Transceiver operation

The apparatus that comprises an aircraft station is:

• an antenna system and feedline coaxial cable
• a radio transmitter/receiver unit or transceiver with modulating, transmitting, receiving, demodulating and power amplification circuits, plus mounting for the
• operator controls and displays
• a speaker/earphone and circuits to convert electromagnetic waves to sound waves
• a microphone and circuits to convert sound waves to electromagnetic waves
• the necessary interconnection cables, connectors and matching devices.

All the system components must be correctly matched (electrically) to each other and to any separate cockpit intercommunication unit installed in a two-seat aircraft.

Transmission

Amplitude modulation [AM] of the fixed RF carrier wave, rather than frequency modulation [FM], is used in the aviation band to impress the voice information on the carrier wave generated by the transceiver. AM occupies less bandwidth than FM, consequently the AM channel spacing in the aviation COMMS band is only 25 kHz

When the transceiver is powered up and the pilot speaks into the microphone while depressing a 'press-to-talk' [PTT] button, the transmitter circuits amplify and broadcast, via the antenna system, the selected output frequency — 126.7 MHz for example — modulated with the audio frequencies from the microphone. This may also include the cockpit background noise. The low-fidelity R/T audio frequencies added are in the range 50 Hz to 5000Hz; much the same as the domestic AM radio broadcast or the public telephone system

The transmission power of handheld transceivers is usually around 1 to 1.5 watts carrier wave. Fixed-installation transceivers are around 4 to 8 watts carrier wave.

Some hand-held transceiver suppliers quote the peak envelope power [PEP] output which, for ordinary speech, is probably around three times the carrier wave value. The peak envelope power of an AM signal occurs at the highest crest of the modulated wave

Reception

An aircraft antenna continually collects all passing RF energy in the band for which it is designed, which at any time will normally consist of many transmissions

The receiver tunes out all transmissions on all frequencies except one — the selected, or active, frequency. Signals on this frequency are demodulated to isolate the voice information from the carrier, amplify it and pass to the speaker system to convert to the sound waves heard in the earphones or speaker

Setting and changing frequencies

The frequencies required are usually entered into a VHF transceiver via an electronic keyboard, concentric rotatable knobs, toggle buttons or a set of thumbwheels

There may be a switch to set channel steps at either 25 kHz or 50 kHz. Most transceivers allow the user to set one frequency into the unit as the active frequency and to set a second frequency as the standby frequency. All transmission and reception is done on the active frequency. Pressing a flip-flop, or similar switch, causes the standby frequency to become the active, and the active to become the standby

Thus, normal procedure prior to take-off is to set the airfield frequency as the active and the flight information area [FIA] frequency as the standby

When departing the airfield area, pressing the flip-flop will make the FIA frequency active for the required listening watch. On return to the airfield area pressing the flip-flop again restores the airfield frequency to active.

Generally when selecting, keying or dialling another frequency during flight the new frequency changes the stand-by frequency.

Features common to most transceivers

a number of memory positions (5–50) allows storage of frequently used airfield/FIA and other frequencies an associated fast-scanning function of those stored frequencies instant access to the emergency/distress frequency of 121.5 MHz high and low transmit power settings for hand-held transceivers, giving a choice of minimum battery drain or maximum range hand-held transceivers are usually supplied with adapter(s) to connect the unit to the aircraft's COMMS (and NAV) antenna(s) hand-helds usually have key locking facilities to prevent inadvertent frequency changes or transmissions Hand-helds may also provide access to the 200 channels in the NAV band between 108.00 and 117.975 MHz, which gives a limited VOR capability if the transceiver can be adapted to a NAV dipole antenna. The main advantage provided by this facility is access to any ATIS or AWIS frequencies between 112.1 and 117.975 MHz.

