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Old 4th September 2005, 05:02 AM   #1
diyAudio Member
Join Date: May 2005
Location: Mumbai
Default My Fm Transmitter Query

Hello friends.

i have a 1WATT FM Transmitter schematic, to which i design a PCB using Express PCB soft..

at the test, transmission clairity is very good but only for an OVER THE AIR distance of 50-55 mtrs max.. surely much less than my expectation of around 500-600 mtrs atleast...

i used a dipole antenna for transmission with a decent RG8 cable. And the class C amplfier 2N3866 doesnt disspiate significant amount of heat. using a scope i observe no amplification of the osc. freq. arcoss its o/p.. can anyone suggest some modifications to be to the design..

i have posted the blurred schematic of my Tx.. I feel the need for greater inductance across Class C biasing.. and removing Trimmer 2; the one just before the Tx o/p..

Please advice..


Expectation leads to disappointment...
More expectations lead to more disapoinments.
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Old 4th September 2005, 06:42 AM   #2
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Join Date: Mar 2003
Location: Willmar, Minnesota
What are the element lengths of your dipole antenna? Is the antenna near anything which might detune it? Are you sure your output impedance matches the antenna's input impedance?

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Old 4th September 2005, 07:03 AM   #3
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Join Date: May 2005
Location: Mumbai

The total dipole length is around 2 meters (1 + 1 mtr). Impedance of the cable is 75 Ohms, as rated with the o/p impd. of the transmitter.. The tx. is tuned to freq. of 102.3 MHz.

And if i use a simple 100cm aerial, then too the range is around 15-20 meters.. not much diff. as compared to the dipole range..

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Old 6th September 2005, 07:12 AM   #4
diyAudio Member
Join Date: May 2002
Location: Switzerland
Some thoughts: Wavelength at 100 MHz is 3 meters approx so a half-wavelength dipole is 1.5 meters and a quarter-wavelength aerial is around 75 cm.

When using the dipole then use is vertically, since your receiver's antenna is most probably also vertically polarised.

Have you ever checked the output power of your TX into a resistive load ?


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Old 6th September 2005, 07:49 AM   #5
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Get yourself a VSWR meter so you can tune your antenna properly.
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Old 9th September 2005, 11:13 AM   #6
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Join Date: May 2005
Location: Mumbai
Originally posted by TwoSpoons
Get yourself a VSWR meter so you can tune your antenna properly.

Could you elaborate more on that pls..
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