RF Amp/Antenna Interface Theory

Discussion in 'Wireless & RF Design' started by fester225, Feb 16, 2011.

1. fester225 Thread Starter New Member

Nov 11, 2008
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When you apply an RF signal to the base of an antenna, power gets radiated, but how? There is no current flow since there is nowhere for the current to go.

It is common to put a capacitor in series with the final stage of a transmitter. This prevents any current at all from moving through the antenna, so how does it radiate energy?

2. t_n_k AAC Fanatic!

Mar 6, 2009
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Your statement that there is no current is incorrect. The radio frequency voltage and current varies along the length of the antenna.

Have you Googled RF antenna theory?

3. nsaspook AAC Fanatic!

Aug 27, 2009
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Last edited: Feb 17, 2011
4. t_n_k AAC Fanatic!

Mar 6, 2009
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You also seem to have misunderstood the fact that energy can pass through a series capacitor under AC conditions. A series capacitor will block the continuous flow of energy from a DC source. Perhaps that is what you are basing your ideas on.

5. fester225 Thread Starter New Member

Nov 11, 2008
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It seems the antenna acts like a (low block) capacitor, whose value varies with the length of the antenna. When the voltage to the antenna changes, the electrons in it compress and decompress a little. That action produces the electromagnetic field.

How's that?

6. t_n_k AAC Fanatic!

Mar 6, 2009
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If you are genuinely interested in gaining some understanding in this area I think you need to do some more reading on antennas. Doesn't need to be as complex as Maxwell's electromagnetic theory.

It would help if you had some basic appreciation of RF propagation and radio wave transmission. It's a reasonable jump from basic circuit theory to RF concepts.

The ARRL (for instance) has made some great practical contributions to this field. See if you can find something from that knowledge base or any of the very useful and comprehensive radio frequency websites that abound.

Bertus might be your goto man.

Apr 5, 2008
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8. nsaspook AAC Fanatic!

Aug 27, 2009
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The part of an (tuned/matched) antenna that radiates (forward/real) power acts like a resistor, whose value varies with the length of the antenna.
Using a slightly twisted example of circuit charge/current theory you can think of the RF energy as a constant current source "battery" (say 1 amp when fully charged) that produces a voltage to maintain the current at a set level (say 1 volt : 1W) in a 1 ohm resistance. In this (Mythic) example resistor the measured value is 1 ohm from the center to the outside at one end (base) and 10 ohms at the other end (top). If the length of the resistor is long so the current moving in the center from the base to the top can only flow into a small slice of the resistor at a time (moving at about light speed) the resistance seen by the constant current source will change as it moves causing a change in voltage to maintain the current. At every small slice of the resistor an amount of charge will be used so at the next slice the current source might only produce .5 amp at the point of 5 ohm resistance. ( 2.5 volts : 1.25W) then at the end .1 amp (1 volt : .1W)

As others have said, you need to study basic RF theory to understand what's really happening.

Last edited: Feb 17, 2011
9. fester225 Thread Starter New Member

Nov 11, 2008
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Thanks for the assistance everybody!

Bertus: I went to your referenced site and got the following, for the moment, this does it for me!

http://www.fas.org/man/dod-101/navy/docs/es310/propagat/Propagat.htm
The Dipole Antenna

This is nothing more than a straight piece wire. When
voltage is applied to the wire, current flows and the electrical
charges pile up in either end.....
...When an alternating voltage is applied the antenna, dipole moment oscillates up and down on the antenna, corresponding to the current. The oscillating current creates oscillating electric (E) and magnetic (H) fields which in turn generate more electric and magnetic fields. Thus a outward propagating electromagnetic wave is created.