is it possible to increase signal output of this FM transmitter?

[dennisyuki]

I have this FM transmitter.
http://www.monoprice.com/Product?c_id=108&cp_id=10844&cs_id=1084406&p_id=10402&seq=1&format=2

I know the FCC limits the signal output of these devices for retail sales.  But I was wondering is it possible for the consumer to modify the circuit after it is purchased to increase the signal output.

What I want is to have the transmitter over power the weak radio station that is playing on a specfic freqency on the radio.   

This product works great but in the area where I live practically every radio frequency has a station playing on it.  so sometimes the weak radio signals messes with my transmitter signal.   I just want to overpower the outside signals that my radio is picking up. 

Is this possible?  If so, how.  If not, why?   

[AG6QR]

It's possible, but illegal, to connect an amplifier to the output stage of the transmitter.  I won't give any more direction along this avenue, because I don't advocate overpowering licensed broadcasters, even in a small area.  Nor do I advocate violating the FCC rules regarding unlicensed transmitter output power.

What is most likely legal is to improve the antenna coupling between the transmitter and intended receiver.  Done correctly, this won't increase the radiated power of your transmitter, but it can still allow it to be received.  The simplest solution is to bring your transmitting and receiving antennas closer together.  Assuming you're using this transmitter in a car, as it suggests, then a variation would be to use a "y" adapter like the following to connect your receiver to two antennas

http://www.vetco.net/catalog/product_info.php?products_id=8819&gclid=CKTxy5ziy8ICFUKUfgodmgcAiA

One side of the Y can go to your existing vehicle antenna, and the other can go to a length of wire which you wrap around the transmitter's cord.  That would probably provide enough coupling so that the transmitter can be clearly heard.

[Dave Turner]

Why would you want to 'overpower' a legitimate signal?

[Rick Law]

Why would you want to 'overpower' a legitimate signal?

The OP was quite clear about why.  He has one of those gizmo that uses FM to play it's audio.  I have a couple of MP3 player that uses FM via low power broadcast.

I am also in an area saturated with weak stations from afar.  I too wouldn't mind to take control inside the space within my car to play my MP3 players.  When the signal is unwanted, this legit station is intruding into my personal space.

I think the OP has similar application in mind instead of anything naughty.

[babysitter]

Back in the CD Changer days, there was a device which was better suited for this job inside cars:

A little FM Transmitter that went into the Cable from the Antenna to the radio. A tiny relay switched from Antenna-only to Transmitter-only reception. I had such a module from scrapheap, made by a reputable source, I think a Matsushita brand.

The local radio would have to cross that relay.

[GEuser]

Just put a decent antenna on it imo .

The antenna in it is probably designed so that it is the 15ft limit .

Same goes for those weather wireless gadgets and the likes , some around only 10 metres distance , poke on a good radiating element cut for the frequency (a bit of wire) and 30+ metres .

[nixfu]

Just be sure you do realize that those federal regulations apply to the consumer as well as the manufacturer. 

BTW, the  biggest FCC fines given to individuals of $10-20,000 happen for exactly what you want to do... interfere with a commercial radio service.  All it will take is someone who is a fan of the station you are 'overpowering' to complain, and you could be in big hot water.


The safest thing to do is find how maybe to switch it into "EU" mode and get some slightly extend frequencies that are not used in the USA but are used in Europe for commercial FM radio such as 87.1 mhz and use those with a non-modified unit.  Then you will have a clear frequency to use.  This was the trick everyone used with the very popular iTrip.  They were very hard to use in any city setting without using this trick.  I don't see it mentioned on their website above, but since they(monoprice) don't actually manufacture anything themselves, maybe you could find out more about this device by finding the same device on a different location on the web etc.. it usually consists of holding done some combination of buttons when turning on, etc.   For instance, here is the SAME cheap chinese transmitter that monoprice sells on ebay: http://www.ebay.com/itm/FM-Transmitter-3-5mm-Audio-Car-Charger-for-iPhone-4-4G-4S-iPod-Nano-Touch-US-/300964039170

Probably the next safest thing would be to lengthen the antenna wire. This will increase the radiated power a little bit, but would have much less chance of adding additional interference products, harmonics or anything such as that compared to if you went to adding more amplification power or something that actually modified the transmitter signal.   The easiest will be to increase the length of wire on the receiver to add more coupling between the two devices as someone suggested above.   

