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Subelement T4
AMATEUR RADIO PRACTICES
Section T4A
Station setup: connecting a microphone, a power source, a computer, digital equipment, an SWR meter; bonding; Mobile radio installation
Which of the following is an appropriate power supply rating for a typical 50 watt output mobile FM transceiver?
  • 24.0 volts at 4 amperes
  • 13.8 volts at 4 amperes
  • 24.0 volts at 12 amperes
  • Correct Answer
    13.8 volts at 12 amperes

Most (okay, pretty much all) amateur radio systems designed for mobile installation will also run on mobile power, which is 13.8 volts (the nominal voltage of an automotive battery), ±15%. Thus, the correct answer will also contain 13.8 volts, as that's the power supply rating you will need. The two 24 volt answers can be discarded.

As for the amperage, W=V*I, so of the two appropriate voltage answers:

Incorrect: 13.8 * 4 = 55.2 Watts

  • Due to efficiency losses in the radio, This is not enough power for a radio to output 50 watts RF

Correct: 13.8 * 12 = 165.6 Watts

  • This rated wattage allows for losses in the radio and RF Circuitry.

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Which of the following should be considered when selecting an accessory SWR meter?
  • Correct Answer
    The frequency and power level at which the measurements will be made
  • The distance that the meter will be located from the antenna
  • The types of modulation being used at the station
  • All these choices are correct

An SWR meter measures the Standing Wave Ratio in the antenna system; the reason that is useful is that it is an indicator of how closely the impedance of the transceiver matches the impedance of the antenna system -- by the Maximum Power Transfer theorem maximum power is transferred when the effective resistance (impedance in this case) of both systems are equal, which means that the maximum benefit of the antenna system can be obtained when the impedance matches on both sides.

Since we're talking about impedance and power, we only need consider things which affect one or the other. Impedance is basically resistance at a given frequency and power is just whatever the output power are, so the things which the SWR meter might need to care about are going to be those two things.

The distance that the meter will be located from the antenna could affect the reading, but it would affect all SWR meters equally. The type of modulation is going to be immaterial except as it affects the power. Thus those two distracters are just that!

When you buy an SWR meter you'll find that they have power limits (too much power could blow it out or at least give you a useless reading) and frequency limits; most commonly you'll find a SWR meter which works at e.g. 0 to 150MHz and handles 100 Watts; thus if you want to operate on 440 (70cm) or you are using a power amplifier and 500 watts you need to make sure that you buy a SWR meter which can handle the correct frequency and power.

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Why are short, heavy-gauge wires used for a transceiver’s DC power connection?
  • Correct Answer
    To minimize voltage drop when transmitting
  • To provide a good counterpoise for the antenna
  • To avoid RF interference
  • All these choices are correct

Power connections should be:

  • Heavy gauge because heavier gauge wires can conduct more current and have less resistance for a given length.

  • Short as possible because even heavy gauge wire has some resistance, and the longer the wire the more resistance the connection will have.

The more resistance the connection has the more the voltage will drop. So to get the least resistance, and the least voltage drop, you want short heavy gauge wire.

A radio or other electronic device may fail to operate properly or even be damaged if a supply line is so long that the voltage on the other end is below a minimum required level.

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How are the transceiver audio input and output connected in a station configured to operate using FT8?
  • To a computer running a terminal program and connected to a terminal node controller unit
  • Correct Answer
    To the audio input and output of a computer running WSJT-X software
  • To an FT8 conversion unit, a keyboard, and a computer monitor
  • To a computer connected to the FT8converter.com website

Memory aid: FT8 Is a Weak Signal mode, and runs on WSJT software.

Test Aid: The correct answer is the only one answering the question of how the audio input and output are connected, and is the only answer containing the words "audio input and output".

FT8, or "Franke-Taylor design, 8-FSK modulation", is a digital signal designed to take advantage of less-than-optimal propagation and allow for contacts where they could not otherwise be made. The software designed for these weak signal modes is WSJT-X, or "Weak Signal Joe Taylor - 10" (Joe Taylor K1JT being the author of the program). This program uses a computer connected to the audio input and output of any radio to transmit and receive, then decode, or transmit short messages.

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Where should an RF power meter be installed?
  • Correct Answer
    In the feed line, between the transmitter and antenna
  • At the power supply output
  • In parallel with the push-to-talk line and the antenna
  • In the power supply cable, as close as possible to the radio

As RF power in this context only travels through the feed line from the transmitter, “In the feed line between the transmitter and the antenna” is the only logical answer.

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What signals are used in a computer-radio interface for digital mode operation?
  • Receive and transmit mode, status, and location
  • Antenna and RF power
  • Correct Answer
    Receive audio, transmit audio, and transmitter keying
  • NMEA GPS location and DC power

Just remember that a large number of digital modes (almost all of the ones used on HF, if not all) can be encoded and decoded by the simple sound card in a reasonably modern computer. For example, RTTY uses AFSK and can easily be decoded from audio by modern computers. Even Slow Scan TV can be encoded and decoded as audio by desktops, laptops, tablets, and smartphones!

