Measurements and installation in RF
In radio frequency you cannot trust your eyes or the multimeter: everything is measured with specific instruments, and a good part of the job consists of knowing what to measure, with which instrument, and without destroying anything in the attempt.
01The instruments
| Instrument | What it measures |
|---|---|
| Spectrum analyzer | Amplitude as a function of frequency. It is the central instrument: it shows the carrier, the sidebands, the harmonics, the spurious emissions and the noise floor. |
| RF wattmeter | Actual power delivered, typically with directional elements that separate forward power from reflected power. |
| VSWR meter | The ratio between the forward and reflected wave. It tells you whether the antenna and the line are matched. |
| Antenna analyzer | Sweeps in frequency and plots VSWR and impedance. It lets you see where an antenna actually resonates. |
| Dummy load | A noninductive 50 Ω resistor able to dissipate power. It replaces the antenna to test without radiating. |
| RF generator | Delivers a signal of known level and frequency, with adjustable modulation. It is the reference for measuring receivers. |
| Attenuators and couplers | They lower the level by a known amount or take a sample of the signal, so that high power is not brought to the instrument. |
| Frequency counter | Verifies the emission frequency with precision, a legal requirement in licensed services. |
The input of a spectrum analyzer typically withstands +30 dBm (1 W), and its preamplifier much less. Connecting the output of a 25 W transmitter directly to it destroys it on the spot, and the repair costs more than the whole lab.
A suitable directional coupler or power attenuator is always inserted, the total attenuation is verified before turning on the transmitter, and you start measuring with the highest input range.
02Measuring a transmitter
| Measurement | How it is done | What to look for |
|---|---|---|
| Output power | Wattmeter between the equipment and the dummy load. | That it matches the specification, and that it stays stable over time. |
| Frequency | Frequency counter or analyzer marker. | Error within tolerance. An off-frequency transmitter interferes with the neighboring channel. |
| Harmonics | Spectrum analyzer, with attenuation and a low-pass filter. | That they are at least 40 to 60 dB below the carrier. |
| Occupied bandwidth | Analyzer, measuring the width that contains 99 % of the power. | That the emission fits in the assigned channel. |
| FM deviation | Analyzer or deviation meter, with a test tone. | That it does not exceed what is allowed: overdeviating intrudes on the adjacent channels. |
| AM modulation index | Oscilloscope on the envelope, or analyzer. | Close to 80 to 90 %, without overmodulating. |
| System VSWR | In-line VSWR meter, with the antenna connected. | Below 2:1. It is the final check of every installation. |
Harmonics fall at 2f, 3f, 4f… and come from the nonlinearity of the power stages; they are attenuated with low-pass filters at the output. Spurious emissions appear at arbitrary frequencies, generally from internal mixing products or poorly shielded oscillators, and they are corrected by attacking the cause inside the equipment. Telling them apart is the first step of the diagnosis.
03Measuring a receiver
| Parameter | What it means | Typical value |
|---|---|---|
| Sensitivity | The minimum input level that produces a usable output. In FM it is specified with a 12 dB SINAD. | 0.2 to 0.5 µV |
| Selectivity | How much it rejects signals from neighboring channels. | 60 to 80 dB |
| Image rejection | How much it attenuates the image frequency, which the mixer would convert to the same IF as the desired signal. | > 60 dB |
| Dynamic range | Difference between the weakest and the strongest signal it can handle without distorting. | 70 to 90 dB |
| Noise figure | How much noise the receiver itself adds. It determines the floor below which nothing can be received. | 1 to 6 dB |
In a superheterodyne receiver, the mixer converts to the intermediate frequency both the desired signal and another one located two times the IF away: the image. If the input filter does not reject it, a distant station on that frequency shows up on top of the one you want to hear. This is the reason the IF is chosen high and why double-conversion receivers exist.
With a 10.7 MHz IF and a station at 100 MHz, the local oscillator is at 110.7 MHz and the image falls at 121.4 MHz: it must be attenuated before the mixer.
04Measuring the line and the antenna
- VSWR versus frequency: shows where the antenna resonates and whether it is in the right band.
- Return loss, in dB: the same information expressed differently. A return loss of 14 dB is equivalent to a VSWR of 1.5:1.
- Cable loss: measured with the far end open and shorted, or by comparing power at both ends.
- Distance to fault: modern analyzers calculate how many meters away the problem is, just like an OTDR on fiber.
- High and constant VSWR across the whole band: connector or cable problem.
- VSWR with a minimum shifted in frequency: the antenna is the wrong size, or the surroundings detune it.
- VSWR that changes when the cable is moved: loose connector or damaged braid.
- VSWR that gets worse with rain: water inside the coax or the connector.
