to scrape off the coating they have, don't to the brass (or whatever). Quality manufacturers like Amphenol produce connectors with superior machining tolerances and plating consistency.
Installing PL-259 Connectors: Step-by-Step Guide
Required Tools and Preparation
Proper PL-259 installation requires specific tools and preparation. Prep tools are available to prep the coax to the specified length so all that needs to be done is slip the new connector over the RG-8, RG-213 or LMR-400 coax and then crimp the end piece (Center Conductor) and the ferrule over the outer shield.
A small tipped soldering iron is used for the holes and the gun is used for the center conductor. The idea is to supply enough heat to quickly solder the connections but not enough to damage the cable. The cable is quite resistant to heat and you should have no problems making the connections. A good solder connection should be slightly shiny which means it has had enough heat and cooled correctly.
Essential tools include: a high-wattage soldering iron or gun (75-100 watts minimum), rosin-core solder, coax stripping tools, and a sharp knife for precision cable preparation.
Soldering Techniques for Proper Connection
PL259 so the tip is pointing down while you solder it. This way the solder will not run down the center pin towards the main body of the connector. (Note: I have read, but not experienced myself, that there are poorly made PL259's on the market that allowed excessive molten solder to flow through the center pin and into the body of the connector settling such that it created a short between the center pin and the braid at some point inside the connector. As you can imagine this might/will cause a short and damage to the rig. This potential problem is a good reason to use crimp-on connectors requiring the center pin to be crimped not soldered.)
Coat the first 1/2 inch of braid closest to the insulation some resin core flux. Your solder probably has a flux core but this step will help the tinning process with a minimum amount of heat and solder. You want to tin or solder the first 1/2 inch of braid together on all sides (all around).
Instead of assembling the connector and soldering the braid through the little holes I start with just the reducer and solder the braid to that very carefully first so you can inspect for damage. Then I assemble the connector leaving only the center conductor to solder. Otherwise any damage will be hidden inside the connector and you won't know until its too late. I also don't pull a lot of braid back over the reducer, you only really need to bend the braid slightly over the reducer, then solder the shoulder of the reducer where the braid touches, then trim off the excess braid.
Common Installation Mistakes to Avoid
Many hams have varying degrees of success when attempting to install the revered PL-259 solder on connector. Often these attempts end with melted coax and connector and a poor connection that will fail over time. I have installed thousands of these connectors and I must admit that when things go right I consider myself lucky.
The problem is usually the soldering of the shield. Either too much heat and melted insulation or too little heat and cold solder joints. If you do not supply enough heat or jar or shake a solder joint while cooling it will look gray almost crystallized. This is the sign of a poor connection, which should be reheated.
If you did, that's undoubtedly the cause. It may sound like an indictment, but very few amateurs have the tools and/or knowledge to correctly install PL259s. Especially so when a reducer is required.
Testing and Verification Methods
At this point double check with an ohmmeter to make sure the braid has not shorted to the center conductor. If everything is fine proceed to the soldering phase. This basic continuity test prevents catastrophic failures when the connector is first used.
After installation, conduct SWR measurements across the intended operating frequencies to verify proper installation. Poor solderability equates to loose connections! If you suddenly have an RFI or intermittent SWR problem, the first place to look is at the coax connections!
Compatibility and Mating Connectors
SO-239 Socket Specifications
The PL-259 male connector mates with the SO-239 female socket. The jack-female is often referred to as a SO239. The SO-239 is the chassis-mount receptacle that accepts the PL-259 plug, creating a threaded connection that provides mechanical security.
A double-ended SO-239 connector is designated as an SO-238. These barrel connectors are commonly used to join two coaxial cables with PL-259 plugs installed.
Adapter Options and Conversions
The amateur radio market offers numerous adapter options for interfacing PL-259 connectors with other connector types. When you need to connect an N-type outdoor antenna to a PL-259-ported transceiver, or to RP-SMA WiFi equipment, you need a cross-connector cable.
Common adapter configurations include PL-259 to N-type, PL-259 to BNC, and PL-259 to SMA variants. However, each adapter introduces additional impedance discontinuities and potential failure points in the RF path.
Frequency Limitations and Applications
UHF connectors are used for frequencies from DC to a maximum of 300 MHz due to non-constant impedance across the length of the cable. Poor quality connectors will support substantially less than 300 MHz. For HF, 2 meter and even 70cm, they are 'good enough' for most hobbyists.
I know the conventional wisdom is don't use PL-259's above 2 meters but I saw nothing on the analyzer that said this was true below 1 GHz. Now that doesn't preclude a bad connector design or poor female connectors on the radio unit but I saw no reason not to use the PL-259 crimp on at UHF.
Mechanical Durability Considerations
The PL-259 has been around forever and properly installed is simple, rugged, and reliable. The key is "properly installed". The threaded coupling provides excellent pull strength and resistance to accidental disconnection, making PL-259 connectors suitable for permanent installations.
I know there are the old timers who scoff at a crimp on connector but in the cell phone industry we have found that crimp on
Recommended Comments