Coax & Connectors

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Coax & Connectors

Post by RF-Bot » Mon Apr 23, 2018 1:00 am

Submitted By: Jason.


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Picture it: You have just purchased a new base station scanner. Rushing home, you cannot wait to remove your new toy from it's packaging, and set it up on your desktop. Let's see, plug in the power cord, screw in the supplied telescopic antenna, and program in a few frequencies. Voices issue forth from the speaker, but not as clearly as you had hoped for, especially on the simplex channels.

So what can you do to improve the situation? An external antenna would help, so next day, you rush back to the store and purchase a suitable antenna, after having given some consideration to the choices available of course. But have you considered the coax cable and connectors that you will use to connect the antenna and the radio? If you go ahead and blindly choose your cable and connectors, you may very well compromise your whole antenna system.

All transmission lines have certain characteristics, one of which will be of interest to most when selecting which type to use: loss. As a radio wave passes along a transmission line, be it balanced pair line, coax or waveguide, some of that radio signal will be lost. This applies to both receive and transmit. The greater the loss, the less radio signal there will be available for the antenna to radiate or for your receiver to hear. Other factors to consider are: shielding, velocity factor and characteristic impedance.

Coax, or more correctly co-axial cable is by far the most popular choice of transporting signals, be they received or transmitted, between the radio and antenna between 25 and 2000 MHz. So why is 'coax' the popular choice? The loss of coax is quite acceptable, given that you match the correct coax to the operating frequency, and the cable is easy to install, since it has none of the problems that you have with balanced pair lines. Lets take a look in more detail the differences between balanced pair line and coaxial cable.

Balanced pair lines consist of two, sometimes four, parallel wires that run between the antenna and radio. Interfacing the balanced pair line to the radio is a 'balun' (short for balanced-unbalanced) which converts the low impedance coaxial connector output to a higher impedance suitable for driving balanced pair. At the antenna end, there may or may not be a balun, depending on the design of the antenna. If the antenna is of a high impedance design, and is close to the impedance of the balanced pair, then no balun is needed. On the other hand, if the antenna is of low impedance design, such as a simple dipole, then a balun will be needed. The two conductors of the balanced pair are held apart at a constant distance along the length of the line by some form of spacers. The dielectric, that is, the material that is between the two conductors can be air, plastic, foam or many other non-conductive materials, with air dielectrics realising the least 'loss'. As the signal travels along the transmission line, some of the signal is 'lost' due to the resistance in the cable, the capacitance between the cables, and many other factors. This loss must be kept to a minimum if any signal is to be left at the end of the transmission line for the antenna to radiate.

With balanced pair lines, RF currents travel along both conductors roughly out of phase, hence the term balanced line. Balanced pair lines are best used at low frequencies (below say 20 MHz) since their losses increase as frequency increases. They must also be kept away from any conductive materials, and avoid sharp bends. They do have the advantage, however, of being able to handle very high power levels, such as those used in shortwave broadcast stations, and hence balanced pair transmission lines are used extensively in the broadcast industry. Parrots have a hard time with such systems, however. If they land on one side of the balanced pair, they are quite safe. Parrots have a habit of wiping their beak after landing, and the closest thing to one conductor of a balanced pair is... the other conductor! Our poor parrot now becomes a short circuit to the RF current, and, well, no more parrot is usually the result.

The most common example of this type of feedline is the good 'ol TV ribbon. When used correctly and careful attention is paid to all considerations, balanced pair line can have lower loss than coax up into the lower parts of VHF, say 150 MHz.

Coax has several advantages, and disadvantages over balanced pair lines. The main advantages are that the feedline can be routed any which way from the transmitter to the antenna, without needing to keep it away from things metallic like you have to with balanced pair lines. Coax can be buried, cable-tied direct to masts or used in a mobile installation, all situations where balanced pair line cannot be used. Coax is also shielded, in that external magnetic and electric fields are excluded from interfering with the signals being carried by the coax. The current going out to the antenna is carried by the centre conductor, with the return current going by the inner surface of the shield. The centre conductor and shield are separated by a dielectric, just like the two conductors in balanced pair. And just like balanced pair, the loss of coax increases with frequency, but nowhere near as quick as it does with balanced pair. As frequency increases, the skin affect comes into play, and it is the outer 'skin' of the conductors which carry most of the RF current. This will in turn mean that there is more resistive loss in the conductor due to the smaller effective amount of metal carrying the RF. This is something to be aware of when considering coax with high frequencies in mind.

