L-Band IF Application Guide
Page L-Band - 10
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)
Estimated
Cost/ft.
(USD)
RG58 (50
Ω
) 1,000
.19
21
70
$0.39
RG59 (75
Ω
) 1,000
.25
18
70
0.39
RG6 (75
Ω
) 2,200
.25
10
>90
0.89
RG11 (75
Ω
) 2,200
.405
5 >90
0.89
Times LMR-240 (50
Ω
) 5,000
.24
9.2
>90
0.47
Times LMR-300 (50
Ω
) 5,000
.30
6.8
>90
0.53
Times LMR-400 (50
Ω
) 5,000
.405
4.8
>90
0.64
Belden 9913 (50
Ω
) 5,000
.405 5.2
>90
0.60
Times LMR-600 (50
Ω
) 5,000
.59
3.1
>90
1.30
RG214 (50
Ω
) 5,000
.405
10.1
>90
1.70
3/8”LDF (50
Ω
) 5,000 .44 4.1 >90 1.89
1/2”Superflex (50
Ω
) 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. Both of these examples assume that the DC loss of the BUC
power is within tolerance. For many receive applications RG6 cable is a good choice.
The typical DC resistance of 0.405 inch class 50 Ohm cables such as RG214 or LMR-400 is
approximately 0.3 Ohms per 100 feet including both the center conductor and sheild. Thus at a
250 foot run the total resistance is 1 Ohm, which is a 1 Volt drop per Amp of BUC current draw. It
would be reasonable to then assume an approximate 2 Volt drop in the DC Voltage available at
the BUC input ( 22 Volts for a 24 Volt supply at the modem).
4.0
Interoperability Between 70 MHz and L-Band Modems
Not only is the design and operation of the PSM-4900L modem closely based on that of the PSM-
4900 70 MHz modem, but the units are fully interoperable. Thus a typical system configuration
with one or more “Hub” stations utilizing 70 MHz as the IF and many remotes utilizing both 70
MHz and L-Band equipments works well without consideration to the particular equipment at any
site. New sites in an existing system may be added using either L-Band or 70 MHz as the IF link
frequency.
5.0 Specifications
The specifications for the PSM-4900L are shown below.
