Does anyone have a simple idea or plan on how to build a simple FM field strength meter that would be tunable to a particular frequency (ie, using an FM pocket radio)? I'm looking to build something that would show relative values to determine when I am getting closer or farther from the source of an illegal radio station.
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How to build FM field strength meter?
- Thread starter radiofish
- Start date
A
Andy_Brown
Guest
> Does anyone have a simple idea or plan on how to build a
> simple FM field strength meter that would be tunable to a
> particular frequency (ie, using an FM pocket radio)? I'm
> looking to build something that would show relative values
> to determine when I am getting closer or farther from the
> source of an illegal radio station.
>
http://www.northcountryradio.com/Kitpages/rffsm.htm
<img src=http://www.northcountryradio.com/Kitphotos/rfm-4.jpg><P ID="signature">______________
Electricity is really just organized lightning.
~George Carlin</P>
> simple FM field strength meter that would be tunable to a
> particular frequency (ie, using an FM pocket radio)? I'm
> looking to build something that would show relative values
> to determine when I am getting closer or farther from the
> source of an illegal radio station.
>
http://www.northcountryradio.com/Kitpages/rffsm.htm
<img src=http://www.northcountryradio.com/Kitphotos/rfm-4.jpg><P ID="signature">______________
Electricity is really just organized lightning.
~George Carlin</P>
You want to Direction Find, not do precise signal measurements - here's an idea
> Does anyone have a simple idea or plan on how to build a
> simple FM field strength meter that would be tunable to a
> particular frequency (ie, using an FM pocket radio)? I'm
> looking to build something that would show relative values
> to determine when I am getting closer or farther from the
> source of an illegal radio station.
>
For the purpose you describe an FM Field Strength Meter is NOT what I would suggest. You want to Direction Find (DF) rather than make specific measurements. You just want to home in on the signal.
My suggestion is to find or borrow any kind of receiver with relative receive indication (often called an S-meter). Many scanners, some Ham radios and some other types can receive FM broadcast band and have this relative receive indicator. Even high end receivers (got a power inverter so you can plug a home unit in to your car?).
But that's not all you'll need. Also important (maybe even more so) is a DIRECTIONAL ANTENNA. You can rig a simple beam on foamboard using wire (cut to the right dimensions and etc..). You could wield a 4 or 5 element beam fairly easily (the higher the FM freq the smaller the antenna). You swing the directional antenna around until it POINTS to the strongest signal (but realize that FM signals can bounce - so you may get a few false readings).
LoJack is a direction finding system. You see the police cars with FOUR 1/4 wavelength antennas (strategically positioned) on the roof. The freq is around 170 MHz. To oversimplify, they use the input from the 4 antennas (through a circuit that compares the signal strength and phase) and outputs a DIRECTION to the transmitting signal (or it's reflection). No need for a FM Field Strength meter.
Look around for a local Ham who enjoys DFing (it's a hobby for some people - and a constant signal is almost a "gimmie"). Or do a bit of research on Direction Finding. There are people with experience who do not need a directional antenna - they use their body for a shield of their hand held receiver (and rubber duckie antenna).
Hope this helps you. Good luck
> Does anyone have a simple idea or plan on how to build a
> simple FM field strength meter that would be tunable to a
> particular frequency (ie, using an FM pocket radio)? I'm
> looking to build something that would show relative values
> to determine when I am getting closer or farther from the
> source of an illegal radio station.
>
For the purpose you describe an FM Field Strength Meter is NOT what I would suggest. You want to Direction Find (DF) rather than make specific measurements. You just want to home in on the signal.
My suggestion is to find or borrow any kind of receiver with relative receive indication (often called an S-meter). Many scanners, some Ham radios and some other types can receive FM broadcast band and have this relative receive indicator. Even high end receivers (got a power inverter so you can plug a home unit in to your car?).