Coax, like balanced pair also exhibits a 'characteristic impedance' due to the capacitance and inductance that is distributed along its length. Similarly, it will delay the signal passing through it. When RF passes via a transmission line, the speed at which it travels is reduced compared to the speed at which it travels in air or free space. The percentage of the speed at which RF travels through a given transmission line compared to the speed at which RF travels in free space is called velocity factor. For example, if a given coax reduces the speed of RF by 2/3rds of that in free space, the velocity factor of that coax is 66%. This also means that a given wavelength in the transmission line will be less then what it is in free space. In our example above, one wavelength in the coax will be 66% of what it would be in free space. This 'new' length is called the 'electrical length' of the transmission line. This can also apply to conductors being used as radiating elements in antennae. You will need to be aware of this characteristic if you need to make coax a certain 'electrical length', for example when making phasing harnesses. Air and foam dielectrics tend to have a higher velocity factor, and result in longer electrical wavelengths.

Disadvantages of coax include the relative complexity and cost over balanced pair, and exhibit a higher loss at low frequencies, not that the loss of either is significant at these frequencies. Coax can come in many varieties and of course quality. The ubiquitous RG58 style coax can come in a huge range of varieties. For example, RG58 that you might buy from DSE is fine for general use, but there are so many choices if you look hard enough. Would you like foam, polyethylene or PTFE high temperature dielectric? How about double shielding braids, or an electrostatic foil? Solid or stranded centre conductor? Would you like fire-retardant, UV-resistant or pH neutral (suitable for burying) outer jackets? And of course, colour. What colour jacket would you like? This is not as silly as it sounds. Certain colours tend to be less attractive to birds who may find your coax somewhat edible! Certainly, you may have to look around to find a supplier that offers all these options. But there are some things that you just cannot change with the one type of coax, like characteristic impedance, and losses. RG58 is a 50 ohm coax, and can come in several versions that affect losses. Foam dielectric RG58 will have less loss than it's polyethylene brother, and double shielding and/or electrostatic foil shields will help with external noise affecting the signals passing through the centre conductor. Be aware, though, that while RG58 coax may be specified at 50 ohms, that may vary between manufacturers, versions and even the batches made on different dates. This variance in characteristic impedance is usually less than 10%.

Cheap RG58 may have a low-quality polyethylene dielectric and loosely woven braid with poor shielding characteristics. It will probably be made of lower quality, less durable materials. Hence the cheap coax can be quite 'lossy', have a poor noise immunity due to the poor shielding and not last as long especially if it exposed to the elements. At the other end of the quality spectrum is the very high grade RG58 style coax, like RFI's CellFoil. It has a solid copper conductor (good for the skin affect), has a low loss air-foam dielectric and two shields - one an aluminium foil and the other a well woven copper braid. Its jacket is tough and durable. I love this stuff! It has a loss similar to RG213 and is more flexible and half the diameter. Another coax of similar size is 'sucoflex' style coax. It is used in microwave systems where a flexible cable is needed. How about silver plated oxygen free copper centre conductor for minimum resistive loss, two incredibly tightly woven braids and super-high quality control on all measurements and specifications that affect the characteristic impedance of the coax? If you want to buy this stuff, be prepared to have alot of the folding stuff. I paid over $120 for a 1.5 metre patch cord of this from Huber & Shuner in Sydney. But, you do get an incredibly high quality product at these prices, with specifications guaranteed upto 12 GHz!

RG213 style coax is the next most common coax cable used in 50 ohm systems. The same remarks for RG58 apply here, although there tends to be less 'junk' versions of this around. Again, polyethylene, foam and air dielectric versions can be found with the air dielectric exhibiting the lowest losses. The double shielded version of RG213 is called RG214. Being just a shade over 10 mm in diameter, it is a little stiff but is still convenient to work with and connectors are cheap. RFI has a super-low loss version of RG213 with an air dielectric called 9005, and Belden has its 9913 style coax which is my favourite. It has a solid copper centre conductor, air dielectric, really tightly woven braid and a second aluminium foil shield. I've just used this cable in a commercial radio application at 1GHz, which should give you an idea of just how impressive this coax is, all at a reasonable price and while still using the standard RG213 connectors which are readily available and relatively cheap. There are probably many other RG213 equivalent cables out there which are of similar quality, I singled out these two types because they are easier to find.

