I got to thinking about antennas the other day and realized that I don't really know how they operate from a circuit theory perspective. I've learned the basic theory about electromagnetic waves and how they are created by passing a current through a wire, that passing an ac current produces waves of the desired form/frequency, that kind of thing.
My question is this: In all of the circuit diagrams I see that contain an antenna, the antenna appears to just be left hanging off of the circuit; it's not connected to ground or the rest of the circuit. Like in this simple FM transmitter circuit:
When I look at actual devices that transmit signal via an antenna, there is usually just a single coaxial cable going to the antenna. So, how do they transmit? In order for the antenna to radiate the waves, it has to have current go through it, and if there is no path to ground(or whatever) through the antenna, how does any current get in there?
In short I would say the circuit you show is a very simple transmitter , not obeying effective designing. The simple wire acting as an antenna looks very lossy , though it will work . As this antenna is connected to the Tanks circuit it starts radiating energy as its conducting time varying current and any conductor if gets a time varying current will radiate anergy . The question of GND doesnt comes here . Actually think it in terms of dissipating coulombs field when the charge on the antenna tends to its maximum.
-Adi
Thats not quite accurate. The wire antenna if 1/4λ works like any other antenna. It reflects back to signal ground even if it is the size of a pencil eraser. For above type of circuit, it is wise to use a large groundplane.
The ground acts like the other 1/4λ.
Possibly because you have a low post count, the post does not appear until it has been "passed" by a moderator. Not to worry, it is here now.man, i've tried to reply like 4 times and I seem to keep getting modded or something.
The simple transmitter circuit which you showed earlier may appear to look like your sketch, however:anyway, i'm thinking about these circuits based on what i've learned in my DC circuits course. To me, antennas seem to just be hanging wires and from what i've learned, current won't flow into a dead end like that.
To me it looks like this:
**broken link removed**
the hanging wire in the diagram would be the antenna, but no current should be entering that wire, unless it gets connected to something, right?
man, i've tried to reply like 4 times and I seem to keep getting modded or something. anyway, i'm thinking about these circuits based on what i've learned in my DC circuits course. To me, antennas seem to just be hanging wires and from what i've learned, current won't flow into a dead end like that.
To me it looks like this:
**broken link removed**
the hanging wire in the diagram would be the antenna, but no current should be entering that wire, unless it gets connected to something, right?
you meant monopole, right? so, antennas all have two terminals, but with monopole antennas, the counterpoise is one of them, is that correct? the dipole has two easily identifiable terminals, but the circuit still seems to have hanging wires. what I took away from your previous post is that it's possible for high frequency AC current to flow into a hanging wire(antenna), but it bounces back at the end and creates a standing wave, which gets radiated as electromagnetic waves.Well, in fact the dipole in your case has one side as the wire "antenna" and the other side as your transmitter's circuit board copper (and any wires leading up to the board other than the antenna). These are the two terminals that form your "circuit" if you like.
you meant monopole, right? so, antennas all have two terminals, but with monopole antennas, the counterpoise is one of them, is that correct? the dipole has two easily identifiable terminals, but the circuit still seems to have hanging wires. what I took away from your previous post is that it's possible for high frequency AC current to flow into a hanging wire(antenna), but it bounces back at the end and creates a standing wave, which gets radiated as electromagnetic waves.
RadioRon, please edit your last post again to change your Ohm's law statement. You say I=ER, which would be very interesting
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