When a High Frequency (HF) signal is Frequency Modulated (FM), the bandwidth it gains is not as small as that it develops during Amplitude Modulation (AM). The difference during FM could be higher by many Mega Hertz. That is one reason why FM transmissions, especially TV stations are known in Channels, instead of being identified by their centre transmitting frequencies. For example, Channel 12 refers to a transmission done between the frequencies of 224 MHz and 228 MHz. This frequency spectrum of channel 12 is allotted for TV transmissions only. In Channel 12, 224 MHz side is used for Audio and 228 MHz side for video - picture is Amplitude Modulated and sound is Frequency Modulated.
Quality of audio is always better in FM because it is the amplitude of signals that are easily disturbed by atmospheric distortions. At Frequency Modulations with sound only, as per international rules, the Bandwidth is to be held below 200 KHz. One major drawback of Frequency Modulated signals with carrier frequencies at VHF and beyond, is that these signals are accessible only at a small limited range, because at normal conditions VHF signals propagate at a line of sight rule.
Radio Listeners
Be it AM or FM, the atmosphere is truly filled with many many billions of modulated signals of varying kinds. There are some people, as always, who are keenly interested in listening to as much Radio stations as possible - useful or useless, powerful or weak, English or Malayalam, AM or FM. A Broadcast Commercial Station listener is called a BCL (Broadcast Commercial Listener), while a Ham Radio station listener is called a SWL (Short Wave Listener). There is still another enthusiastic group of listeners called BC Dxers (Broadcast Commercial Dxers). They not only listen but keep track of the Stations general propagation conditions regularly. 'D' here refers to distance, the 'X' here refers to an unknown place (just like in algebra) and the full combination 'Dx' refers to something foreign.
Be it AM or FM, the atmosphere is truly filled with many many billions of modulated signals of varying kinds. There are some people, as always, who are keenly interested in listening to as much Radio stations as possible - useful or useless, powerful or weak, English or Malayalam, AM or FM. A Broadcast Commercial Station listener is called a BCL (Broadcast Commercial Listener), while a Ham Radio station listener is called a SWL (Short Wave Listener). There is still another enthusiastic group of listeners called BC Dxers (Broadcast Commercial Dxers). They not only listen but keep track of the Stations general propagation conditions regularly. 'D' here refers to distance, the 'X' here refers to an unknown place (just like in algebra) and the full combination 'Dx' refers to something foreign.
BC Dxing
Anyone who is engaged in broadcast related activities would naturally have a curiosity or an enthusiasm to know how their signals reach at different points on earth, the comparative quality of their signals at different times of the day/night, the condition of the atmosphere and the influences that affects the signals at that time etc. Many Broadcasting companies appoint official monitors all over the world, just for this sort of observations and study. However, an official permission is not required to prepare a signal report of any Commercial Station and send it to the concerned authorities for their evaluation. According to the prevailing practice, broadcasters receiving such amateur Signal Reports respond with Official Acknowledgement Cards (QSL Cards) and sometimes gifts of varying kinds too. QSL Cards are very dear to BC Dxers. Those in QSL Card hunt/collection have even their own Net programs which provide necessary timely information in this regard. In South India, there is a Regular Net (SSB/AM) of BC Dxers on 40 Meters, from 8.30 AM onwards on all Sundays.
QSL Cards
Anyone who is engaged in broadcast related activities would naturally have a curiosity or an enthusiasm to know how their signals reach at different points on earth, the comparative quality of their signals at different times of the day/night, the condition of the atmosphere and the influences that affects the signals at that time etc. Many Broadcasting companies appoint official monitors all over the world, just for this sort of observations and study. However, an official permission is not required to prepare a signal report of any Commercial Station and send it to the concerned authorities for their evaluation. According to the prevailing practice, broadcasters receiving such amateur Signal Reports respond with Official Acknowledgement Cards (QSL Cards) and sometimes gifts of varying kinds too. QSL Cards are very dear to BC Dxers. Those in QSL Card hunt/collection have even their own Net programs which provide necessary timely information in this regard. In South India, there is a Regular Net (SSB/AM) of BC Dxers on 40 Meters, from 8.30 AM onwards on all Sundays.
QSL Cards
QSL Cards have no specified size or strict format; they need not even be sent by post. In most countries, there are QSL Bureaus exclusively created for the distribution of QSL Cards. QSL Bureaus sort all Cards to one country/area/individual together and forward it in one bundle, through other QSL Bureaus or Clubs and make sure that they are delivered to the right person safely at the lowest possible expenditure, though a little late.
