Campbell Scientific DCP100 GOES Data Collection Platform - Manual (Page 31)

D-1
APPENDIX D. DETAILED FORWARD/REFLECTED
POWER INFORMATION
D.1 IMPEDANCE MATCHING
The reflected power to forward power ratio
shows the degree of impedance match between
the transmitter and the cable/antenna assembly.
The percent of power reflected approximates
the impedance match with the following
equation:
% power reflected = [((ref + 17.4)/(fwd + 17.4))
2
x 100] - 1
This equation is an approximation because
some of the power reflected to the transmitter
can be reflected back to the antenna and then
reflected back to the transmitter. These
multiple reflections can cause incorrect
readings, especially when the reflected
power is large.
Impedance matching is also measured as
reflection coefficient (
Γ
), Voltage Standing
Wave Ratio (VSWR), and Return Loss (RL).
Table D.1-1 correlates values between the
different measurements.
Table D.1-1. Impedance Matching
Correlation
% power ref
Γ
VSWR RL
1 0.1 1.2 20
2 0.14 1.3 17
5 0.22 1.6 13
10 0.32 1.9 10
20 0.44 2.6 7
50 0.71 5.8 3
80 0.89 17.9 1
D.2 CALCULATING POWER-OUT
The amount of power going out of the
transmitter at the BNC connector is
approximated by the following equation:
Approx P
out
= (10 log[((fwd + 17.4) x
0.0100077)
2
x 1000/50]) + 20.8
This equation assumes the dBm is 50 ohms
and the impedance match between the trans-
mitter and the cable/antenna assembly is good
(% power reflected less than 5). Table D.2-1
lists P
out
for various values of forward power.
TABLE D.2-1. P
out
Values
FWD P
out
(dBm)
110 +35.9
130 +37.2
150 +38.3
165 +39.0
175 +39.5
185 +39.9
195 +40.4
205 +40.8
215 +41.1
230 +41.7
250 +42.4
D.3 IMPEDANCE MATCH
DATALOGGER PROGRAM
D.3.1 CR10X, CR10, CR510, AND CR500
This example calculates the percent of power
reflected and the amount of power going out to
the transmitter if the percent of power reflected
is less than 5.
;
*Table 1 Program
01: 10.0 Execution Interval
(seconds)
;USER DEFINED PROGRAM
;Calculate the percent of power reflected with
this equation: % power reflected = [((ref
+17.4)/(fwd + 17.4))^2 x 100] -1
01:
Z=X+F (P34)
1:
5 X Loc [ RefPwr ]
2: 17.4 F
3: 8 Z Loc [ RefPlus ]
02: Z=X+F (P34)
1:
4 X Loc [ FwdPwr ]
2: 17.4 F
3: 9 Z Loc [ FwdPlus ]
03: Z=X/Y (P38)
1:
8 X Loc [ RefPlus ]
2: 9 Y Loc [ FwdPlus ]
3: 10 Z Loc [ Scratch1 ]