Sizing a BUC or SSPA without overpaying

The block upconverter or solid state power amplifier is often the most expensive single item at a remote site, the heaviest thing on the mount, and the largest consumer of power. It is also the component most often specified by rounding upwards.

What sets the required power

Output power is the last number in a chain, not the first. It follows from the link budget, and every term in that budget is a design decision.

• Antenna gain. Gain rises with reflector area and with frequency. A larger reflector directly reduces the amplifier needed for the same link.
• Required data rate and modulation. Higher-order modulation carries more bits per hertz but needs a better carrier-to-noise ratio, so it pushes amplifier power up.
• Forward error correction. A stronger code tolerates a worse carrier-to-noise ratio and reduces the power needed, at the cost of throughput.
• Rain fade margin. The margin you hold for the worst weather at the site, which depends on band and geography far more than most specifications admit.
• Satellite and beam. The operator’s figures for the specific beam over your site, not the peak figure on the coverage map.
• Losses between amplifier and feed. Cable, waveguide and connector losses that are easy to underestimate on a large mount.

Back-off: the number that is quietly missing

An amplifier is only linear up to a point. Run it near saturation and it generates intermodulation products that interfere with neighbouring carriers, which the satellite operator will notice before you do. Real designs run the amplifier backed off from its saturated rating, and the amount depends on how many carriers it is handling.

This is why a rated output figure alone tells you very little. The question is how much usable linear output remains at your operating point.

Why oversizing is not free

A larger amplifier costs more, but the consequences continue after purchase.

• Higher DC consumption, which matters at a site running on generator or solar
• More heat, which means more cooling or a shorter service life
• More weight at the feed, which pushes up mount and positioner requirements
• A larger transit case and a heavier flyaway package
• Worse efficiency when running far below the rated output

This is also where the shared-inbound technology choice comes back. In a system where every terminal must be sized for the widest channel in the network, the smallest site carries an amplifier it will never use. In a system that allocates per terminal, each site is sized for its own maximum rate. We cover that difference in MF-TDMA or Dynamic BoD BM-FDMA.

Before you order

Confirm the band and polarization, the operator’s figures for your beam, the reference point at which output power is quoted, the DC supply available at the site, the maximum ambient temperature, and whether the unit will be feed-mounted or indoors. Our BUC and SSPA range covers all satellite bands, and we would rather quote the right size than the large one.