I guess if would help if you mentioned if this is for a car/auto purpose or a home purpose.

[jeroen79]

Why would you want to 'overpower' a legitimate signal?

The OP was quite clear about why.  He has one of those gizmo that uses FM to play it's audio.  I have a couple of MP3 player that uses FM via low power broadcast.

I am also in an area saturated with weak stations from afar.  I too wouldn't mind to take control inside the space within my car to play my MP3 players.  When the signal is unwanted, this legit station is intruding into my personal space.

I think the OP has similar application in mind instead of anything naughty.

Naughty in this context doesn't require any malevolence towards others.
Using it for your mp3 player in your car is innocent in itself but not taking into account how it affects others -who have greater rights- and only caring about your own pleasure is naughty.

'Personal space' does not apply to the radio spectrum.
The legitimate broadcaster has a right to transmit on a given frequency without interference and you have the right not to tune to that frequency.
This is true even inside your car and your house.

You can transmit illegally but you would need to keep a low profile and not bother others if you don't want to get caught.

[ConKbot]

http://www.minicircuits.com/MCLStore/ModelInfoDisplay?14188207777120.36117321028750493 

But honestly, the best solution would be a "Wired FM modulator" that hooks inline between the antenna and the radio. Its what Babysitter mentioned earlier in the thread.

[Dave Turner]

In the UK in the early 80's after a long 'battle' CB became legal for people without having to take exams. It permitted broadcasting on 40 channels using FM. Note that this was years before mobile/cell phones became the norm. Power was limited to 4 Watts and you were meant to normally limit power to 400 milliWatts where practical, if I recall correctly.

For a few months one could talk over relatively long distances - then came Christmas and it seemed like everyone had one. Then the problems started. Consider an urban environment where there were far more CB users than had been envisaged and the fact that FM reception tends to lock on to the more powerful transmission. AM & SSB transmissions bled over allowing one to hear the weaker signal at the expense of noise.

With FM in local cells possibly as small as a few blocks (to use an Americanism) communication was possible. Further than that communication was sporadic depending on the channels in use. In rural areas and whilst travelling communication was much easier.

Now some people, using a number of methods. boosted the power output of their transmitters which effectively overrode other transmissions. I specifically remember one individual who, after repeatedly requesting certain users to stop this practice tied in a dirty power booster keyed the mike and took the day off. This effectively blocked communications within a substantial area.

The analogy is not exact compared to the originator's question; but it is pertinent!

As it happens I acquired the boosted transmitter which still in my possession and has not since been used other than with either a dummy load or an attenuator.

I for one do not approve of blasting the available spectrum.

[SeanB]

I know of one CB operator who had a linear amplifier that had an unfortunate property of cooking birds sitting on it when keyed. He was a little over the legal power level.............

[Dave Turner]

Fast food?

[SeanB]

The dog did love hid instacooked meals though.

[Richard Crowley]

is it possible for the consumer to modify the circuit after it is purchased to increase the signal output.

Yes, it is certainly possible. But it is almost impossible that it would be legal (or even "decent" if you don't care about the law).

What I want is to have the transmitter over power the weak radio station that is playing on a specfic freqency on the radio. 

And that is explicitly illegal in most civilized jurisdictions, most certainly in the USA.

This product works great but in the area where I live practically every radio frequency has a station playing on it.  so sometimes the weak radio signals messes with my transmitter signal.   I just want to overpower the outside signals that my radio is picking up. 

