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Fm transmitter (BH1415F) range boost options

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New Member
Hi, I've been trough lots of fm transmitter discussion but i'm looking more for a cookbook approach to solving my problem. I'm looking for input on solutions and correction when my assumptions are wrong :)

Goal: I want to extend the range of a centronix fm transmitter for car radios built around the BH1415F chip. I am aware of the legal aspects of such undertaking.

The broadcast should be well received to regular car type quality receivers or portable mp3 receivers within 100 meters in an open space. The sound quality will be dvd. And i want to do this cheaply, less than 50$. I have a 'scope and electronics experience (logic, audio, opto, ) but no RF experience.

The way i see it, there is either some antenna configuration that makes this range possible, or i build a RF amplifier which takes the ouput from the BH1417F chip). Still i want to minimize RF energy going outside of the receivers area.

Question1 : How do i calculate how much RF energy needs to be broadcast at this range (100m)? Is there a cookbook approach to this? From this i can determine how much amplifying in need.

BH1415F info:
Specs (click here to download) say the RF power (Transmission output level) is typically 100dBuV. If i convert this to milliwatts (converter) i gives me 20.00046. So this is the RF power going out of pin 11. Is my logic correct? And how sensitive are cheap car radio, and how does it relate?

Question 2: Can a passive or inexpensive active antenna amplify the BH1415F signal to the level required?

Question 3: If an (cheap) antennae cannot do it then i found various transmitters schematics but they are probably either too powerful or involved.
I am open to suggestions as to which design to use. I have thought of using part of Audioguru's fm transmitter (mod4) for this job, or this Ipod 1w xmitter which seems a bit overkill.

Question 4:
If i use Audioguru's circuit i would need to couple BH1415F pin 11 (apparently expecting 50ohms impedance) to Q3. Here i am totally at a loss on how to calculate and implement the impedance matching. I'm looking for pointers or a cookbook approach / reference on soving this.. i'm getting back into electronics after 25 years and those particular memories have been recycled long ago :)

Thanks for you patience and help.

mvs sarma

Well-Known Member
BTW, how many watts is Audioguru's mod4 transmitter?

Range depends on the issue of plain rural ,open area or congested concrete forest of City area.

As AudioGURU himself told, it covered an range of almost 2KM across an open area,with a good Receiver, even with a nominal antenna. Wattage emitted , depends more on impedance matching of the antenna.
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Well-Known Member
Most Helpful Member
The FM stereo transmitters made for cars have an attenuator at their RF output. The Micromitter project and kit has an attenuator that reduces the output to 10uW so that there is less interference to nearby radios and so the output power is legal.
You can remove the attenuator (it is on the circuit board) then the range will be maybe 500m.

My FM transmitter is not stereo so its range is much more.

FM stereo (it is old) is not DVD (it is new) quality.
The frequency response is 50Hz to 15kHz, not 20Hz to 20kHz.
The signal to noise ratio is 60dB, not 110dB.
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New Member
Thanks for your clarifications

Thank you all for your input!

I had not realized Mod4 was mono, had not thought about the frequency response (FM quality will do) and never thought the micrometter would get 500m without the attenuator.

I now have my plan: properly understand antenna matching, then modify the attenuator in my centronic fm transmitter to get 100m. The output is about the same for both chip.

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