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14. Transmitter Measurement

Definitions

  • Decibel is used as a comparison. It can either be a comparison between two measurements or between one measurement and a standard unit.
  • 0 dBW does not mean there is zero power, but it means the power is exactly the same as the reference, 1 Watt, in this case
  • 3dB down is the bandwidth usually quoted to be the frequencies between the half-power points
  • Two Tone Generator comprises two audio oscillators whose out puts are combined into a single output to be connected to the microphone input of the transmitter under test
  • Mean power output of an SSB transmitter fed with two tones will be 50% of the Peak Envelope Power
  • Effective radiated power (ERP) is an IEEE standardized definition of directional radio frequency (RF) power transmitted from a theoretical half-wave dipole antenna
  • Peak envelope power (PEP) is the highest envelope power supplied to the antenna transmission line by a transmitter during any full undistorted RF cycle or series of complete radio frequency cycles
  • Net happens when more than two Amateurs are in conversation over the radio
  • Radio transeiver is a combination of a radio transmitter and a radio receiver
  • PTT (Push To Talk) or MOX (Manually Operated Transmit) or VOX (Voice Operated Transmission) are 3 different types of modes to change from listen to speak (receive to transmit)

Diagrams

  • Log vs linear scale

Formula

  • Gain = 10 log (output power / input power) where Gain is in dBW, output power and input power is in Watts
  • Gain = 20 log (output voltage / input voltage) where Gain is in dBV, output voltage and input voltage is in Volts
  • Gain = 20 log (output current / input current) where Gain is in dBV, output current and input current is in A
  • Gain = Gain(1) + Gain (2) - Cable loss where Gain is total gain of 2 amplifiers put together in series one after another
  • dBW = 10 log Watts
  • 3db down on X = X/2 where X is power in Watts

Notes

Decibel

  • It can either be:
    • comparison between two measurements
    • comparison between one measurement
    • standard unit
  • BEL was named after the inventor of the telephone Alexander James Bell
  • it was found that this unit was too large, so it was divided by ten
  • Hence deci Bel and is usually abbreviated to dB
  • human ear operates in a logarithmic fashion
  • Decibel therefore follows a logarithmic law
  • 3dB is doubling of power
  • -3dB is halving of power
  • Advantage: Losses and gains can be added / subtracted
  • decibel is not just used for power comparisons and measurements but current and voltage too
  • Decibels are often used as the scale for response measurements

Examples of dBW values

dBW Watts
-9dBW 0.125 Watts (or 125mW)
-6dBW 0.25 Watts (or 250mW)
-3dBW 0.5 Watts (or 500mW)
0dBW 1 Watts
+3dBW 2 Watts
+6dBW 4 Watts
+9dBW 8 Watts
+12dBW 16 Watts
  • +dBW is a power greater than the reference
  • -dBW is a power less than the reference

Watts and dBW conversions

  • dBW = 10 log P
  • P = 10 ^ (dbW/10)
dBW Watts
-20 dBW 10mW
0dBW 1W
3dBW 2W
6dBW 4W
9dBW 8W
10dBW 10W
13dBW 20W
14dBW 25W
15dBW 32W
16dBW 40W
20dBW 100W
22dBW 160W
26 dBW 400W
30 dBW 1KW
40 dBW 10KW

Ratios

Amount dB
2X 3dB
4X 6dB
8X 9dB
10X 10dB
100X 20dB
1000X 30dB

Amplitude modulation

  • fully amplitude modulated by a sine wave audio tone
  • radio frequency is very high compared with the audio frequency
  • individual RF cycles may merge into each other on the screen and just appear as a green "modulation envelope"
  • carrier now varies between zero and twice the amplitude of the original (unmodulated) carrier

Single Side Band

  • SSB transmission only contains one side band
  • other sideband has been eliminated and the carrier has been suppressed
  • very important to ensure that the various frequencies contained in the remaining sideband do not inter-modulate (mix) with each other
  • other unwanted frequencies (intermodulation products) would be generated and transmitted
  • very undesirable and must be avoided at all costs otherwise considerable interference will be caused

Two tone test

  • Both of the frequencies must be in the normal microphone input frequency range
  • The tones should have the same amplitude but must not be harmonically related
  • 700Hz and 2kHz would be suitable frequencies

three main types of radio communication:

  • Speech - via microphone
  • Morse - via Morse key or keyer (automatic or semi-automatic)
  • Digital - via keyboard on teleprinter or computer Procedures for each of these types will be explained.