Your adjective "practically" suggests that there are "some" channels that are interference-free.  It is not clear why you don't simply select a free channel.
If you had revealed what city you are in, it is trivial to look up all the FM channels and see what the power and distance is to every transmitter in your market.

[dennisyuki]

I live in a big metropolitan area in the southeast.  I feel that every channel has something on it or even a strong static that interferes with the fm transmitter's signal quality.  I do not know what the wattage is on the transmitter or even if it is set at its max allowable consumer level wattage.  But I just though I could increase the signal strength just enough to cover my "personal space" in my car, as another poster called it. My goal is not to broadcast to a large geographic area or cause problems to others around me, just get a clearer signal in my car.   Plus I want to learn about FM signal strength circuitry.

I like the ideas others have posted about splitting the antenna line.  That is basically what I am trying to do is get the signal picked up stronger by the antenna.  And moving the antenna line closer to the transmitter might work.

But,  I guess the best way to fix this problem is to add an AUX line to my radio, but I dont want to mess with pulling out my radio at this moment.   

is it possible for the consumer to modify the circuit after it is purchased to increase the signal output.

Yes, it is certainly possible. But it is almost impossible that it would be legal (or even "decent" if you don't care about the law).

What I want is to have the transmitter over power the weak radio station that is playing on a specfic freqency on the radio. 

And that is explicitly illegal in most civilized jurisdictions, most certainly in the USA.

This product works great but in the area where I live practically every radio frequency has a station playing on it.  so sometimes the weak radio signals messes with my transmitter signal.   I just want to overpower the outside signals that my radio is picking up. 

Your adjective "practically" suggests that there are "some" channels that are interference-free.  It is not clear why you don't simply select a free channel.
If you had revealed what city you are in, it is trivial to look up all the FM channels and see what the power and distance is to every transmitter in your market.

[Richard Crowley]

If you transmitter can't even overcome open-channel background noise, it may be broken, or else you need much better coupling to your antenna.

[janoc]

Um, guys, wouldn't it be just simpler to use, you know, a piece of wire between whatever your are playing the music from and your stereo?

Hacking in an AUX input will likely take much shorter than fudging with this gadget - and with better audio quality and no risk of fines too!

And if you really really must have wireless, get a stereo that has Bluetooth connection. The BT transmitter dongles cost peanuts online.

[edavid]

Um, guys, wouldn't it be just simpler to use, you know, a piece of wire between whatever your are playing the music from and your stereo?

Hacking in an AUX input will likely take much shorter than fudging with this gadget - and with better audio quality and no risk of fines too!

I guess you have never pulled an OEM stereo out of a dashboard and taken it apart to figure out where to connect an input.  It is anything but simple, in fact it is a huge hassle, and the input part is often impossible unless you resort to the blank CD trick.

OP, the first thing to do is to try a different transmitter.  I have had success with Griffin brand, but I'm sure there are other good ones.  If you do need an inline FM modulator, try to find a Kenwood.

[Rick Law]

Um, guys, wouldn't it be just simpler to use, you know, a piece of wire between whatever your are playing the music from and your stereo?

Hacking in an AUX input will likely take much shorter than fudging with this gadget - and with better audio quality and no risk of fines too!

I guess you have never pulled an OEM stereo out of a dashboard and taken it apart to figure out where to connect an input.  It is anything but simple, in fact it is a huge hassle, and the input part is often impossible unless you resort to the blank CD trick.

OP, the first thing to do is to try a different transmitter.  I have had success with Griffin brand, but I'm sure there are other good ones.  If you do need an inline FM modulator, try to find a Kenwood.

If only a 3.5mm jack is standard on the front of car stero...

I have an old car with cassette tape player.  I ended up abandoning the FM broadcast MP3 player and use a cassette audio adaptor - a 3.5mm jack audio input into a fake cassette that sends the signal to the tape head.

But car with cassette player is rarer than hens teeth these days.