I understand things like waveforms and frequencies quite well, but I can't find something that will adequately explain how we get the radio waves in the air and get it back out. In a receiver, does the antenna receive a tiny voltage that is amplified to be useful? And how do we make a line voltage or waveform come out of an antenna? My brain still only thinks that it would act like an open circuit, but it obviously doesn't. What part of a radio transmitter makes it able to turn voltage signals into radio waves?
I understand things like waveforms and frequencies quite well, but I can't find something that will adequately explain how we get the radio waves in the air and get it back out. In a receiver, does the antenna receive a tiny voltage that is amplified to be useful? And how do we make a line voltage or waveform come out of an antenna? My brain still only thinks that it would act like an open circuit, but it obviously doesn't. What part of a radio transmitter makes it able to turn voltage signals into radio waves?
One more interesting fact. Antenna designers that are designing transmitting antennas and those that are designing receiving antennas are really one and the same person. It turns out that in most common antennas if they are good at transmitting, they are also good at receiving, as long as the frequency in each direction is about the same.
Start with basics... A frequency has a wavelength. It is determined by the speed of light which is also the speed of an electromagnetic wave. The speed of light is 300,000,000 meters per second. This is what is used to cut an antenna to resonance (the transmit/receive) frequency. So if I want an antenna to resonate at 7 MHz, the frequency wave length will be 40 meters. 300 / 7 = 42.86 meters.
Normally we do not use a full wavelength but a half wavelength will also resonate at the given frequency. We call this a dipole for 1/2 wavelength. There are several reasons for using a dipole as opposed to a full wavelength. It has to do with the phase relationship between voltage and current along an antenna wire. I won't get too elaborate but you may want to source some diagrams showing the voltage/current relationships through various wavelengths. One very obvious reason for using a dipole is that if the dipole is center fed meaning the two transmission line wires are connected to two 1/4 wavelength pieces of wire not connected to each other, than we have a very high current at this point and a very low voltage which means it is a low impedance. In a transmitter with any significant power this is desirable because we do not want a high impedance at the output of the final amplifier or it will be highly prone to feeding back into one of the driver stages or even the oscillator. So quite often you will see transmission lines rated at about 50 ohms to match the antenna impedance.
On the receiver side the high current at the center feed point of the dipole antenna also has proven to be optimal for maximum sensitivity.
Right matching 75 ohms, the impedance of a half-wave cennter fed dipole is much easier to match to a piece of 50 ohm coax. the wavelength is in freespace, on pcb's, the wavelength decrease as the inverse of the square root of the dielectric constant.
for a half-wave, the current is strongest at the center and zero at the ends, this makes sense since the antenna is open-ended and you get a reflection. a dipole antenna, is thus a standing-wave antenna.
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