Meters: how each connects
| Instrument | Measures | Connection |
|---|---|---|
| Voltmeter | electric potential | in parallel with the component |
| Ammeter | electric current | in series with the component |
| Ohmmeter | resistance | across an unpowered component |
The connection follows from what is being measured. Voltage is a difference between two points, so the meter bridges them. Current flows through, so the meter must be in the path.
A multimeter measures voltage and resistance — and current, on most, through separate terminals.
How an ohmmeter works is worth knowing because it explains its hazards: by applying a small current and measuring the resulting voltage. The meter is a source, not a passive observer.
Two ways to destroy a multimeter
Which of the following can damage a multimeter?
Attempting to measure voltage when using the resistance setting.
On resistance, the meter supplies its own small current and expects to see only its own voltage. Applying an external supply drives current back into a circuit designed for microamps.
The other classic is the mirror image: leaving the probes in the current terminals and measuring voltage. The current input is nearly a short circuit, so across a supply it becomes exactly that, and the internal fuse — if you are lucky — is what stops it.
And when measuring in-circuit resistance: ensure the circuit is not powered. Both for the meter’s sake and because a reading taken on a live circuit is meaningless anyway.
A useful ohmmeter trick
What reading indicates that an ohmmeter is connected across a large, discharged capacitor?
Increasing resistance with time.
The ohmmeter’s small current charges the capacitor. As it charges, less current flows for the applied voltage, so the meter reads a rising resistance. Watching that climb is a quick, no-extra-equipment test that a large capacitor is not short-circuited.
It is also a reminder that the ohmmeter is a source — the same reason you must never connect one to a charged capacitor.
Dummy loads
What is the primary purpose of a dummy load?
To prevent transmitting signals over the air when making tests.
What does a typical RF dummy load consist of?
A 50-ohm non-inductive resistor mounted on a heat sink.
Every word of that description earns its place. 50 ohms so the transmitter sees a perfect match. Non-inductive so it stays 50 ohms at RF rather than becoming reactive. On a heat sink because every watt you transmit becomes heat — the power law again, and a 100 W test is a 100 W heater.
Tuning up into a dummy load is the courteous default, and it is not a transmission, so no identification is required.
Antenna analysers
Which of the following is used to determine if an antenna is resonant at the desired operating frequency?
An antenna analyzer.
An analyser sweeps a range and shows impedance and SWR against frequency, so you see not just whether the antenna is resonant but where — and therefore which way to cut. An SWR meter with a transmitter tells you the value at one frequency and requires transmitting to get it.
Reading SWR
1:1 indicates a perfect impedance match between antenna and feed line. 4:1 indicates an impedance mismatch — nothing more specific than that.
A directional wattmeter can determine SWR, by measuring forward and reflected power separately and comparing them.
Solid-state transmitters reduce output as SWR rises to protect the RF output amplifier transistors. Reflected power appears as voltage and current stress on the finals, and a transistor has no thermal margin to spare.
Power lost in a feed line is converted into heat. It does not disappear and it does not radiate usefully; it warms the cable.
Coaxial cable failure
Which of the following causes failure of coaxial cables?
Moisture contamination.
Water wicks along the braid, ruins the dielectric, and raises loss permanently. It is why sealing outdoor connectors is not optional.
Why should the outer jacket of coaxial cable be resistant to ultraviolet light?
Ultraviolet light can damage the jacket and allow water to enter the cable.
The chain is UV → cracked jacket → water → dead cable. Cable sold for outdoor use has a UV-resistant jacket; cable sold for indoor use does not, and it fails in a few seasons on a roof.
Foam-dielectric coax has less loss per foot than solid dielectric, because air is a better dielectric than polyethylene. The trade-off is that foam is more easily crushed and wicks water faster once breached.
Soldering
Which of the following types of solder should not be used for radio and electronic applications?
Acid-core solder.
Acid flux is for plumbing. In electronics it corrodes joints from the inside for years afterwards. Use rosin-core.
What is the characteristic appearance of a cold tin-lead solder joint?
A rough or lumpy surface.
A good joint is smooth, shiny, and concave where it wets the lead. A cold joint is dull, grainy, and blobby — solder that was melted but never properly wet the metal. It may conduct today and fail intermittently for months, which makes it one of the most annoying faults there is.
The cause is almost always insufficient heat in the joint: heat the work with the iron and feed solder to the work, not to the iron.
Check yourself
- You want to measure the current a radio draws. How do you connect the meter?
- You connect an ohmmeter to a large capacitor and the reading climbs steadily. Faulty?
- Your coax is five years old on a roof and loss has crept up. Likely cause?
Answers
- In series, in the path the current takes — and with the probes in the current terminals, not the voltage ones.
- No — that is exactly what a good capacitor does as the meter’s small current charges it.
- Moisture, most likely after UV damage to the jacket cracked it open.