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  • Everything you wanted to know about coaxial connectors but were afraid to ask



    By Carl G. Lodström Recycled Goods Ventura. CA 93001 2014

    The DIN (Deutsches Institut für Normung) connector. It is really not a coaxial connector, but it is so common, and there are a few important things about it, so we will cover it here! Pictured is a 5-pin, spread 240°. They come in 3 (and other) pin numbers and angles. 3 and 5-pin, with a 180° spread, is a common audio connector, number 1 and four below: 

        210° 360° 180° 360° 240°   

          Varieties with different number of pins and different spreads. For the 4 and 5 pin, an angle must be called out! (Digi-Key, do you hear me?!)

    Socket & plug for loudspeakers – and - the common 5-pin 180° Notice that the location of three of the pins in the 5-pin plug are sidentical to that of a 3-pin plug! So they are compatible for mono applications! The “lost pins” are the right side channels on record and playback (input and output).

     These connectors are very versatile. A 5-pin plug handles left and right channels, input and output, on a tape recorder for example, all with one single connector and cable! That beats four RCA plugs that may well be mixed up!

     The loudspeaker plug, for connecting external speakers, turns off the internal speaker if it is plugged in one way and not if it is plugged in the other way! Clever!

     These days, for computer keyboards and such, there are mini-versions of the DIN connectors too.


    True coaxial, or “RF” connectors

     For electronics the BNC is the most common connector:

    A couple of male connectors.  a common T-junction adapter

    A common T-junction adapter mounted on a typical cable, Females on the straight-through   

    For an overview, and more detailed facts, about RF connectors:

     where there also is a link to just about each type and an explanation of how their names came about.

    A BNC-F-F “barrel” for joining two cables with BNC-M connectors on the ends. A SMA-F-F “barrel” mounted on a SMA-M to BMC-M adapter.
    A TNC-M connector attached to a RG-142 cable.   adapter The TNC-F side of an angular adapter.

    This connector is often used in applications with a lot of vibrations where a BNC may come loose. The difference from a BNC is in the mantel only. The central parts are identical.

     There are limits to how heavy things one can hang on a BNC (or on a TNC) connector. Where mechanical strength is required the N-connector is a good choice. It is also good up to 12.4 GHz.

    The N-M mounted on a 141 Semirigid cable.  The N-F connector end restingon a SMA connector.

    Of course, the N connector can be mounted on cables that would be too large and heavy for the smaller SMA, BNC and TNC connectors. All connectors also come in a flanged version that can be mounted on a panel, or in it (then called a “bulkhead connector”), with a lock nut or with a flange. We will soon see some.

     For coverage of what are not just connectors, but Adapters, see page 16!

     A very common connector is the “PL 259”. Generally it is not used on serious equipment, as it has no match to 50 Ω, whereas the rest of the connectors mentioned here do.

     At the time it was designed, WWII, 70 years ago, measuring cable impedances and reflexes was not so easy and it was “good enough” for much of whatever it was used for. It is sometimes called “UHF connector”. At the time UHF was everything above 30 MHz. Now that is called HF to 30 MHz, VHF to 300 MHz and UHF is 300 MHz and up. In round numbers, the PL259 is OK to 30 MHz, questionable above 100 MHz and should not see serious use above 300 MHz.



    Flanged connectors, that may or may not have a connector on the backside.

    The upper row are “C-connectors”, about equivalent to the N-connector, good to 12 GHz, but for that they have a bayonet connection (like the BNC) enabling quick connect and disconnect. They are professional grade, expensive and not very common.

     The lower row is the flange version of the very common socket “SO-259” accepting the plug “PL-259”. This connector is not “professional grade” but very common among ham operators (at frequencies below a few hundred MHz) and CB radio enthusiasts. Not the sharpest knife in the drawer, but sometimes it may be proper to use them anyway! When the customer is likely to only have cables with this “UHF” connector, as it also is called, it creates more problems for the customer to make, or to procure adapters, or cables with N or C connectors, especially if it is not really needed. Also: one can plug a banana contact in this connector. It was often used on old oscilloscopes for example.


