Quality connectors for coaxial cables
On this page we want to show you what types of connectors for coaxial cables are available, which mounting options are available to you and we give some insights about the different applications. The goal is to make it easier for you to choose the right coaxial connector for your personal requirement.
Connectors for coaxial cables are used to make the connection between the cable and a device or antenna. Just as there are many different coaxial cables, there are correspondingly many types of plugs. On the one hand, they differ in diameter - which must fit the cable exactly - and on the other hand, there are differences in the application area. Most of the time, however, the type of connector is precisely specified, namely by the device we want to connect - the radio or the antenna.
Technical properties of a coaxial plug
The essential technical characteristics of a coaxial connector are easily listed:
- The impedance
- The maximum permissible frequency
- The manner how the connector is mounted to the cable
The impedance is the alternating current resistance of an electrical component. In other words, the value that results from the ratio between an alternating voltage (the high frequency voltage) and the current that then flows.One must also consider the phase relationship between voltage and current. Coaxial cables and connectors that have the same impedance as the connected radio and antenna are the basis for optimal signal transmission. As with cables, almost exclusively plugs with 50 Ohm impedance are used in amateur radio. This is given by the equipment and antenna technology.
But what about the attenuation? This was so important for cables, why isn't it also specified for the connector? Usually there is little choice in connectors. The mating side determines the type of connector; the cable determines the diameter. Under these circumstances, there are very few cases where we have different plugs with different attenuation to choose from. Therefore, we typically add an attenuation of approx. 0.2 to 1 dB for each plug, depending on the type and frequency.
Coaxial connectors can usually only be used up to a certain, highest frequency. For simple connectors, this is a few hundred MHz. Very precisely manufactured connectors with a suitable surface coating (silver, gold or other alloys) are suitable up to the GHz range..
The coaxial connector must of course be fitted cleanly and precisely to the cable. Not doing this properly will result in an abrupt transition from one impedance to another, which leads to reflections and poor matching. The result is signal loss and possibly interference due to transition resistance, and even strong heating. Further, moisture can penetrate into incorrectly mounted connectors, which then leads to a gradual deterioration of the signal.
When we talk about how a connector is mounted, we have to distinguish which conductor we are talking about, the inner or outer conductor of a coaxial cable
The connector assembly
- Solderless connections
With some connector types (e.g. PL) have their inner conductor permanently installed in the connector body, in which case soldering is the only available method. There are other plug types with a loose inner contact, which hare also soldered. This is easy to do if you can conveniently install the plugs at your workplace.This becomes much more difficult when installing a cable in a vehicle or on top of the antenna mast.
Especially if the cable must be installed at first without a connector. For this reason (installation far from the power supply, at inconvenient places), crimping technology has become established, especially in the professional sector. Here, we place the centre contact on the inner conductor and then press it on with precisely defined pressure using a special crimping tool.
This results in a cold weld with very good contact properties. After a little practice, crimping is usually faster and more reliable than soldering. It is important to use the contact that exactly matches the diameter of the inner conductor and the crimping tool that exactly matches the diameter. For some connector types, it is even easier to use solderless contacts for the inner conductor. With this assembly technique, the contact is equipped with a metallic feathering on the inside, which fits exactly onto the inner conductor. For large connectors (N, 7/16" etc.) this assembly is now the standard. The contact is very good, but what is important here is the very precise machining of the cable. The lengths and distances must fit very precisely so that the inner contact sits well in the insulator of the body. The advantage of solderless mounting is of course the simplicity, but also the better flexibility, for example with rotatable antennas or with expansion of the cable due to temperature changes.
The outer conductor of a coaxial plug is either crimped or clamped, in a few cases also screwed and soldered. The professional sector uses crimping technique more often, at least for relatively thin cables up to approx. 7 mm, occasionally also up to 10 mm diameter. For crimping, the body of the connector has a cylindrical tube on the cable side, often with a roughened surface. For assembly, insert the correctly stripped cable with the dielectric into this small tube; the braiding is placed over the tube on the outside. Next, place a metallic sleeve over the cable's shielding braid, then press this sleeve with a crimping tool. Just as with the inner conductor, the result is a very good contact and good strain relief - provided the diameter and type of tool (stepped, straight) fits exactly. Finally, it is good practice to protect the sleeve with a bend protector or heat shrink tubing.
Most connectors commonly used in amateur radio use clamping as a means to connect the outer conductor. In this case, we push a sleeve under the exposed outer conductor. Then we push and secure this sleeve into the body of the plug with a rubber ring and a nut on the cable. The rubber ring also provides a seal against moisture.
With very simple plugs (e.g. PL), the braiding is folded back over the outer sheath and the body of the plug is screwed onto it. For this purpose, the plug body is equipped with a coarse thread on the inside. This type of assembly may be sufficient for shortwave and very simple conditions, but it is neither waterproof nor very durable.
For special cables with a rigid outer conductor (Cellflex, Semi Rigid), the plug is screwed or soldered onto the cable as a whole. These mounting techniques are rarely used in amateur radio, and we will not discuss then further at this time.
No matter how we mount a plug on the coaxial cable - if it used outdoors, we must protect it additionally against water ingress. This also applies to the types that are already waterproof in themselves (e.g. N connectors), better safe than sorry.
Use good self-vulcanising rubber tape for this weather and water protection. This material is temperature-resistant and is placed around the plug end like plasticine. If you have a heat gun, you can also use shrink tubing. It is best to use one that has adhesive on the inside.
A brief introduction to different types of coaxial connectors
For coaxial cable, a huge number of connector types exist. Keeping track of them is sometimes difficult, so we hope to provide some guidance with this short table.
