Hi. Is the voltage applied to an antenna, any antenna whatsoever, always alternating? That is, the voltage going from say positive, then negative, then positive. Thanks. Rich

 

The voltage between the antenna feed and ground may or may not have a DC component. It would make no difference.
If the voltage varies between -5V and +5V it would produce the result as if the voltage varied between 100V and 110V. In both cases the AC component is 10V and that is the bit which matters.

 

Are there any products or systems in the wireless world, where the rf excitation to the antenna is not ac? But, varying dc?

I suppose at the receiving end, the signal from the antenna will always be ac? Well, I know, it is.

 

One may ask why I query what signal is suitable to be fed to an antenna. Well, I'm looking at Pulse Amplitude Modulation. Where there are two types: Single polarity & Double polarity. Not entirely sure what the consequences are for modulation of an rf carrier. I suppose, none. Both produce the same result in terms of modulation. It seems.

Edit. I think I'll start a new thread on PAM modulation.

 

Are there any products or systems in the wireless world, where the rf excitation to the antenna is not ac? But, varying dc?

I suppose at the receiving end, the signal from the antenna will always be ac? Well, I know, it is.

Isn't varying DC called AC? Anyway, you need a time-variant voltage to excite an antenna.

 

Isn't varying DC called AC?

Would you call 100V ± 5V AC?

 

Would you call 100V ± 5V AC?

If the ± 5V is a tolerance, then "no". If the ±5V is an AC voltage on top of 100V, then "yes". If you can pass a signal thru a capacitor, then it is an AC signal.

 

If the ± 5V is a tolerance, then "no". If the ±5V is an AC voltage on top of 100V, then "yes". If you can pass a signal thru a capacitor, then it is an AC signal.

In that case there is no such thing as DC as all DC will include some noise.

 

In that case there is no such thing as DC as all DC will include some noise.

Most batteries output fairly pure DC.

 

Most batteries output fairly pure DC.

Plus noise. There is no escaping noise.

 

Are there any products or systems in the wireless world, where the rf excitation to the antenna is not ac? But, varying dc?

I suppose at the receiving end, the signal from the antenna will always be ac? Well, I know, it is.

Sure, the transmit antenna could be a tuned coil from the dc power supply to a transistor collector like was common some older RF remote controls. RF with a DC component passes through the coil/antenna but only RF is transmitted.