    The SMA connectors

     SMA connectors have gained a lot of popularity. They are reasonably priced, easy to mount, especially males on the .141 Semi Rigid cable, and it is a good performer up to some 18 GHz. There above it soon falls apart, but so do most other connectors!

         It is so cleverly designed that the center pin is the extended center conductor of the cable! So, by stripping the cable to different lengths for the mantle, insulator and the center conductor, the connector is almost made already! The center conductor is just rounded off a little on the tip.

     A compatible connector is the “3.5 mm” connector. It works well up to 26.5 GHz. In a pinch it mates with a SMA connector. Notice that one cannot see the dielectric in these! In the SMA there is visible, white, Teflon around the center pins, here it is air. The 3.5 mm connectors are not inexpensive!

    It is important, for SMA and these connectors, that the nut be properly torqued, with a 5/16 inch wrench, 3-5 in·lbf (0.3 to 0.6 N·m) for brass, and 7-10 in·lbf (0.8–1.1 N·m) for stainless steel connectors. Proper means: not too hard!

    When turning the nut, it is also important that the remainder of the connector is not allowed to rotate; otherwise premature wear of the connector will result. Furthermore, the connector should be carefully inspected before each use, and any debris cleaned with the tip of a clean rag, a cotton swab or with compressed air. When properly handled, a SMA connector should meet specified performance through 500 mating cycles. Smaller than the 3.5 mm, but similar in appearance, are the 2.92 mm, 2.4, 1.85 and 1.0 mm connectors, which do not cross-mate with SMA. These smaller than SMA connectors can be used up to frequencies of 40 and 70 GHz.

     The F-connector.

     This is the connector of choice for mating cable TV (CATV & CCTV) equipment. You certainly recognize it.

    The male part is designed to be crimped onto a RG-59 cable. By stripping off the layers of cable isolation and shield to the right lengths, the cable center conductor becomes the center pin in the connector, keeping the cost for this connector way down. The design of the crimp body is unusually good and clever, making for one of the few reliable crimped coaxial connectors in the market! But only if the proper crimping tool was used!


    On top of this, the RG-59 is a very good cable, with low loss, lower than the RG-58, and very well shielded for just about no signal leakage and mechanically strong. RG-59 cables, and the F-connectors, have 75 Ω characteristic impedance. To a 50 Ω device, the mismatch is almost without consequence and the lower loss in this cable may even make up for the small mismatch loss. In addition, if the other end is connected to a simple dipole antenna, 75 Ω is close to its impedance (73 Ω) so no matching is then necessary at that end, saving components and losses. RG-58, RG-8, RG-142 and RG-174 are 50 Ω cables.


    The proper tool for crimping F-connectors, and an unused connector body.


    The properly crimped F-connectors are useable to 1 GHz. This is as high (800-some MHz) as TV transmissions and cable go anyway, and a TV is not a very demanding piece of equipment. It in itself is probably not very well matched to 75 Ω anyway!

     For example: use of proper power splitters to split the signal between more than one TV usually works very well. Just splitting up the cable and joining two cables to one is not good and will result in serious reflections of the signal. This will be visible as “echoes” or multiple edges on the left and right sides of images on the screen. It is not science, but treat it with a little bit of respect!

     The “7 mm” connector

    The APC-7, also called the “7 millimeter connector”, is a very high precision connector, good for use to 26.5 GHz. It is often used on instrumentation where cost is not much of an issue.


    It is hermaphroditic, that is: there are no males and females! All connectors can mate to all others. The threaded sleeve, visible in the right picture, fits in the threaded body in the left picture! But the sleeve can be retracted, and another can be presented from the left connector!

    The mating surfaces must be wiped clean before mating, and the connectors should be rotated counter clockwise slightly just when the body is tightened, by hand is enough! The resulting wiping action, of the gold plated surfaces onto each other, will greatly reduce the contact resistance. Only one of the two threaded rings should be tightened! If both are tightened the connectors will actually be forced apart again! I have seen this error cause problems several times.

