NJRC BUC and LNB

1.8M Patriot Antenna, 5 Watt BUC

20 Watt C-Band BUC

Prodelin Antenna with BUC

Connection from the indoor equipment (modem and router) to the outdoor equipment at the antenna normally involves two inter-facility (IFL) cables. For L band, 75 ohm impedance cables with F connectors are common for consumer grade equipment. For L band BUCs, 50 ohm cable is sometimes used and with 70 MHz systems 50 ohm cables are normal.

There is more dB attenuation loss on L band cables, particularly on very long lengths, but you can compensate with slope amps. You can alternatively use fiber optic to transmit either 70 MHz or L band. With fiber distances of several km are possible.

When clients use standard LMR400 and RG6 cables we suggest that they limit their cable runs to less than 60 meters in order to minimize cable losses.

For both BUCs and LNBs there is often a DC supply via the cable. This has implications for earth (ground) loops and corrosion, cables with very thin center conductors or high DC resistance with film screen/outers are not suitable.

With L-Band, a wider range of frequencies can cause interference, so be careful if you are locate near high power transmitters. At 70 MHz watch out for local FM radio stations at 88-108 MHz to avoid these problems good screening is essential.

The best cables for any cross site application are foam filled Heliax, with solid copper outers, but cost rules this out for consumer installations. The more practical cable is LMR400 for the transmit portion and something similar to RG6 on the receive side.

If you are considering multi-carrier operation with a BUC make sure it is suitable. You don't want unexpected interaction between the carriers. As an example it was noted that the DC power current taken by a 2 watt BUC varied according to the carrier on/off. Operation of multiple carriers through this type of BUC would be doubtful as TDMA interruptions of one carrier may well cause amplitude/phase/frequency hits on the other.

A client recently asked why BUCs and LNBs use a Local Oscillator (LO) frequency. The Local Oscillator (LO) frequency is what drives the mixer. The output frequency is different from the input frequency. The difference is the LO frequency. In the case of a Ku band BUC, the LO is normally 13.05 GHz so an L band input at 1 GHz comes out at 14.050 GHz. In the case of a C band BUC, LO frequencies of 4.9 and 7.375 GHz are used. In the case of 7.375 GHz the output is inverted so you need to set the modem modulation for inverted spectrum.

Some BUCs are manufactured with different LO frequencies and so be sure to check with the BUC supplier to be sure that you have the correct value for the BUC that you are installing.

In the case of BUCs, the LO frequencies are stabilized by a 10 or 50 MHz reference which you need to transmit up through the cable to the BUC. This reference frequency must be accurate and with low phase noise. Check with a spectrum analyzer that you are not sending spurious low frequencies into the cable (between say 10 kHz and 2 MHz), for example from a noisy switch mode power supply or voltage regulator that is faulty and oscillating. This applies to both LNBs as well as BUCs.

If you have very long cable run and erratic BUC current, consider putting the DC supply close to the BUC. Chirp on the front end of TDMA bursts is a possibility. There are also problems for the LNB as the return outer conductor drops varying voltages and these are superimposed on the LNB supply volts. The BUC and LNB are often partially connected to one another and to the earth ground at the antenna, so there is scope for strange voltages. Lightning and safety rules come first and the consequences of this may mean that you need an extra very thick earth (ground) cable between the antenna and indoors.

Note that 70 MHz transceivers can only transmit and receive over a small part of the satellite bandwidth, so if you want to transmit carriers 200 MHz apart you need two TX cross site cables and two up-converters. Alternatively you can get transceivers with 140 MHz center frequency for a much wider range of transmission frequencies.

If you get a transceiver make sure you understand how the frequencies are calculated. Sometimes a carrier at 70 MHz counts as zero frequency when added to the up-converter front panel readout setting, or sometimes 70 MHz.

Check first so that you don't transmit on the wrong frequency!

Back to Explanation of L-Band BUCs compared with 70 MHz Transceivers