# RF excitation of an antenna

Discussion in 'Wireless & RF Design' started by richard3194, Nov 30, 2017.

1. ### richard3194 Thread Starter Active Member

Oct 18, 2011
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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

2. ### AlbertHall AAC Fanatic!

Jun 4, 2014
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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.

3. ### richard3194 Thread Starter Active Member

Oct 18, 2011
141
3
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.

Last edited: Nov 30, 2017
4. ### richard3194 Thread Starter Active Member

Oct 18, 2011
141
3
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.

Last edited: Nov 30, 2017
5. ### SLK001 Senior Member

Nov 29, 2011
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Isn't varying DC called AC? Anyway, you need a time-variant voltage to excite an antenna.

6. ### AlbertHall AAC Fanatic!

Jun 4, 2014
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Would you call 100V ± 5V AC?

7. ### SLK001 Senior Member

Nov 29, 2011
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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.

8. ### AlbertHall AAC Fanatic!

Jun 4, 2014
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In that case there is no such thing as DC as all DC will include some noise.

9. ### SLK001 Senior Member

Nov 29, 2011
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Most batteries output fairly pure DC.

10. ### AlbertHall AAC Fanatic!

Jun 4, 2014
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Plus noise. There is no escaping noise.

11. ### nsaspook Expert

Aug 27, 2009
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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.