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What cable/connectors to use between my radio and antenna?

Didier Juges, KO4BB

(c) June 2004


This article refers to data from the RadioWorks on-line catalog, so you may want to log on to their web site at www.RadioWorks.com.
Click on Catalog 2004, then on Parts then on Coaxial Cable (page 62-64 in their 2004 catalog).

I have no financial interest in RadioWorks, but I have been a satisfied customer for some time and I recommend them. Besides, their web site is well laid out and contains a lot of useful information, in addition to detailled specifications and prices for their products. I have also found the people there to be very helpful and friendly on the telephone.

Where to start?

The main things you need to be aware of when selecting a transmission line between your radio and your antenna are:

  1. The type of cable you need depends on the frequency of operation, the power level, the length of cable you need (you need better cable if you have a long run for instance) and whether the cable will be installed inside, outside, buried in the ground, and also will depend on whether the cable will be subjected to frequent bending (such as the cable that connects to an antenna on a tower with a rotator), or if the cable will lay on the ground where it may be stepped on by people.

  2. The type of coax cable you use will (to a large extent) dictate the type of connector you use.

  3. Even for a given cable type, there are many variations on connector styles, and it is easy to be confused and not buy the right part unless you know exactly what you need.

Selecting the cable

Aside from the Characteristic Impedance, which for Ham applications will be 50 ohms most of the time, the next main specification parameter of coaxial cable is the attenuation for 100 feet of cable. The next critical parameter is the power handling. Finally, the environmental specifications and mechanical specifications are also important, but not equally in all cases.

For instance, there are 2 basic types of coax cable in use in amateur operations. Go to the RadioWorks catalog page 62. You will see the 3 types to the left are about 1/4 inch in diameter and the others are about 0.4 inch. If you look at the table below the cable types, you will see that the attenuation per 100 feet for the smaller cables is generally much greater than for the other cables. So the smaller cables are OK for short runs, or for low frequency antennas. I typically use RG-8X for short interconnect cables inside the shack, and for temporary use in the field when the runs are short or the frequency low (as on HF). At higher frequency (such as 2m, or 144 MHz, and higher), the larger cables will always be better.

For instance, if you need 50 feet from your VHF radio to the antenna (make sure to measure the actual length you need, including bends and turns, as you can easily underestimate by 50% if you just measure the straight line distance), the 1/4 inch type cable will loose about 1.85 dB (one half of 3.7 dB) of the power fed to it. That's about one third of your signal, both on transmit and receive. That's too much for most installations as far as I am concerned.

Rule of thumb: 1 dB means you loose 20% of your signal. With 2dB, you loose 36% of your signal and with 3dB of loss, you loose 50% of your signal (that's a whole half). Even worse, with 10 dB, you would loose 90% of your signal, a pretty good dummy load :-)

On the other hand, if you use RadioWorks' SuperCable, you will loose 1.15 dB for 50 feet, which is negligible for all practical purposes. For 50 feet, even RG-213 is OK (1.45 dB loss) at VHF.

If you look at the specs closer, you will see a parameter called Velocity Factor (VF). Typically, the cables with lower VF are more lossy, but that's because they use solid insulation, instead of a low density foam or mostly air (with small spacers to keep the center conductor roughly "centered" inside the outer conductor), so they are also considerably more rugged (such as you can step on them without causing damage). The cables with higher VF are better, from a loss standpoint, but they need to be protected against mechanical abuse. They are also typically much more difficult to deal with when it's time to install connectors, and they have a tendancy to trap water inside if not terminated properly, which will ruin the cable very quickly.

Another fine point of the spec are the shields. The number in the table represents how much of the cable is covered by the shield. For instance, cable with one 95% shield means that the shield covers 95% of the cable surface. That means that signals can get in and out of the cable though that 5%, and that increases the loss. High quality cable has two shields, with one being rated 100% because it usually is a foil that completely covers the insulation inside the cable, while an outside braid gives the cable better power handling.

Connectors

Cables are useless if they don't have connectors at the ends. Installing connectors on coax cable is more an art than a science, and it takes a lot of practice. It is perfectly possible (and quite common) to ruin a good cable by improperly installing connectors. Unless you have lots of practice and the right tools, you will either burn (or melt) the cable by applying too much heat, or make a cold solder joint that will fail shortly, typically right after you have installed the antenna on the tower, or just before a contest when it's raining :-)

Connector styles will vary also. For 1/4 inch cable, you can use BNC connectors, which are very convenient (they are small and easy to connect/disconnect). You can also use PL-259/SO-239 sets, with the proper adapters. Most mobile and fixed radios use these larger types of connectors, while most HTs use BNC (newer, very small HTs also use SMA).

For short cable runs at home, I use crimp type BNC connectors with RG-8X cable. The center pin is soldered, but the braid is held to the connector body with a crimped ferrule. They are a good compromise between cost, performance and ease of installation. You need to have the right crimp tool, which you will amortize quickly as you will find it easy to make just the right cables you need.

It is best to buy the connectors and cable from the same place at the same time to make sure they are made for each other. Different vendors often have slightly different dimensions for what seems like the same product, so interoperability is not guaranteed.

BNC connectors are not recommended for outdoor use, but even PL-259/SO-239 need to be protected from the rain or else the rain will get inside the connector (bad) or even inside the cable (very very bad).

