Transmission line can be various impedences depending on a number of things. Normally the transmission line is the same impedence as the reciever or transmitter, effectively transferring that devices in/out to the antenna. If the antenna is not 50 ohms, then a Balun/transformer is required to convert it. In some cases the transmission line may be at the impedence of the antenna with a balun/transformer at the Rx/Tx.
Common Transmission lines are 50 ohm, 75 ohm, 95 ohm, 300 ohm, and 600 ohms.
Common transmitter/reciever impedences are 50 ohm, 75 ohm, and 300 ohm
Antenna impedences are totally variable with a ground plane being around from 35 to 60 ohms, a dipole 50 to 100 ohms, a folded dipole around 200 to 400 ohms... depending on design.
I echo what Gadget says as well. In fact the 50-ohm level of impedance is actually designed at that level because it yeilds the best voltage & current characteristics throughout the line. Do some research on sites such as epanorama They have an extensive tutorial on the physics of feedlines.
I copied the jist of it here;
Why 50 ohm coax ?
Standard coaxial line impedance for r.f. power transmission in the U.S.
is almost exclusively 50 ohms. Why this value was chosen is given in a
paper presented by _Bird Electronic Corp._ Standard coaxial line
impedance for r.f. power transmission in the U.S. is almost exclusively
50 ohms. Why this value was chosen is given in a paper presented by Bird
Electronic Corp.
Different impedance values are optimum for different parameters. Maximum
power-carrying capability occurs at a diameter ratio of 1.65
corresponding to 30-ohms impedance. Optimum diameter ratio for voltage
breakdown is 2.7 corresponding to 60-ohms impedance (incidentally, the
standard impedance in many European countries).
Power carrying capacity on breakdown ignores current density which is
high at low impedances such as 30 ohms. Attenuation due to conductor
losses alone is almost 50% higher at that impedance than at the minimum
attenuation impedance of 77 ohms (diameter ratio 3.6). This ratio,
however, is limited to only one half maximum power of a 30-ohm line.
In the early days, microwave power was hard to come by and lines could
not be taxed to capacity. Therefore low attenuation was the overriding
factor leading to the selection of 77 (or 75) ohms as a standard. This
resulted in hardware of certain fixed dimensions. When low-loss
dielectric materials made the flexible line practical, the line
dimensions remained unchanged to permit mating with existing equipment.
The dielectric constant of polyethylene is 2.3. Impedance of a 77-ohm air
line is reduced to 51 ohms when filled with polyethylene. Fifty-one ohms
is still in use today though the standard for precision is 50 ohms.
The attenuation is minimum at 77 ohms; the breakdown voltage is maximum
at 60 ohms and the power-carrying capacity is maximum at 30 ohms.
Another thing which might have lead to 50 ohm coax is that if you take a
reasonable sized center conductor and put a insulator around that and
then put a shield around that and choose all the dimensions so that they
are convenient and mechanically look good, then the impedance will come
out at about 50 ohms. In order to raise the impedance, the center
conductor's diameter needs to be tiny with respect to the overall cable's
size. And in order to lower the impedance, the thickness of the
insulation between the inner conductor and the shield must be made very
thin. Since almost any coax that *looks* good for mechanical reasons just
happens to come out at close to 50 ohms anyway, there was a natural
tendency for standardization at exactly 50 ohms.
Let's not forget that the purpose of the transmission line is to conduct reactive power from one place to another, and that the efficiency will be greatly improved if the line has the same characteristic impedance as the source/load.
Hi,
In fact, what I cannot understand why 30ohm is the optimum resistance for maximum power carrying, 77ohm for attennuation and 60 ohm for voltage breakdown (what does this mean?). Are they related with cable heat or loss or sth. else?