But that's not all you'll need. Also important (maybe even more so) is a DIRECTIONAL ANTENNA. You can rig a simple beam on foamboard using wire (cut to the right dimensions and etc..). You could wield a 4 or 5 element beam fairly easily (the higher the FM freq the smaller the antenna). You swing the directional antenna around until it POINTS to the strongest signal (but realize that FM signals can bounce - so you may get a few false readings).
LoJack is a direction finding system. You see the police cars with FOUR 1/4 wavelength antennas (strategically positioned) on the roof. The freq is around 170 MHz. To oversimplify, they use the input from the 4 antennas (through a circuit that compares the signal strength and phase) and outputs a DIRECTION to the transmitting signal (or it's reflection). No need for a FM Field Strength meter.
Look around for a local Ham who enjoys DFing (it's a hobby for some people - and a constant signal is almost a "gimmie"). Or do a bit of research on Direction Finding. There are people with experience who do not need a directional antenna - they use their body for a shield of their hand held receiver (and rubber duckie antenna).
Hope this helps you. Good luck
Re: You want to Direction Find, not do precise signal measurements - here's an idea
Thanks for the suggestion. I've used a doppler direction finding device from Ramsey Electronics. It is complicated to use and gives sketchy results, plus it can quickly turn into a nightmare when the magnetic mount antennas start popping off the top of the car.
Typically the unlicensed broadcaster's location can be narrowed down really quickly based on intial complaints by driving in that direction using a car radio. They are easily found driving North-South and finding the peak of signal, then going East-West and finding that peak. This usually narrows it down to a few blocks so a meter of some sort would be great.
Any way to build an S meter into a pocket FM radio or digital tuning FM radio? I guess an S-meter is what I need.
> > Does anyone have a simple idea or plan on how to build a
> > simple FM field strength meter that would be tunable to a
> > particular frequency (ie, using an FM pocket radio)? I'm
> > looking to build something that would show relative values
>
> > to determine when I am getting closer or farther from the
> > source of an illegal radio station.
> >
>
> For the purpose you describe an FM Field Strength Meter is
> NOT what I would suggest. You want to Direction Find (DF)
> rather than make specific measurements. You just want to
> home in on the signal.
>
> My suggestion is to find or borrow any kind of receiver with
> relative receive indication (often called an S-meter). Many
> scanners, some Ham radios and some other types can receive
> FM broadcast band and have this relative receive indicator.
> Even high end receivers (got a power inverter so you can
> plug a home unit in to your car?).
>
> But that's not all you'll need. Also important (maybe even
> more so) is a DIRECTIONAL ANTENNA. You can rig a simple
> beam on foamboard using wire (cut to the right dimensions
> and etc..). You could wield a 4 or 5 element beam fairly
> easily (the higher the FM freq the smaller the antenna). You
> swing the directional antenna around until it POINTS to the
> strongest signal (but realize that FM signals can bounce -
> so you may get a few false readings).
>
> LoJack is a direction finding system. You see the police
> cars with FOUR 1/4 wavelength antennas (strategically
> positioned) on the roof. The freq is around 170 MHz. To
> oversimplify, they use the input from the 4 antennas
> (through a circuit that compares the signal strength and
> phase) and outputs a DIRECTION to the transmitting signal
> (or it's reflection). No need for a FM Field Strength meter.
>
>
> Look around for a local Ham who enjoys DFing (it's a hobby
> for some people - and a constant signal is almost a
> "gimmie"). Or do a bit of research on Direction Finding.
> There are people with experience who do not need a
> directional antenna - they use their body for a shield of
> their hand held receiver (and rubber duckie antenna).
>
>
> Hope this helps you. Good luck
>
Thanks for the suggestion. I've used a doppler direction finding device from Ramsey Electronics. It is complicated to use and gives sketchy results, plus it can quickly turn into a nightmare when the magnetic mount antennas start popping off the top of the car.
