FM transmitter

Thread Starter

Stuart66

Joined Jun 3, 2022
1
Hi guys
I’m an absolute amateur trying to rig something for the weekend.
I’ve seen that you can hack FMT for cars to increase the range. Basically you take out a resistor and add a bigger aerial.
I’m attempting this with a Scoshe fmt4 but cannot see what is what!
many help gratefully received?
cheers Ess
 

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Audioguru again

Joined Oct 21, 2019
4,811
If you increase the range of an FM transmitter then you need a license from the government to operate a certified transmitter on a vacant frequency selected by them. The low range ones do not need to be certified and do not need a license.

The reason is that it is illegal to cause interference to anybody listening to a certified and licensed radio station.
 

Papabravo

Joined Feb 24, 2006
18,839
Hi guys
I’m an absolute amateur trying to rig something for the weekend.
I’ve seen that you can hack FMT for cars to increase the range. Basically you take out a resistor and add a bigger aerial.
I’m attempting this with a Scoshe fmt4 but cannot see what is what!
many help gratefully received?
cheers Ess
I saw those same instructions in a rest area toilet stall. What a coincidence!
Is it possible that it was just a cruel joke?
 

Papabravo

Joined Feb 24, 2006
18,839
"The Micromitter" project and Kit from Silicon Chip magazine in Australia has an RF attenuator on its antenna connection to make its short range legal.
I guess if you built the kit and forgot those 3 resistors, or substituted alternate values, you might have a barely plausible defense. Taking them off of a commercial product and replacing them with jumpers or alternate values would be risky. No plausible defense about not knowing what you were doing.
 

sparky 1

Joined Nov 3, 2018
718
By not hacking the delicately tuned FM transmitter additional problems will be reduced. Even if it is possible it may not be recommended.
There are some who over rate their hacking skills, a closer look usually shows a significant reduction in signal quality.
You can design a boost circuit however there are ready made amplifier stage modules that will increase gain slightly for extended range,
 
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Audioguru again

Joined Oct 21, 2019
4,811
The laws are made to protect people, do not be illegal.
Hacking the existing low range transmitter to be overloaded and increase its range might cause harmonics interference on 911 emergency wireless phone calls and police, ambulance and fire department communications.
Other problems might be with wireless burglar alarms and garage door openers. Interference to wireless security cameras?
 

col_panek

Joined Oct 30, 2015
13
I hacked one of those car FM transmitters once. Opening it up, I found a pad labeled "ANT" which had an inch of wire feeding it. Not sure whether they just forgot the wire or not. I checked it with a spectrum analyzer a few feet away and it was 20 dB lower than a broadcast station 25 miles away. When I soldered on a quarter wave wire it came up to comparable to the broadcast station, I plugged it into my laptop audio, using a USB for power (the internal regulator takes the car 12V or the USB 5V down to 3.3V, although the light doesn't work). Presto, instant radio station, in stereo and crystal controlled. Pretty good technology for ten bucks. I think I used it at a party once.

I checked with a portable receiver and it faded out in a hundred feet. Before the antenna mod, it wouldn't even reach my car radio because the antenna is on the roof. I think there was a rudimentary LP filter, but the harmonics were undetectable anyhow.

If you don't have a spectrum analyzer handy, best to leave the modding to professionals who can carefully calculate and measure field strength.
 

bassbindevil

Joined Jan 23, 2014
463
The Belkin Tunecast transmitters are often so weak they don't even work with a car radio antenna 6 feet away. There's little chance of interfering with aircraft if you bypass the inductor going to the antenna wire. If you google, you'll find a lot of interest in modifying them for use with holiday light displays set to music.
 
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