DB Demystified
Decibels Demystified
Some of my students get rather confused with this so I thought a hint sheet may help:
dBi: Gain verses Isotropic resonator. An isotropic antenna is a small antenna with no feed in free space which radiates well in all directions
dBd: Gain verses a Reverence Dipole with a classic donut pattern of radiation.
- dBi = dBd + 2.15
- dBd = dBi – 2.15
ERP : Effective Radiated Power, the real numbers to concern yourself with when building antennas.
Gain: uses a log 10 scale
- 3dB = 2 fold improvement
- 6dB = 4 fold improvement
- 10dB = 10 fold improvement
- 20dB = 20 fold improvement
Gain = Power x Gain – Feed loss
Coax dB Loss per 100 Feet using common coax types:
|
dB Loss / 100 feet |
1.0 |
10 |
50 |
100 |
200 |
400 |
900 |
1000 |
3000 |
5000 |
|
6A, 212 |
.26 | .83 | 1.9 | 2.7 | 4.1 | 5.9 | 6.5 | 9.8 | 23.0 | 32.0 |
|
8 MINI, 8X |
1.1 | 2.5 | 3.8 | 5.4 | 7.9 | 8.8 | 13.0 | 26.0 | ||
|
LMR -240 |
.24 | .76 | 1.7 | 2.4 | 3.4 | 4.9 | 7.5 | 7.9 | 14.2 | 18.7 |
|
8, 8A, 10A, 213 (RG8/8A hard to find ) |
.15 | .55 | 1.3 | 1.9 | 2.7 | 4.1 | 7.5 | 8.0 | 16.0 | 27.0 |
|
9913, 9086, 9096 |
0.9 | 1.4 | 1.8 | 2.6 | 4.2 | 4.5 | 13.0 | |||
|
4XL8IIA, FLEXI 4XL |
0.9 | 1.4 | 1.8 | 2.6 | 4.2 | 4.5 | 13.0 | |||
|
LMR-400 |
.9 | 1.2 | 2.5 | 4.1 | 4.3 | |||||
|
LMR-500 |
.7 | 1.0 | 2.0 | 3.2 | 3.4 | |||||
|
LMR-600 |
.6 | .8 | 1.4 | 2.5 | 2.7 | |||||
|
8214 |
.60 | 1.2 | 1.7 | 2.7 | 4.2 | 7.8 | 14.2 | 22.0 | ||
|
9095 |
1.0 | 1.8 | 2.6 | 3.8 | 6.0 | 7.5 | ||||
|
9, 9A, 9B, 214 |
.21 | .66 | 1.5 | 2.3 | 3.3 | 5.0 | 7.8 | 8.8 | 18.0 | 27.0 |
|
11,11A,12,12A,13,13A, 216 |
.19 | .66 | 1.6 | 2.3 | 3.3 | 4.8 | 7.8 | 16.5 | 26.5 | |
|
14, 14A, 217 |
.12 | .41 | 1.0 | 1.4 | 2.0 | 3.1 | 5.5 | 12.4 | 19.0 | |
|
17,17A,18,18A, 218, 219 |
.06 | .24 | .62 | .95 | 1.5 | 2.4 | 4.4 | 9.5 | 15.3 | |
|
55B, 223 |
.30 | 1.2 | 3.2 | 4.8 | 7.0 | 10.0 | 14.3 | 16.5 | 30.5 | 46.0 |
|
58 |
.33 | 1.2 | 3.1 | 4.6 | 6.9 | 10.5 | 14.5 | 17.5 | 37.5 | 60.0 |
|
58A, 58C |
.44 | 1.4 | 3.3 | 4.9 | 7.4 | 12.0 | 20.0 | 24.0 | 54.0 | 83.0 |
|
59, 59B |
.33 | 1.1 | 2.4 | 3.4 | 4.9 | 7.0 | 11.0 | 12.0 | 26.5 | 42.0 |
|
62, 62A, 71A, 71B |
.25 | .85 | 1.9 | 2.7 | 3.8 | 5.3 | 8.3 | 8.7 | 18.5 | 30.0 |
|
62B |
.31 | .90 | 2.0 | 2.9 | 4.2 | 6.2 | 11.0 | 24.0 | 38.0 | |
|
141,141A, 400, 142, 142A |
.30 | .90 | 2.1 | 3.3 | 4.7 | 6.9 | 13.0 | 26.0 | 40.0 | |
|
174 |
2.3 | 3.9 | 6.6 | 8.9 | 12.0 | 17.5 | 28.2 | 30.0 | 64.0 | 99.0 |
|
178B,196A |
2.6 | 5.6 | 10.5 | 14.0 | 19.0 | 28.0 | 46.0 | 85.0 | 100 | |
|
188A, 316 |
3.1 | 6.0 | 9.6 | 11.4 | 14.2 | 16.7 | 31.0 | 60.0 | 82.0 | |
|
179B |
3.0 | 5.3 | 8.5 | 10.0 | 12.5 | 16.0 | 24.0 | 44.0 | 64.0 | |
|
393, 235 |
.6 | 1.4 | 2.1 | 3.1 | 4.5 | 7.5 | 14.0 | 21.0 | ||
|
402 |
1.2 | 2.7 | 3.9 | 5.5 | 8.0 | 13.0 | 26.0 | 26.0 | ||
|
405 |
22.0 | |||||||||
|
LDF4-50A |
.06 | .21 | .47 | .68 | .98 | 1.4 | 2.2 | 2.3 | 4.3 | 5.9 |
|
LDF5-50A |
.03 | .11 | .25 | .36 | .53 | .78 | 1.2 | 1.4 | 2.5 | 3.5 |