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  • jalves61 6:07 pm on January 17, 2020 Permalink | Reply
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    ARRL Examination Study (Part I) 

    The ARRL (American Radio Relay League) is an organization for amateur radio enthusiasts. In order to communicate using HAM radio, at least a technician license must be obtained. The following post is meant as a useful information guide for those wishing to obtain a license.

    The ARRL provides a complete manual as a study reference for HAMs. The book is divided into nine chapters: Basic info about ARRL, Radio and Signals, Circuit components, propagation and antennas, Amateur radio equipment, HAM communication, License regulation, operating regulation and safety. The questions come directly from each chapter (35 total, 26 to pass).

     

    For Radio and Signal fundamentals, it is important to know basic properties of waves including wavelength, speed of propagation, the relation between wavelength and frequency, identifying frequency bands, the frequency ranges of various bands used by HAMs and so forth. The fundamental equation for propagation of waves is c = fλ. Because radio waves are being transmitted by antennas through air, the speed of propagation is 300 million meters/sec. This is a constant value and therefore if frequency is increased, the wavelength decreases proportionally. This speed value is roughly equivalent to the speed of light in a vacuum. The property of radio waves used to identify different frequency bands is wavelength. HAMs tend to use the frequencies occupied by bands MF through UHF. It is important to know the frequency ranges of these bands.

    bands.PNG

    In this section, it is important to know prefixes for the SI unit system, so conversions between various values can be made. The following table should be committed to memory.

    SI.PNG

    The next section deals with modulation, which is a necessary function to transmit the correct signal to receiver. It is important not to set a transmit frequency to be at the edge of any band to allow for transmitter frequency drift, allow for calibration error, and so that modulation sidebands do not extend beyond the band edge. It is important to know about FM deviation (which is dependent on amplitude of the modulating signal) and that if the deviation is increased, the signal occupies more bandwidth. Setting a microphone gain too high could cause the FM signal to interfere with nearby stations. It is important to know the types of AM modulation (Double Sideband, Single Sideband, etc) and which modulation technique is best for various frequency bands. “Continuous wave” (Morse code-esque) modulation occupies the lowest bandwidth, followed by SSB modulation. The various advantages to certain modulation techniques should be understood. For example, SSB is preferential to FM because it occupies less bandwidth and has longer range. The bandwidth for each modulation technique is shown below.

    bandwidths.PNG

    The final section of Chapter two deals with radio equipment basics. A repeater should be understood to be a station that retransmits a signal onto another channel. The following is an image of a transceiver, which transmits and receives RF signals using a TR switch to switch between each function. A repeater uses a duplexer in place of this switch to transmit and receive simultaneously.

    transciever

     
  • jalves61 12:00 am on January 13, 2020 Permalink | Reply
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    The Human Ear 

    The Human ear is important to the study of acoustics because it is inborn pressure sensor. It is one of the most sensitive parts of the human body and its job is to sense pressure changes in air and convert these to electrical signals that the brain can process as “sound”. Humans can hear roughly between 20 Hz to 20 kHz but this range decreases with age. The human ear can sense sound intensities from 1 W/sqm to 1 trillionth of a W/sqm. What most people intuitively perceive as music loudness, pitch and timbre roughly corresponds to amplitude (or sound intensity, which is proportional to the square of amplitude), frequency and waveform shape. Of course, these are not one to one relationships because if a tone is too high in frequency (ultrasound) or too low (infrasound) it will effect the perceived loudness because it will not be heard at all, for example.

    The human ear consists of three main parts: inner ear, middle ear and outer ear. The outer ear consists of the pinna, auditory canal and eardrum. The pinna (the only visible part of the ear) serves as a guide to guide pressure waves into the ear canal. The ear canal is filled with air which is necessary because sound needs a medium such as air to transmit pressure waves. The waves reach the conically shaped eardrum, which vibrates and sends signals to the brain to process.

    ear.png

    The middle ear consists of several dense bones (ossicles) called the hammer, anvil and stirrup. These are elastically connected and serve to transmit and amplify sound from the outer to inner ear. These bones are necessary because the pressure waves are being transferred to a different medium (air to ear fluid called endolymph) and require an impedance matching network to transmit sound effectively. This is not unlike the soundboard of a guitar (for impedance matching to air) or an electrical impedance matching network design for maximum power transfer from a source to a load.

    The inner ear contains the cochlea and the semicircular canals. The cochlea contains thousands of tiny hair cells that are stimulated by the vibrations of sound. The semicircular canals contribute to our sense of balance, but not the sensation of hearing. The inner ear fluid causes the hairs in the cochlea to bend, which are converted to electrical pulses and sent to the brain. These are sent to the auditory nerve and are interpreted as sound.

    The following diagram depicts the human ear as a passive electrical circuit using the “impedance analogy”. The eardrum middle ear section is shown to be a transformer to match the outer ear to the middle ear. There could also be another transformer between the middle ear and the cochlea, as stated before. Without going into excruciating detail, it is important to show that the human ear is not all different from an electrical circuit in the sense that it impedance matches and transforms/transduces different forms of energy.

    analogear.png

     

     
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