dB and aerials and whatnot.

[David Paste]

I "know" that the decibel is a measure of power but that is about it. I
also know (without the quotes) that I know next to bugger all.

I know that the attenuation of FM radio signals (and UHF TV signals) is
greater per unit distance in a coax cable than in air.

If I run a coax from a half wavelength dipole near the top of the house to
my radio in the cellar, it provides a better signal than an aerial in the
cellar, but the signal is still hissy.

So how does one work out the level of signal needed at the aerial so a
hiss-free signal will be delivered by the radio? I assume it'll require an
amp sat by the aerial.

How do you measure these mysterious dBs and what do they represent in
actual terms?

Thanks for your tolerance.

[David Woolley]

Post by David Paste
I "know" that the decibel is a measure of power

It's a measure of a power ratio, not power. dBm is a measure of power;
based on the ratio to 1 milliwatt.

Post by David Paste
So how does one work out the level of signal needed at the aerial so a
hiss-free signal will be delivered by the radio? I assume it'll require
an amp sat by the aerial.

Define hiss-free. No real signal will be hiss free.

What is the nature of the signal. FM and AM behave differently, and
digital tends to fail in ways other than hiss.

Which frequency band?

An amplifier may not be sufficient. You may need a better aerial.

To determine the gain needed, assuming that noise received through
aerial is not the dominant cause, you need to know the noise figure of
the receiver, and of the amplifier, the length of the cable and its
loss, per unit length, at the frequency in question. All in dB, cable
length * loss per unit length - the gain of the amplifier + noise figure
of amplifier, needs to be several dB (depends on your definition of
noise free, but probably upwards of 6dB) greater than the noise figure
of receiver. This is basically so that the noise figure of the
amplifier dominates. (Someone please check this, as I'm not sure I have
this right).

Note that too much gain can cause other types of problem. Also a better
aerial is generally always a better solution.

Post by David Paste
How do you measure these mysterious dBs and what do they represent in
actual terms?

Many you look up from data sheets, although the figures may not be
published for domestic use receivers.

The formal definition is 10 * log (power out / power in), where log is
the base 10 logarithm. dBm is 10 * log (power / 0.001), where power is
measured in watts. Measuring dBm requires calibrated receivers. 3dB is
close to factor of two and is generally taken as being a factor of 2.

[David Woolley]

All in dB, cable length * loss per unit length - the gain of the
amplifier + noise figure of amplifier, needs to be several dB (depends
on your definition of noise free, but probably upwards of 6dB) greater
than the noise figure of receiver.  This is basically so that the noise
figure of the amplifier dominates.

Note that this doesn't guarantee a very low hiss. Other factors that
have to be taken into account are the actual noise figure of the
amplifier, which needs to be as low as possible, and the antenna. No
amplifier can get rid of the noise that is coming through the antenna.
Only a better antenna can do that. What it does is to make sure that
you have enough gain to compensate for the cable and to make sure the
the amplifier is the limiting factor for the noise coming from the
system itself. Generally amplifiers have

[Woody]

Post by David Paste
I "know" that the decibel is a measure of power but that is about it. I
also know (without the quotes) that I know next to bugger all.
I know that the attenuation of FM radio signals (and UHF TV signals) is
greater per unit distance in a coax cable than in air.
If I run a coax from a half wavelength dipole near the top of the house to
my radio in the cellar, it provides a better signal than an aerial in the
cellar, but the signal is still hissy.
So how does one work out the level of signal needed at the aerial so a
hiss-free signal will be delivered by the radio? I assume it'll require an
amp sat by the aerial.
How do you measure these mysterious dBs and what do they represent in
actual terms?
Thanks for your tolerance.

A decibel is a RATIO, nothing more. However it is more often than not
used as a ratio of powers, hence the sound brigade have adopted dB as an
absolute sound level, which I suppose could be interpreted as such given
a reference.

On hi-fi tuners you will often find the signal quoted as dBuV which is
dB above a 1uV reference. In signal voltage every 6dB increase is a
doubling, 20dB increase is ten times, and odd things come up like 10dB
is roughly three times voltage.

