IMPROVING RADIO MIC PERFORMANCE: THE CABLE FACTOR
by Edwin J. Somers, CAS

As promised, the next few articles will discuss ways to improve radio mic performance. One factor and the focus of the first article, was on using a better antenna to increase range. Now we will be discussing how to get the radio signal from the antenna to the receiver.

The importance of using the proper transmission line cannot be stressed strongly enough. The term "transmission line" refers to any cable system used to transfer an electrical signal between pieces of equipment, a good example of which is an antenna cable. It is possible to use several different kinds of cable to deliver the signal from the antenna to the receiver; however, for radio mic applications, coaxial cable is the most effective. Coaxial refers to a design where there is a center conductor surrounded with an insulating spacer, which in turn, is surrounded with a shield. The main advantages of this design are convenience, and even more important, immunity from interference.

One of several issues with coax is characteristic impedance. For maximum transfer of energy, the impedance of the coax must match the antenna and the receiver. Almost all communications equipment is designed to be 50 W (Ohms) with the exception of television, which is either 300 W or 75 W. Video cable is not suitable because it is 75 W, and using it for radio mics is one of the more common mistakes.

What would be the effect of using the wrong impedance coaxial cable? Unfortunately, wherever there is a change in impedance, there will be a reflection of the signal at that point. Reflections cause loss of signal strength and ringing. A good example is with television; reflections show up as ghosts in the image.

Another issue is design loss. Different models of coaxial cable exhibit different degrees of signal loss, and it is frequency dependent. The higher the frequency, the greater the loss. With VHF wireless, coax was not a major issue; however, with UHF it is.

Let's look at some specific examples.

Manufacturers normally rate their cable by so many dBm of signal loss per unit length at specific frequencies. The most popular coax is RG-58. It is less than 1/4 inch in diameter, light, flexible, and inexpensive. Unfortunately, it is very lossy. A 100' length of RG-58 coax at 400 MHz will attenuate the signal 11.5 dBm. At 700 MHz, the loss for 100' is 17 dBm. You should realize that for every 3 dBm of loss, you cut your signal in half.

That's a rather large amount of loss, isn't it? Let's look at the types of coax suitable for our application.

The final issue is signal loss per unit length. All coaxial cable attenuates radio signals - the longer the cable the greater the loss. Since the loss is linear, if you double the length, you've doubled the loss.

So, what can we deduce from all this glark? You should use the shortest cable you can get away with, and you should use the lowest loss cable you are willing to put up with. You will notice from the chart that the low loss coax is thicker and, of course, stiffer than the small stuff. The low loss coax is available with BNC, PL-259 and "N" connectors. Location Sound stocks International Wire & Cable 9096 coax, and we can make cables in any desired length.

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