A receiver can only work with the signal its antenna delivers. In between there's cable, connectors, sometimes an amplifier and almost always a distribution unit. Each piece adds or subtracts decibels, and many audio dropouts that seem to come «from the receiver» start in that chain.
The signal budget
antenna gain + amplifier gain − cable loss − connector loss ≈ 0 dB
Shure states that its receivers perform best when the sum of gain and cable loss is around 0 dB. A system with a net loss hears less; one with excess gain also amplifies noise and interference.
Reading the numbers: −3 dB is half the power, −6 dB a quarter and −10 dB a tenth. A cable that loses 12 dB delivers less than a tenth of what the antenna picked up.
Coaxial cable
Coax loss depends on cable type, length and frequency: the higher the frequency, the greater the loss. It's worked out by multiplying the manufacturer's loss per 100 ft (30.5 m) by the actual length.
| Cable | At 450 MHz | At 900 MHz | Use |
|---|---|---|---|
| LMR-400 (low loss, 10 mm) | 2.7 dB | 3.9 dB | Long fixed runs |
| RG-58 (thin, 5 mm) | About 3 to 4 times more than LMR-400 | Same | Short runs only; the exact figure varies by manufacturer |
- Quality matters as much as length: according to Shure, 50 ft of light-grade cable may need more gain than 100 ft of low-loss cable.
- Every connector and adapter adds some loss and a point of failure. Outdoors, moisture inside a connector is a classic cause of loss that appears «out of nowhere».
- Real calculations use the cable maker's datasheet at the working frequency, not a generic table.
Amplifiers: make up, don't reach
An antenna amplifier doesn't make the system reach further. It amplifies everything the antenna picks up, including interference and ambient RF noise, and can't separate the transmitter's signal from the rest. Its job is to make up for cable loss.
- It goes next to the antenna, before the cable: amplifying after the loss no longer recovers the signal-to-noise ratio. That's why active antennas have the amplifier built in.
- Its gain is set to the calculated loss: with −6 dB of cable, +6 dB of gain.
- On very short runs there's too much gain: the excess can overload the receivers and cause intermodulation inside the rack itself. Some active antennas have an overload indicator; if it lights up, lower the gain or move the antenna away from the transmitter.
- Active antennas are usually powered through the same coax from the receiver or distribution unit. Check that this unit supplies the voltage the antenna needs.
Antenna distribution
With several receivers you don't use two antennas per receiver: a distribution unit takes one pair of antennas (A and B) and splits their signal to all of them. This avoids forests of antennas interfering with each other, and many models also power the receivers.
- A passive two-way splitter loses at least 3 dB per output, because it halves the power; a four-way one, at least 6 dB. Active distribution units make up for that loss.
- When cascading (one distribution unit feeding another), follow the manufacturer's gain guidance: stacking active stages without control leads to overload.
- Frequency range: the distribution unit and antennas must cover the range of every connected receiver.
Placement
- Line of sight: the human body absorbs UHF. Antennas go up, above people and the camera crew.
- The two diversity antennas are separated by at least a quarter wavelength (about 12 cm at 600 MHz); more separation helps indoors, with lots of reflections.
- Directional antennas point at the action area, not at the furthest transmitter, and with the same orientation (polarization) as the transmitter antennas.
- Away from metal surfaces, from in-ear and IFB transmit antennas, and from digital equipment that generates RF noise.
| Symptom | Likely cause | What to check |
|---|---|---|
| Dropouts as the actor moves away | Net loss in the antenna chain | Cable length and type, connectors, amplifier gain |
| Noise or dropouts with the transmitter close to the receiver | Overload from too much signal | Excess gain, antenna too close to the action |
| Noise on several channels when everything is on | Intermodulation on set or in the rack | Frequency plan, gain, in-ear antennas next to the receive antennas |
| Dropouts on a single channel | Interference on that frequency or a poorly placed bodypack | Scan that frequency, transmitter antenna position on the body |
Sources and recommended reading
Technical documentation from manufacturers and regulators. Links open external sites.
- UA874 active directional antenna: user guide · Shure ↗
- LMR-400 low loss coaxial cable: datasheet · Times Microwave Systems (via Fairview Microwave) ↗
- Wireless RF: antennas and distribution · Sennheiser ↗