Exciting and symbolic comments like 'Good,' 'Excellent,' 'Bad,' or 'Fading' are not that what are expected through a Signal Report. A signal report is prepared at International Standards and at maximum precision. SIO code is the format for BC Dxers (Ref. C-3/1). Here, the word 'Interference' refers to all signals that affect the clarity of the modulation. In SIO, 555 means a powerful Station in its' full strength and clarity.
C-3/1 | S = Signal Strength | I = Interference | O = Overall Merit |
5 | Excellent | Nil | Excellent |
4 | Good | Slight | Good |
3 | Fair | Moderate | Fair |
2 | Poor | Severe | Poor |
1 | Barely Readable | Extreme | Unusable |
In HAM (Amateur) Radio, RST code is the standard (ref. C-3/2). In ordinary HAM Radio chats, only R and S factors are mentioned and signal reports are exchanged like 59+, 57, 46 etc. At the same time, when it come to Morse Code, the quality of tone also is exchanged. There the report comes like 599, 589 etc.
C-3/2 | R = Readability | S = Strength | T = Tone |
1 | Not Readable | Very weak | - |
2 | Barely Readable | Weak | - |
3 | Poor | Barely Copiable | - |
4 | Readable | Copiable | - |
5 | Fully Readable | Average | Very Bad |
6 | - | Good | Bad |
7 | - | Reasonable | Tolerable |
8 | - | Fine | Not Clear |
9 | - | Excellent | Good |
Hams usually prepare the RS (Readability and Strength) report from his/her experience; only a few looks at the 'S' Meter (Strength Meter) on the front panel of the Tx (Transmitter). The maximum power a 'S' Meter reads in digit is '9'. Signal strength beyond that is seen in 'db' (decibel). That is, if the meter shows 20 db it will be mentioned as a 59+ signal and in cases it is more than 20 db, '59++' is used to inform that.
Even though 'SIO' and 'RST' codes are very popular, it is to be noted that they are not enough for an elaborate evaluation and split study of all varying features and characteristics of a signal. In similar critical cases there is the 'SINPO' code for help (ref.C-3/3).
Signal Strength | Interference | Noise | Propagation Discussion | Frequency Fading | Modulations | Overall Rating | ||
Quality | Depth | |||||||
5 | Excellent | Nil | Nil | Nil | Nil | Excellent | Maximum | Excellent |
4 | Good | Slight | Slight | Slight | Slight | Good | Good | Good |
3 | Fair | Moderate | Moderate | Moderate | Moderate | Fair | Fair | Fair |
2 | Poor | Severe | Severe | Severe | Severe | Poor | Poor & Nil | Poor |
1 | Barely Audible | Extreme | Extreme | Extreme | Extreme | Very Poor | Not at all Satisfactory | Unusable |
S | I | N | P | F | E | M | O | |
C-3/3 SINPO CODE | ||||||||
A QSL Card from an average Ham band listener naturally contains a lot of information. Day and exact time of listening, place of hearing, details of the receiver used, specifications of the antenna assembly, self - introduction (in case of fresh contacts) with all necessary profile/bio-data information etc. Always time should be noted in UTC (Universal Time Coordinated) at 24 hours mode (avoiding AM and PM). UTC is Greenwich Mean Time only. Some listeners add pictures too (shack photograph, own picture etc.) in their Cards.
International Telecommunications Union has divided the globe into Zones and Regions with signifying numbers to them. India is in Region 3 and our Zone is No. 22. These details mostly show the location roughly. Hams and SWLS use longitude and latitude for better location identification purposes - it is sharper.
Any broadcaster is permitted to use only that frequency or frequency range/band that is allotted to him/her by ITU (International Telegraph Union). The International Telecommunication Union is a specialised agency of the United Nations (UN) that is responsible for issues that concern information and communication related technologies. ITU which was founded in 1865, insists that every Radio Transmitter should have a unique identification code or an open name of its' own. This ITU regulation was in force from the beginning of the previous 20th century itself.
Earlier, Radio Stations had an option to decide its own name. Slowly, as the number of Radio Stations increased, ITU rules and disciplinary codes also changed accordingly. It ended up with Prefixes to every country. 'VU' goes to India and '457' goes to Sree Lanka. The codes that follow these prefixes exactly show who the operator is. The station code of a VU station is decided by the Union Ministry of Communications. The station name in full is precisely called the 'Call Sign'. There are Call Books that contain all the names of Ham stations within a country. On the QSL Card, a BC Dxer however, has to write the full name of his country, not any of the codes.
Generally, QSL Cards are not sent immediately after listening a station. In most cases relevant details and report are prepared in a special register for that (log book/record), immediately after the chat/conversation. But cards won't move out of the shack until it grows into a pretty good number or the listener finds it convenient to send it out. In case QSL cards are sent at a later date, it is recommended to include that station's later report also, if happened to hear the same station again with a different signal report. Showing serial no. reference in the log book/QSL Card naturally will tell how experienced a particular listener is. In case of Hams, they are bound by law to keep a log book with all contacts marked.
