True or false

The term "characteristic impedance" means when the line is terminated in that impedance, it will be the same all along the line. That means if it is terminated in any other load, the impedance will VARY along the line!

 

Since those who claim to have knowledge have disregarded answering or responding to this I will interpret that as a deliberate attempt to misinform people or allow people to remain in the dark which is just as bad as being deceitful.

So here is the truth.

If the terminating impedance is equal to the characteristic impedance, 50 ohms in our example, the input impedance is 50 ohms regardless of the length of the line and the frequency neglecting losses. This is known as a matched line.

If you have an unknown piece of coaxial cable and some way of measuring impedance at RF such as the Millen impedance bridge or the Heathkit noise bridge you can determine the characteristic impedance as follows. Connect a resistor of a value that you are sure isn't the same as the line impedance. Start at a low frequency and work your way up until the input impedance becomes resistive and read the bridge. Apply the equation given above. I have measured the characteristic impedance of some Radio Shack audio cable as 25 ohms.

As you might imagine shorted transmission lines can be, and are, used in place of tuned circuits in VHF and UHF equipment. 1/4 wave length transmission lines can be used to transform impedances in a single frequency or narrow band system. 1/2 wave or multiples there of lines can be used to reflect the terminating impedance to the line input regardless of the impedance of the load or the characteristic impedance of the line. Within reason. This is known as a mismatched line. Such a line has standing waves on it.

What is a Standing Wave.
Tie a rope to some stationary object. It's better if the rope is vertical so a tree limb is probably called for. Hold the free end of the rope in your hand and make a single motion in one direction. You will see a wave go up the rope and when it reaches the top it will return back down the rope. This is an example of a single pulse being reflected.
If you wave the rope in a sinusoidal motion you will send waves up the rope that will be reflected back to you. If you have good eye hand coordination you can find a frequency at which the middle of the rope is almost stationary while the rest is waving about.

If you send radio waves up a transmission line and the termination is matched to the characteristic impedance of the line all of the energy will be absorbed. If you could measure the voltage on the center conductor of a coaxial line at random points along its length you would measure a constant voltage all the way along.

If there is a mismatch some of the energy will be reflected back along the line. The return wave combines with the incident wave and in some places they reinforce and in others they partially cancel. If you could measure the center conductor voltage along such a line you would see peaks and valleys.

Mismatched Systems and SWR.
When there is a mismatch between the load impedance and the characteristic impedance of the line the term SWR comes into use. It stands for Standing Wave Ratio. The SWR is the ratio of the maximum voltage to the minimum voltage. Some engineering texts use the phrase Voltage Standing Wave Ratio abbreviated VSWR. SWR can be defined in terms of maximum and minimum currents in the conductors of the line so we will use the more general, and shorter, term SWR.
A mismatched transmission line is not automatically a bad thing. Suppose after arriving home with that roll of coaxial cable you bought for a song at a ham fest you find that it has a characteristic impedance of 125 ohms. All you have to do to use it on your 40 meter dipole is to make it 1/2 wavelength or 1 wavelength, taking velocity factor into account and the impedance at the transmitter end will be the same as the impedance of the antenna feed point.

 

The length of cable is determined by the velocity factor of the cable he's using. He was using Belden RG58 when he was in Lodi Ohio.
He's using LMR 240 cable now. Mark would tell you to go to the truck stop and purchase a 12 foot piece of cable and you will be very close to what he sold you when he was in Lodi Ohio.

If you're interested he can send you a piece of LMR 240