question about magnetic fields

I studied that an equale magnetic field take place if a flow passes the wire so .
is it possible to transmit a triangle wave?
why we should have a sinus wave for carrier?
why the last output is a reflected wave like image below (why * = - **)?

can we send a non-reflected?

You can transmit an approximation of triangle wave. You can see the triangle wave as a sum of infinite sinusoid having the frequency (1, 2, 3, 4 ... infinite) times the triangle wave frequency (see Fourier series). This means you'll need an infinite bandwidth to transmit a perfect triangle wave while you'll need a very narrow bandwidth to transmit a sinewave. Of course is not possible to transmit over infinite bandwidth so only an approximate triangular wave can be actually transmitted.
To have a quite accurate approximation of a triangular wave you'll need at least 5 harmonics, that means a bandwidth of 4*(triangle wave frequency).