Coaxial cables are the most popular form of transmission line for getting our signals to and from our antennas. There are many types of cable to choose from and it can be confusing to chose the best one. In this article, we’ll cover the most common choices of cable to get you started. We’ll focus on the most popular cables, with 50 ohm impedance to match the output impedance of our transceivers.
Here’s the really simple, short story:
|RG-58 type||0.194 in||Standard cable for mobile installations|
|RG-8X type||0.242 in||Larger and lower loss than RG-58 but still convenient for shorter cable runs and jumpers,Up to 50 feet in length at 50 MHz or below (Rule of Thumb)
Up to 25 feet in length at 146 MHz (Rule of Thumb)
|RG-8U type||0.405 in||General purpose coaxial cable, best for long cable runs|
At one time, RG-58, RG-8X and RG-8U were military standards but now these terms are used rather loosely and refer primarily to the size of the cable. Accordingly, I added “type” to the term to indicate that it is not a precise standard.
All three of these cable types will handle 100W or more at frequencies below 500 MHz, which covers most ham transceivers. If you are running more than 100W, you should check the power specification of the cable you are using. Times Microwave Systems has a very handy online calculator for coaxial cable specifications, which I used for the calculations in this article.
All coaxial cables will attenuate the signal as it travels down the cable and the signal loss can be significant. For example, 3 dB of signal loss means that you lost half of the transmit power as it propagates down the line. This loss applies for both transmit and receive… we’ll get less power out to the antenna and we’ll have less signal showing up at the receiver.
The cable loss will be determined mostly by the size of the cable (bigger is better), the dielectric used in the cable (the insulator between the center conductor and the shield) and the frequency of operation. As an example, consider a 100 foot run of cable for use at 146 MHz, which is high enough in frequency and a long enough run such that we’ll see some significant losses. According to the Times Microwave calculator, 100 feet of RG-58 style cable produces a loss of 5.5 dB, which means that only 28% of the power gets through the cable. (The percent power delivered is shown as Cable Run Efficiency in the calculator.) This is not good, so we would rarely (never?) want to use RG-58 for that long of a cable run.
Changing the able to RG-8X drops the loss to 4.5 dB, which is only a minor improvement. (4.5 dB loss corresponds to 36% of the power makes it through.) However, using RG-8U type cable decreases the loss to 2.4 dB (58% of the power makes it through the cable), so clearly the larger cable size has an advantage. Now let’s change the dielectric. LMR-400 is a popular cable that has the same diameter as RG-8U but with a lower loss dielectric (Foam PE). The 146 MHz loss through 100 feet of this cable is 1.5 dB, or 0.9 dB better than ordinary RG-8U. A loss of 1.5 dB means that we still lose 30% of the power.
Now let’s see what happens when we change the frequency of operation. If we use our 100 foot run of LMR-400 on the 20m band (14 MHz), the loss is only 0.5 dB. This means that 90% of our signal power makes it through the cable. You can use the Times Microwave System calculator to try out different combinations of cable length, cable style and operating frequency.
There are a few other cable specifications that you may be concerned about, depending on application. Cables with solid center conductors are less flexible than those with stranded center conductors. The dielectric material and the outer insulating jacket can also affect the flexibility of the cable. For portable operations, I make it a point to get cable that is rated “flexible” because it is easier to handle and deploy. Direct burial cable has a durable outer insulation that will withstand being buried in the ground. The type of outer shield used in a cable can vary widely, with some cables providing much more shielding and isolation than others.
This is a quick introduction to choosing the right cable for your amateur radio station. I hope it points you in the right direction. Its always a good idea to buy quality cable from a reputable supplier and to read the specifications for that exact cable type.
73, Bob K0NR