Transmission Lines
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.
TO TRANSFORM OR NOT TO TRANSFORM
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)
BALUNS
The primary purpose of a balun (contraction of balance/unbalanced) is to feed a balanced antenna
system such as a dipole with a unbalanced feed line such as coax or vice versa.
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).
The simplest balun of all is made by coiling 4 to 6 turns of coax cable into a loop 6 inches to 1 foot in
diameter,( the larger the diameter of the coax cable, the large the diameter of the loop so that the cable
is not stressed and deformed) located at the feed point, and securing them with electrical tape or
nylon ties. This acts as an RF CHOKE, choking off current flow on the shield.
Often a "lumped constant" balun is the answer to a particular problem.
(especially for multiband antennas such as Windoms ) They are readily available
and can be purchased at a very reasonable price at most "HAM" stores. Check the power rating
and frequency range to make sure that it is suitable for your purpose. A "full legal power" or a
balun rated at 2KW is usually only a few dollars more then one rated at 300 watts.
They're not as much fun as building your own, or as much work. (hihi)
Baluns may also be configured to provide impedance matching as a step up transformer such as
matching 75 ohm coax to a folded dipole of 300 ohm input impedance.(4:1) They may also be
reversed to provide a step down match although this is seldom practiced. They may also be stacked
,one balun connected to another balun, between the antenna system and feed line to provide even
higher transformer ratios. (4:1 X 4:1= 16:1)
MORE TO COME