Typically the unlicensed broadcaster's location can be narrowed down really quickly based on intial complaints by driving in that direction using a car radio. They are easily found driving North-South and finding the peak of signal, then going East-West and finding that peak. This usually narrows it down to a few blocks so a meter of some sort would be great.
Any way to build an S meter into a pocket FM radio or digital tuning FM radio? I guess an S-meter is what I need.
> > Does anyone have a simple idea or plan on how to build a
> > simple FM field strength meter that would be tunable to a
> > particular frequency (ie, using an FM pocket radio)? I'm
> > looking to build something that would show relative values
>
> > to determine when I am getting closer or farther from the
> > source of an illegal radio station.
> >
>
> For the purpose you describe an FM Field Strength Meter is
> NOT what I would suggest. You want to Direction Find (DF)
> rather than make specific measurements. You just want to
> home in on the signal.
>
> My suggestion is to find or borrow any kind of receiver with
> relative receive indication (often called an S-meter). Many
> scanners, some Ham radios and some other types can receive
> FM broadcast band and have this relative receive indicator.
> Even high end receivers (got a power inverter so you can
> plug a home unit in to your car?).
>
> But that's not all you'll need. Also important (maybe even
> more so) is a DIRECTIONAL ANTENNA. You can rig a simple
> beam on foamboard using wire (cut to the right dimensions
> and etc..). You could wield a 4 or 5 element beam fairly
> easily (the higher the FM freq the smaller the antenna). You
> swing the directional antenna around until it POINTS to the
> strongest signal (but realize that FM signals can bounce -
> so you may get a few false readings).
>
> LoJack is a direction finding system. You see the police
> cars with FOUR 1/4 wavelength antennas (strategically
> positioned) on the roof. The freq is around 170 MHz. To
> oversimplify, they use the input from the 4 antennas
> (through a circuit that compares the signal strength and
> phase) and outputs a DIRECTION to the transmitting signal
> (or it's reflection). No need for a FM Field Strength meter.
>
>
> Look around for a local Ham who enjoys DFing (it's a hobby
> for some people - and a constant signal is almost a
> "gimmie"). Or do a bit of research on Direction Finding.
> There are people with experience who do not need a
> directional antenna - they use their body for a shield of
> their hand held receiver (and rubber duckie antenna).
>
>
> Hope this helps you. Good luck
>
A
Andy_Brown
Guest
Re: You want to Direction Find, not do precise signal measurements - here's an idea
>
> Any way to build an S meter into a pocket FM radio or
> digital tuning FM radio? I guess an S-meter is what I need.
>
>
> > > Does anyone have a simple idea or plan on how to build a
S metering is built around the front end of a tuner. It is not
"easy" to jury rig one on an existing front end without care or
the antenna/first stage will not work very well. BTDT
That's why I suggested the 70 buck kit in my first post.
In combination with the home brew yagi previously discussed,
you would get the job done.
Or spend that much on an inverter and use a tuner with a meter built in,
although consumer stuff with a "s" meter won't perform anywhere near as well.<P ID="signature">______________
Electricity is really just organized lightning.
~George Carlin</P>
>
> Any way to build an S meter into a pocket FM radio or
> digital tuning FM radio? I guess an S-meter is what I need.
>
>
> > > Does anyone have a simple idea or plan on how to build a
S metering is built around the front end of a tuner. It is not
"easy" to jury rig one on an existing front end without care or
the antenna/first stage will not work very well. BTDT
That's why I suggested the 70 buck kit in my first post.
In combination with the home brew yagi previously discussed,
you would get the job done.
Or spend that much on an inverter and use a tuner with a meter built in,
although consumer stuff with a "s" meter won't perform anywhere near as well.<P ID="signature">______________
Electricity is really just organized lightning.
~George Carlin</P>
DFing without an S-meter
> Typically the unlicensed broadcaster's location can be
> narrowed down really quickly based on intial complaints by
> driving in that direction using a car radio. They are
> easily found driving North-South and finding the peak of
> signal, then going East-West and finding that peak. This
> usually narrows it down to a few blocks so a meter of some
> sort would be great.
