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 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 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 54.0 83.0 59, 59B .33 1.1 2.4 3.4 4.9 7.0 11.0 12 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 24.0 38.0 141,141A, 400, 142, 142A .30 .90 2.1 3.3 4.7 6.9 13 26.0 40.0 174 2.3 3.9 6.6 8.9 12.0 17.5 28.2 30 64.0 99.0 178B,196A 2.6 5.6 10.5 14.0 19.0 28.0 46 85.0 100 188A, 316 3.1 6.0 9.6 11.4 14.2 16.7 31 60.0 82.0 179B 3.0 5.3 8.5 10.0 12.5 16.0 24 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 26.0 26.0 405 22 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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