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Decibels, μV and dBμV

Brief explanation of the Decibel (aerial trade wise)

Graph of dB to ratio

Graph of dB to 1 mV

The Decibel (or dB for short) is a much misunderstood unit, it is simply a ratio.

For an amplifier it is the ratio between the input signal level and the output signal level.

In the case of aerial gain it is (or should be) measured in dBd, that is gain as compared to (i.e. the ratio to) just a half wave dipole on its own. Some aerial manufacturers quote dBi, which is gain relative to a theoretical isotropic source. I`m not really sure what the latter is, and, to be perfectly frank, I`m not that bothered either, because aerial gain in the real world should be measured in dBd anyway. I object to manufacturers trying to make their aerials seem higher gain than they really are by quoting dBi, and hoping the customers don`t

know the difference.

To convert dBi to dBd deduct 2.15.

In fact manufacturers shouldn`t just quote peak gain anyway

(whether in dBi or dBd) because it`s potentially misleading....

The dB is a logarithmic scale and this can be very helpful helpful

in the aerial game, big differences in signal can be worked with relatively simply. The latter can be achieved by using dBμV instead of mV. i.e a unit which is dB relative to one microvolt.

Since 0dB means no increase or decrease, then 0dBμV = 1μV    That is to say : 0dB x 1μV  

Other levels are shown in the table on the left. By using dBμV simple addition / subtraction can be utilised to calculate signal levels throughout a system, including, crucially, at each TV input.

The preferred range at the TV input is 60 to 80dBμV, so aiming for 70dBμV makes sense.

As an example, if one has 80dBμV at the aerial (a strong signal area) and one loses 3dB through the cable, then utilises a 6 way splitter (loss 10dB) one can easily calculate the output by simple subtraction, thus 80 - 3 - 6 = 71dBμV, i.e. it should work fine !

Amplified systems work in the same way, e.g. input signal 65dBuV (medium signal area), then gain through the amplifier 15dB, loss through cable 3dB, then through an 8 way

splitter (loss 12dB). Thus  65 + 15 - 3 - 12 = 65dBμV, i.e. it should work fine !

When it come to amplifiers, or attenuators, the ratio is the output compared to the input.

Examples (shown on the graph below) could be :

A 14dB increase will give an output level of 5 from an input level of 1.  

In the case of an aerial amplifier, a 1mV input will give a 5mV output.

Minus 6dB ( = attenuation) makes the output level half of the input.

If using a 6dB attenuator, a 1mV input would result in a 0.5mV (500μV) output.               μ