Car fm transmitter boost help

Thread Starter

Német Tamás

Joined Mar 3, 2016
Hello i need some help here is a car radio transmitter , i want to boost the signal and i cant find the resistor witch lower the signal.
somebody can help me?




Joined Apr 5, 2008

It looks like the circuit is on 2 PCBs.
I can not see the display cable on the first picture.



Joined Apr 30, 2011
No, you can't boost the transmitter by removing the imaginary resistor which limits its power. These things aren't made with extra power which is secretly suppressed by a special circuit.
Actually, most of them kind of are. Most are based on a Rohm BH1417F chip (datasheet linked below) which has an intrinsic ability to drive up to 450mW to the antenna but to qualify under FCC rules for sale in the USA, they have a pi or T attenuation network on the output that typically reduces the output power to a pitiful 1% of that potential. Combine that with a very non-optimal antenna, often just a few inches of wire, and you have a transmitter that's so weak that it has to be within a few feet of the receiving antenna to be received clearly. By bypassing and/or removing attenuator components, the effective range can easily be extended to over 100 feet and the signal is strong enough to overcome interference from distant broadcast transmitters.

Having said all that, the modification is illegal and may not work on all transmitters, especially those not based on the aforementioned chip.
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Joined Apr 30, 2011
It's not always possible and sometimes the critical component is a capacitor, not a resistor. The device you have may not be a good candidate for modification. If you have an oscilloscope, you can identify the RF output and go from there.


Joined Nov 4, 2013
I see one connection point labeled ANT. That's the antenna output so follow that back through surface mount resistors and see what sort of circuit it is using.

If it just uses a simple resistor voltage divider or power limiting setup somewhere between the ANT connection point and whatever part of the circuit is generating the FM signal it may be possible to simply bypass that or change out the resistors to substantially increase the antenna power.

I've done that to many wireless remote devices over the years and it works well.
I have a wireless remote power outlet switch unit that when I got it was useless at anything over 30 feet so I took it apart and found to used a simple resistor power limiting system on the transmitter. When I was done it would easily do 500 feet on a a good battery and all I did was change out two 10K resistors for 470 ohm ones Wasn't surface mount though.


Joined Nov 30, 2010
I guess the strantor method worked. Say it can't be done and a dozen people will pop up with how to to it.:D