The BNC plug was patented as early as 1951. BNC stands for Bayonet Neill-Concelman after its two inventors Paul Neill and Carl Concelman and the bayonet instead of screw fixing. It has a defined characteristic impedance and clean contact also on the ground/shield side. The system is well suited for thin cables and allows quick fastening/unfastening. However, the BNC connector should be used only up to a certain cable diameter, to not overload it mechanically. In the past, many amateur radios, usually handheld radios, had a BNC socket, but today this socket is normally used in amateur radio more for IF outputs or similar. Unfortunately, BNC connectors are not waterproof; they are not intended for outdoor use.
This is the oldest high-frequency connection system: The banana plugs, which can still be found today on universal measuring instruments, laboratory and radio power supplies, were once also used on radios for antenna connection. Around 1930, solid shielding was added for professional applications. This made the connectors much more suitable and reliable for radio transmissions. The system is robust and easy to connect due to its size. The plug is also called PL-259 due to a listing in the army requirements, the socket SO-239 (PL = ‘plug’, SO = ‘socket’). Another name for this connector system is "UHF", although it is not suitable for today's UHF radio operation. The reason for this name is that back in 1930s, the engineers considered frequencies above 30 MHz as "UHF", ultra-high frequencies. Today we define UHF as the range from 300 to 3000 MHz. The problem with the PL connector is a lack of standardisation of the mechanical and electrical parameters - the characteristic impedance is not well defined and not constant at 50 or 75 ohms as it is in modern systems. This makes it quite suitable for HF up to 30 MHz, but only conditionally suitable for VHF and no longer usable for UHF.
If you want the stability and size of the PL/SO system, but fixing by screwing with a system that is also suitable for higher frequencies, you should go for the ‘N type’ connector. Electrically it is comparable to BNC, has a well-defined characteristic impedance and clean contact also on the ground/shield side. The sockets are larger than PL/SO and relieve the connection mechanically. This makes this connector system suitable for cables with a larger diameter. N connectors are therefore among the most commonly used connectors in radio technology, they are available for practically all cables. The advantage is the robustness and simple assembly without special tools, apart from crimping pliers for crimp connectors. N-connectors are also largely waterproof when mounted correctly.
The 1960s showed an increased need for smaller connectors for very high frequencies like microwaves, so the SMA connector (Sub-Miniature version A) came to market. Today this connector type is employed for applications up to 14 GHz, in some cases models exist, which are used up to 25 GHz. By its nature, SMA is not suitable for particularly thick cables, but it has replaced the BNC connector in all applications for high and highest frequencies and is also used for internal connections in radio equipment. Today, antennas for handheld radios also mostly use SMA connectors, sometimes reversed for mechanical reasons (socket on antenna, plug on device). Most SDR receiver also use the SMA connector. Although the SMA connection is reliable, it is not weatherproof and the plugs do not support a very large number of mating cycles. Depending on the coating, SMA is usually suitable for up to 500 connect/disconnect cycles. Any use beyond this number of cycles can lead to deteriorated performance (higher insertion loss, corrosion etc.).
The 1960s showed an increased need for smaller connectors for very high frequencies like microwaves, so the SMA connector (Sub-Miniature version A) came to market. Today this connector type is employed for applications up to 14 GHz, in some cases models exist, which are used up to 25 GHz.
By its nature, SMA is not suitable for particularly thick cables, but it has replaced the BNC connector in all applications for high and highest frequencies and is also used for internal connections in radio equipment. Today, antennas for handheld radios also mostly use SMA connectors, sometimes reversed for mechanical reasons (socket on antenna, plug on device). Most SDR receiver also use the SMA connector.
At WiMo, many car antennas, switches and filters use the FME connector. With FME, the distinction between plug and socket becomes a little bit confusing: the FME socket is the part on the cable that has an external thread; the inner conductor is female. An FME plug, on the other hand, is the counterpart on the device or antenna that has an inner thread and the centre conductor is a male pin.
Of course, there are many more types of connectors, each application area seems to invent its own connector type. The particularly simple and inexpensive F-connector is used for 75 ohm satellite systems. Wifi cards make use of extremely small plugs, MMCX, MC-Card, SSMB are examples. In contrast to this are the rather large 7/16" (7/16 inch) connectors. Some applications at very high power and at high frequencies require this type of connector.
Fields of application for coaxial connectors
To classify connectors for coaxial cables, we need to look at a few fields of application.
An important area is measurement technology. Here we want to quickly disconnect and reconnect a connector, and do that very often. For this purpose, the BNC connector with bayonet lock is ideal. This connector is available up to the lower GHz range and for almost every cable type (diameter). For very high frequencies, most devices in measurement technology use "N" plugs, and some commercial radios (mostly military) occasionally use “C type” plugs.
For us radio amateurs the most defining factor is whether the cable is used indoors or outdoors. Outside, of course, a cable must be weatherproof; it is mandatory that we prevent the ingress of water into the plug. Certain types of plugs are largely waterproof when mounted correctly. This is the case, for example, with N plugs or PL plugs, which are constructed in the same way as N plugs on the cable side. Other connector types are not waterproof due to their design and should be avoided outdoors, e.g. BNC or SMA connectors.
Another area of application is the internal wiring of devices for do-it-yourself construction. Since this usually involves low power in a small space, we usually prefer miniature connectors. This is for example SMA or SMB, but also MCX or similar very small connector types. These connectors are usually only available for very thin cables. On the positive side, these connectors are available up to the upper SHF range.
With all connectors, you should pay attention to how many mating cycles they can handle. With some plugs, this is often only 30 to 50 mating cycles, i.e. far too few to be used constantly in daily use.
In this text we have shown what is important for coaxial connectors - quality, technical parameters and a careful selection depending on the application. Taking these simple principles into account, you will have "peace of mind" for a long time with coaxial connectors at your own station and can transmit without worry, even at the highest frequencies and with high power.
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