The VSWR measured at the equipment and the one measured at the base of the antenna are not the same: the cable attenuation masks the reflection and makes the installation look better. Reference measurements are taken as close as possible to the antenna, and where each value was measured is documented.
05Safety and equipment protection
- Do not stay in front of a directional antenna with the transmitter on. At sites with several active antennas, request a shutdown before climbing.
- Respect the safety distances for radio-frequency exposure set by the regulations in force. RF radiation is not ionizing, but it heats tissue.
- Work at height with a harness, a double lanyard and never alone. Never climb with wind or a storm in sight.
- Discharge the power-supply capacitors before touching a high-power unit: large transmitters work at lethal voltages.
- Never transmit without an antenna or a dummy load.
- Attenuate before connecting any instrument to a power output.
- Grounding of the mast and a surge arrester on the line.
- Connectors sealed against water, and proper torque when tightening: overtightening ruins the connector.
- Electrostatic discharge: low-noise amplifiers with gallium arsenide transistors are destroyed by a spark you cannot even feel.
A well-delivered site includes: measured frequency and power, VSWR with the date and the measurement point, output spectrum, received level of the link, photos of the sealing and of the pointing, and the list of equipment with serial numbers. That folder is what makes it possible, two years later, to know whether something changed or whether it was always like this.
06In the lab
With a dummy load, an attenuator and a spectrum analyzer, measure power, frequency, occupied bandwidth and harmonic level of a low-power transmitter. Repeat with and without a low-pass filter at the output and compare the level of the second harmonic.
With an RF generator modulated with a 1 kHz tone, lower the level until the audio signal is no longer acceptable. Record that level in dBm and convert it to microvolts. Compare two different receivers and the manufacturer's specification.
With an antenna analyzer, sweep an antenna of known band and plot VSWR against frequency. Identify the resonant frequency and the useful bandwidth. Then modify the antenna —shorten it, bring a metal object near it— and observe how the curve shifts.
Prepare several cables with known faults —loose connector, cut braid, poorly made splice— and have them diagnosed by measuring VSWR and loss, without opening them. It is the exercise that most resembles real work: one symptom and one instrument.
07Common errors
| Error | Consequence |
|---|---|
| Connecting the transmitter straight to the analyzer | The instrument input is destroyed. Always use an attenuator or coupler. |
| Transmitting without an antenna or load | The transmitter's final stage is destroyed. |
| Measuring VSWR at the equipment and not at the antenna | Cable loss masks the reflection: the installation looks better than it is. |
| Confusing a harmonic with a spurious emission | The problem is sought where it is not: one is solved with a filter and the other inside the equipment. |
| Measuring power with a multimeter | It does not measure RF. The values are made up. |
| Overdeviating in FM to “sound louder” | The adjacent channels are intruded on and the signal is distorted in the receivers. |
| Not documenting the commissioning | At the next problem there is nothing to compare against, and the whole diagnosis starts from zero. |
08Self-assessment
What must be inserted before connecting a spectrum analyzer to a transmitter?
A power attenuator or a directional coupler, verifying the attenuation before turning on. The instrument input tolerates only very small powers.
What is a dummy load for?
To test a transmitter without radiating: it is a noninductive 50 Ω resistor able to dissipate the power of the equipment.
Difference between a harmonic and a spurious emission.
A harmonic falls at an exact multiple of the operating frequency and comes from the nonlinearity of the stages; a spurious emission appears at an unrelated frequency, generally from internal mixing products.
How is the sensitivity of an FM receiver specified?
As the input level, in µV or dBm, that produces a 12 dB SINAD at the audio output.
What is the image frequency and where does it fall?
It is the frequency that the mixer converts to the same IF as the desired signal. It is two times the IF away from the signal: with a 10.7 MHz IF and a station at 100 MHz, the image is at 121.4 MHz.
What does a high and constant VSWR across the whole band indicate?
A cable or connector problem, not an antenna problem. If the problem were sizing, a minimum shifted in frequency would appear.
Why is the VSWR measured at the equipment better than the one measured at the antenna?
Because the cable attenuation also reduces the reflected wave. The worse the cable, the better the VSWR looks: it is a dangerous mirage.
What precautions does working at a site with several active antennas require?
Request the shutdown of the transmitters before climbing, respect the safety distances, do not stand in front of directional antennas and use a harness with a double lanyard.
Convert a return loss of 14 dB to VSWR.
|Γ| = 10−14/20 = 0.2, so VSWR = (1 + 0.2)/(1 − 0.2) = 1.5:1.
What must the commissioning documentation include?
Measured frequency and power, VSWR with the date and measurement point, output spectrum, received level, photos of the sealing and of the pointing, and the equipment with its serial numbers.