>
I have used Larson Mag mounts and Doppler DFing. It takes a bit of practice. I can do it without the Doppler. I have no problem with Larson Mag mounts coming off the roof and I've seen LoJack units running Magnetic mount antennas. An old trick was to use one mag mount off the back corner of the roof - the bigger ground plane area (opposite corner of the roof) would be the direction of the strongest signal on my S-meter (or by ear for loudest signal).
With the new information you've added (already narrowing your target to a neighborhood) here's another idea.
When I DF using my handie talkie (or hand held Bearcat scanner) and I'm in the neighborhood of the target - I remove the receive unit mounted antenna. Without an antenna I have to be really close to hear the signal. The closer I get the louder the signal until it's FULL QUIETING (no background noise). If your car radio uses a whip antenna, unscrew it and remove it while cruising that neighborhood. You could home in on the transmitter that way.
I do have an attenuator I can insert in the antenna line and add attenuation in several steps but I haven't used it in years (left over from 2-meter repeater DFing). Then there are those near field frequency counters with signal strength meter (like Opto-Electronics http://www.optoelectronics.com/ ) - you might be able to borrow one from a local Ham.
Seems like you'll get more hints here to help you. Good luck.
AL
> Typically the unlicensed broadcaster's location can be
> narrowed down really quickly based on intial complaints by
> driving in that direction using a car radio. They are
> easily found driving North-South and finding the peak of
> signal, then going East-West and finding that peak. This
> usually narrows it down to a few blocks so a meter of some
> sort would be great.
>
I have used Larson Mag mounts and Doppler DFing. It takes a bit of practice. I can do it without the Doppler. I have no problem with Larson Mag mounts coming off the roof and I've seen LoJack units running Magnetic mount antennas. An old trick was to use one mag mount off the back corner of the roof - the bigger ground plane area (opposite corner of the roof) would be the direction of the strongest signal on my S-meter (or by ear for loudest signal).
With the new information you've added (already narrowing your target to a neighborhood) here's another idea.
When I DF using my handie talkie (or hand held Bearcat scanner) and I'm in the neighborhood of the target - I remove the receive unit mounted antenna. Without an antenna I have to be really close to hear the signal. The closer I get the louder the signal until it's FULL QUIETING (no background noise). If your car radio uses a whip antenna, unscrew it and remove it while cruising that neighborhood. You could home in on the transmitter that way.
I do have an attenuator I can insert in the antenna line and add attenuation in several steps but I haven't used it in years (left over from 2-meter repeater DFing). Then there are those near field frequency counters with signal strength meter (like Opto-Electronics http://www.optoelectronics.com/ ) - you might be able to borrow one from a local Ham.
Seems like you'll get more hints here to help you. Good luck.
AL
C
CW
Guest
Re: You want to Direction Find, not do precise signal measurements - here's an idea
> S metering is built around the front end of a tuner. It is
> not
> "easy" to jury rig one on an existing front end without care
> or
> the antenna/first stage will not work very well. BTDT
Incorrect!! In a FM radio, it is usually a simple amplifier tapped off the receiver's last IF stage after the final filter and then the voltage is rectified to produce a voltage proportional to signal (In FM radios, the signal will saturate the meter quickly due to the limiters...where as in AM and other modes, the S Meter is much more "lazy" and is usually driven off the AGC circuit..so it is more precise in signal changes than FM S meters...)
HOWEVER a simple "quieting" circuit can be made for FM radios and is a noise based circuit (basically the beginnings of a squelch circuit)...only needs to be connected to the high side of the volume control...but the signal must be noisy to get a usable reading in directional finding.
