Transmission lines (TL) have many uses other than simply transferring RF power from one point to another.

Impedance matching, baluns and filters are probable the most common of these.

Determining the electrical length of a transmission line is simple if you know it's (VF) Velocity Factor.

492/Fmhz = 1/2 wavelength in free space multiplied by the (VF) = 1/2 **electrical** wavelength.

Of course divide this by 2 to get 1/4 **electrical **wavelength.

A few common Velocity Factors are:

450 ohm ladder line: 0.98

RG-58 coax: 0.66

RG-8 coax: 0.66

RG-8X coax: 0.78

RG-6: 0.78

1/2 and multiples of 1/2 **electrical wavelengths** are very useful while tuning antennas since they simply

reflect the input impedance to the output end.(no transformer action) If the antenna is properly tuned

using a section of TL that is a multiple of 1/2 electrical wavelengths long you will not encounter or will

at least minimize the common problem of having the VSWR change as the length of the feed line

between the RF source and the antenna changes.

A 1/4 wavelength section (or odd multiple of 1/4 wave(1/4,3/4,5/4) of TL act as a impedance transformer. By

taking the square of the impedance and dividing it by the known impedance at one end, the impedance that it will

match can be found. Examples:

450 ohm ladder line: 450*450 = 202500 if hooked to 50 ohm coax at one end a perfect match is to be had to

an impedance of 4050 ohms.( 202500/50 = 4050) .(not an unusual impedance for a "E" fed antenna system)

Since VSWR is a relative value a ratio of 2:1 will be reach at 8100 ohms and 2025 ohms. All values of resistive

feed point impedance between these extremes will be less then 2:1.

75 ohm coax: 75*75 = 5625 divided by 50 ohms = 112.5 ohms ( a pretty good match to the impedance of a

quad loop antenna.) SWR will be 2:1 at 1/2 and at double 112.5 ohms.

Many single band mobile antennas use an odd multiple of 1/4 wavelength feed line to reduce the VSWR

swings across it's frequency range.

While there are several reasons to specify the length of a section of TL to be used with an antenna, impedance

matching is the most common. (32 to 34 feet of 450 ohm on the G5RV- 70 feet of coax is also often prescribed

by it's users for the best match)

Why is this a problem?

Coax cable is considered to have power flowing only on the inner conductor with the shield as ground

reference.(unbalanced) When coax is used to feed a balanced antenna system such as a dipole, not only is the

radiation pattern skewed unpredictably, but the energy radiated by the antenna system induces current into the

shield making meaningful SWR measurements impossible to obtain, thereby making it impossible to properly

tune the antenna system. This problem may be overcome by either **shielding the shield** from the energy

radiated by the antenna (shield balun) or **canceling this unwanted energy with equal but opposite energy **

(folded coax balun).