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Correct answer: relative reflected voltage

An SWR/VSWR meter measures forward and reflected signals in a transmission line.

When switched to:

• forward → indicates relative forward voltage (or power)
• reverse → indicates relative reflected voltage

Reflected voltage corresponds to the portion of the signal that is not absorbed by the antenna and is returned toward the transmitter.

• It does not directly display power in watts.
• Forward voltage is measured in the forward position.
• It does not directly measure reflected power in dB.

Therefore, the reverse position indicates relative reflected voltage.

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Correct answer: a DC ammeter

A small hand-held transistorised receiver operates from a DC power supply (such as a battery).

To measure the current drawn from this supply, a DC ammeter is connected in series with the power supply line.

• A power meter measures RF output power.
• An RF ammeter measures radio-frequency current.
• An electrostatic voltmeter measures voltage, not current.

Therefore, the correct instrument is a DC ammeter.

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Correct answer: a low value resistance

When measuring current, an ammeter is connected in series with the circuit.

A good ammeter has:

• very low internal resistance
• minimal effect on circuit current

Therefore, in the circuit it behaves like a small (low value) resistance.

• It is not an insulator (which would block current).
• It is not a perfect conductor (zero resistance is ideal but not real).
• While it does draw a tiny amount of power, that is not its intended function.

Therefore, the meter acts as a low value resistance.

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Correct answer: in series with one of the receiver power leads

To measure current, the meter must be placed in series with the load so that all of the supply current flows through the meter.

In a receiver power circuit, this is done by opening one of the power leads (positive or negative) and inserting the ammeter in series at that point. The same current flows in both leads, so measuring either lead gives the correct value.

• in parallel with both receiver power supply leads would short the supply and could damage the meter or power supply.
• in parallel with one of the receiver power leads would not measure load current and may create a short circuit.
• in series with both receiver power leads is unnecessary and impractical, since the same current already flows through each lead.

Therefore, the current meter should be placed in series with one of the receiver power leads.

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Correct answer: a very low internal resistance

An ammeter is connected in series with the circuit to measure current flow.

To avoid affecting the current it is measuring, the ammeter must introduce as little additional resistance as possible.

A very low internal resistance ensures:

• minimal voltage drop across the meter

• negligible change to the circuit current

• A high or infinite resistance would significantly reduce current flow.

• Matching the resistance of other components is unnecessary.

Therefore, a good ammeter should have a very low internal resistance.

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