Something which you may notice at this stage is that as your choice of coax gets physically larger, the power handling capabilities increase, and the loss decreases.

For the ultimate in low loss feedline for two-way radio applications, there is one word that is synomonous with high performance and quality. Heliax. Andrew Antennas makes this series of coax and waveguide in several versions. Heliax is a very high quality coax, which is favoured in commercial, broadcast, and military radio applications, and even in low frequency microwave applications. It has been used by the US Army for its Patriot missile system, and the MilStar series of communication satellites, as well as alot of shipboard and airborne radar and communication systems. Yep, it really is that good. Being a somewhat special cable, you need to match it with some very high quality connectors, which are also available from Andrew. Also note that you would need the 'Easiax' tool to strip this cable for termination. Trademarks of the Heliax range are low attenuation, high power handling capability, and low noise due to a solid copper shield, and a very durable sheath, which can be ordered in a fire retardant version. Generally, Heliax can have 2 dielectrics - foam or a spiral air spacer.

The cables with an air dielectric should be pressurised with dry air to prevent moisture ingress and to help reduce tarnishing, which could cause problems. The advantages of using an air dielectric however are the realisation of slightly lower loss and a higher power handling capability. For most of our hobby or low to medium power applications, foam dielectric will be quite OK and it negates the requirement for a pressurising system. Foam dielectrics are also more resistant than solid type dielectrics to having water ingress and having that water travel by capillary action down the coax.

There is one more type of coax that Andrew sells under the brand name of Radiax. This coax is designed to 'leak' RF along its length. This can be handy in tunnels, mine shafts etc, where conventional antenna systems will not provide the required coverage. The Sydney harbour tunnel is a great example of where this system is used to provide communication.

When selecting a high quality cable like Heliax, there are a few considerations to look into. First, loss. Generally the larger the cable, the less the loss, and the greater the cost. What power will you be loading up the cable with? Again, larger cables will be able to handle higher power. At very high power levels, you will need to take into account the affects of modulation. Flexibility? The FSJ series of Heliax is more flexible than its LDF series brothers. And cost. You will always be balancing the cost against the performance in the back of your mind. As with everything, the law of diminishing return applies. The more you spend on cable and connectors, the less advantage you will gain relative to the outlay, so long as your specifications are met. For example, if you need a well shielded cable such as RG 214, you may do well considering the Heliax FSJ1 or LDF2 cables. RG 214 costs about $25 per metre, while the FSJ1 and LDF2 cables have comparable shielding qualities and better loss figures while costing less than $16 per metre. The connectors, however are more expensive at between $40 and $90 each. Still, when using a long run, it may be more economical using the Heliax cable.

Some hints on coax use would not go astray right about now. Coax has a minimum radius bending diameter. If you bend the cable to sharply, you would kink the coax, which upsets the characteristic impedance at that point. As a general rule, you should not bend the cable in a radius less than 10 times its diameter. If, for example, you have a 10 mm RG213, its minimum bending radius should be kept below 100 mm if possible. Anchoring the coax is another critical point. Make sure your cable is secured to your mast/structure using cable ties or special coax hangers or clips. This does two things: first it takes the stress of the weight of the coax off the antenna connector, and second, it secures the coax so that it cannot flap about in the breeze. I've seen a radio system fail due to the wind blowing an unsecured section of coax back and forth - with the result of a micro fracture in the centre conductor.

Having made your choice of coax, what about connectors? It's been said before, and I will say it again: cheap connectors equal cheap performance. $2 PL259 connectors might be fine if you are only looking for average performance on 27 MHz CB, but use them in a medium power or UHF situation, and you will wish you had spent the extra money. Different types of connectors have different specifications, too. To a certain extent, your choice of connectors will be determined by the equipment and antennae you use. If your radio equipment had a flying coax lead, then changing the connector on the end of this is no problem. Even radios with fixed type connectors can be changed, as thousands of Philips FM320 & 620 owners have proved. But, usually, you probably would not want to go butchering your radio to put a new RF connector on just to gain 0.1 dB by changing from a SO239 to an N type. However, if your radio uses an obscure or lossy type of connector, such as a Motorola (car radio) or Belling-Lee type, then there would be plenty of justification in changing these.