There are many different models of PL-259 connectors, the most common type of connector used on ham equipment up to VHF/UHF.
The main considerations are the finish and the insulator. The silver plated types are so much easier to solder that it would be inconceivable to recommend anything else. Yes, silver oxidizes, but the silver oxyde is a good conductor, so it should not significantly affect the performance of the connector.
The other consideration is the insulator material. Teflon has lower loss but melts easier, so you may cause more damage when soldering the connector. If you are not experienced, and if you operate at VHF and below, bakelite will be a better choice.

At my station, all the rigs have SO-239 sockets, but I have installed PL-259-to-BNC adapters on the rigs, and I use short RG-8X cables with BNC connectors to go from the radio to an interface panel near the hole in the wall through which the cables go. That allows me to quickly change antennas, or to disconnect the radios from the antennas directly at the panel when I don't use the station. A disconnected antenna is much safer than any kind of lightning protection device you can buy :-) Then, from the interface panel, I use 0.4" diameter cable (low loss for the VHF antennas, RG-213 type for the beam HF antenna) and SO-239/PL-259 connectors.

My house was hit by lightning in 1995. More precisely, lightning hit a tree in the yard, which had grown over the underground utility cable (which included the power cable, cable TV and telephone cable). It caused $7,000 of damage inside the house, all to electrical appliances, TVs, telephones, VCRs, computers, stereo, garage door opener, washing machine and so on. The only equipment that was not damaged was the ham station, which was completely disconnected from the antennas and from AC power at the time.

Connector installation

I said before outside connectors need to be protected from the rain. That seems obvious, but corrosion of the connector is actually the least of our problems, since it is visible (in most cases) and relatively easy and inexpensive to replace a connector, unless it is atop the tower (I tend to ignore these details because my tower folds :-)

The more serious problem is that if water (or moisture) is allowed to get inside the cable, it will propagate through the coaxial cable's outside braid or even the center conductor, over long distances over time and ruin the whole cable. If you use a very low loss air insulated cable (such as LMR-400 or equivalent), water will easily get inside the cable itself and cause greatly increased losses. Once the cable has absorbed moisture, there is no repair procedure other than replace the entire section, from connector to connector.

You can use electrical tape to protect connectors that are outside. Make sure the tape is installed properly. RadioWorks has a page describing the proper installation of tape. They also have other products to make an even more durable protection for outdoor connections.

This is the reason why, for outside cable runs, I strongly recommend that you buy pre-made cable runs with professionally installed connectors, unless you know exactly how to install connectors.

VSWR

VSWR is a measure of how well two devices are impedance matched to each other. Typical ham radio equipment is designed for 50 ohm load impedance, so we hams use 50 ohm cables and build or buy antennas that are specified for 50 ohm. While most cable has a flat impedance over frequency (they measure 50 ohm at all frequencies you are likely to use), the same is not true of the antennas.

A 1.0:1 VSWR is a perfect match. That means the load impedance is exactly 50 ohms. A 2.0:1 VSWR is obtained when the load impedance is either 25 ohms or 100 ohms.

Because most ham transceiver will deliver full power with a load VSWR of up to 2.0:1, this value is usually considered the limit for acceptable operation. Many hams prefer to keep their VSWR below that however, but for all practical purposes, it is unnecessary to spend time or money trying to get much below a VSWR of 1.5:1. The benefits will be hard to measure and even harder to hear.

On the other hand, coaxial cable losses increase rapidly, for a given frequency of operation, when the antenna VSWR exceeds 2.0:1. This can even, in some extreme cases, result in the coaxial cable burning, even when running 100 W. Using a higher grade of cable will definitely improve things, but even high quality coaxial cable becomes very lossy when VSWR exceeds 3.0:1 at higher HF frequencies (or VHF and higher). I remember seing a good article in a magazine about this, and if I find the link, I will add it here.

If you have a badly matched antenna, using an antenna tuner is the usual way to keep the transceiver happy. However, the antenna tuner, if it is installed at the output of the transceiver, will only keep the transceiver happy, but it will do nothing for the coaxial cable going to the antenna. The best place for a tuner is at the antenna feed point. Several compagnies (SGC, LDG and Icom are a few of them) offer remote antenna tuning units designed to be installed at the antenna feed point, even if that is outside.

Conclusions:

  • For a typical 2m station with a 50W transceiver or so, and an outdoor or attic mounted antenna less than 30 feet from the radio, it's OK to use small cable (RG-8X type). If you need a longer run than that, or run higher power, go with larger cable. RG-213 would be OK up to 50 to 60 feet, after that you need the good stuff, or be prepared to have significant losses.

  • For a typical HF station (160 to 10m, 100W), you can use RG-213 if you have something like a tri-band beam, and RG-213 or RG-8X for lower band antennas, up to 100 feet. For longer runs, upgrade to the next quality up of cable. For higher power, use at least RG-213.

  • All this assumes the antennas are fairly well matched. If there is significant VSWR (> 2.0:1 at the antenna), cable losses increase rapidly and you need to either match the antenna better, or use higher grade cable and an antenna tuner. Because cable losses increase so rapidly with frequency, antenna tuners are not used at VHF and above. For these frequencies, the antennas have to be naturally matched.

  • Unless you are an old hand at this (and if you are, you probably did not read that far), buy your cable in the right length and have RadioWorks install the connectors for you. They have a small charge for the service, and they do a wonderful job.

  • I have always found answers by calling RadioWorks on the telephone. They are very helpful and will answer your questions and recommend the best product. I have no financial interest in RadioWorks, but I am (and the local club is too) a very satisfied customer. Their cables are professionally made and fairly priced. Particularly, make sure you tell them what you want to do so you buy connectors that are designed to go together.


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