Headsets

The cockpits of powered recreational aircraft are notoriously noisy and those close to a high rpm two-stroke engine are the worst. Propeller tip speeds may approach Mach 0.8 and generate noise at fairly high frequencies while the engine produces noise in the low to middle frequencies. External airflow noise may or may not be significant depending on the existence and effectiveness of cockpit sealing. In all, the cockpit noise level may approach 100 dB and long-term exposure to noise above 90 dB will damage hearing. Also, noise and vibration add to pilot fatigue and the low-frequency engine noises below 300 Hz are particularly fatiguing Consequently all pilots must wear some form of hearing protection — which may be incorporated within a good quality protective helmet

Headsets serve a dual purpose in providing hearing protection whilst improving communications. The basic headset consists of two earphones with some physical sound sealing capability plus a directional microphone mounted on an adjustable boom, so that it can be positioned within 1–3 cm in front of — and square on to — the pilot's lips when transmitting. The headset cables are jacked into the transceiver input/output sockets or patched via a cockpit intercom unit

Standard headsets may not be able to be used with hand-held transceivers without an adapter device

Additional facilities — such as individual volume control on each earphone with an electronic noise reduction system and cockpit noise cancelling microphones — are

available. You can get headsets specifically designed for two-stroke engine noise reduction

Normal headsets rely solely on passive noise reduction — creating a physical barrier around the ear to attenuate noise — which usually works quite well for middle

to high-frequency sound but doesn't block low-frequency engine noise and background rumble

Active noise reduction technology uses electronics to determine the amount of low-frequency (50–600 Hz) engine and other noise entering the system and then generating out-of-phase noise, in the same frequency range; this counters the background noise and leaves a soft 'white' noise in the headphones. But the technology doesn't significantly affect the higher-frequency noise

Microphones

How They Work

A microphone is an example of a transducer, a device that changes information from one form to another. Sound information exists as patterns of air pressure the microphone changes this information into patterns of electric current. The recording engineer is interested in the accuracy of this transformation, a concept he thinks of as fidelity

A variety of mechanical techniques can be used in building microphones. The two most commonly encountered in recording studios are the magneto-dynamic and the variable condenser designs

THE DYNAMIC MICROPHON

Transceiver operation

The apparatus that comprises an aircraft station is:

• an antenna system and feedline coaxial cable
• a radio transmitter/receiver unit or transceiver with modulating, transmitting, receiving, demodulating and power amplification circuits, plus mounting for the
• operator controls and displays
• a speaker/earphone and circuits to convert electromagnetic waves to sound waves
• a microphone and circuits to convert sound waves to electromagnetic waves
• the necessary interconnection cables, connectors and matching devices.

All the system components must be correctly matched (electrically) to each other and to any separate cockpit intercommunication unit installed in a two-seat aircraft.

Transmission

Amplitude modulation [AM] of the fixed RF carrier wave, rather than frequency modulation [FM], is used in the aviation band to impress the voice information on

the carrier wave generated by the transceiver. AM occupies less bandwidth than FM, consequently the AM channel spacing in the aviation COMMS band is only 25 kHz

When the transceiver is powered up and the pilot speaks into the microphone while depressing a 'press-to-talk' [PTT] button, the transmitter circuits

amplify and broadcast, via the antenna system, the selected output frequency — 126.7 MHz for example — modulated with the audio frequencies

from the microphone. This may also include the cockpit background noise. The low-fidelity R/T audio frequencies added are in the range 50 Hz to

5000Hz; much the same as the domestic AM radio broadcast or the public telephone system

The transmission power of handheld transceivers is usually around 1 to 1.5 watts carrier wave. Fixed-installation transceivers are around 4 to 8 watts carrier wave.

Some hand-held transceiver suppliers quote the peak envelope power [PEP] output which, for ordinary speech, is probably around three times the carrier wave value.

