DB Demystified

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
Frequency Mhz
Cable Type

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

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