RF Antennas, Splitters and Active Combiners: Polarisation, LPDA Paddles and Helical Antennas.

RF Antennas, Splitters and Active Combiners: Polarisation, LPDA Paddles and Helical Antennas

The performance of a radio-frequency system depends on more than transmitter and receiver quality. The decisive factor in coverage and electromagnetic link stability is the antenna distribution system. Omnidirectional whip antennas mounted at the rear of metal racks introduce RF shadowing, cable clutter and massive losses.

This guide examines RF front-end engineering in detail: antenna types, including omnidirectional antennas, directional LPDA paddles and circularly polarised Helical antennas; propagation physics; $50,\Omega$ coaxial cables; line amplifiers, or boosters; and active RF routing through distribution splitters for receivers and active combiners for in-ear (IEM) transmitters.

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1. RF Antenna Types and Polarisation Patterns

Every antenna in a wireless audio system must be matched to the standard impedance of $50,\Omega$, unlike domestic video/TV, which uses $75,\Omega$. Choosing the right antenna depends on its role, microphone reception or IEM transmission, and the venue’s topology.

   [ Omnidirectional Dipole ]      [ Directional LPDA Paddle ]       [ Helical    (Circular) ]
            ||                            / |                             ( ( ( ( ( ( @
            ||                           /  |                             ( ( ( ( ( ( @
       Coverage  360º                   /___|                             ( ( ( ( ( ( @
     Gain    : 0–2.15 dBi            Gain    : 5–7 dBi                 Gain    : 9–12 dBi

1.1 Omnidirectional Dipole Antenna (Whip / ½ Wave)

• Radiation Pattern: A $360^\circ$ toroid in the horizontal plane.
• Gain: $0\text{ dBd}$ ($2.15\text{ dBi}$).
• Recommended Use: Small stages, or where microphones move around the antenna through $360^\circ$ without a clear front. Mount it away from metal structures or walls, at least $\lambda / 2 \approx 30\text{ cm}$.

1.2 Directional LPDA Antenna (Log-Periodic Dipole Array / “Paddle”)

• Radiation Pattern: Directional cardioid/supercardioid, with approximately $90^\circ - 120^\circ$ forward coverage and $-15\text{ dB}$ rear rejection.
• Gain: $5\text{ dBi} - 7\text{ dBi}$.
• Recommended Use: Microphone reception from FOH or the sides of the stage. Forward gain compensates for coaxial cable losses, and rear rejection eliminates interference sources behind the antenna.

1.3 Helical Antenna (Circular Polarisation)

• Radiation Pattern: A narrow, high-gain directional beam of $60^\circ$, with gain of $9\text{ dBi} - 14\text{ dBi}$.
• Polarisation: Helical circular.
• Why It Matters for IEM: In a handheld microphone or instrument bodypack, the antenna remains vertically linearly polarised most of the time. However, in-ear receiver bodypacks hanging from the performer’s belt constantly change spatial orientation as the musician moves or leans. If a transmitting antenna is vertically linearly polarised and the bodypack becomes horizontal, polarisation mismatch occurs, causing a signal drop of up to $-20\text{ dB}$. Circular polarisation emits the electromagnetic field in a continuous corkscrew, delivering identical power regardless of the bodypack antenna’s orientation.

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2. $50,\Omega$ Coaxial Cables and Line Loss Calculations

High-frequency RF signals in the UHF range experience severe attenuation as they travel through coaxial cables. $50,\Omega$ cables, such as RG-58, RG-213 and LMR-400, are mandatory. Never use $75,\Omega$ RG-59 television cable, because it causes impedance mismatch and reflected waves (SWR).

   [ Signal Loss by Coaxial Cable Length at            600 MHz ]
   RG-58 (Thin/Flexible)  : ~32 dB / 100 m  ---> Unsuitable for runs   > 10 m
   RG-213 (Pro Standard)  : ~19 dB / 100 m  ---> Suitable for runs     < 15 m
   LMR-400 (Low Loss    ) : ~10.8 dB / 100 m ---> Recommended for long runs       (20-50 m)

2.1 RF Link Budget Equation

To ensure that the signal reaches the receiver with sufficient margin:

$$P_{\text{rec}} = P_{\text{trans}} + G_{\text{antena}} - L_{\text{cable}} + G_{\text{booster}}$$

Where:

• $P_{\text{rec}}$: Power at the receiver input (dBm).
• $G_{\text{antena}}$: Antenna gain (dBi).
• $L_{\text{cable}}$: Total coaxial cable loss (dB).
• $G_{\text{booster}}$: Line amplifier gain in dB, set only to compensate for the exact loss $L_{\text{cable}}$, never to overamplify signals from the air.

Overamplification Warning: Adding a $+12\text{ dB}$ line amplifier, or booster, to a short cable with only $2\text{ dB}$ of loss will overload the receiver’s front-end, generating internal intermodulation and blocking the system.

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3. Active RF Routing: Reception Splitters vs IEM Combiners

Operating multiple channels without filling the stage with dozens of antennas requires dedicated RF distribution equipment.

   [ Receiver Antenna Distribution System               (Active Splitter) ]
   Aerial A (LPDA) ---> [ Distributor Active A ] ---> RX1 A, RX2 A, RX3 A, RX4 A
   Aerial B (LPDA) ---> [ Distributor Active B ] ---> RX1 B, RX2 B, RX3 B, RX4 B

   [ IEM Transmitter Combiner System        (Active Combiner) ]
   IEM TX1 (50mW) --\
   IEM TX2 (50mW) ---+---> [ Active IEM Combiner   ] ---> Single Helical Antenna 
   IEM TX3 (50mW) --/

3.1 Active Splitters for Microphone Receivers

• An RF Antenna Distribution System, such as the Shure UA844+ or Sennheiser ASA 214, takes the pair of True Diversity antennas, A and B, and amplifies them internally with unity gain of $0\text{ dB}$ using broadband, low-noise-figure dividers. It distributes the signal to 4 or 5 dual-channel rack receivers.
• It also supplies Bias DC, $12\text{ V}$ direct current, through the coaxial cable to power line amplifiers or active antennas with an integrated booster.

3.2 Active Transmitter Combiners for IEM

• IEM transmitters emit a high-power radio signal of $50\text{ mW}$. If the RF outputs of 4 IEM transmitters are connected using reversed passive splitters, each transmitter’s energy enters the final stage of adjacent transmitters directly, immediately generating destructive third-order intermodulation products.
• An Active Transmitter Combiner, such as the Shure PA421B or Sennheiser AC 41, uses high-power isolation stages between RF inputs, with isolation $> 30\text{ dB}$, and sums the 4 or 8 signals into one coaxial output connected to the helical antenna. It also supplies DC power to the rack transmitters themselves through integrated umbilicals.

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Antenna and RF Infrastructure Setup Checklist

• Antennas positioned at a minimum height of $2.5 - 3\text{ metres}$ above the floor to clear audience heads and human barriers.
• Minimum separation of $1.8 - 2\text{ metres}$ maintained between Antenna A and Antenna B in the True Diversity system.
• Directional LPDA or helical antennas aimed directly at the performer’s working area.
• Coaxial cable type verified: constant impedance of $50,\Omega$ and properly crimped BNC connectors.
• Line boosters set exclusively to the dB value needed to compensate for coaxial cable attenuation.
• IEM transmitters connected exclusively to an active transmitter combiner, never to a receiver distributor or passive T-connectors.