The peak envelope power of an AM signal occurs at the highest crest of the modulated wave

Reception

An aircraft antenna continually collects all passing RF energy in the band for which it is designed, which at any time will normally consist of many transmissions

The receiver tunes out all transmissions on all frequencies except one — the selected, or active, frequency. Signals on this frequency are demodulated to isolate

the voice information from the carrier, amplify it and pass to the speaker system to convert to the sound waves heard in the earphones or speaker

Setting and changing frequencies

The frequencies required are usually entered into a VHF transceiver via an electronic keyboard, concentric rotatable knobs, toggle buttons or a set of thumbwheels

There may be a switch to set channel steps at either 25 kHz or 50 kHz. Most transceivers allow the user to set one frequency into the unit as the active frequency and to set a second frequency as the standby frequency. All transmission and reception is done on the active frequency. Pressing a flip-flop, or similar switch, causes the standby frequency to become the active, and the active to become the standby

Thus, normal procedure prior to take-off is to set the airfield frequency as the active and the flight information area [FIA] frequency as the standby

When departing the airfield area, pressing the flip-flop will make the FIA frequency active for the required listening watch. On return to the airfield area pressing

the flip-flop again restores the airfield frequency to active.

Generally when selecting, keying or dialling another frequency during flight the new frequency changes the stand-by frequency.

Features common to most transceivers

a number of memory positions (5–50) allows storage of frequently used airfield/FIA and other frequencies

an associated fast-scanning function of those stored frequencies

instant access to the emergency/distress frequency of 121.5 MHz

high and low transmit power settings for hand-held transceivers, giving a choice of minimum battery drain or maximum range hand-held transceivers are usually supplied with adapter(s) to connect the unit to the aircraft's COMMS (and NAV) antenna(s) hand-helds usually have key locking facilities to prevent inadvertent frequency changes or transmissions

Hand-helds may also provide access to the 200 channels in the NAV band between 108.00 and 117.975 MHz, which gives a limited VOR capability if the transceiver can be adapted to a NAV dipole antenna. The main advantage provided by this facility is access to any ATIS or AWIS frequencies between 112.1 and 117.975 MHz.

Headsets

The cockpits of powered recreational aircraft are notoriously noisy and those close to a high rpm two-stroke engine are the worst. Propeller tip speeds may approach Mach 0.8 and generate noise at fairly high frequencies while the engine produces noise in the low to middle frequencies. External airflow noise may or may not be significant depending on the existence and effectiveness of cockpit sealing. In all, the cockpit noise level may approach 100 dB and long-term exposure to noise above 90 dB will damage hearing. Also, noise and vibration add to pilot fatigue and the low-frequency engine noises below 300 Hz are particularly fatiguing

Consequently all pilots must wear some form of hearing protection — which may be incorporated within a good quality protective helmet

Headsets serve a dual purpose in providing hearing protection whilst improving communications. The basic headset consists of two earphones with some physical sound sealing capability plus a directional microphone mounted on an adjustable boom, so that it can be positioned within 1–3 cm in front of — and square on to — the pilot's lips when transmitting. The headset cables are jacked into the transceiver input/output sockets or patched via a cockpit intercom unit

Standard headsets may not be able to be used with hand-held transceivers without an adapter device

Additional facilities — such as individual volume control on each earphone with an electronic noise reduction system and cockpit noise cancelling microphones — are

available. You can get headsets specifically designed for two-stroke engine noise reduction

Normal headsets rely solely on passive noise reduction — creating a physical barrier around the ear to attenuate noise — which usually works quite well for middle

to high-frequency sound but doesn't block low-frequency engine noise and background rumble

Active noise reduction technology uses electronics to determine the amount of low-frequency (50–600 Hz) engine and other noise entering the system and then

generating out-of-phase noise, in the same frequency range; this counters the background noise and leaves a soft 'white' noise in the headphones. But the technology doesn't significantly affect the higher-frequency noise

Microphones

How They Work

A microphone is an example of a transducer, a device that changes information from one form to another. Sound information exists as patterns of air pressure

the microphone changes this information into patterns of electric current. The recording engineer is interested in the accuracy of this transformation, a concept he

thinks of as fidelity

A variety of mechanical techniques can be used in building microphones. The two most commonly encountered in recording studios are the magneto-dynamic and

the variable condenser designs

THE DYNAMIC MICROPHON

In the magneto-dynamic, commonly called dynamic, microphone, sound waves cause movement of a thin metallic diaphragm and an attached coil of wire

