COAX CABLES

 

Amature Radio Trader March 1997

One of the most important parts of any amateur radio station is the coax cable. Its importance is as obvious as its function, to carry the RF signals to and from your antenna. In a perfect world, the coax would have no signal loss. However in real life, signal loss is an, unavoidable problem. It is not however an unbeatable one.

It is important to remember that not only is your transmitted signal attenuated by coax loss, the signal you are receiving also suffers. When you use a 40 watt transmitter on VHF, 3 db loss is not critical to solid communications. If the signal you are receiving is not very strong, then that type of loss might make it unreadable.

The obvious solution is to make the coax run as short as possible. This is just not possible in some situations, which forces us to use the most efficient coax we can find.

Three main factors will effect the amount of signal loss you will experience: the band you will be operating on, the type of coax you will be using and the length of your coax.

The “coax db attenuation’ table lists the seven most popular coax types and the four frequency bands you may be operating on. The db loss figure is found by finding the intersection of the two factors. These loss figures assume you are using 100 feet of coax. If you are only using 50 feet, divide the loss figure in half.

Compare the current type of coax you are using and determine the amount of gain you will realize if you change to a lower loss coax. If the net gain is less than 2 db, you may not feel the money and time expense of replacing the coax is worthwhile.

The “db to percentage loss/gain chart” will show you another perspective on the effect of signal loss. It is very obvious that a good amount of your signal is lost even under the best of conditions.

The coax signal loss can be offset with a gain antenna. If you are using a 3db gain antenna and have 2db loss in your coax, you will end up with a net gain of 1db. Using the db power chart, 1db equals 21% gain. Using a 25 watt transmitter, your ERP will be 30 Watts!!

Remember to keep the number of coax connectors to a minimum because they also increase signal loss.

TNX to J. Henning for the db formula. 73 and have some fun.

Percentage change from a decibel value (db has a positive sign for gain and a negative sign for loss) [(10 x db/10)-1]x100