Technical Differences between L-Band BUCs or 70 MHz Transceivers.

During the previous few years we have been selling a great many L-Band modems and Block Up Converters (BUCs). Orders for this newer technology has surpassed sales of the traditional modem and transceiver package, mainly because of price. A typical VSAT installation has the antenna mounted outside, with the transceiver mounted as close to the antenna feed as possible. The modem and other electronics are connected by cable from the operations center out to the antenna. If this cable had to carry the original RF frequency of 4 to 6 GHz it would need to be thick and very expensive. In order to cut down costs, satellite engineers designed transceivers that would take the high frequencies from the antenna and then convert them into a much lower frequency. They called this new frequency “Intermediate Frequency (IF)“. IF frequencies can be carried over long distances between the antenna and indoor equipment using cheap cables.

C band LNB(4GHz) and BUC (6 GHz) manufactured by JRC. Note the rectangular waveguide flanges and dimensions.

To illustrate the difference between the two technologies let us first review the 70 MHz modem and transceiver combination, using C-Band as my example, but everything remains true for KU band as well.

The receive frequency of 4 GHz is collected by the antenna and fed to an LNA and then passed to the receive port of the transceiver. Typically the transceiver is mounted on the antenna and so only a short length of thick cable is required between the LNA and the transceiver. Once the signal gets to the transceiver it is converted from high frequency to an Intermediate Frequency (IF) of 70 MHz and then carried by standard Coax cable the long distances from the antenna to the indoor equipment. Detailed VSAT technical information for Engineers

The transceiver contains all the frequency conversion and input/output power controls that are needed to send and receive a satellite signal. The transceiver must be waterproof and impervious to heat and cold. All of these features add to the purchase price of the transceiver, which is typically comprised of a power supply, an SSPA, an up converter and a down converter.

In an effort to reduce the price of the transceiver, a new scheme was developed that uses a higher Intermediate frequency in the range of 900 to 1500MHz. Even though much higher than 70MHz, this L-Band frequency, as it was named, can still be carried long distances over cheap cable.

The other change was to replace the transceiver with a much simpler and cheaper unit called a Block Up Converter or BUC. The BUC makes a simple frequency conversion and then acts as an amplifier sending the signal up to the satellite. The BUC is a very dumb device compared to a transceiver.

The power supply, radio power settings and frequency selections are now moved into the L-Band modem. The modem sits indoors and does not require the same environmental protection. The penalty is that the L-Band modem now costs more than a similar 70 MHz modem. Even so, the cost of the combination BUC, LNB and L-Band modem is many thousands of dollars less than the cost of the older transceiver system.

An Anacom or Codan 20 Watt C-Band Transceiver will cost $15,845 and above, with a matching satellite modem costing $3000 and upwards. A typical 20W C-Band BUC will cost $6550 with the L-Band modem costing $4250, at the other end of the scale a 5 watt BUC can cost less than $1200 compared with the price of an Anacom 5 watt transceiver at $9663.

As the sales of BUCs continue to increase the manufacturers are decreasing the prices for their C-Band and KU band BUCs.
Prices for a KU BUCs are slightly different than for C-Band, but the same ratio of price savings can be expected.

More considerations regarding L-Band or 70 MHz for cross site cabling at VSAT installations.