Comtech EF Data SLM-5650A Vipersat - Manual (Page 136)

Signal Power Level Considerations
C-16 Vipersat SLM-5650A User Guide
level is then calculated based on the BUC’s P
in
max minus the transmit cable
loss.
The levels shown in the example above assume a transmit cable loss of about
12 dB. Apply this calculated value to the returned Max Power value to deter-
mine if one, the maximum power is within gain/loss limits and two, the total
range is achievable.
Power Considerations
The amount of power is defined by the link budget. As a rule of thumb, there
should be at least 2 dB of headroom (margin) left when operating at the highest
bit rate that the remote is expected to transmit at C-band. Ku‐band should typi-
cally have 5 or 6 dB of headroom to compensate for rain fade. Transmission
equipment works best when it is operated mid range. Typically, any BUC larger
than 8 W will need an external power supply.
Cabling Considerations
L-band frequencies are much more susceptible to signal loss over cable runs
than IF frequencies in the 70/140 MHz range. For example, RG214 is
commonly used for IF frequency runs. At 70 MHz, the loss per 100 ft of cable is
approximately 1.7 dB. However, at 1.2 GHz (L-band), the loss is more than
8 dB. A higher quality cable such as LMR400, which exhibits approximately
4.5 dB of loss per 100 feet, is recommended. Note, however, that in some
cases—such as short cable runs—additional losses may be required to maintain
that balance between min/max ranges.
It is much simpler to add/subtract excess cable to increase/decrease losses for
proper adjustment. The alternative of inserting an attenuator that can pass
power to the BUC is considerably more expensive.
The transmit cable must carry a heavy current on the order of 1 to 5 amps to
power the BUC/PA. The DC resistance and cable voltage drop must allow this
gear to receive their minimum voltage, plus enough margin for variation with
time and temperature.
The transmit and receive cables must be separated and definitely not bundled
directly together with cable ties, especially on longer runs because of the
tremendous difference between the transmit and receive levels. This is made
worse on long cable runs because the modem end will have higher transmit
levels and the receive end will have lower levels than on a short run. The better
cables in this regard have double shielding (two braids or a braid/foil combina-
tion) and a shielding efficiency of 100 dB or better.