Information About Coax Cables

Most coaxial cables used for amateur radio are either approximately 1/4 inch thick (such as RG58 or RG8X) or nearly 1/2 inch thick (RG8 and others). Of course the thicker cables are not as easy to bend, but typically they have lower ratings in signal loss (which is good). One would find that a thicker cable would be inconvenient to handle when connected to a hand-held radio, whereas it would easily be connected to a mobile radio used as a base station.

The following table indicates the cable loss in dB per 100 feet of cable at various frequencies with the data taken from coax loss a website which features a calculator to obtain cable loss at any frequency.

Frequency, MHz 3.5 7 14 21 28 50 146 440
RG58A loss 0.75 1.07 1.53 1.90 2.21 3.03 5.52 10.63
RG8X loss 0.60 0.85 1.21 1.50 1.74 2.36 4.23 7.92
LMR240 loss 0.46 0.64 0.91 1.12 1.29 1.73 2.98 5.23
RG213 loss 0.27 0.39 0.57 0.71 0.83 1.16 2.20 4.50
RG8/9913 loss 0.23 0.32 0.46 0.57 0.66 0.89 1.55 2.82
LMR400 loss 0.23 0.32 0.45 0.56 0.64 0.87 1.50 2.66

From this table one can see that there is not much concern about cable loss below 50 MHz. This may be some incentive for upgrading of the license to be able to operate on the HF bands.

Another point of interest in designing antennas is that the velocity factor is different for various coax cables. RG58 foam coax has a velocity factor of 0.535. RG58A, RG58B, RG58C, RG213, RG214, and RG8 have a velocity factor of 0.66 . The velocity factor for RG8X is 0.79. 9914 cable has a velocity factor of 0.78, and 9913 cable has a velocity factor of 0.84 .

Velocity factor is used to determine the length of cable to be used for a specific wavelength, such as 1/2 wavelength. For antennas that are not resonant (SWR exists) it is adventageous to use a feed line that is a multiple of 1/2 wavelengths long.

The formula for determining the length of a half-wavelength of cable is:

L = VF x 492 / f

where L is the physical length of one half wavelength, VF is the velocity factor, and f is the frequency of interest.

Return to W7NAT Home Page Home Page