    In the pictures above we can see how the center conductor is protruding slightly. It is spring-loaded and retracts when mated. Many tests have proven these to be the best type of connectors for measurements and lab work. The lowest loss, the best match, the best repeatability, but all for a very pretty penny! These connectors must be treated with care, or they will no longer be among the best!

     The General Radio GR 874 connector

     GR 874 is another hermaphroditic connector.

    The GR 874 connector and a GR 874 to N-F adapter


    These connectors are also hermaphroditic. There are no males or females of this kind; they can all mate with all others. It is a very good connector. General Radio used them on their instruments for audio, RF, at least up to 3 GHz, if not higher. They may look funny, but they are actually very good!

     Some Unusual Connectors.

     The BNC comes in a few flavors. Two (at least) for high voltage and one for differential signals.

     Pictures of two HV versions coming soon!

     Here: a normal BNC receptacle on the left and the quite unusual “Differential BNC receptacle” to the right thereof.

    The “UHF connector” also comes in a dual, differential, version. It is sometimes called a “200 Ω UHF connector” but, just as with the regular UHF connector, controlled impedance is not the strong side of the 259 connectors! For many applications this is not important, so a simple and strong design, at a moderate price, is more important!

    A normal BNC-F connector A 75 Ω BNC-F connector

     Another flavor of BNC (and N-connectors and a few more) is the 75 Ω variety, as opposed to the regular 50 Ω version. They are different only in that the center pin is thinner and one have to look out for this! See the pictures above! The 75 Ω connector in itself is not a problem, not until one tries to mate it with a 50 Ω male. The center pin is too thick, and the fingers in the female center pin may be permanently deformed or broken. Even worse is the 93 Ω version, sometimes used on instruments for nuclear research. Take a look at the female pin before mating! It may save your connector from destruction.

    The tri-axial BNC is quite unusual. As you can see, it will not mate with a regular BNC. If nothing else, it has three, not two, pins in the bayonet.


    The normal and the “Tri-axial” BNC.


    The tri-axial BNC is quite unusual. AS you can see, it will not mate with a regular BNC. If nothing else, it has three, not two, pins in the bayonet.


    Big connector!


    I do not recall what these are called, somehow the name “7/16 inch connector” comes to mind, but I am not sure that is right. I will update the document when I find out. Compare to the BNC connectors!



    These are for power levels of several kW, where N (or 259) connectors are not large enough. This may also be called for by a thicker coaxial cable, which cannot be adapted to fit the smaller connectors. These connectors are found in broadcast transmitters, particle accelerators and such equipment.



    Case in point: Imagine the small N-connector on the left fitted to this 1.1 inch diameter coaxial cable! The weight of the cable alone would risk breaking the connector!


    When connectors are not connectors!

     There are a lot of adapters, and other stuff, around and we will just take a quick look so we are a little familiar with them when they show up!

     Adaptors are needed when we want to mate a cable with one kind of connector to another cable or instrument with another kind of connector. It is also needed when we want to join two cables with the same kind of connectors. The cables often have BNC-M on the ends, so a BNC-F-F “barrel” will handle it. This is a short adapter with female connectors at both ends.






    BNC barrels.


    On the left: a F-F barrel that can join two cables, as they normally have M connectors.


    In the middle: a M-M barrel for joining BNC-F connectors.


    On the right: another F-F barrel, but Nickel plated. The other two are Silver plated. This is an important difference that we will return to.



    On the left: a TNC and a N barrel, both with M connectors and there are of course the same with F on both ends, but I do not have any such TNC. On the right: N barrels F-F, the one with a nut is a bulk head connector for mounting through a chassis.


    Small barrels and other unfamiliar animals!



    Here we have some T-adapters. From left to right: SMC-M on all three, SMB-M on the left and right and SMB-F on the middle one. Then two T’s with SMA connectors for size comparison.


