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Subel. J
6 questions
Subelement J

MF-HF Equip. and Comms:

Section 81

MF-HF Frequency-Simplex, Duplex, Half-Duplex

How are paired SITOR (NBDP) frequencies normally used?

  • Correct Answer
    These are normally used for ARQ communications with coast radio stations.
  • These are normally used for FEC communications with coast radio stations.
  • These are normally used only for Distress communications to limit channel interference.
  • These are normally used for DSC communications with coast radio stations.

SITOR is short for the SImplex Teletype Over Radio. It is used for sending text messages.

SITOR uses 7-bit characters as a bit stream with transfer rate of about 100 baud. It uses the FSK (frequency-shift key) wave modulation, just like the older RTTY. But, SITOR also has error detection, data redundancy to catch some errors, and requests for resending if errors in receiving a text message.

It comes in A and B "flavors." SITOR-A uses the automatic repeat request (ARQ) when errors detected which cannot be fixed. The B version just send each character twice, to assure that transmission was correct. If character unreadable, the second one is used. If both characters are bad, then it knows that there is an error. SITOR-B uses FEC, forward error correction, by sending additional data to be used to catch errors.

The NBDP, or Narrow Band Direct Printing is a telex machine, not for voice but for texting. Sometimes texting is clearer when voice is not send clearly or the receiver does not understand English well.

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How are paired SSB frequencies normally used?

  • These are normally used for FEC communications with coast radio stations.
  • Correct Answer
    These are normally used for J3E communications with coast radio stations.
  • These are normally used for ARQ communications with coast radio stations.
  • These are normally used for DSC communications with coast radio stations.

Paired SSB frequencies normally used for J3E communications with coast radio stations.

In J3E mode, the J stands for a single sideband SSB (no carrier), 3 stands for a single channel for analog data or information, and E stands for voice or sound in a telephony mode, not for telex as data.

Wikipedia: Types of Radio Emissions

"The ITU defines J3E for a SSB and independent-sideband amplitude modulated emissions for telephony or multi-channel voice frequency telegraphy."
ITU Class. of Emissions for Radio Communications

FCC defines types of messages for types of stations and appropriate channels to use.

FCC Ship Radio Stations Definitions

47 CFR § 80.359 - Frequencies for digital selective calling (DSC).

Table of paired frequencies and entities-very good

Also, an excellent article and paired tables are on Blue Seas site MF & HF-SSB Duplex Channels

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For general communications purposes, paired frequencies are:

  • Normally used with private coast stations.
  • Normally used between ship stations.
  • Correct Answer
    Normally used with public coast stations.
  • Normally used between private coast and ship stations.

Normally used with public coast stations.

47 CFR § 80.359 - Frequencies for digital selective calling (DSC).

Table of paired frequencies and entities-very good

Also, an excellent article and paired tables are on Blue Seas site MF & HF-SSB Duplex Channels

For the table of classes of emissions § 80.207, see Code of Federal Regulations Title 47 Telecommunications

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For general communications purposes, simplex frequencies are:

  • Normally used with public coast stations for routine telephone communications.
  • Normally used between ship stations as well as public coast stations simultaneously.
  • Normally used with public coast stations for routine SITOR (NBDP) communications.
  • Correct Answer
    Normally used between ship stations and private coast stations or for ship-ship communications.

Generally, simplex is used for ship to ship calls.
Also, the Channel 16 works in simplex mode so that any vessel can hear and talk to any other vessel within range.

Under normal weather conditions, you can expect about 20 nautical miles coverage.

A nautical mile is the distance of one minute of latitude. So, when you use it, you tell how far on a globe or how fast you are going, considering the curvature of Earth.

The length of one nautical mile is 1,852 meters, or 6,076 feet, or 1.151 miles. When you quote speed on a ship or airplane in knots, you are in fact stating it in nautical miles per hour.

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An ITU simplex channel frequency assignment is defined as:

  • Correct Answer
    Transmit and receive frequencies must be identical.
  • Transmit and receive frequencies must be different.
  • Transmit and receive frequencies may be different, depending on whether communications are ship-shore or ship to ship.
  • Transmit and receive frequencies are different regardless of emission mode.

Simplex – transmission occurs between two transmitters directly using the same radio frequency. Thus, only one party can talk. If you try to talk on your mobile phone while the other party talks, and you are not heard, you are in simplex mode.

See an excellent article on Simplex at
MF & HF-SSB Simplex Frequencies

If you want to get deep level info, the ITU document is very comprehensive (pdf)
ITU-R F.746-4 RADIO-FREQUENCY CHANNEL ARRANGEMENTS FOR RADIO-RELAY SYSTEMS

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An ITU duplex channel frequency is defined as:

  • Transmit and receive frequencies may be identical if communications are ship-shore.
  • Correct Answer
    Transmit and receive frequencies must be different.
  • Transmit and receive frequencies must be identical.
  • Transmit and receive frequencies may be different if communications are ship-ship.

In Duplex mode of sending and and receiving communications, a radio transmits on one frequency and receives on a defined paired another frequency.

ITU has tables of defined pairs of frequencies for sending and receiving.

For example, for Channel 16, if you select channel 1601, then the sending frequency is 17,242.0 kHz while the receiving frequency is 16,360.0 kHz.

Excellent info on MF & HF-SSB Duplex Channels

If you want to get deep level info, the ITU document is very comprehensive (pdf)
ITU-R F.746-4 RADIO-FREQUENCY CHANNEL ARRANGEMENTS FOR RADIO-RELAY SYSTEMS

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