How much power can I receive from a FM station

[Allen]

I am designing a FM radio receiver. Can anyone tell me show much power
can I receive from a FM station that is in a acceptable range? I need
that value to decide the gain of all the stages. Thanks.

Allen

 

[Tom Bruhns]

I am designing a FM radio receiver. Can anyone tell me show much power
can I receive from a FM station that is in a acceptable range? I need
that value to decide the gain of all the stages. Thanks.

Allen

Usually FM receivers are designed with excess gain, since there is no
need for amplitude linearity. You didn't say what sort of FM station
you want to receive. FM signals vary greatly in the bandwidth they
occupy: communications channels (police, fire, pagers, etc) are
typically only a few kHz wide, and FM broadcast is typically about
150kHz wide. That makes a difference: since you'll pick up about the
same amount of noise per unit bandwidth (atmospheric noise; galactic
noise; ...) for signals at approximately the same carrier frequency,
the total noise in the channel varies with bandwidth. So for wider
bandwidth, it takes more signal to rise above the noise and be
useable. A reasonable goal for FM broadcast at 100MHz would be to be
able to receive signals greater than -165dBm/Hz bandwidth, or about
-114dBm. That's about half a microvolt at 75 ohms, I believe.

You may wish to think about a bunch of other things before going very
far with your project: how will you avoid distortion from big signals
when you are trying to listen to small ones? How will you filter out
frequencies (channels) other than the one you want to listen to? How
will you maintain phase linearity so you achieve low distortion?

If you don't get discouraged along the way, and you keep your eyes and
mind open, you'll discover that there's a lot more to designing a good
receiver than initially meets the eye. And I want to make it clear
that I do NOT want to discourage you, because it's by doing things
like that that you will learn, possibly learn a whole lot.

Cheers,
Tom

 

[Robert Baer]

I am designing a FM radio receiver. Can anyone tell me show much power
can I receive from a FM station that is in a acceptable range? I need
that value to decide the gain of all the stages. Thanks.

Allen

Define "acceptable range".
Define how much power you want.
I know this Q regards an FM station, but in the SF bay area, near the
Dumbarton bridge, an AM station (KGO) broadcasts (i think) 100KW, a
"clear channel" station.
In the Los Altos hills, one can string up a 100 foot horizontal
antenna and connect it to a (partly tuned circuit may not be needed)
flashlight lamp and watch it flicker according to the amplitude (but
always lit).
Why i said "partly tuned circuit may not be needed", is that i do not
know exactly how that lamp was connected to the antenna.

 

[mpm]

I am designing a FM radio receiver. Can anyone tell me show much power
can I receive from a FM station that is in a acceptable range? I need
that value to decide the gain of all the stages. Thanks.

Allen

Well, you don't care as much about power received as voltage applied
to the receiver's antenna terminals. Another poster asked some other
relevant questions, so I'll wait to hear those answers before
proceeding.

-mpm

 

[ChairmanOfTheBored]

Well, you don't care as much about power received as voltage applied
to the receiver's antenna terminals. Another poster asked some other
relevant questions, so I'll wait to hear those answers before
proceeding.

-mpm

Antennas gather microvolts. The resulting power, if one wants to call
it that would be in femtowatts.

 

[Jamie]

Antennas gather microvolts. The resulting power, if one wants to call
it that would be in femtowatts.

ha, haven't seen that spelt out in years! use to seeing as "f" reference

 

[mpm]

Antennas gather microvolts. The resulting power, if one wants to call
it that would be in femtowatts.

Yeah, I was trying to save everyone the whole uV/m discussion again.
Some people think they can charge 9-volt batteries using FM Radio
Waves a half mile from the tower.

But in the OP's case, unless one knows the antenna efficiency, and the
field (power) into which it will be placed, it's really not possible
to accurately answer his question directly. I do know the FCC
Blanketing rules for FM extend to about 115 dBu, but that's not likely
to help him very much.

Also, depending the bandwidth, Q might be more important than voltage
anyway.
Depends on the app.