If your antenna comes in a choice of RF connector, then it makes sense to go for the higher quality connector, even if it does mean an extra few dollars. Generally, the higher quality connector would be more water-proof than the lower cost one, which is important if the antenna is to be out in the elements.


Now let's take a quick tour around some of the more common connector types.




SO-239 & PL-259
Also called UHF connectors, very common, easy to assemble, and cheap. Also, despite what the manufacturers try to tell you, the design is not inherently 50 ohms impedance. They are also not designed to be waterproof, although you should not rely on any connectors properties alone for waterproofing. Many junk versions out there, and you will have to go looking for the high quality ones if you are locked into using them. Good for HF, OK for VHF, passable for UHF, forget it for anything above 500 MHz.


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BNC & TNC
Again, very common, a bit more difficult to assemble, although I think the crimp types are a breeze. Excellent characteristics up to UHF. Not much more waterproof than the PL259 types, but then again, I've never seen these types used outdoors. Yet again, junk versions are available, and high quality types are not too hard to obtain. Good quick locking/removal in the BNCs. I like these connectors - I've standardised all my connectors as far as possible in my shack to be of the BNC type.


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N-Type
Common, and high quality. A little more difficult to assemble, but the rewards are worth it. These types exhibit excellent impedance matching, and are usable up to microwave frequencies. More waterproof than the others, but I still would not rely on the connector alone for waterproofing. I've only seen 'low quality' rather than 'junk' versions of this connector about. Again, you get what you pay for. Rugged, able to handle high power and probably the best choice overall.


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SMA
Small, expensive for the size, not too common on non-microwave equipment, but brilliant in the quality and impedance matching department. Delicate connector, needs to be done up using a torque wrench to ensure it is not over tightened. Not designed to be an outdoor connector. Is usually only seen on microwave communication equipment. I've seen them in use up to 30 GHz!


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Motorola Type
As used on your car radio. Yuk! Nothing more to say here!


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Beling-Lee
As used on your TV. And that's where it should stay!


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DIN 7/16th
Common on commercial equipment out of Europe. Great high power characteristics, and good in the impedance and weatherproofing department. A high-power rival to the N type. A big connector compared to the others above.


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Mini UHF
Probably only seen on mobile phone equipment. A better connector than the PL259, even though it is smaller. It is an indoor, low power connector only.


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FME Nipple
As used on Ericsson mobile equipment. A good pressure-quick connect (or rather contact) connector, but it needs to be kept super clean to work properly.


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F-Type
75 ohm low loss connector. Easy to assemble and not waterproof. Strictly low power.


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EIA Flange
The domain of broadcast equipment. High power, top quality. I would hate to terminate one of these suckers! Comes in sizes from 7/8 inch up to 6 1/8 inches.


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There are other connectors which I've not had much experience with and which are not common such as the SC, LC and HN series, which all look similar to the TNC types to me. And again, I wont mention waveguide connectors, more correctly called flanges.


Good characteristics to look for in a connector are:
  • Silver plating on all metal surfaces. Silver is a very good conductor, and with the skin affect at high frequencies, most of your signal will flow through this silver plating.

  • PTFE insulation might be looked at if you are using the larger cables. This high temperature material will not deform when heated. Your large cable will take awhile to heat up to soldering temperature, and a standard insulating material might deform under this extended heating period.

  • Good waterproofing grommets and seals. This will help keep out the water which can kill an otherwise good connector.


Generally, if you stick to the brand names, you will be buying a good connector. Huber & Shuner, Andrew, Greenpar/MaCom/James Hardie and Amphenol all make high quality connectors. Sure, they may cost more, but they are worth it in the long run, believe me.

Choosing and buying the right connector is only half the story. Terminating and installing the connector is the other half. This is the area where you determine the success or otherwise of the system. Using the correct solder, the correct size iron, the correct tools are all important. Taking time to assemble correctly the connector, inspecting it every step of the way in good light will help ensure the end product is as high quality as the manufacturer intended it to be. When striping the cable, use a sharp razor blade or a purpose made stripping tool. Take time to experiment with a stripping tool to get it set right. If it comes set from the factory, leave it alone! Either way, when you strip the cable ready to accept a termination, you need to ensure that the braid is not cut, where you remove the jacket or sheath, and when you strip the dielectric, the centre conductor should not be scored or nicked. If the dielectric doesn't come away easily, you can try squeezing it with a pair of pliers in two dimensions to help ease it off the centre conductor, or you could try removing it in shorter lengths. I used to assemble coax connectors as part of my profession, and at one time, we used a thin 'blade' of metal with a slot cut in it, attached to our soldering irons. This was used as a hot knife to remove the dielectric, with the centre conductor passing through the slot of the blade. This method was used until we found that a) the fumes were not especially good for your health and b) it was very messy to use on foam dielectrics.

