RF Antenna Systems & Distribution Physics
1. Antenna Transduction and Polar Patterns In professional wireless deployment, the antenna is the primary transducer between the electromagnetic field and the electronic signal. We primarily utilize two types: the Omnidirectional Dipole and the Log-Periodic Dipole Array (LPDA).
Dipole Antennas: Radiate energy in a 360-degree toroidal (donut) pattern. While useful for short-range coverage, they offer 0 dB of passive gain and are susceptible to interference from all directions.
LPDA (Paddle) Antennas: Utilize multiple elements to create constructive interference in a single forward direction. This provides "Forward Gain" (typically 5–7 dB), which effectively increases the "reach" of the receiver without needing active amplification. Understanding the "Front-to-Back Ratio" of your paddle is essential for rejecting noise sources like LED walls located behind the antenna position.
2. Diversity Logic and Phase Centers Diversity reception is the industry standard for combating "Multipath Fading." Multipath occurs when the RF signal reflects off metal surfaces, arriving at the antenna at different times and phases. If the direct and reflected waves arrive 180° out of phase, they cancel out, creating a "null."
Space Diversity: By placing two antennas (A and B) at least one wavelength apart (approx. 60cm at 500MHz), we ensure that the probability of both antennas being in a null simultaneously is mathematically remote ($P \approx 0.001$).
3. Active vs. Passive Distribution and TOI In a rack of 12 wireless channels, you cannot use 24 antennas. We use an Antenna Distribution System (Distro).
Passive Splitters: Every 2-way split results in a 3.5 dB loss of signal power ($10 \log_{10}(0.5) \approx -3.01$ dB plus insertion loss).
Active Distros: Use a pre-amplifier to compensate for this loss. However, these amplifiers introduce Third-Order Intercept (TOI) issues. If the incoming signal is too hot (e.g., a transmitter is 1 meter from the antenna), the active distro itself will generate intermodulation distortion (IM3) products before the signal reaches the receiver.
4. Cable Loss and Unity Gain Coaxial cable is a "leaky" pipe. For a 30-meter run of RG-8X, you may lose 6 dB of signal. To combat this, we use In-Line Boosters. A critical engineering failure is "Over-Boosting." If you have 6 dB of cable loss and you set your booster to +12 dB, you have raised the noise floor of your entire system by 6 dB. The goal is Unity Gain: The booster should only replace what the cable lost.
II. Practical Lab: The Antenna Signal Chain
Tool: Wireless Workbench (WWB) / Master Formula Table.
Tasks: * Calculate the total loss for a 40-meter run of LMR-400 cable at 600 MHz.
Model a "Master/Slave" distribution system for 32 channels.
Using the IMD Calculator, determine the IM3 products for two transmitters at 550.00 and 552.00 MHz and check if they land on your distribution amp's noise floor.
III. Daily Assessment (Monday)
Q1: If an antenna has 6 dB gain and the cable has 4 dB loss, what is the net gain?
Q2: Why does "High Power" on a transmitter often lead to worse performance in a distribution system?