PSM-2100L Satellite Modem Addendum
Page L-Band - 8
the receive LNB has a gain of 60 dB and the Bias-T/Mux has 6 dB of conversion loss then the
cable can have a maximum loss of 19 dB (60 – 35 – 6 dB). In formula form this is:
Loss(cable max) = Gain(LNB) – 35 – Loss(misc)
in dB
Or for the transmit side the formula would be:
Loss(cable max) = Gain(PA) – 35
in dB
Notice that we are assuming no miscellaneous losses in the transmit side, but there may be other
losses such as a splitter or output sample port used.
Trying to buy too cheap a cable will only result in problems that are more expensive to fix than
using the proper cable to begin with. Remember that the L-Band design allows for moderately
inexpensive cable in exchange, especially as compared to the typical requirement for either
expensive outdoor converters or very expensive heliax / waveguide with indoor converters.
The PSM-2100L provides a 50 Ohm impedance on its cable connections. Most BUCs are 50
Ohm, but many LNBs provide a 75 Ohm impedance and use Type “F” connectors. We can supply
a receive “Bias T” that includes an integrated 75 to 50 Ohm converter (minimum loss pad). When
the 75 to 50 Ohm converter is included the receive input connector (from LNB) can also be a type
“F”.
Several cable types are shown below with typical maximum recommended frequency, size,
losses per 100 feet at 1.2 GHz, shielding efficiency, and relative approximate costs per foot.
Recommended cables are shown with asterisks. Since maximum loss is preferred to be 20 dB or
less, then generally the cable size is chosen to keep the cable loss well below that point. 10 to 15
dB is probably a better design guide considering that other connection losses are inevitable. DC
resistance for the transmit cable should also be considered with respect to BUC current
draw/voltage drop. Also consider that in areas where temperature change is high a lower loss
cable should be chosen to minimize absolute transmit power variation.
Typical Coaxial Cable Characteristics
Cable Type
Max. Freq.
(MHz)
O.D. (in.)
Loss/100
feet (dB)
@ 1.2 GHz
Shielding
Efficiency
(dB)
Cost/ft.
(USD)
RG58 (50)
1,000
.19
21
70
$0.39
RG59 (75)
1,000
.25
18
70
0.39
Times LMR-240 (50)
5,000
.24
9.2
>90
0.47
Times LMR-300 (50)
5,000
.30
6.8
>90
0.53
Times LMR-400 (50)
5,000
.405
4.8
>90
0.64
Belden 9913 (50)
5,000
.405
5.2
>90
0.60
Times LMR-600 (50)
5,000
.59
3.1
>90
1.30
RG214 (50)
5,000
.405
10.1
>90
1.70
3/8”LDF (50)
5,000
.44
4.1
>90
1.89
1/2”Superflex (50)
5,000
.52
4.2
>90
1.89
Note that the common RG214 type cable is not only more expensive, but also higher loss than
several other available cable types. The maximum length that RG214 would be used assuming
the approx 15 dB loss criteria would be 150 ft or 50 meters. Times LMR-400 cable would be
usable over 300 ft. At less cost.
4.0
Interoperability Between 70 MHz and L-Band Modems
Not only is the design and operation of the PSM-2100L modem closely based on that of the PSM-
2100 70 MHz modem, but the units are fully interoperable. Thus a typical system configuration
