“THE DOCTOR IS IN” [THE ARRL LETTER]

This week, ARRL Letter readers are in luck! The ARRL’s very own Doctor, author of the popular QST column “The Doctor Is IN,” answers a question from his mailbag:

Question – Ray Fritts, KA8SYX, of Jacksonville, Florida, asks: If a piece of coaxial cable has a specified loss figure in dB per 100 feet at a given frequency, does that mean that the loss in a different length of the same cable that is a fraction of 100 feet long is the same fraction of loss?
For example, I have a type of coax that has a loss of 6 dB per 100 feet at 150 MHz. I have a 15 foot length I want to use as a feed line for my 2 meter mobile SSB transceiver. Does that mean that my feed line would have a loss of about 0.9 dB, not including SWR and connector insertion? I am particularly interested in the loss in received signal. Is my math correct, or is there a different method to determine the amount of signal lost in a coaxial cable when the length is different from that for which the published loss figures are expressed?

The Doctor Answers – Your calculations are right on. That’s all there is to it. But do keep in mind just a few potential pitfalls:

Published cable loss data is for new cable. If used indoors in a non-hostile environment, it will stay close to new for many years. If the jacket allows moisture or moisture vapor to penetrate, it can degrade from subsequent corrosion. I have been amazed to find that the copper in some old cables that have been used outdoors has turned black from corrosion, and likely is no longer acting like a shield at all.

You are correct that the loss increases with an SWR higher than 1:1. For your receive case, keep in mind that the SWR is determined by the input impedance of the receiver — not the antenna impedance. Sometimes receivers aligned for minimum noise figure do not have an impedance of 50 Ohms. Check your receiver specs.

Do you have a question or a problem? Send your questions to doctor@arrl.org; or to “The Doctor,” ARRL, 225 Main St, Newington, CT 06111 (no phone calls, please).

http://www.arrl.org/arrlletter/07/1012/

This entry was posted on October 15, 2007 at 7:22 am and is filed under ARRL Letter. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.