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Brief explanation of the Decibel (aerial trade wise) (including definition of dBμVs and dBmV)
Before we go any further we must clarify the difference between decibels power wise
(e.g. Watts = Volts x Amps) and decibels level wise (e.g. Volts), the two differ by a factor
of two. In the aerial game we`re interested in signal level, specifically at the input to the TV
tuner. For level a doubling equates to 6 dB, whereas for power doubling equates to 3 dB.
All dB levels quoted on this site are for signal level.
The Decibel (or dB for short) is a much misunderstood unit, it is simply a ratio.
For an amplifier (or splitter) 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....
Either way 0dBd or 0dBi does not mean that no signal would be received by the aerial.
0dBd means that the aerial only picks up the same amount of signal as the dipole
would on its own (i.e. with no reflector or any directors), 0dBi means it`s 2.15dB down.
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 dBμVs
instead of mVs, 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.
Worked Examples of System Planning
These examples assume good signal quality.
The preferred range at the TV input is 60 to 80dBμV on analogue (or 45 to 60dBμV digital).
For FM some sources quote 60 to 75dBμV and for DAB 40 to 70dBμV, but most FM tuners will work perfectly well (in stereo) on signals down to 50dBμV and some down to 40dBμV or even less. Arguably one should aim for 55dBμV (FM) and 45dBμV (DAB).
For analogue TV aiming for 70 to 75dBμV makes sense.
As an example, if one has 85dBμV 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 : 85 -
Amplified systems work in the same way, e.g. input signal 60dBμV (weakish signal area),
then gain through the amplifier 20dB, then the loss through cable 3dB, then through a
4 way splitter (loss 8dB). Thus : 60 + 20 -
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.
(graphic courtesy of Bill Wright)
Also see Wikipedia and Astrosurf articles on the decibel
If you`ve found this site informative and, hopefully, interesting as well,