Question 1

What is free-space path loss?

Accepted Answer

FSPL is the attenuation of radio energy between two isotropic antennas in free space with no obstructions. It increases with both frequency and distance, following an inverse-square law.

Question 2

When should I use FSPL vs. a more complex propagation model?

Accepted Answer

FSPL is a best-case baseline. For real-world links add terrain, foliage, rain, and multipath fading corrections (e.g. ITU-R P.676 for atmospheric gases, Okumura-Hata for urban cells).

Question 3

What is EIRP?

Accepted Answer

Effective Isotropic Radiated Power is the product of transmitter power and antenna gain referenced to an ideal isotropic radiator. It represents the power that would need to be radiated by an isotropic antenna to produce the same signal strength in the direction of maximum gain.

Question 4

How much link margin do I need?

Accepted Answer

A common rule of thumb is 10-20 dB of margin for outdoor links to account for fading, rain attenuation, and component aging. Safety-critical links may require 30 dB or more.

Question 5

Why is a real dipole shorter than the calculated length?

Accepted Answer

The theoretical length assumes an infinitely thin conductor. Real wire has finite diameter, which increases capacitance at the ends and lowers the resonant frequency. A typical shortening factor is 0.95 to 0.98.

Question 6

What is the gain of a half-wave dipole?

Accepted Answer

A half-wave dipole has approximately 2.15 dBi gain (0 dBd). It radiates in a toroidal pattern perpendicular to the element axis.

Question 7

Why is 50 ohms the standard impedance for RF traces?

Accepted Answer

50 ohms is a compromise between minimum attenuation (~77 ohms in air) and maximum power handling (~30 ohms). Most RF test equipment, connectors (SMA, N-type), and coaxial cables are standardised at 50 ohms.

Question 8

How accurate are the Hammerstad-Jensen equations?

Accepted Answer

The closed-form approximation is typically within 1% of full-wave electromagnetic simulation for W/h ratios between 0.1 and 100. For very narrow or very wide traces, use a 2.5D EM solver.

Question 9

What does 0 dBm mean?

Accepted Answer

0 dBm equals exactly 1 milliwatt. It is the reference point for the dBm scale. Every 3 dB increase doubles the power, so 3 dBm = 2 mW, 10 dBm = 10 mW, 30 dBm = 1 W.

Question 10

Why is 50 ohms used as the default impedance?

Accepted Answer

50 ohms is the standard characteristic impedance for most RF systems, coaxial cables (RG-58, LMR-400), and test equipment. Telecom systems sometimes use 75 ohms instead.

Question 11

Why does the first stage dominate system noise figure?

Accepted Answer

In the Friis formula, each subsequent stage's noise contribution is divided by the cumulative gain of all preceding stages. A high-gain LNA in the first position reduces later stages' contributions to negligible levels.

Question 12

What is a good noise figure for an LNA?

Accepted Answer

Modern LNAs achieve 0.3-1.5 dB noise figure depending on frequency. Sub-1 dB is typical for satellite and radio astronomy receivers. Consumer Wi-Fi front-ends are usually 2-4 dB.

Question 13

What VSWR is acceptable for most RF systems?

Accepted Answer

A VSWR below 2.0:1 (return loss > 9.5 dB) is acceptable for many systems. For precision work, VSWR below 1.5:1 (return loss > 14 dB) is preferred.

Question 14

What is the difference between return loss and mismatch loss?

Accepted Answer

Return loss measures how much power is reflected back toward the source. Mismatch loss measures how much less power reaches the load compared to a perfect match. At VSWR 2:1, return loss is 9.5 dB but mismatch loss is only 0.51 dB.

Question 15

When should I use a Pi attenuator vs a Tee attenuator?

Accepted Answer

Both provide the same attenuation and impedance match. Pi pads are preferred when you need lower shunt impedance (good for high-frequency decoupling). Tee pads are preferred when lower series resistance is needed.

Question 16

What are the power handling limits of a resistive attenuator?

Accepted Answer

Power handling depends on resistor wattage ratings. At low attenuation, most power passes through R2; at high attenuation, R1 and R3 dissipate most power. Use appropriately rated resistors for your application power level.

Question 17

When should I use a Tee attenuator vs a Pi attenuator?

Accepted Answer

Both provide identical attenuation and impedance matching. Tee pads have lower series resistance and higher shunt resistance, making them better suited for applications where DC bias must pass through the attenuator.

Question 18

Can I cascade multiple attenuator pads?

Accepted Answer

Yes. Because each pad is impedance-matched, cascading N pads of X dB gives N*X dB total attenuation. This is useful when standard resistor values don't match a single-pad design.

Question 19

Why is 60% Fresnel zone clearance the standard?

Accepted Answer

At 60% clearance of the first Fresnel zone, diffraction loss is negligible (< 0.5 dB). Below this threshold, signal attenuation increases rapidly. Many link planners use 60% as the minimum and target 80-100% for critical links.

Question 20

How does frequency affect Fresnel zone size?

Accepted Answer

Higher frequencies produce smaller Fresnel zones because wavelength is shorter. A 900 MHz link has a first Fresnel zone roughly 3x wider than a 5.8 GHz link at the same distance, making lower frequencies more tolerant of near-path obstructions.