A magnet produces a magnetic field which surrounds the coil, and motion of the coil within this field causes current to flow. The principles are the same as those that produce electricity at the utility company, realized in a pocket-sized scale. It is important to remember that current is produced by the motion of the diaphragm and that the amount of current is determined by the speed of that motion. This kind of microphone is known as velocity sensitive

THE CONDENSER MICROPHONE

In a condenser microphone, the diaphragm is mounted close to, but not touching, a rigid backplate. (The plate may or may not have holes in it.) A battery is connected to both pieces of metal, which produces an electrical potential, or charge, between them. The amount of charge is determined by the voltage of the battery, the area of the diaphragm and backplate, and the distance between the two. This distance changes as the diaphragm moves in response to sound. When the distance changes, current flows in the wire as the battery maintains the correct charge. The amount of current is essentially proportioinal to the displacement of the diaphragm, and is so small that it must be electrically amplified before it leaves the microphone

A common varient of this design uses a material with a permanently imprinted charge for the diaphragm. Such a material is called an electret and is usually a kind of plastic (You often get a piece of plastic with a permanent charge on it when you unwrap a record. Most plastics conduct electricity when they are hot but are insulators when they cool.) Plastic is a pretty good material for making diaphragms since it can be dependably produced to fairly exact specifications. (Some popular dynamic microphones use plastic diaphragms.) The major disadvantage of electrets is that they lose their charge after a few years and cease to work Carbon microphone

A carbon microphone use a capsule or button containing carbon granules pressed between two metal plates. A voltage is applied across the metal plates,

causing a small current to flow through the carbon. One of the plates, the diaphragm, vibrates in sympathy with incident sound waves, applying a varying pressure to the carbon. The changing pressure deforms the granules, causing the contact area between each pair of adjacent granules to change, and this causes the electrical resistance of the mass of granules to change. The changes in resistance cause a corresponding change in the current flowing through the microphone, producing the electrical signal. Carbon microphones were once commonly used in telephones; they have extremely low-quality sound reproduction and a very limited frequency response range, but are very robust devices

Using the squelch control

All transceivers have some form of ON/OFF/TEST/VOLUME control. As aircraft cockpits are very noisy, the output volume control must be set fairly high

This of course amplifies the weak atmospheric background radio frequency noise — the hash — which is always there when no strong transmissions are being heard on the active frequency; this hash can be quite annoying

The 'squelch' or 'gain' or 'RF gain' or 'sensitivity' control is an adjustable filtering device which, for operator comfort, can be set just to filter out the hash but still allow any strong signals to be switched through. The squelch control should only be switched on and adjusted when contact with the active frequency has been established. Otherwise, when the signal is weak, there is a high risk of also filtering out the active frequency transmissions which, in effect, turns the receiver off

Some transceivers have an automatic gain control. In which case, pressing the test facility will override the squelch, allowing the background hash to be heard

3.3 Wave length and antennas

It is stated in the electromagnetic spectrum section that the frequency in MHz = 300/wavelength in metres — or restated, the wavelength in metres = 300/MHz.

Thus the wavelengths involved in the aviation VHF COMMS band, 118.00 to 136.975 MHz, are from 2.54 metres to 2.19 metres and the mid-point is about 2.37 metres

The Multicom frequency — 126.7 MHz — has a wavelength of 300/127.6 = 2.35 metres. Wavelength is important as the efficiency of the antenna (a passive electrical conductor that radiates the signal energy when transmitting, or collects signal energy when receiving) partly depends on its length relative to the frequency wavelength.

Most ineffective radio installations are because of ineffective antenna installations and/or RF interference generated by the engine ignition system or the aircraft's electrical components.

Dipole antennas

Aircraft COMMS antennas are usually dipoles or monopoles. A dipole is an antenna that is divided into two halves insulated from each other. Each half is connected to a feedline (coaxial cable and RF BNC series bayonet connectors) at the inner end, which routes the RF energy between the antenna and the transceiver. The length of each half is about 5% less than the mid-point quarter-wave — usually about 56 cm, or 22 inches. (The mid-point quarter-wave is 2.35/4 =59 cm.) Rather than being set out end-to-end horizontally, each half is canted up about 22.5° to form an internal angle of around 135°, which prevents a deep "null" zone off both ends. NAV or COMMS dipoles may be mounted within the fuselage if the aircraft is not metal-skinned or metal-framed. A NAV antenna must be horizontally polarised; i.e. mounted horizontally.

