Feed line loss and velocity factor

What it is

No feed line delivers all the power you put in. Some is turned into heat in the conductors and the insulation; that is loss (attenuation). Separately, a wave travels slower in a cable than in air; that ratio is the velocity factor. One tells you how much power arrives, the other how long to cut things.

Loss

Loss is quoted in dB per 100 feet, so total loss = rate × length ÷ 100. Because dB add, you can add the loss of a cable, a connector and a switch. Convert back with power left = 10^(−dB ÷ 10): 1 dB leaves about 79%, 3 dB leaves half, 10 dB leaves one tenth.

Loss grows with frequency, roughly with its square root at HF and VHF and a little faster above that, as the insulation starts to absorb too. So a run that wastes almost nothing on 80 m can lose half your power on 70 cm. Fatter cable, foam or air insulation, and solid shields all reduce loss.

Receive loss counts too. A dB lost on the way in is signal that never reaches the radio, and at VHF and above, where outside noise is low, it directly worsens signal-to-noise ratio. This is why VHF and UHF stations use short runs of low-loss cable, and why a few dB of loss matters far more at 70 cm than at 40 m.

Loss also changes how SWR looks. If the antenna is mismatched, reflected power travels back along the line and is attenuated again, so the SWR at the radio reads better than at the antenna. A low reading at the radio can hide a bad antenna and a lossy cable. See SWR and reflections.

Velocity factor

Waves in a cable slow down because of the insulation. The fraction of free-space speed is the velocity factor (VF), and it shortens the wavelength inside the cable by the same fraction. Physical length of a given electrical length = free-space length × VF.

Line	Typical VF
Solid-plastic coax	about 0.66
Foam coax	about 0.8
Twin-lead	about 0.8
Window line	about 0.9
Open-wire line	0.95 or higher

A quarter wavelength of 0.66 coax at 14.2 MHz is 246 × 0.66 ÷ 14.2 ≈ 11.4 feet (3.5 m), against 17.3 feet in free space. Use the VF printed in the datasheet, and measure the finished piece with an antenna analyzer, because real cable varies. Cable lengths matter for stubs and matching sections and for phasing lines.

In practice

• Shorter is better. Put the radio close to the antenna, or a remote tuner or amplifier at the antenna.
• Use better cable where frequency is high. On HF, a thin cable is often adequate; on VHF and up, use the fattest practical line.
• Water in coax, corroded connectors and kinks all add loss. A once-good cable can lose much more after years outside.
• Open-wire line avoids most of the loss for a given length, even with a high SWR. See Open-wire and ladder line.

Related

• Coaxial cableConstruction, types and choosing a coax.
• Open-wire and ladder lineLow-loss balanced feed lines.
• SWR and reflectionsStanding waves, reflection coefficient and return loss.
• Characteristic impedanceWhat Z0 means and why it must match.
• Stubs, quarter-wave sections and the gamma matchMatching with pieces of transmission line.
• DecibelsThe logarithmic scale for gain, loss and signal strength.