> S metering is built around the front end of a tuner. It is
> not
> "easy" to jury rig one on an existing front end without care
> or
> the antenna/first stage will not work very well. BTDT
Incorrect!! In a FM radio, it is usually a simple amplifier tapped off the receiver's last IF stage after the final filter and then the voltage is rectified to produce a voltage proportional to signal (In FM radios, the signal will saturate the meter quickly due to the limiters...where as in AM and other modes, the S Meter is much more "lazy" and is usually driven off the AGC circuit..so it is more precise in signal changes than FM S meters...)
HOWEVER a simple "quieting" circuit can be made for FM radios and is a noise based circuit (basically the beginnings of a squelch circuit)...only needs to be connected to the high side of the volume control...but the signal must be noisy to get a usable reading in directional finding.
A
Andy_Brown
Guest
Re: S meter
> > S metering is built around the front end of a tuner. It
> is
> > not
> > "easy" to jury rig one on an existing front end without
> care
> > or
> > the antenna/first stage will not work very well. BTDT
>
> Incorrect!! In a FM radio, it is usually a simple amplifier
> tapped off the receiver's last IF stage after the final
> filter and then the voltage is rectified to produce a
> voltage proportional to signal (In FM radios, the signal
> will saturate the meter quickly due to the limiters...where
> as in AM and other modes, the S Meter is much more "lazy"
> and is usually driven off the AGC circuit..so it is more
> precise in signal changes than FM S meters...)
>
> HOWEVER a simple "quieting" circuit can be made for FM
> radios and is a noise based circuit (basically the
> beginnings of a squelch circuit)...only needs to be
> connected to the high side of the volume control...but the
> signal must be noisy to get a usable reading in directional
> finding.
>
No, not incorrect. Perhaps I should have said in a quality tuner ...
needless to say most of the schematics I have laying around do not support your claim (or mine actually). Most of them show meters being tapped at the first IF at the first filter. My point was that you can't easily build a decent indicator circuit and paste it into a tuner without having problems if the original design does not support that upgrade. Front ends in consumer devices are often sealed units and appear on schematics without detail.
Here is an example of a receiver with a pin out for RF strength right out of the front end
http://www.sbszoo.com/ve6atv/docs/23cmRxDwg.pdf
Here is a classic ... the Dynaco FM5 tuner ... with the sample being derived after the first IF amp ...
http://home.insightbb.com/~dunn.greg/FM5/schem.jpg
Thanks for your comments. I thought the emphasis in the post was a quick affordable reliable solution, and I maintain modifying a tuner circuit to drive a meter is not as easy as a FIM kit.
A.<P ID="signature">______________
Electricity is really just organized lightning.
~George Carlin</P>
> > S metering is built around the front end of a tuner. It
> is
> > not
> > "easy" to jury rig one on an existing front end without
> care
> > or
> > the antenna/first stage will not work very well. BTDT
>
> Incorrect!! In a FM radio, it is usually a simple amplifier
> tapped off the receiver's last IF stage after the final
> filter and then the voltage is rectified to produce a
> voltage proportional to signal (In FM radios, the signal
> will saturate the meter quickly due to the limiters...where
> as in AM and other modes, the S Meter is much more "lazy"
> and is usually driven off the AGC circuit..so it is more
> precise in signal changes than FM S meters...)
>
> HOWEVER a simple "quieting" circuit can be made for FM
> radios and is a noise based circuit (basically the
> beginnings of a squelch circuit)...only needs to be
> connected to the high side of the volume control...but the
> signal must be noisy to get a usable reading in directional
> finding.
>
No, not incorrect. Perhaps I should have said in a quality tuner ...
needless to say most of the schematics I have laying around do not support your claim (or mine actually). Most of them show meters being tapped at the first IF at the first filter. My point was that you can't easily build a decent indicator circuit and paste it into a tuner without having problems if the original design does not support that upgrade. Front ends in consumer devices are often sealed units and appear on schematics without detail.