The two halves of a COMMS dipole antenna can be end-to-end vertically mounted with a centre feedline and built into the fin of a fibre-reinforced composite aircraft — but not if it is carbon fibre. Similarly a half-wave dipole antenna might be used on a trike where the longer length can be mounted vertically end-to-end and strapped to the king - post.

The telescopic 'rabbit's ears' antennas used with the old black and white TVs were dipoles — as channels (frequencies) were changed the length was adjusted to maintain the half-wavelength dimension.

Monopole or whip antennas

The most common recreational aircraft COMMS antenna — the monopole — is just one half of a dipole; i.e. quarter-wavelength. (To calculate antenna quarter-wavelength in centimetres, divide 7130 by the frequency; i.e. 7130/126.7 = 56 cm.) Thus the monopole is usually about 56 cm long, mounted vertically (vertically polarised) — normally on the top of the fuselage (away from the undercarriage legs) — with the feedline conductor to/from the transceiver connected to the bottom end of the antenna. The 56 cm length should provide very good mid-frequency reception and reasonable reception at the lower and upper ends of the COMMS band and, usually, increasing the thickness of the antenna element increases its effectiveness. The antenna element may be enclosed within a streamlined fibreglass fairing to add structural strength. To replace the other half of the dipole a conductor system is placed just below the antenna to serve as an earth ground — a ground plane, ground screen or at least four ground radial strips or rods, connected to the coax cable shielding. The radius of the ground equals the length of the antenna; i.e. 56 cm. In a metal-skinned aircraft the fuselage acts as a ground plane, which is electrically insulated from the antenna by a very small gap.

The photo shows the ground plane, in Leo Powning's Jodel project, mounted under the ply turtle deck (looking aft). The centre plate and four 25 mm wide radials are cut from light gauge aluminium sheet sold in hardware stores. Total dimension from the antenna socket to the end of each radial is 57 cm — about the mid-point of the COMMS band. The sloped radials provide an antenna impedance of approximately 50 ohms. The 50 ohms coax connecting the antenna is attached to the turtle deck formers with plastic P clips.

Transmission/reception pattern

Because of antenna characteristics and airframe shielding, the radiation/reception pattern of the antenna will be weaker in some directions and may even exhibit null zones.

The easiest way to check this is to tune in the continuous broadcast — at a reasonable (say 30 nm) distance — from a known ATIS, AWIS or AERIS location, then circle while listening to the signal strength. A few turns should be sufficient to plot the directions, relative to the aircraft's longitudinal axis, from which signal strength weakens and/or reduces to nil.

Because the attitude of the aircraft also affects transmission/reception, it is advisable to first fly non-banked turns to ascertain the normal pattern then fly banked turns to check the consequent effects.

Impedance matching

All VHF transceivers are designed for a standard load (impedance) of 50 ohms. Ideally the coaxial cable, BNC connectors and antenna match that 50 ohm impedance all the way; then all the transmission power sent to the antenna will be radiated as RF energy. However, the resonant frequency of any antenna will match only one frequency, and the COMMS operational frequencies range over 19 MHz. So for most transmission frequencies the antenna will exhibit positive or negative reactance (or impedance), which results in the phenomenon known as 'stationary' or 'standing' waves in the feed line and reduces the output of the antenna. Also the incoming signals will be weaker.

The RF performance of the antenna system is expressed in terms of the voltage standing wave ratio [SWR or VSWR]. A perfect (but most unlikely) antenna system would have a SWR of 1:1 but generally a SWR less than 2:1 results in quite acceptable performance and limits transceiver overheating. The Microair 760 — described in the next module — requires a SWR between 1.3:1 and 1.5:1. If the transmission performance is okay then the reception performance should also be okay

אנטנות

אנטנה (בעברית: מְשׁוֹשָׁה) היא מתמר, רכיב חשמלי או מתקן המורכב ממוליכים המיועד לשדר ו/או לקלוט גלי רדיו.