The SWL version of the same QSL Card is given in pic: C-3/5A. This can be prepared in boxes drawn in a simple note book. If it is Ham Station that is heard, the full call sign of the station should be given. Beyond all the details given in a QSL Card, the listener may specify the name in which he loves to be called. Many Hams are called in the very combination of their station licence; it is like calling someone TG or LN. Fortunately, I got a better combination (VU2JIM) with my VU Prefix and in Ham circles, my handle is 'Jim'. One side of the QSL Card is completed with Handle, full address, phone numbers, mail IDs etc.
Sl No. | Date | Starting Time | Ending Time | Freq. | Mode | Stations | Report |
001 | 17.03.2017 | 14.10 | 15.20 | 7040 | AM | …………….. | 59+ |
C-3/5 A | |||||||
QSL | Remarks | Wx | |||||
Received | Sent | Unusually strong signal today | Sunny Bright | ||||
On the next side, write the address of the person to whom the QSL card is sent. There is scope for conveying a few personal information too, along with questions if there are. If QSL cards are sent to persons, it is part of curtsy that we include enough stamps also for a reply QSL Card. Unless one signal report is different from another, a Ham need not be sent another QSL Card. No. '73' used every time signifies love and regards'. '73' comes from what is known as the 'Phillips Code', a series of numeric messages conceived for the purpose of cutting down transmission time on the old land telegraph systems when sending text that is basically the same. 'Wx' refers to atmospheric situations and 'Rx' to receivers ('Tx' - Transmitter). With necessary stamps attached, together with all the cards, anybody can pass it to the QSL Bureaus, if they are at reach.
Exchange of QSL Cards is actually a traditional ceremony that strengthens the bond of friendship in Ham Radio circles. This is not a rule but a practice. In fact, this is perhaps the most expensive side of this unique hobby.
Shortcuts in Communication
In all sorts of Radio transmissions, except commercial, importance is for maximum data exchange in minimum time. Where it is data transfer that is more important, certain codes that further accelerates data exchange speed also are used. One example is telegraph in which number/word codes are used. In communication electronics there are codes using English letter combinations, generally called 'Q' codes (because all of them begins with 'Q'). For example QTR for exact time, QSP for a message, QRX for waiting, QSL for understood QSO for conversation and so on. 'Q' codes could be questions or answers, depending on the nature of the situation. Now we know why an acknowledgement card is called 'QSL Card'. 'QSL Card' simply refers to a token of acknowledgement. There are a pretty good number of Q Codes in Ham Terminology. This does not mean that Q Codes are the only short cuts Hams regularly use. Look at the below chart:
Shortcuts in Communication
In all sorts of Radio transmissions, except commercial, importance is for maximum data exchange in minimum time. Where it is data transfer that is more important, certain codes that further accelerates data exchange speed also are used. One example is telegraph in which number/word codes are used. In communication electronics there are codes using English letter combinations, generally called 'Q' codes (because all of them begins with 'Q'). For example QTR for exact time, QSP for a message, QRX for waiting, QSL for understood QSO for conversation and so on. 'Q' codes could be questions or answers, depending on the nature of the situation. Now we know why an acknowledgement card is called 'QSL Card'. 'QSL Card' simply refers to a token of acknowledgement. There are a pretty good number of Q Codes in Ham Terminology. This does not mean that Q Codes are the only short cuts Hams regularly use. Look at the below chart:
Terminology | Meaning |
Home brew | Self made equipment |
Shack | Work Station |
Ham | Licensed Amateur Radio Operator |
Rag chewing | On the air chit-chat |
Ticket | License |
Rig | Wireless Equipment |
Harmonics | Children |
Old Man | All licensed Male Hams are called with a prefix OM |
YL/ Young Lady | Lady Radio Amateur |
SWL | All those who have not yet received a license |
Crystal Controlled | Married |
Signing Off | Closing the contact |
Going QRT/Pulling the Big Switch | Closing the station |
LL (Lima Lima) | Landline |
QTH | Working QTH = Office, Home QTH = House |
GM/GE (mostly in Morse Code) | Good morning/God Evening |
Eyeball QSO | Meeting face to face |
Dx/VU land | Foreign country/ India |
Silent Key | Die |
Field Day | Out door transmission/reception experiments |
4 wheeler/2 wheeler/multi wheeler | Car/Bike/Train |
Qualities of a Receiver
For an Amateur Radio hobbyist, SWL or HAM, the most precious of assets would be an ideal receiver. It was in 1925 that the first commercial receiver came out. The interesting thing was that all manufacturers crowded around ‘higher sensitivity’. Their advertisement strategy made the then customers believe that it is highest sensitivity that any receiver most requires. It is only with fear that we could remember those ‘very sensitive’ roaring receivers, which accepted all atmospheric distortions without prejudice to any. Thank God, they are gone!
