1. What Exactly Does a Link Budget Calculate?

The core purpose of a link budget is to calculate one key metric: how much signal power remains after the signal travels from the signal source, through cables, RF components and free space, before reaching the mobile phone.

There are two critical reference points:

-Antenna Port Power: The signal power at the antenna input. This value determines the radiation strength of the antenna.

-Mobile Received Power: The signal power that arrives at the mobile phone after free-space propagation. This value decides whether users can achieve stable communications.

Inaccurate link budget calculations render all subsequent design work purely speculative.

2. From RRU to Antenna Port: Losses of Components and Feeders

All losses in this segment are tangible, including cables, connectors, power dividers and couplers, each with a specific loss value.

2.1 Feeder Loss

Feeder loss depends on two factors: cable diameter and frequency. Thicker cables deliver lower loss, while higher frequencies bring greater loss.

Loss of various feeders at different frequency bands (dB per 100 meters)：

Feeder Specification	800MHz	1800MHz	2600MHz	3500MHz
1/2" Super Flexible Feeder	6	7.5	8.9	11.5
1/2" Standard Feeder	4.8	6.5	7.8	10.2
7/8" Feeder	3	4.2	5.4	7

Feeder loss = 40 × 7.8 / 100 = 3.12 dB.

If the design calculation uses 6 dB per 100 meters by mistake, the loss will be underestimated by 0.72 dB. A 0.7 dB deviation over a 40-meter link seems trivial, yet small errors accumulate at every subsequent stage and result in a large total power deficit.

2.2 Power Divider Loss

Loss of power dividers consists of two components: theoretical distribution loss and insertion loss.

2-way divider: Theoretical loss 3.0dB + Insertion loss 0.1–0.5dB = Total 3.1–3.5dB

2.3 Coupler Loss Loss calculation for directional couplers is split into two paths:

- Through port: Only insertion loss (0.1–0.2dB) is deducted, excluding coupling loss. - Coupled port: Sum of coupling loss plus insertion loss. Loss values of commonly used couplers:

Coupling Value	Through Port Loss	Coupled Port Loss
6dB	0.1dB	6.2dB
10dB	0.1dB	10.3dB
15dB	0.15dB	15.4dB
20dB	0.2dB	20.5dB

Practical Case：Five 10dB couplers are cascaded on the main trunk line, with each coupler bringing 0.1dB insertion loss on the main path, totaling 0.5dB for all five units. However, if you mistakenly treat the coupling value as insertion loss and deduct 10dB per coupler (50dB in total for five), your calculation will be completely wrong. Only the insertion loss should be counted for the main line of a coupler; the coupling value is not included.

2.4 Connector and Jumper Loss

This is the most easily overlooked loss component. Each N-type connector has an insertion loss of roughly 0.05–0.1 dB, and each jumper cable contributes 0.2–0.5 dB loss.

With dozens of connectors in one system, the total accumulated loss can reach 1–2 dB.

Practical Case：In one project, only feeder and component losses were included in the link budget, while connector losses were ignored. On-site testing showed received power was 2.1 dB lower than calculated values.

After troubleshooting, the discrepancy was traced to accumulated loss from 12 connectors, each with 0.1–0.2 dB loss.

3. From Antenna to Mobile Phone: Free-Space Propagation Loss

After signals radiate from the antenna, they keep attenuating while traveling through the air. This attenuation is known as path loss, which accounts for the largest loss component in a link budget.

4. Complete Link Budget Calculation Example (End-to-End)

Scenario：A single floor of an office building covers an area of 40m × 20m with a floor height of 3.5m. One RRU with 10dBm output power is deployed to cover the entire floor via a passive DAS system.

Step 1: Calculate Feeder Loss

Main trunk: 20m 1/2" standard feeder at 1800MHz, loss = 6.5dB per 100m

Step 2: Calculate Component Loss

Cavity 2-way power divider: Theoretical distribution loss 3dB + Insertion loss 0.1dB = 3.1dB

Step 3: Calculate Antenna Port Power

Antenna Port Power = RRU Output Power − Main Trunk Loss − Power Divider Loss − Branch Feeder Loss + Antenna Gain

Step 4: Calculate Free-Space Path Loss (15m to the farthest point)

For 1800MHz, distance d = 0.015km

Step 5: Calculate Mobile Phone Received Power

Received Power at Mobile = Antenna Port Power − Free-Space Path Loss

5. Five Most Common Mistakes in Link Budget Calculation

6. Practical Empirical Values for Direct Application

①Loss per N-type connector: roughly 0.05–0.1 dB

②Loss of 1/2" feeder: approx. 0.65 dB per 10 meters at 1800 MHz, 0.78 dB per 10 meters at 2600 MHz

③Signal attenuation per cavity 2-way power divider: 3.1 dB; per microstrip 2-way power divider: 3.5 dB

1. ④Typical coverage radius of ceiling-mounted antennas: 10–20 meters (varies with transmit power and frequency band)
2. ⑤Recommended antenna port power: ≥ 0 dBm (coverage radius shrinks sharply below 0 dBm)
3. ⑥Basic communication threshold for mobile received power: ≥ −105 dBm (4G/5G industry standard)

7. Contact Information

Anhui Luxun Electronic Technology Co., Ltd. specializes in the R&D, production, sales and after-sales service of RF passive components and antennas, including power dividers, tappers, directional couplers, hybrid couplers, combiners, duplexers, attenuators, dummy loads, connectors, cables, isolators and circulators.

Thank you for your time. Please feel free to reach us via phone or email. Anhui Luxun is ready to provide professional support for you.