When FM radio was being created the BBC decided that a solid stereo
signal would be 1mV at the receiver input (60dBuV) and a fringe signal
would be 250uV or 48dBuV. Most modern receivers would be happy with a
signal around 100uV and in many cases would work happily at 50uV.

Going to the power ratio side, there is a formula for working out a free
space signal path loss:
36 + 20log(distance in miles) + 20log(frequency)
If you were using Holme Moss on 100MHz from 30 miles (as I am) this
would calculate in round figures to:-
36 + 30 + 40 = 106dB
250kW = +54dBW or 84dBm. i.e. 54dB above a reference of 1W or 84dB above
a reference of 1mW. So I would be getting 84-106=-22dBm or about 17mV.*
However you then have to add any receive aerial gain and deduct any
aerial cable loss. Note that dB is a relative measurement but dBm or dBW
is an absolute measurement.

*Quick way to work it out. 0dBm is about 223mV of signal. -40dBm would
be 100 times down on this so about 2.23mV. -22 is +6dB applied 3 times
which is, give or take, 18mV.

HTH

[charles]

Post by Woody
When FM radio was being created the BBC decided that a solid stereo
signal would be 1mV at the receiver input (60dBuV) and a fringe signal
would be 250uV or 48dBuV. Most modern receivers would be happy with a
signal around 100uV and in many cases would work happily at 50uV.

Actually, stereo was not involved - it wasn't around. The primary service
area was actually defined as 1mv/m. What that gave the receiver was
entirely dependent on the aerial used.

[Jim Lesurf]

Post by Woody

Post by David Paste
I "know" that the decibel is a measure of power but that is about it.
I also know (without the quotes) that I know next to bugger all.
I know that the attenuation of FM radio signals (and UHF TV signals)
is greater per unit distance in a coax cable than in air.

A decibel is a RATIO, nothing more. However it is more often than not
used as a ratio of powers, hence the sound brigade have adopted dB as an
absolute sound level, which I suppose could be interpreted as such
given a reference.

However for an FM tuner you can assume that a dipole has a 'gain' of..
0 dBd where the final 'd' means wrt a dipole. :-) That IIRC translates
to about +3dB relative to an 'isotropic' antenna which is in practical
terms an, erm, theoretical reference level. 8-]

If you have space, get a 3 or 4 element 'Yagi' array as it will give you
some gain and hence a bigger signal. This tends to be preferred to a preamp
because a preamp will add noise from its own internals. If the Yagi doesn't
do the job, then add a low noise to its output - preferrable with just a
short cable run from the antenna. i.e. at the 'input' end of the cable.

You can get antennas with more than 3 or 4 elements and these nominally can
provide more gain - and thus get you more signal. However this starts to
run into snags.

They are bigger, so need more room. Heavier, so that may need to be allowed
for when installing what holds them in place.

Less well known is that the more gain /directionaltiy the antenna has, the
most 'sensitive' its actual behaviour is to what is around it. This
*includes* directions to its sides, above, and below. So that can upset
performance if you install in a loft space and mean bigger may not be
better.

Also, depending where you are 'sky noise' can be mainly man-made and if
that is 'in the beam' your output will contain more noise along with more
signal if you get a higher gain antenna. 8-]

As others have said, you tend to need a good signal as well as a good tuner
for best results. And it can depend a lot on where you are.

Jim

[Woody]

Post by Jim Lesurf

Post by Woody

Post by David Paste
I "know" that the decibel is a measure of power but that is about it.
I also know (without the quotes) that I know next to bugger all.
I know that the attenuation of FM radio signals (and UHF TV signals)
is greater per unit distance in a coax cable than in air.

A decibel is a RATIO, nothing more. However it is more often than not
used as a ratio of powers, hence the sound brigade have adopted dB as an
absolute sound level, which I suppose could be interpreted as such
given a reference.

