Wednesday, January 11, 2012

Feed Line: Introduction to coax

Two rules to keep in mind.  One, keep it as short as practical.  And two, buy the best quality your budget will afford. 

I’ve heard a number of new hams ask, “What kind of coax should I buy?”

First, for almost any ham antenna, 50 Ohm coax is about the only requirement.  The next consideration is loss.  The very lowest loss type of coax, usually called “hard line”, is extremely expensive while higher loss small diameter coax is less expensive.  Generally speaking, larger the diameter coax is the lower loss.  The amount of loss also changes with the type of insulating material between the outer conductor (shield) and the center conductor.  This is taken into account on coax loss specification tables.

However, the greatest contributor to coaxial line loss is frequency of operation. 

Here are a couple of loss tables for some common 50 Ohm coax.

This one is for Belden 9258 (RG-8x) which is .242 inch diameter.  Belden is a cable manufacture.

Frequency (MHz)                  Nom. Attenuation (dB/100 ft.)

1                                              0.3

10                                            0 .9

50                                            2.1

100                                          3.1

200                                          4.5

400                                          6.6

700                                          9.1

900                                          10.7

1000                                        11.2

This one is for Belden 8214 (RG-8/U) which is .403 inch diameter.

Frequency (MHz)                  Nom. Attenuation (dB/100 ft.)

1                                              0 .1

10                                            0.5

50                                            1.2

100                                          1.7

200                                          2.6

400                                          3.9

700                                          5.6

900                                          6.5

1000                                        7.0

Note that RG-8x is about half as big around as RG-8.  But RG-8 has about half the loss at all frequencies.  Because of the weight and flexibility differences, RG-8 is used mostly for home installations while RG-8x is a good choice for portable operations. 

Follow this link to an abbreviated table for most coax types.

For RG-8x see.  This is a complete specification.

If you already have the coax, check it carefully for the manufacture and numbers printed on the outer cover.  Try a Google search with the manufacture name and numbers, at least type in the RG type followed by the word spec.  You may have to look at a number of “hits” to find what you’re looking for.  Don’t give up easily.

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