 

[Jeff Liebermann]

I am designing a FM radio receiver. Can anyone tell me show much power
can I receive from a FM station that is in a acceptable range? I need
that value to decide the gain of all the stages. Thanks.
Allen

Easy enough to calculate. The maximum EIRP that FCC will tolerate for
FM broadcasters is 100,000 watts EIRP or +80dBm. That's your starting
point.

As you move way from the transmitting antenna, the field strength goes
down by the square of the distance. Assuming your out of the near
field, the free space path loss is:

FSPL = 20 log(distance) + 20 log(freq) + 32.44
where:
free space path loss = dB
distance = km
frequency = MHz

So, at 100MHz this reduces to:
FSPL = 20 log(distance) + 72.44

Since you didn't bother supplying any numbers with your question, I'll
assume that you're about 1km away from the transmitter. That
conveniently works out to:
FSPL = 72.44 dB

Therefore, the signal received by a handy dandy isotropic antenna will
be:
+80dBm - 72.44 dB = +7.6dBm

Unfortunately, my stock of isotropic antennas are somewhat depleted.
However, I do have a 1/2 wave dipole with a gain of 2.15dBi. That
will yield a signal level at the antenna terminals of:
+80dBm - 72.44 + 2.15 = +9.7dBm

Converting to milliwatts across 50 ohms,
9.7dBm = 9.3 milliwatts.

There are some additional losses involved. Most big FM stations
broadcast about 75% of their power horizontally polarized, and 25% of
their power vertically polarized. You'll see anywhere between -1.25
and -6.0 additional cross polarization loss. There will also be some
minor transmission line losses between your dipole and whatever you're
trying to drive.

 

[Jeff Liebermann]

Easy enough to calculate. The maximum EIRP that FCC will tolerate for
FM broadcasters is 100,000 watts EIRP or +80dBm. That's your starting
point.

(...)

Oh crap. I forgot to check if this is someones homework assignment.
Yep. Boston University.
RDNS = edns03.bu.edu. [192.12.188.130]

 

[Eeyore]

I am designing a FM radio receiver. Can anyone tell me show much power
can I receive from a FM station that is in a acceptable range? I need
that value to decide the gain of all the stages. Thanks.

You need *variable gain* to cope with the wide range of signals you're likely to
receive. The gain required for any given station is determined by the AGC
(automatic gain control) circuitry.

It seems your question should be "what's a typical minimum signal that's worth
trying to receive" and it's normally given in microvolts not watts.

If this is for ordinary commercial FM 88-108 MHz, I'd look at some typical
receiver specs to get a feel for it.

Graham

 

[Eeyore]

Easy enough to calculate. The maximum EIRP that FCC will tolerate for
FM broadcasters is 100,000 watts EIRP or +80dBm. That's your starting
point.

As you move way from the transmitting antenna, the field strength goes
down by the square of the distance. Assuming your out of the near
field, the free space path loss is:

FSPL = 20 log(distance) + 20 log(freq) + 32.44
where:
free space path loss = dB
distance = km
frequency = MHz

So, at 100MHz this reduces to:
FSPL = 20 log(distance) + 72.44

Since you didn't bother supplying any numbers with your question, I'll
assume that you're about 1km away from the transmitter. That
conveniently works out to:
FSPL = 72.44 dB

And at 30km which is IME about as far as you want to get for decent FM
reception ~ 42dB

Therefore, the signal received by a handy dandy isotropic antenna will
be:
+80dBm - 72.44 dB = +7.6dBm

Unfortunately, my stock of isotropic antennas are somewhat depleted.
However, I do have a 1/2 wave dipole with a gain of 2.15dBi. That
will yield a signal level at the antenna terminals of:
+80dBm - 72.44 + 2.15 = +9.7dBm

Or ~ -20dBm @ 30km.

Converting to milliwatts across 50 ohms,
9.7dBm = 9.3 milliwatts.

Which is 682 mV

Or 21.5 mV @ 30km.

Now scale for those transmitters that transmit rather less power. 100kW is
unknown in the UK for FM. 10kW would result is a signal of 2.15 mV @ 30km.