Here is an example of a receiver with a pin out for RF strength right out of the front end
http://www.sbszoo.com/ve6atv/docs/23cmRxDwg.pdf
Here is a classic ... the Dynaco FM5 tuner ... with the sample being derived after the first IF amp ...
http://home.insightbb.com/~dunn.greg/FM5/schem.jpg
Thanks for your comments. I thought the emphasis in the post was a quick affordable reliable solution, and I maintain modifying a tuner circuit to drive a meter is not as easy as a FIM kit.
A.<P ID="signature">______________
Electricity is really just organized lightning.
~George Carlin</P>
T
THEInsultant
Guest
> > Does anyone have a simple idea or plan on how to build a
> > simple FM field strength meter that would be tunable to a
> > particular frequency (ie, using an FM pocket radio)? I'm
> > looking to build something that would show relative values
>
> > to determine when I am getting closer or farther from the
> > source of an illegal radio station.
> >
>
> http://www.northcountryradio.com/Kitpages/rffsm.htm
>
Get a scanner like this:
http://www.universal-radio.com/catalog/widerxvr/2488.html
Then, get a paper clip. Use the radio with it's antenna in weaker areas. Take the radio and put it about a foot from your stomach. Rotate yourself until you hear a "swish" in the audio. That's the null. Opposite of the direction you find that null you'll most likely find the direction of the station your trying to find. Sometimes stuff will bouce off other objects, so if you dont get a distict null, try a different location. (Sometimes you can walk several feet away and get a different reading, etc. Sometimes you need to go a few blocks away and try again) The distance from your body effects things greatly in the null dept., so you'll have to play with the distance a bit until you get the most noticable dip. Be sure to keep it consitant thoughout your reading once you find the proper distance.
Ok.. When you get in close, switch out the paper clip. You need to just flatten the clip out so it's straight and put it into the antenna connector on the radio. Repeat the "look for a swish" deal. If you get in really close you might have to take the clip out totally. (readings aren't as precise this way usually though looking for the null)
Honest to God, I've had better luck with doing it the "hard way" by just using my body as a way to find the null than setting up antennas or using a doppler B.S. You can jack around with all the stuff you have to lug around to do it the other way, or you can simply get real good at doing it with the minimal stuff. I prefer the minimal stuff. One huge advantage is you can keep the radio with you and look for problems wherever you are, quickly, without having to get other stuff out.
Hope this helps.
<P ID="signature">______________
--- THE Insultant ---</P>
> > simple FM field strength meter that would be tunable to a
> > particular frequency (ie, using an FM pocket radio)? I'm
> > looking to build something that would show relative values
>
> > to determine when I am getting closer or farther from the
> > source of an illegal radio station.
> >
>
> http://www.northcountryradio.com/Kitpages/rffsm.htm
>
Get a scanner like this:
http://www.universal-radio.com/catalog/widerxvr/2488.html
Then, get a paper clip. Use the radio with it's antenna in weaker areas. Take the radio and put it about a foot from your stomach. Rotate yourself until you hear a "swish" in the audio. That's the null. Opposite of the direction you find that null you'll most likely find the direction of the station your trying to find. Sometimes stuff will bouce off other objects, so if you dont get a distict null, try a different location. (Sometimes you can walk several feet away and get a different reading, etc. Sometimes you need to go a few blocks away and try again) The distance from your body effects things greatly in the null dept., so you'll have to play with the distance a bit until you get the most noticable dip. Be sure to keep it consitant thoughout your reading once you find the proper distance.
Ok.. When you get in close, switch out the paper clip. You need to just flatten the clip out so it's straight and put it into the antenna connector on the radio. Repeat the "look for a swish" deal. If you get in really close you might have to take the clip out totally. (readings aren't as precise this way usually though looking for the null)
Honest to God, I've had better luck with doing it the "hard way" by just using my body as a way to find the null than setting up antennas or using a doppler B.S. You can jack around with all the stuff you have to lug around to do it the other way, or you can simply get real good at doing it with the minimal stuff. I prefer the minimal stuff. One huge advantage is you can keep the radio with you and look for problems wherever you are, quickly, without having to get other stuff out.