However for an FM tuner you can assume that a dipole has a 'gain' of..
0 dBd where the final 'd' means wrt a dipole. :-) That IIRC translates
to about +3dB relative to an 'isotropic' antenna which is in practical
terms an, erm, theoretical reference level. 8-]
If you have space, get a 3 or 4 element 'Yagi' array as it will give you
some gain and hence a bigger signal. This tends to be preferred to a preamp
because a preamp will add noise from its own internals. If the Yagi doesn't
do the job, then add a low noise to its output - preferrable with just a
short cable run from the antenna. i.e. at the 'input' end of the cable.
You can get antennas with more than 3 or 4 elements and these nominally can
provide more gain - and thus get you more signal. However this starts to
run into snags.
They are bigger, so need more room. Heavier, so that may need to be allowed
for when installing what holds them in place.
Less well known is that the more gain /directionaltiy the antenna has, the
most 'sensitive' its actual behaviour is to what is around it. This
*includes* directions to its sides, above, and below. So that can upset
performance if you install in a loft space and mean bigger may not be
better.
Also, depending where you are 'sky noise' can be mainly man-made and if
that is 'in the beam' your output will contain more noise along with more
signal if you get a higher gain antenna. 8-]
As others have said, you tend to need a good signal as well as a good tuner
for best results. And it can depend a lot on where you are.

+1 Jim - except the OP should NEVER consider one of those 'ring' dipoles
- they are a total waste of space.

It is also worth considering using a single vertical half-wave dipole as
they tend to work better as an omni, and as most FM transmissions these
days are either mixed or vertical there will be little degradation. It
also has the advantage that a vertical VHF/FM dipole usually works quite
well with DAB.

[Brian Gregory]

Post by Woody
It is also worth considering using a single vertical half-wave dipole as
they tend to work better as an omni, and as most FM transmissions these
days are either mixed or vertical there will be little degradation. It
also has the advantage that a vertical VHF/FM dipole usually works quite
well with DAB.

Vertical is the preferred orientation of VHF radio aerials in many
locations such as where I am near Reading. All the local radio is
vertical or mixed and our local BBC relay station is too. However aerial
fitters continue to fit those stupid horizontal halo aerials on blocks
of flats for FM.

[charles]

Post by Brian Gregory

Post by Woody
It is also worth considering using a single vertical half-wave dipole
as they tend to work better as an omni, and as most FM transmissions
these days are either mixed or vertical there will be little
degradation. It also has the advantage that a vertical VHF/FM dipole
usually works quite well with DAB.

Vertical is the preferred orientation of VHF radio aerials in many
locations such as where I am near Reading. All the local radio is
vertical or mixed and our local BBC relay station is too. However aerial
fitters continue to fit those stupid horizontal halo aerials on blocks
of flats for FM.

and, invariably, fit them with the nul towards the transmiter

[Ian Jackson]

Post by charles

Post by Brian Gregory

Post by Woody
It is also worth considering using a single vertical half-wave dipole
as they tend to work better as an omni, and as most FM transmissions
these days are either mixed or vertical there will be little
degradation. It also has the advantage that a vertical VHF/FM dipole
usually works quite well with DAB.

Vertical is the preferred orientation of VHF radio aerials in many
locations such as where I am near Reading. All the local radio is
vertical or mixed and our local BBC relay station is too. However aerial
fitters continue to fit those stupid horizontal halo aerials on blocks
of flats for FM.

and, invariably, fit them with the nul towards the transmiter

Horizontal halos don't have a null in the horizontal directions. Their
reception diagram is a reasonably constant -3dB circle, relative to a
dipole 'broadside', and this isn't affected by how it is mounted on the
vertical mast (which is often running up its centre). While a vertical
dipole itself receives equally from all directions, if it's mounted on
the side of a mast its directivity is certainly affected.

[David Woolley]

Post by Jim Lesurf
That IIRC translates
to about +3dB relative to an 'isotropic' antenna which is in practical
terms an, erm, theoret

[Max Demian]

Post by David Paste
I "know" that the decibel is a measure of power but that is about it. I
also know (without the quotes) that I know next to bugger all.
How do you measure these mysterious dBs and what do they represent in
actual terms?

As its name might suggest, a decibel is a tenth part of a Bel, which is
named after the telephone pioneer Alexander Bell. (He lost an el in the
process for some reason.)