A 25 W BUC and a 16 W BUC can deliver nearly the same usable power. That sounds wrong until you read how each one is rated. On a datasheet, the wattage can refer to saturated output, the 1 dB compression point, or a linear output tied to a specific ACPR figure. Compare the labels alone and you can easily pay for power your link cannot use, or fall short of the power it needs. This article works through BUC sizing in order: start from the required EIRP, add the margin the link needs, then match the result to the right rating.
Step 1: Work Out the Power the Link Needs
BUC sizing starts with the uplink EIRP from your link budget, which normally comes from the satellite operator or the network designer for your carrier. Subtract the antenna gain, then add the feed loss and the margin:
P_BUC (dBm) = EIRP (dBW) + 30 - G_tx (dBi) + L_feed (dB) + M (dB)
G_tx (dBi) = 10 log10[ efficiency x (pi x D / wavelength)^2 ]For example, take a single Ku-band carrier that needs 45 dBW of clear-sky EIRP. Assume 14.25 GHz, 60% aperture efficiency, 0.5 dB of feed loss, and 6 dB of uplink power control range for rain fade. These values are assumptions for this example only. Your rain margin should come from your site’s climate and availability target, typically using a model such as ITU-R P.618.
| Antenna | Tx gain | Clear-sky power at BUC | With 6 dB margin |
|---|---|---|---|
| 0.96 m | 40.9 dBi | +34.6 dBm (2.9 W) | +40.6 dBm (11.5 W) |
| 1.2 m | 42.8 dBi | +32.7 dBm (1.9 W) | +38.7 dBm (7.4 W) |
| 1.8 m | 46.4 dBi | +29.1 dBm (0.8 W) | +35.1 dBm (3.2 W) |
The antenna is the strongest lever in this table: moving from 0.96 m to 1.8 m cuts the required BUC power by more than 70%. A larger dish also has a narrower beam, which helps with the off-axis EIRP density limits set by satellite operators and regulators. A very small antenna cannot always make up its gain with a bigger BUC, so check those limits before settling on the antenna and BUC pair.
If the BUC carries more than one carrier, the required power must be the total for all carriers. Two equal carriers need 3 dB more than one, and they also generate intermodulation products, so more back-off may be needed. Where a datasheet gives an IM3 figure, check it at your planned total power. The NJT8319, for example, specifies IM3 of -28 dBc typ. at a total power of up to +39 dBm, which is 3 dB below its P1dB.
Step 2: Know What the Wattage Means
The result from Step 1 is a linear power requirement, because the carrier must stay within its spectral mask. As an amplifier approaches saturation, it compresses the signal and spectral regrowth spills into adjacent channels. BUC ratings describe different points on this curve:
- Saturated output (Psat): The maximum power the amplifier can reach. It is not a linear operating point for a modulated carrier, so check the output at which the ACPR is specified.
- Output at 1 dB compression (P1dB): The point where the gain has dropped 1 dB from its linear value. It is a useful reference, but it does not by itself tell you the spectral performance of your carrier.
- Rated or linear output: The power available at a stated ACPR, measured under a stated modulation condition. This is the closest to what the link needs.
The Nisshinbo Micro Devices Ku-band BUCs we handle include all three styles, which makes them a useful illustration:
| Model | Label | Rated by | Linearity reference (typ.) | DC power (typ.) | Weight |
|---|---|---|---|---|---|
| NJT8304 | 4 W | P1dB +36 dBm min. | ACPR -26 dBc @ +35.5 dBm | 28 W @ +36 dBm | 500 g |
| NJT8376 | 6 W | P1dB +37.8 dBm min. | ACPR -26 dBc @ +37 dBm | 34 W @ +37.8 dBm | 540 g |
| NJT8306 | 6 W | P1dB +37.8 dBm min. | ACPR -30 dBc @ +37.8 dBm | 40 W @ +37.8 dBm | 1.2 kg max. |
| NJT8334 | 8 W | Rated +39 dBm | +39 dBm @ -26 dBc +38 dBm @ -30 dBc | 70 W @ rated | 1.8 kg |
| NJT8336 | 16 W | Rated +42 dBm | +42 dBm @ -26 dBc +41 dBm @ -30 dBc | 110 W @ rated | 1.8 kg |
| NJT8319 | 16 W | P1dB +42 dBm min. | ACPR -28 dBc @ +41 dBm | 160 W @ +42 dBm | 2.4 kg |
| NJT8370 | 25 W | Psat +44 dBm min. @ +25 °C | ACPR -30 dBc @ +42 dBm | 180 W @ +42 dBm | 2.5 kg |
| NJT8371 | 40 W | Psat +46 dBm min. @ +25 °C | ACPR -30 dBc @ +44 dBm | 220 W @ +44 dBm | 4.2 kg |
Read across the table and the labels stop lining up. The 25 W NJT8370 quotes its ACPR at +42 dBm, which is about 16 W, and the 40 W NJT8371 quotes its figure at +44 dBm, which is about 25 W. Meanwhile, the NJT8336 reaches +41 dBm at -30 dBc with a label of only 16 W. Its GaN HEMT and linearizer are what allow it to run so close to its rated output.