Graham

 

[ChairmanOfTheBored]

Easy enough to calculate. The maximum EIRP that FCC will tolerate for
FM broadcasters is 100,000 watts EIRP or +80dBm. That's your starting
point.

(...)

Oh crap. I forgot to check if this is someones homework assignment.
Yep. Boston University.
RDNS = edns03.bu.edu. [192.12.188.130]

That does not mean that it is someone's homework assignment,
presumptuous dipshit.

 

[Tom Bruhns]

On Nov 10, 10:30 pm, Eeyore <[email protected]>
wrote:
....

And at 30km which is IME about as far as you want to get for decent FM
reception ~ 42dB

Why on earth would you say that 30km is "about as far as you want to
get for decent FM reception"?? I know "decent" is subjective, but the
stations I most commonly listen to are transmitting from between 100km
and 150km away, and I'm quite pleased with reception, even in the car
with a simple whip antenna. With a Yagi well up in the air, and
transmitting antennas on high hilltops, reception at 200km isn't
unreasonable. Perhaps signals don't propagate as well where you are??

Cheers,
Tom

 

[Jeff Liebermann]

Which is 682 mV

Or 21.5 mV @ 30km.

Yep, into 50 ohms. However, the question was how much power, not
voltage, could be received. If I increase the receive antenna gain to
a small yagi antenna, I can probably get 6dB more gain out of the
system. That will yield 15.7dBm or 37.2 milliwatts. That should be
enough to ran a flea power FM receiver.

If you want more voltage, a tuned RF step up transformer will do the
trick.

Now scale for those transmitters that transmit rather less power. 100kW is
unknown in the UK for FM. 10kW would result is a signal of 2.15 mV @ 30km.

Graham

The OP is in Boston, Mass, USA. 100Kw EIRP is legal but not very
common. Most run about 5,000 to 20,000 watts EIRP. The maximum in
most metro areas is 50,000 watts EIRP.

<http://en.wikipedia.org/wiki/Effective_radiated_power>

 

[Jeff Liebermann]

You need *variable gain* to cope with the wide range of signals you're likely to
receive. The gain required for any given station is determined by the AGC
(automatic gain control) circuitry.

Wrong. FM broadcast receivers do not have AGC circuitry (unless
you're trying to keep the group delay through the receiver constant).
FM receivers run with the front end gains at roughly maximum gain,
followed by IF stages running as limiters. There is no amplitude
component to the signal so there is no need to keep the signal in the
linear part of some mythical AM detector.

It seems your question should be "what's a typical minimum signal that's worth
trying to receive" and it's normally given in microvolts not watts.

FM receivers can be designed that use very little power. They can
drive something like a piezo earphone, which also is very low power.
You don't need watts.

If this is for ordinary commercial FM 88-108 MHz, I'd look at some typical
receiver specs to get a feel for it.

Blundering through various car stereo specs, I see 10-13dBf for
"usable sensitivity". 14-18dBf for 50dB quieting. 1dBf is about
0.274uV into 75 ohms. 15dBf would seem to be a good minimum operating
point, which is about 8.7uV.

I don't see what the receiver sensitivity has to do with this. The
problem is to deliver enough power to run the radio. It would be a
fair assumption that the signal level necessary to supply this power,
is far higher than the basic receiver sensitivity.

 

[Eeyore]

Eeyore wrote:
...

Why on earth would you say that 30km is "about as far as you want to
get for decent FM reception"??

From my own experience in the UK.

I know "decent" is subjective, but the
stations I most commonly listen to are transmitting from between 100km
and 150km away, and I'm quite pleased with reception, even in the car
with a simple whip antenna. With a Yagi well up in the air, and
transmitting antennas on high hilltops, reception at 200km isn't
unreasonable. Perhaps signals don't propagate as well where you are??

Maybe we don't use such powerful transmitters as a rule ? I see the BBC do have
some 250kW transmitters on FM but they are quite rare. Plus there aren't many
large areas of unobstructed landscape such as wheat plains here.

I am forever disappointed by the FM reception I get.