Therefore, the only answer here that makes any sense is receive audio, transmit audio, and push-to-talk, the last one so computer software can activate and deactivite the transmitter without the need to do so manually.

Often even a computer's microphone and speaker next to the microphone and speaker of the radio is sufficient to encode and decode digital signals as audio, but obviously this method is more error prone so using audio cables is highly preferable for avoiding errors due to noise in the room.

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Which of the following connections is made between a computer and a transceiver to use computer software when operating digital modes?
  • Computer “line out” to transceiver push-to-talk
  • Computer “line in” to transceiver push-to-talk
  • Correct Answer
    Computer “line in” to transceiver speaker connector
  • Computer “line out” to transceiver speaker connector

Memory Aid: Line-in Listens to a speaker (Line in, Listen).

With digital modes, your computer needs to listen in order to decode the signal.

Push-to-Talk signals do not connect to Line signals, so both of those answers can be disregarded.

"Line Out" would connect to the transceiver's Mic In connection and not the speaker.

Therefore, think of the audio path for the correct answer: Audio Out of the receiver (normally via the speaker) needs to go into the computer to be decoded.

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Which of the following conductors is preferred for bonding at RF?
  • Copper braid removed from coaxial cable
  • Steel wire
  • Twisted-pair cable
  • Correct Answer
    Flat copper strap

RF current tend to flow along the surface of conductors. To provide the best conductor for RF current flow you need a large surface area vs a large cross area. So Flat straps make for better bonding than round wire.

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How can you determine the length of time that equipment can be powered from a battery?
  • Divide the watt-hour rating of the battery by the peak power consumption of the equipment
  • Correct Answer
    Divide the battery ampere-hour rating by the average current draw of the equipment
  • Multiply the watts per hour consumed by the equipment by the battery power rating
  • Multiply the square of the current rating of the battery by the input resistance of the equipment

This formula gives you a good idea of roughly how long your battery would be able to perform. As an example, if you have a 24AH battery and a radio that draws 1A on receive and 12A on transmit:

24AH / 1 A = 24 hours of receive-only.

and

24AH / 12A = 2 hours of transmit-only

You can see by this that you have somewhere between 2 and 24 hours of use for the given setup, depending on how much you transmit, and at what power output (if adjustable).

NOTE: This formula does not take into consideration how much charge you plan to leave on your battery before recharging, heat/cold exposure, and other environmental factors that may affect operational capacity of the battery.

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What function is performed with a transceiver and a digital mode hot spot?
  • Correct Answer
    Communication using digital voice or data systems via the internet
  • FT8 digital communications via AFSK
  • RTTY encoding and decoding without a computer
  • High-speed digital communications for meteor scatter

Digital Mode hot spots are semi-automatic transceivers which provide a gateway between radio frequencies and other stations across the internet.

As an example, you could program your DMR radio to use a DMR hotspot and then you use DMR on your radio however you would normally use it and as long as you are in range of that hotspot you can communicate with other DMR stations across the internet. This becomes even more interesting when you realize that some of those stations may be using not hotspots but full DMR repeaters, which enables some of the following interesting scenarios:

  • You go out of town and take your hotspot; you can then call across the hotspot to talk to people who are using a DMR repeater near your house.
  • You are located in an area with few active repeaters, so you use your DMR hotspot to let you communicate with people on an active repeater in another area
  • You set up a talk group with the other people in the online ham radio group you are a member of; anyone with a hotspot can get on and talk to anyone else with a hotspot or local DMR repeater who has their radio on and connected.

While I've used DMR for these examples similar things can be done with most digital modes including D-Star and System Fusion and some hotspots can even work on any of those modes.

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Where should the negative power return of a mobile transceiver be connected in a vehicle?
  • Correct Answer
    At the 12 volt battery chassis ground
  • At the antenna mount
  • To any metal part of the vehicle
  • Through the transceiver’s mounting bracket

A mobile transceiver can draw many Amperes of current when transmitting. That current needs to come directly from the battery. Using another electrical path could reduce the current available and hinder the tranceiver's performance.

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What is an electronic keyer?
  • A device for switching antennas from transmit to receive
  • A device for voice activated switching from receive to transmit
  • Correct Answer
    A device that assists in manual sending of Morse code
  • An interlock to prevent unauthorized use of a radio

An electronic keyer is the modern fancy replacement for the traditional telegraph key, or in other words a device that assists in manual sending of Morse code.

Electronic keyers can have a variety of functions including:

  • Separate keys for sending dots and dashes at a specified rate.
  • The ability to set a limited transmission rate (in WPM) while allowing the operator to "get ahead" of the current transmission by buffering up additional code or word spaces to be sent in sequence after previously entered code is sent.
  • The ability to continually send dots or dashes by holding down a key so that fewer hand movements are required to transmit a series of dots or dashes.
  • The ability to combine some functions with the use of a tradtional key that is plugged into the electronic keyer.

Often they are hardware devices but they can also be computer or smartphone software applications that provide these and other functions.

Electronic keyers.... They just might save you from carpel-tunnel syndrome!

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