One caution: an ACPR value only means something together with its test condition. The NJT8334 and NJT8336 datasheets state QPSK at 1 Msps with a 1 MHz offset. For a strict comparison between series, ask for the figures under the same modulation condition, ideally your own.
Step 3: Match the Requirement to a Shortlist
Now compare the requirement from Step 1 with the linear output at the ACPR your network demands. For our example, the first candidates would be:
| Antenna | Required | If -26 dBc is acceptable | If -30 dBc is required |
|---|---|---|---|
| 0.96 m | +40.6 dBm | NJT8336 (+42 dBm) | NJT8370 (+42 dBm) NJT8336 (+41 dBm, thin margin) |
| 1.2 m | +38.7 dBm | NJT8336 (+42 dBm) NJT8334 (+39 dBm, thin margin) | NJT8336 (+41 dBm) |
| 1.8 m | +35.1 dBm | NJT8376 (+37 dBm) NJT8304 (+35.5 dBm, thin margin) | NJT8306 (+37.8 dBm) NJT8334 (+38 dBm) |
Notice how the same 1.2 m station can move to a different model simply because the ACPR requirement changes by 4 dB. Similar output can also come at a very different cost: at 1.8 m and -30 dBc, the NJT8306 draws 40 W typ. and weighs 1.2 kg max., while the NJT8334 draws 70 W typ. and weighs 1.8 kg. The linearity values above are typical figures, so leave some margin instead of designing right up to them, and confirm the guaranteed limits with the datasheet.
Step 4: Look Beyond Output Power
Once two or more models meet the requirement, the rest of the datasheet usually decides the selection:
- DC power: At +42 dBm, the NJT8336 draws 110 W typ. and the NJT8319 draws 160 W typ. In continuous operation, that 50 W difference comes to about 440 kWh per year, per station. It also affects your supply sizing and any solar or battery system.
- Power supply and voltage: The input range varies: +12 to +30 VDC for the NJT8304, NJT8376 and NJT8306, +18 to +54 VDC for the NJT8334, +36 to +54 VDC for the NJT8336, and +36 to +60 VDC for the NJT8319, NJT8370 and NJT8371. Check the maximum consumption and the voltage drop along the cable run, then choose the supply. Our VSAT page lists the 150 W, 250 W and 500 W AC/DC units.
- Temperature: Separate the operation guarantee from the performance guarantee. The NJT8336 operates up to +75 °C ambient, but its performance is guaranteed up to +60 °C. For the NJT8319, the performance guarantee ends at +55 °C. Size the system for the conditions at your hottest site.
- IF drive level: Make sure the modem can drive the BUC through the cable. With its 62 dB minimum gain, the NJT8336 needs about -20 dBm at its IF input to reach +42 dBm, and the IF cable loss must be added on top of that.
- Size and weight: These matter most on portable, vehicle and maritime antennas, where the BUC is often mounted on the boom or feed.
Summary: A Sizing Checklist
- Get the required EIRP from the link budget and calculate the power at the BUC, including feed loss.
- Add rain margin or the uplink power control range for your site, and the total power for multi-carrier operation.
- Check the operator’s off-axis EIRP density limits for your antenna.
- Confirm whether the wattage on the datasheet means Psat, P1dB or a rated linear output.
- Compare linear output at the same ACPR and under the same modulation condition.
- Finish with DC power, supply voltage, the temperature guarantee, IF drive level, and weight.
If you send us your required EIRP, antenna size, carrier conditions and site environment, we will review the suitable BUC, LNB and power supply combination with our technical department. The full lineup and datasheets are available on our VSAT page.
*Specifications, designs and service contents described are subject to change without notice. Please confirm the details with our sales department. The sizing example is for illustration only and does not replace a full link budget.*