What about those crimp connectors I mentioned above? As I said above, I assembled coax connectors as part of my job, and when we changed over to crimp connectors, I was a bit sceptical. After a while, I was won over - crimp connectors are very quick and easy to assemble, have no trade-off in performance, and to my eyes, look neater. They are also just as durable, given a proper 'boot' and proper assembly. When assembling crimp style connectors, debate rages whether you should leave some braid exposed and 'hanging out' where the crimp tube meets the ferrule. I prefer to cut this excess braid off, so that there is no braid exposed at all. Not only does it look neater, you wont prick your finger on a bit of loose braid every time you pick it up. When crimping, only crimp tube the width of the crimper's jaws, with the crimp butting up as flush to the ferrule as you can get. Double crimping is not a good idea. While actually performing the crimp, hold the crimp tool with the connector still in it up to the light. If you can see light through the jaws of your crimp tool dies or around the edges of your freshly made crimp, then it is time to replace the crimper dies. Crimp in a smooth fashion, not letting go of the pressure half way through the procedure. Only ease up on the pressure once the crimp is complete.

Doing a pre-solder trial assembly of your connectors will help if you are unsure of how it will all go together. Once satisfied that it will all go together OK, use some thin solder (electronic grade only with a flux core) and an appropriate sized soldering iron to make well the connection. How much solder is enough? Just a 'dob' will not do, and over-soldering can short out the connector or make it impossible to assemble. Hold the connection still while soldering, and do not blow on the connection to cool it down quickly. Doing either of these things can result in a 'cold' or 'fractured' joint. Some excess solder on the exterior of a join (on BNC or N type connectors) can be carefully scraped away with a sharp razor blade, but this should be avoided if possible.

Once assembled, you will need to test the cable; inner conductor to inner conductor at each end should show full continuity (on the lowest ohms scale of your multimeter), outer conductor to outer conductor, and no continuity between inner and outer, using the highest ohms range on your multimeter. The cable should be disconnected at both ends from any equipment during these tests. If you are really keen, you could use a megger or ionisation tester to see if there is any breakdown between inner and outer conductors. Do this test only after the initial continuity tests, since the application of high voltage can temporarily fix intermittent solder joins, and then go faulty later on. Definitely make sure that no equipment is connected to the cable under scrutiny for this test. Oh, and keep your hands clear! After the ionisation test, short the inner and outer conductors of the cable using an insulated probe or wire. This is to drain away any charges stored by the cables' capacitance after the application of high voltage.

When doing up the connection between to RF connectors or RF adaptors, hold the body of the connector or adaptor and do up the locking or screw part of the connector. This is to prevent the centre pin of any connector from turning when in contact with it's mating centre pin on the other connector. This is especially applicable to BNC and N type connectors. Push connectors together perfectly in line, otherwise bent centre pins may result.

If used outdoors, connectors need to be weatherproofed. The application of self-amalgamating tape over the connectors and the edges of overlapping tape 'moulded' together will keep any moisture on the right side of the connectors - the outside! Next, a double layer of black electrical and preferably UV resistant tape will help prevent the self-amalgamating tape from weathering. If in a really exposed situation, you may wish to have some flexible electrical conduit or even 'anaconda' to cover the connectors and cable. A cheap way of waterproofing connectors is to smear them with a good layer of petroleum jelly once they are done up. This really is only a stop-gap measure, however.

The use of RF adaptors should be kept to a minimum, as if I need say it anyway. I once tried using a couple of right angle adaptors to get my coax through a tight corner - with the result that the signal through them was down by about 1 dB. No need for me to say what happened to these adaptors....

Well, that's about all the advice I can give on the various components of an antenna feedline. Your feedline and any connectors really are the vital lifeblood 'veins' of any radio system. Select, install and maintain them well, and you will reap the rewards of an antenna system that should serve you well for many, many years.

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