· בשידור, המוליכים מתמרים את הזרם חשמלי הנכנס לאנטנה, לקרינה אלקטרומגנטית.

· בקליטה, זרם חשמלי נוצר במוליכים של האנטנה, כיוון שהם נמצאים בשדה אלקטרומגנטי משתנה (השראה אלקטרומגנטית).

איפיון

תורן עם אנטנות לטלוויזיה, בקצה העליון אלמנט דיפול מקופל לקליטת רדיו FM.

תכונות האנטנה מורכבות ממספר תחומים:

תכונות פיזיות ומרחביות (הקשורות בכיוון אליו האנטנה משדרת/קולטת), ומהוות פרטים בעקום הקרינה של האנטנה.

רוחב אלומה

כיווניות

עוצמת אונות צד

יחס פנים לאחור

תכונות חשמליות ואלקטרומגנטיות (הקשורות ביכולות ההקרנה של האנטנה):

תחום תדר פעולה.

קיטוב (אופקי / אנכי / אלכסוני / מעגלי וכו')

יעילות / נצילות

יחס גלים עומדים (יג"ע)

היגב

שבח / הגבר

אימפדנס כניסה או יציאה (כניסה בשידור, יציאה בקליטה)

תחום תכנון האנטנות ומאפייניהן נחשב מורכב וייחודי ומצריך הבנה רבה בתחום פיזיקה של גלים ופתירת משוואות מסובכות. מדידות של המאפיינים של אנטנה קיימת מתבצעים במטווח אנטנות

סוגי אנטנות -

קיימים סוגים שונים של אנטנות, בהם:

· אנטנת דיפול - אנטנה הבנויה משני חוטי (או מוטות) מתכת הנמצאים בציר אחד, כאשר במצב שידור תפוקת המשדר מוזנת ביניהם במרכז האנטנה. עקום הקרינה של האנטנה הוא כלל כיווני, במישור הניצב לאנטנה. כאשר אורך האנטנה הוא חציאורך הגל, שבח הדיפול הוא ‎2.2 dBi.

· אנטנת מונופול - בנויה מחוט (או מוט) מתכת מעל משטח אדמה מתכתי אינסופי. בפועל ניתן להסתפק במשטח קטן יותר. לפי שיטת השיקופים, עקום הקרינה של אנטנה זו זהה לאנטנת דיפול. כאשר אורך האלמנט הוא רבע אורך גל, שבחהמונופול הוא ‎6 dBi.

· אנטנת שופר - מבנה דמוי משפך. בנויה מגלבו מורחב בקצהו ומשמשת בעיקר לתדרים מעל 1 ג'יגה־הרץ.

· אנטנה לוג-פריודית - אנטנה רחבת סרט בו ישנם מספר אלמנטים בעלי אורך שונה שכל אחד מהם תוהד בתדר שונה.

· אנטנת יאגי - משמשת בעיקר לתדרים נמוכים ומורכבת ממספר אלמנטים הניקראים דירקטורים ורפלקטורים.

· אנטנה ביקונית - מורכבת משני אלמנטים בצורת חרוט מלא או ממוטות משדרת אלומה כלל כיוונית בציר האזימוט ואלומה כיוונית בציר ההגבהה.

· אנטנת מחזר - נקראת גם אנטנת reflector או צלחת. שימושית בעיקר בתדרי מיקרוגל. בעלת מבנה דמוי צלחת פראבולית, שבמרכזה זן. משדרת אלומה צרה / שבח גבוה.

· אנטנות מודפסות - אנטנות שנכנסו לשימוש בשנים האחרונות ובהן האלמנטים מודפסים על מעגל מודפס כך שניתך להדפיס מספר גדול של אלמנטים ולהשיג שבח גבוה.

ICAO - עבור לארגון התעופה הבנ"ל