Hope this helps.
<P ID="signature">______________
--- THE Insultant ---</P>
C
CW
Guest
Re: S meter
> No, not incorrect. Perhaps I should have said in a quality
> tuner ...
> needless to say most of the schematics I have laying around
> do not support your claim (or mine actually). Most of them
> show meters being tapped at the first IF at the first
> filter.
You can tap at any point really...MOST FM radios use the point in the last IF before the detector.....I have plenty of schematics to back me up on that...
> My point was that you can't easily build a decent
> indicator circuit and paste it into a tuner without having
> problems if the original design does not support that
> upgrade. Front ends in consumer devices are often sealed
> units and appear on schematics without detail.
Again, the FRONT END (RF AMP, MIXER) is NOT the place for a S Meter point...not enough gain to work properly.....(to get a 1ma meter to react, you need a LOT of gain...that is only done in the IF section of the rcvr where it has 80-120db of gain typical!)
> Here is an example of a receiver with a pin out for RF
> strength right out of the front end
>
> http://www.sbszoo.com/ve6atv/docs/23cmRxDwg.pdf
An amateur TV rcvr that uses a module in its "front end" that is actually a total rcvr is NOT what he was using and is NOT a typical FM receiver...if you LOOK, the output is a BASEBAND (BB) which is out of a detector (hence AFTER the IF stage)...the signal output on this module is NOT in the front end but in its IF inside the module...does not make your case at all. Rockwell Collins microwave radios use the same scheme (I have worked on them for 25+ years) and the rcvr signal metering is in the IF stage where there is sufficient gain.
> Here is a classic ... the Dynaco FM5 tuner ... with the
> sample being derived after the first IF amp ...
>
> http://home.insightbb.com/~dunn.greg/FM5/schem.jpg
If there is ample gain, it works BUT the S meter will not be very accurate as it will take a LOT of signal to get the meter to register. Again, late in the IF is best as the meter will be more sensitive to weaker signals.
> Thanks for your comments. I thought the emphasis in the
> post was a quick affordable reliable solution, and I
> maintain modifying a tuner circuit to drive a meter is not
> as easy as a FIM kit.
Again, I have added lots of S meters to FM radios....its not a big deal if you know where to tap...using a HIGH impedance, medium gain ckt at the 455 kHz or 10.7 MHz IF freq in normal FM rcvrs is not a problem and works fine. You evidently have not done S meter additions to scanners or other FM type rcvrs (narrow or wideband) which have been posted on the internet for years.
If it was not that easy, why are they there and why do they work?
I rest my case.
C.
> No, not incorrect. Perhaps I should have said in a quality
> tuner ...
> needless to say most of the schematics I have laying around
> do not support your claim (or mine actually). Most of them
> show meters being tapped at the first IF at the first
> filter.
You can tap at any point really...MOST FM radios use the point in the last IF before the detector.....I have plenty of schematics to back me up on that...
> My point was that you can't easily build a decent
> indicator circuit and paste it into a tuner without having
> problems if the original design does not support that
> upgrade. Front ends in consumer devices are often sealed
> units and appear on schematics without detail.
Again, the FRONT END (RF AMP, MIXER) is NOT the place for a S Meter point...not enough gain to work properly.....(to get a 1ma meter to react, you need a LOT of gain...that is only done in the IF section of the rcvr where it has 80-120db of gain typical!)
> Here is an example of a receiver with a pin out for RF
> strength right out of the front end
>
> http://www.sbszoo.com/ve6atv/docs/23cmRxDwg.pdf
An amateur TV rcvr that uses a module in its "front end" that is actually a total rcvr is NOT what he was using and is NOT a typical FM receiver...if you LOOK, the output is a BASEBAND (BB) which is out of a detector (hence AFTER the IF stage)...the signal output on this module is NOT in the front end but in its IF inside the module...does not make your case at all. Rockwell Collins microwave radios use the same scheme (I have worked on them for 25+ years) and the rcvr signal metering is in the IF stage where there is sufficient gain.
