Why Do I Need a Transmitter Circuit before An Antenna?

Transmitter circuit sample

Reason 1
Baseband is a low frequency. We cannot transmit directly to antenna. When we converted the baseband to high frequency by modulating the RF frequency, the antenna become small and the power become less. By modulation with other RF for other baseband we can have several channel.

Reason 2
Message signal (baseband signals) have low frequencies, signal with low frequencies cannot travel longer distance, to make message signals travel longer distance we use a technique called modulation implemented in the transmitter. 

Modulation is a process in which the characteristics like frequency, amplitude and phase of a carrier signal (high frequency signal) is changed according to the message signal. 

In modulation, we mix the message signal with the carrier signal. Since the frequency of the carrier is high, the message signal can travel through longer distances.

A radio transmitter consists of several elements (power supply, oscillator, modulator, amplifier) that work together to generate radio waves that contain useful information such as audio, video, or digital data. 

The amplifier stage amplifies the modulated carrier wave to increase its power. The more powerful the amplifier, the more powerful the broadcast.

The size of antennas depends on the frequency in which they are tuned.

Reason 3
Frequency division multiplexing. Since all baseband signals are of same frequency range, without multiplexing they will mix with each other producing garbage at the receiver.

Feasibility of antenna size. Since baseband signal is of low spectral range, the antenna size without modulation would have been impractically large. It is so because the antenna size depends on the wavelength.

Narrowbanding. Without modulation, the antenna size size would have to adjusted continuously.

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