1) Sensitivity is never a factor that can be ignored, but a good ‘front end’ need not necessarily be best in sensitivity. Sensitivity should be taken as the capacity of a Radio to pick up weak signals, always avoiding maximum distortion elements. Since the noise figure that forms at the Radio front end and in the following RF stages are critical, it is always better to avoid high sensitivity (and thus high noise) receivers. The fact we many times ignore is that weak signals can easily be amplified in the latter stages.
2) Selectivity, the next top feature of any Radio Receiver, is not limited to the activity of sorting out signals, as we generally assume it to be. It is receiving the right signal in its’ appropriate bandwidth and clarity, avoiding all other signals including harmonics and images. Here, filter circuits are critical. There are generally four types of filter circuits:
1) Low pass filters that let in only signals below a definite frequency,
2) High Pass Filters that let in only signals above a definite frequency,
3) Band stop Filters which block signals of a definite frequency range and
4) Band pass Filters that let in only a definite range of RF signals. Depending upon the size of frequency range, they are further divided into two - Narrowband and Broadband.
Unless we keep the Q (quality) factor of the tuned circuits in a Receiver at a satisfactory level, increasing the ‘selectivity’ of a Receiver is impossible. An ideal receiver should not only be able to reject all unwanted signals but also be able enough to pull in weak signals without any damage to it. Coming to its’ IF stages, a model receiver with ideal selectivity factors should have strictly defined bandwidth too. The bandwidth of such an IF stage should be 800 Hz for CW, 2.5 Khz for SSB and 3.5 Khz for AM. In double conversion receivers, which propose to avoid all image signals, much care is required in the oscillator stages that handle multitudes of signals.
3) Fidelity is the next best quality of any receiver. Fidelity hints at the quality of the output audio signal. Hi Fi (High Fidelity) output requires amplifier stages that function exactly at the proposed bandwidths, frequencies and configurations. Any signal, High Frequency or Low Frequency, varies in power as it goes through a processing stage. Even though power is always measured in Watt, in the case of audio, what we consider is how powerfully it appears to us. Audio power is measured in db (decibel). Decibel is a logarithmic unit used to express the ratio of two values of a physical quantity. Even if a 1W audio signal is amplified 20 times, what we experience could be just a multiplication with 2. We record it with a 2 db note. Even though strength is generally measured through 'S' meter, there is no internationally approved style or method. According to the 'S' meter, the dial is divided into 9 segments of 6 db each.
Communication Radio Receivers
The simplest of Radio Receivers are Crystal radios. It can be made with a few inexpensive parts, such as a piece of wire for an antenna, a coil of copper wire for adjustment (optional), a capacitor (optional), a crystal detector, and an earphone. Crystal sets produce rather weak sound and must be listened to with sensitive earphones, and can only receive stations within a limited range.
Generally, Communication Radio Receivers are classified into three. They are:
1) Tuned Radio Frequency Receivers (TRF)
2) Direct Conversion Receivers (DC) and
3) Super Heterodyne Receivers (SHR).
Tuned Radio Frequency Receivers
A few selective stages, a detector and an audio amplifier together make a complete simple Radio. Though they were widely used in the early stages of the Radio, now TRF (Tuned Radio Frequency) Receivers are not in use at all. The facts that TRF receivers are good for AM and CW signals only and mutual interference possibility between various stages are higher, added to the failure of TRF receivers.
DC Direct Conversion Receivers
DC Receiver is a simple improved version of TRF receiver. This is popular among Radio Amateurs. There are two important reasons for this. The first is that in a DC receiver AM, SSB and CW signals are equally audible without any artificial signal injection. One another reason is that the same oscillator frequency can be used at transmission also.The simplicity of DC receivers and its discipline in keeping bandwidth always at minimum are added attractions.
DC Receivers have its' on draw backs too. In the case of CW signals, unwanted sidebands are not eliminated in a DC Receiver. Even though DC Receivers pull in signals with poor stability they are not as audible as they are in improved versions of Receivers. In DC Receivers, unless the signal is distortion free, clear and stable it is difficult to demodulate the signal successfully. The worst problem is the interference of other powerful commercial stations. Many times they might appear comparatively stronger than the stations we actually tune to. Using a very highly selective front end we can overcome this interference issues though at a considerable stage gain loss. Using a common ground for all the stages in a Radio also causes interferences from other BC Stations. In spite of all these, it is to be noted that most of the commercial Transceiver manufacturers still use DC receivers at the receiver side.