> Here is a classic ... the Dynaco FM5 tuner ... with the
> sample being derived after the first IF amp ...
>
> http://home.insightbb.com/~dunn.greg/FM5/schem.jpg
If there is ample gain, it works BUT the S meter will not be very accurate as it will take a LOT of signal to get the meter to register. Again, late in the IF is best as the meter will be more sensitive to weaker signals.
> Thanks for your comments. I thought the emphasis in the
> post was a quick affordable reliable solution, and I
> maintain modifying a tuner circuit to drive a meter is not
> as easy as a FIM kit.
Again, I have added lots of S meters to FM radios....its not a big deal if you know where to tap...using a HIGH impedance, medium gain ckt at the 455 kHz or 10.7 MHz IF freq in normal FM rcvrs is not a problem and works fine. You evidently have not done S meter additions to scanners or other FM type rcvrs (narrow or wideband) which have been posted on the internet for years.
If it was not that easy, why are they there and why do they work?
I rest my case.
C.
C
cranium
Guest
Re: You want to Direction Find, not do precise signal measurements - here's an idea
I witnessed an engineer hone in on a illegal radio station once by driving his car with a retractable antenna in the trunk...we drove to where the signal was good and rove n/s and e/w until we got full quieting with the antenna fully down.
Worked! Much like an earlier poster had suggested.
I witnessed an engineer hone in on a illegal radio station once by driving his car with a retractable antenna in the trunk...we drove to where the signal was good and rove n/s and e/w until we got full quieting with the antenna fully down.
Worked! Much like an earlier poster had suggested.
L
Les
Guest
recipe
What you want to do is very much like the amateur radio sport "Fox Hunting".
Lots of good ideas on the web if you try "ham radio fox hunt" in any
decent search engine.
Here's one design for a nice directional antenna that you could couple
to a portable FM receiver just by fully retracting the built-in antenna
and winding a couple of turns of one of the antenna leads around a small
stub of it and (if you can) grounding the other to any metal part of the
case:
<a target="_blank" href=http://www.qsl.net/n6bg/thunt/yagi.html>http://www.qsl.net/n6bg/thunt/yagi.html</a>
I've used this in fox hunts with good results.
Yes, the dimensions are on the short side for FM but even so it's
pretty good at FM frequencies. If you want perfection just ask and
people with better memories than have I will surely provide the
formula for determing the exact dimesions for the partiular station
you're seeking. But, be ye warned, there's a good chance the
elements will be as tall as you are!
<P ID="signature">______________
Is THIS what the congressman meant when he suggested bombing Mecca?
<a target="_blank" href=http://www.city-data.com/zips/92254.html>http://www.city-data.com/zips/92254.html</a></P>
What you want to do is very much like the amateur radio sport "Fox Hunting".
Lots of good ideas on the web if you try "ham radio fox hunt" in any
decent search engine.
Here's one design for a nice directional antenna that you could couple
to a portable FM receiver just by fully retracting the built-in antenna
and winding a couple of turns of one of the antenna leads around a small
stub of it and (if you can) grounding the other to any metal part of the
case:
<a target="_blank" href=http://www.qsl.net/n6bg/thunt/yagi.html>http://www.qsl.net/n6bg/thunt/yagi.html</a>
I've used this in fox hunts with good results.
Yes, the dimensions are on the short side for FM but even so it's
pretty good at FM frequencies. If you want perfection just ask and
people with better memories than have I will surely provide the
formula for determing the exact dimesions for the partiular station
you're seeking. But, be ye warned, there's a good chance the
elements will be as tall as you are!
<P ID="signature">______________
Is THIS what the congressman meant when he suggested bombing Mecca?
<a target="_blank" href=http://www.city-data.com/zips/92254.html>http://www.city-data.com/zips/92254.html</a></P>
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