    The SMB is a “push-on” connector that goes “click” when mated. It can be pretty hard to pull off, so by no means should it be pulled by the cable!


    The SMC if easily pushed on and then a threaded collar is tightened by hand. You can see the threads on the leftmost T.


    Here: some SMC barrels. One F-F and one M-M. The latter is really a “bulkhead” connector, made for to be mounted through a sheet of metal, but the nut and the lock washer are missing here. This is one way to lead a coaxial cable through a wall in an instrument. A bulk head connector is put in the wall and a cable is attached to both the inside and outside ends.



    Here: barrels of the very common SMA connector. Gold plated Male, female and a stainless female bulk head connector.




    Here: “Connector Savers”! Some connectors get mated and unmated all day long for testing things. Test cables, with an SMA connector on the end, for the HP 8510 Vector Network Analyzer cost $5000 each in 1986! When the connector is worn out, the cable is scrap! Therefore adapters like the ones above are put on the cable connector and left on. If the cable had a male, the end will still be male and so on. The adapter is then mated and unmated all day long, but not so the cable end connector. When the adapter is worn out it can be replaced for a few dollars!


    In the picture above we see two each, stainless steel and gold plated (over stainless steel probably), versions. Almost all small connectors are made of stainless, the strength of which is desirable. Stainless is not a very good conductor, and it forms a protective oxide layer (that is why it is stainless!). Gold is not a very good conductor either, but it does not oxidize, so it generally makes better contact than stainless.


    Copper is better and best of all is silver! Peculiarly enough, the tarnish on silver (not an oxide, but a sulfide, Ag2S) is also conductive, so even when silver plated connectors look horrible, they are still the best! This is more true the higher the frequency. Above 1 GHz the difference is considerable but already at 10 – 100 MHz silver plating of RF components are common and worthwhile. At swap meets for ham radio buffs it is interesting to see who grabs what from the bins of used connectors and adapters! OM (Old Man) grabs the silver plated connectors; the newbie’s grab the shiny, nickel plated, stuff!


    The “gold” connectors we see in today’s HiFi and video “industry” are probably not gold plate. The shine is not of the right color; it is probably some shiny brass plating. Notice that on “the real stuff”, the silver or gold on the connectors is never polished! Even if it was real gold, it still does not justify an $80 price for an audio cable with RCA plugs! This is worse than used car business.




    Sometimes there is a need for to make a signal weaker (besides for some other reasons that are too complicated for to go into here). An “Attenuator” is put in the line. They have connectors too, of course, often different sex on each end, so they are easy to include in an existing circuit, or to stack several of on each other. Sometimes they have lost their label, and look just like a regular connector saver!


    See the one on the left below! It is an attenuator! What gives it away is its length! A connector saver would have no reason to be this long.



    To the right thereof are two attenuators. They are clearly marked with what they are and how many dB they attenuate. The fourth device, gold plated (but not always so) is a detector! They are quite expensive ($50 - $200). They convert an RF signal to a DC signal, just like the old crystal receiver you had as a kid! It is the same thing, but fancier, more sensitive and much more expensive. The two rightmost ones are other common models of SMA attenuators. Of course, attenuators and detectors come in all shapes and in other connector families too. I have just chosen to show some SMA versions here. N and BNC versions are very common too.



    BNC and N attenuators.

     From top to bottom: 20 dB with BNC F&M, 3 dB with N F&M, 6 dB with N M&F and 10 dB BNC F&M. So they can take many shapes!

     Sometimes the signal to be attenuated may be very strong. Then there are Power Attenuators! The one above attenuates the input signal to 1:100 (20 dB) and can handle up to 100 W on the input (on the right side). With 100 W in, 1 W comes out the left side! This can be used for to measure higher power levels.

     Connectors that do not go to anything!


