FM Transmitter signal boost

[Dr_Taco_Cat]

Hello, I am trying to increase the range of my belkin FM transmitter. It's called a belkin Tunecast auto, the model number is F8Z343, and i'm trying to increase it's broadcast range. It's range is very poor, only about 10 feet or so, and its signal strength is also poor, even on a almost clear station. My music gets staticy when the hosts radio station has even a tiny amount of incoming signal strength. I normally use the station 107.3 FM which is always static in my area, but when I get 15 minutes out of my area the station has some strength. ( The station sounds horrible, and is almost all static, but it still interferes with the transmitter) After some research I found how other people have modded similar belkin transmitters, but when I opened mine it had 2 different antennas. I have no idea which antenna to modify, one is labeled RX_ANT and the other is TX_ANT. The RX_ANT has a resistor connected to it so i'm assuming its the one I should be using, but I figured I would ask first to be 100% sure. My plan is to remove the resistor, increase the antenna lead and hopefully increase it's range and strength.

The TX_ANT has a white wire looping around the front of the transmitter and the RX_ANT has a black wire going up to the Ipod connector.

Hopefully the pictures will help, and thanks for any help you can give me.

[Richard Crowley]

It feels like we've been through this already before.
Why does your transmitter have a "receive antenna"?
What does "wire looping around the front of the transmitter" mean?
If you are getting broadcast interference on 107.3 why don't you use a different channel?
Why do you need more than 10 feet of range?
You seem to want more out of this technology than is available within the law.

[richms]

RX antenna is for the free channel scanning capability which claims to scan the band and find the best channel to use.

Adding a piece of wire to the TX antenna will get you more range. The name brand transmitters tend to operate legally within what power they are allowed to transmit with.

Cheap ones off ebay out of china usually have a much much better range than the name brand ones but you are transmitting with more power than you are allowed to. Buy noone seems to care. I got a $4 one off aliexpress thats also a MP3 player off a USB drive or small SD card (cant handle more than 2 gigs) and it has a fairly decent range. Will get 50 meters on a totally clear channel, and if I put it ontop of a full power FM station (I am in a bit of a signal black hole here) will overpower it for about 10 meters, so it has a lot of power behind its signal.

Sounds aweful, but on a car stereo with no better option like a 3.5mm or bluetooth then its a huge step up from having to listen to commercial radio.

So to sum up, before bothering to mod your belkin, look for a cheap chinese transmitter that will probably do what you want with no modifications needed.

[Dr_Taco_Cat]

It feels like we've been through this already before.
Why does your transmitter have a "receive antenna"?
What does "wire looping around the front of the transmitter" mean?
If you are getting broadcast interference on 107.3 why don't you use a different channel?
Why do you need more than 10 feet of range?
You seem to want more out of this technology than is available within the law.

What do you mean by "It feels like we've been through this already before."
I have no idea why there is a receive antenna, but I assume its for scanning for available channels.
Under the plastic in the front of the case a white wire loops around the screen and buttons.
I have tried all the other channels and 107.3 is the clearest within a 20 mile radius of me.
I don't need more then 10 feet of range, but I need more signal strength.

I think my transmitter has a weak output. I have picked up other peoples car FM transmitters on the same channel and its crystal clear plus it completely washes my transmitter out.

[Zero999]

This question is asked fairly frequently here. People are reluctant to help because it's probably illegal but so is exceeding the speed limit by 5 miles per hour on a big wide road when no one's around and the speed limit is stupidly low anyway.

Yes, the RX antenna is probably to scan the band for a space.

Cheap Chinese transmitters often have poor sound quality. They're often mono and lack pre-emphasis so the treble is lost and it sounds like an AM radio station. You could probably make a better sounding transmitter yourself.

A ¹/₄ wavelength antenna would be optimal. The wavelength of 107.3MHz is 2.8m and a quarter of that is 70cm or 27¹/₂in.

[richms]

This looks the same as the one I got that has fantastic range and strength.

more signal will give more range and be able to overpower more noise. I find with 30something full power stations on the FM band it takes a decent amount of power to get thru all the general noise over the whole band. Places with fewer stations will probably be ok with those low power ones from the big brands.

http://www.ebay.com/itm/4in1-Car-MP3-Player-Wireless-FM-Transmitter-Modulator-USB-SD-CD-MMC-Remote-HT-/281532076247?pt=LH_DefaultDomain_0&hash=item418c9e90d7

[SeanB]

If buying one of those for the car get the RDS version, looks the same but has the nice advantage of putting the song info ( off the MP3 Idtag) on the RDS display of any car radio.

[katzohki]

There are some legal requirements that you must meet in the US for these. The rules are covered under the Part 15 of the FCC rules. One thing worth pointing out is that the minimum fine is $10000. Increasing the antenna length is one thing you could do, shoot for 1 wavelength long. Another would be to increase the height of the transmitter antenna (height above average terrain helps in these frequencies).

I don't remember if the spec is ERP or just power to the antenna, but the rating is 100 milliwatts. If your device isn't already putting that much power there are things you could do to boost it up while staying in the legal limit. Look for a power or RF amplifier that has a P1 rating of 20 dbm or less if you want to try to tackle that. And obviously rated for the frequency too.

[Howardlong]

shoot for 1 wavelength long.

...more likely by quite a long way not to be 1 wavelength, the output is likely to be a low impedance drive expecting something like lambda/4. 1 lambda is not going to be a good match, it'll look like an open circuit without matching, so most of your transmit power will be reflected back into the unit rather than be radiated at all (other than as heat).

[Zero999]

shoot for 1 wavelength long.

...more likely by quite a long way not to be 1 wavelength, the output is likely to be a low impedance drive expecting something like lambda/4. 1 lambda is not going to be a good match, it'll look like an open circuit without matching, so most of your transmit power will be reflected back into the unit rather than be radiated at all (other than as heat).

I agree. Try ¹/₄ lambada first, then connect another ¹/₄ to the 0V rail, as near to the antenna output as possible, ( making ¹/₂ wavelength centre fed dipole) if needed.

[katzohki]

Fair enough! 1 Wavelength isn't terribly long at those frequencies so it's easy enough to try a few times. Not too terrible short either.

[Zero999]

Fair enough! 1 Wavelength isn't terribly long at those frequencies so it's easy enough to try a few times. Not too terrible short either.

Longer isn't always better though. The optimum length antenna is resonant and the frequency of interest.

[katzohki]

Fair enough! 1 Wavelength isn't terribly long at those frequencies so it's easy enough to try a few times. Not too terrible short either.

Longer isn't always better though. The optimum length antenna is resonant and the frequency of interest.

I meant multiples / fractions of the wavelength are easy to deal with for experimenting. 3MHz is a bit large to work with easily, 30GHz is a bit small... etc