|Gain x2||+3dB||Loss ½||-3dB|
|Gain x 4||+6dB||Loss ¼||-6dB|
|Gain x 10||+10dB||Loss 1/10||-10dB|
|Gain x 100||+20dB||Loss 1/100||-20dB|
|Gain x 1000||+30dB||Loss 1/1000||-30dB|
|Gain x 10000||+40dB||Loss 1/10000||-40dB|
Example 400Watt = 4 x 100 watts = + 6dB + 20dB = +26dBW
1000watts = +10dB + 100dB = +30dBW
Amplifiers and Antennas produce a positive gain in +dB
Feeders, filters and attenuators produce a negative gain or loss –dB
Antenna gains are quoted in two quantities which need to be understood.
dBi = Gain in reference to a isotropic antenna (A ideal theoretical antenna of no use in practise) .
dBD = Gain in reference to a dipole
dBD= dBi gain – 2.15
The most practical unit is dBD since the effective radiated power erp = power x gain in dBD.
e.g A Beam antenna of 10dBd gain is fed with 100 watts what is the erp = 10 x 100
= 1000 watts.
If a similar antenna gain was quoted as 10dBi then the erp would be much reduced since dBD = dBi-2.15 = 7.05dBD and erp = 7.05 x 100 = 700.35 watts.
Some manufactures quote dBi instead of dBD because it is a higher numerical value of gain and it looks like you are getting more for your money, but you are not.
Actual useful gain on 2m = (dBi – 2.15) = 7.5-2.15 = 5.35 dBD and on 70cms = 7.15dBD
Simple HF dipole
A simple dipole can be made for any band but it is best to use a half wave of the lowest band possible and the higher bands with are multiples of half wavelength can also be used.
Open feeder or 300Ω – Ribbon cable direct to balanced input on ATU