    Sometimes we find connectors that have only one end! They can only connect themselves to a port! Then it is almost certainly a termination, also called a “load”. The idea is to absorb the signal from whatever it is connected to, and to give no reflex. If a 52 Ω cable is “terminated” with a 52 Ω resistor, all the power in the cable is “terminated”, absorbed, turned into heat, and none is reflected. Like a sooted surface for light! We have a perfect load. The same is (almost) the case when a 75 Ω cable (like RG-59 or RG-6) is terminated with a dipole antenna of 73 Ω. A very good load, nearly perfect.

    Some common terminations in the picture above, left to right: N-M, SMA-M, BNC-M and SMA-M. Happened that they have all male connectors, but there are terminations with female connectors too!

    Here is one! A TNC-F termination, for a little more power, 5 – 10 W maybe, is pictured on the left.  This power attenuator can also be used as a power load!  Any attenuator that has more attenuation than 15 dB, or so, it will make a fine load!


    The particular one has 20 dB attenuation (which is 100 times damping of the power or a 20 dB loss). When the attenuated signal reaches the other end, it is reflected from the open (or shorted) end. It is dampened another 100 times on the way back and from the input end it now looks like only 1/100th times 1/100th = 1/100.000 returned! Called: a 40 dB Return Loss, RL. 20 dB one way and another 20 dB for the reflex! Notice that RL is expressed in dB, not –dB. It is already called a loss. A 40 dB RL constitutes a very good load! This is worst case! If one puts another load (may have to be able to handle 1W) on the output of the attenuator, even less will be reflected. We are reaching, or may even have reached, the limit where theoretical and actual components leave each other in the dust! Even a good load may not have a better RL than 25 - 30 dB and at high frequencies 40 dB RL is not that easy to measure! Neither is it easy to make one that good, especially not for GHz operation.


    But beware! They are not always loads! There are short, open and calibrated mismatches too! If you see no center pin at all, it is an open. If you see the center pin coming up from the metal in the lid, then it is a short. If it looks normal but have for example 100 Ω it may be a faulty 50 Ω load or an intentional mismatch. If it is 75 Ω, it is probably a load for a 75 Ω system.

    A far out adapter!

     Not all signals are conducted in coaxial cables! Waveguides are commonly used for microwaves. The losses (at these very high frequencies) are much lower in WG than in a coaxial cable. But only about one octave of frequency (like 7 to 14 GHz) can be conducted in any one wave guide size, so one need one size for each frequency band. In a coax there is no such limitation.

    So, there are a lot of devices for waveguides too! There are a plethora of adapters, antennae, detectors, attenuators and what not. We will leave them aside and only mention the transition between wave guide and coax.


    Here: an adapter for the popular “X-band” (10 GHz, most radar stuff) adapting between coax and wave guide. The signal, or power, can flow either way. This makes for an easy transition from a cable to an antenna, that at this frequency can look like a rectangular funnel! The crystal detectors become more sensitive in a WG as the voltage (for a given, weak, signal) is higher than in a coaxial cable! It is easy and inexpensive to terminate a waveguide (for small powers) with an adapter like this and a coaxial load. Some WG detectors look like this also.

     The hole is (almost) always rectangular, with a 1:2 side aspect ratio. Here it is 0.90” wide, which makes it a “WR90” guide. On top is a N-F connector. A waveguide detector has a detector housing here instead of the N-connector. It is a little bit longer and usually has a BNC-F connector on top


     They may look like adapters but they are not! If they have a legible label, it is easily determined. They are usually longer than an adapter would have to be just for to go from one kind of connector to another. Detectors usually have different family connectors on either end. Notice the exception on the left below!


    It is a detector with SMA M & F connectors! The RF enters on the SMA-M and the detected signal comes out on the SMA-F. Without a label it could be mistaken for an attenuator. Detectors can be destroyed by strong signals (> 2 V) and electrostatic discharges, so be careful!

    A detector with same kind of BNC-F connectors in and out! It also has a schematic diagram, telling what is inside, and that it works to “1,000 MC”, that is 1 GHz.

    Want to know more about some connector or component? Look it up on Wikipedia for a start!

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