When the center conductor is small, the coax cannot pass much power before that small wire melts.  That thin wire also has a lot of inductance and resistance, so you also lose a lot of signal before it gets to the antenna.  When the center conductor is large , the coax can handle a lot of power, but all that metal makes a good capacitor.  The high capacitance of the coax again makes it lose much of the signal.

OK, we don’t want the center conductor to be too big, and we don’t want it to be too small, where do we end up?
If you take a standard one-inch diameter standard-size coax, then the lowest loss balances out at 75 ohms.  Isn’t it amazing!  The TV cable companies have put up miles and miles of the lowest loss coax because of its size.  Someone did their homework!

For maximum power handling, 30 ohm coax will handle more power than any other coax for its size.  But 30 ohm coax is rarely used.  So why, then do we have 50 ohm coax?

The idea was to split the difference between best loss and best power, 30 and 75 ohms, and make 52 ohm coax.
Most companies now are calling it 50 ohm.

There are several ways in the making of coax cable.  The shield can be a solid tube or woven braid of wire.  And the center conductor can be solid wire, or made from dozens of smaller wires.  The woven braid shield is the most commonly seen coax.  The woven braid is flexible, easy to manufacture, and easy to use.  But some signal does leak out between the wires in the braid.  Not much; we’re talking - 80dB to -100dB or .00000001 percent, but in critical applications, this is too much leakage.  In cable TV, commercial broadcasters and cellular systems, solid shield coax is normally used.

Some companies build the center conductor into an aluminum tube, others wrap the center with a thin copper foil then weld the seam.  In either case, a 100 percent solid metal shield is formed.  Its great coax, until you try to run it through a window, around the rain gutter and twisted around an antenna rotator.
The center conductor can also be solid or stranded.  Solid centers are stronger and can be used as the center pin in a coax connector.  With RG-59, the stiff center conductor of the coax is used as center pin in type “F” connectors.  The stranded centers such as RG-58 and RG-8 make the coax more flexible, but the center conductor has to be soldered or crimped into a connector.  Remember soldering is best.

A coaxial cable is a pair of wires carrying electric currents.  Copper and aluminum are good electrical conductors, so there is not much loss in the resistance of the wires.  But all the energy of a signal is traveling between the center conductor and the shield as electric and magnetic fields.  So all the signal is passing through the plastic insulator.  The fancy name for this plastic is the dielectric.  Dielectrics have loss, so less plastic means less loss.  Here’s where the manufacturing tricks come in.  Air or nitrogen often is mixed with plastic, making something like whipped cream.  This foamed plastic has less plastic in it.  More air, more foams, less plastic, and better coax, to a point.

You’ve got to keep that center conductor in the center of the coax.  Too much and the soft foam won’t hold the center conductor on place.  Many people have had trouble with the inexpensive RG-8X foam. Try running it around a tight corner where the sun can hit it, and you’ll see just what I mean.  The sun’s heat turns the soft plastic into a gooey mess and the center of the coax moves.

If your lucky, the SWR’s just went up a bit, but often the center of the coax will move far enough for the coax to short out.  Now the SWR’s really go up and you could blow a final.
Another big problem with RG-8 “Type” and RG-58 “Type” is that black plastic outer coating.  In some of the cheaper coax, chemicals will leak out of the jacket and seep into the center insulator.  That center foam plastic should look pearly white.  If it has changed to a dirty gray color, it’s time to trash that run of coax.
The dielectric characteristics of the coax have changed, and you don’t have 50 ohm coax anymore.  SWR goes up and the coax gets very, very lossy.

Look for LMR400 when its available.  The black jacked does not contain the plasticisers that contaminate the center insulator, it’s slightly larger giving it less loss, and regular RG-8 PL259 connectors still fit it.  If your lucky and find RG-214, it’s great stuff!  RG-214 has not one, but two layers of shielding and all the wires are silver plated for best conductivity.  Unfortunately regular PL259's do not fit RG-214 coax.