Graham

 

[Rene Tschaggelar]

Easy enough to calculate. The maximum EIRP that FCC will tolerate for
FM broadcasters is 100,000 watts EIRP or +80dBm. That's your starting
point.

As you move way from the transmitting antenna, the field strength goes
down by the square of the distance. Assuming your out of the near
field, the free space path loss is:

FSPL = 20 log(distance) + 20 log(freq) + 32.44
where:
free space path loss = dB
distance = km
frequency = MHz

So, at 100MHz this reduces to:
FSPL = 20 log(distance) + 72.44

Since you didn't bother supplying any numbers with your question, I'll
assume that you're about 1km away from the transmitter. That
conveniently works out to:
FSPL = 72.44 dB

Therefore, the signal received by a handy dandy isotropic antenna will
be:
+80dBm - 72.44 dB = +7.6dBm

Unfortunately, my stock of isotropic antennas are somewhat depleted.
However, I do have a 1/2 wave dipole with a gain of 2.15dBi. That
will yield a signal level at the antenna terminals of:
+80dBm - 72.44 + 2.15 = +9.7dBm

Converting to milliwatts across 50 ohms,
9.7dBm = 9.3 milliwatts.

There are some additional losses involved. Most big FM stations
broadcast about 75% of their power horizontally polarized, and 25% of
their power vertically polarized. You'll see anywhere between -1.25
and -6.0 additional cross polarization loss. There will also be some
minor transmission line losses between your dipole and whatever you're
trying to drive.

I once had a singleboard square inch FM transmitter
with say 10mW on the output and it reached 2 miles
easily. That makes everything minus 70dB


Rene

 

[mpm]

I don't see what the receiver sensitivity has to do with this. The
problem is to deliver enough power to run the radio. It would be a
fair assumption that the signal level necessary to supply this power,
is far higher than the basic receiver sensitivity.

Re-read the original post.
I don't think this is about powering the radio itself. (i.e, power
supply issues)
The question is about gain for the front end and later I.F. stages.
At least, that's how I read it.

 

[mpm]

From my own experience in the UK.


Maybe we don't use such powerful transmitters as a rule ? I see the BBC do have
some 250kW transmitters on FM but they are quite rare. Plus there aren't many
large areas of unobstructed landscape such as wheat plains here.

I am forever disappointed by the FM reception I get.

Graham

To make this simple:

Nothing beats line-of-sight for coverage.

Here in the US, FM radio stations can be licensed for an antenna
height as tall as 2000 feet and 100kW horiz & 100kW vert.. (There are
not too many of them for obvious reasons). Their classification is
"C"

The much more common high-power station will have a center of
radiation of approx 1000 feet and again, transmit 100kW with circular
polarization. These are "C-0" classification.

From there, one drops down to C1, C2, C3, B, etc.., which are all

varients of lower-power, lower altitude stations.

By definition, a Full Class-C has a service coutour of > 83km. (FCC
73.210)
You can also get a feeling for coverage by knowing that the FCC won't
license two class-C's on the same channel if they are closer than
290km (180 miles).

Or better yet, you can just pull up the dBu contours online.
For many lower power classifications, 30km sounds about right.

 

[ChairmanOfTheBored]

To make this simple:

Nothing beats line-of-sight for coverage.

Here in the US, FM radio stations can be licensed for an antenna
height as tall as 2000 feet and 100kW horiz & 100kW vert.. (There are
not too many of them for obvious reasons). Their classification is
"C"

The much more common high-power station will have a center of
radiation of approx 1000 feet and again, transmit 100kW with circular
polarization. These are "C-0" classification.

varients of lower-power, lower altitude stations.

By definition, a Full Class-C has a service coutour of > 83km. (FCC
73.210)
You can also get a feeling for coverage by knowing that the FCC won't
license two class-C's on the same channel if they are closer than
290km (180 miles).

Or better yet, you can just pull up the dBu contours online.
For many lower power classifications, 30km sounds about right.

I'll bet the guy that changes the beacon(s) on the 2k' tower makes a
pretty penny for the job!