VHF FM Transmitter range problem please help

[rohs]

https://i45.tinypic.com/2442quu.jpg

Hi,
I'm using this schme for study, it's used to transmitt audio signal to vhf band 120 Mhz FM modulation to receiver (the receiver is SDR radio in the our lab).

it's works very good, stable frequency, good voice quality, and low power consume.

but the transmittion distance is not more than 40 meter in open field. I need to amplify the R.F power output of this transmitter. but i don't know how. could you please help me?

the antenna is the audio cable itself. it's works great. long cable of about 1.5m . I also tried to use standard antenna and no better result.

pleae note that i need to use this layout, i mean i can change and remove component, but i can't change the layout of the scheme because we already maked the PCB.

note: this transmitter used for study purpose only.

Thank you.

 

[pe2wdo]

Make sure the LC circuits at the amplifier stages are in resonance.
Replace the fixed capacitors over L2, L3 and L4 by variable capacitors of 22pf or 30 pf and tune the circuits at resonance.
If that doesn't do the trick you could replace Q4 for a more powerfull (RF) transistor.

 

[rohs]

the LC at max resonace, i don't have variable capacitor, so i insert ferrit to the coil for the max amplitud. i also tried power RF transistor and no really different found.
maybe i don't understand the operation of this circuit?

Q1 is just preamp.
Q2 is the oscillator at 120MHz, powered by 5v.

Q3 is rf amplifier, class A, but i don't understand why 47p capacitor connected from the base of Q3 to the ground, i know that capacitor is short for high frequecny so this capacitor should short the signal from Q2 to the ground... i don't understand what is the purpose..

Q4 is second RF amplfier, class A, again, but why class A? this Q4 can't work on class C? it's driven by Q3. and why there is again capactor between the base of Q4 the the ground (56p) .

what is the idea of L4 ? it's used to short to ground the harmonics?

how to choose the correct bias for the Q3, Q4 ? what should be? which resistors?

for now, the overall current of this circuit is about 60mA, with good resonance the current is even less, about 40-50mA.
when i just replaced Q4 with power rf transisor, i did'nt found change in the signal amplitud, it's just take more current but did'nt give more RF power, maybe i need to change the bias to the new transisor? but how?
it's very strange but when i used my osciloscope, i didn't found that Q3, Q4 have larger amplitud then the original signal from Q2.

thank you.
it's very important to me.

 

[chuckey]

The first thing to do is to have a proper aerial, at 120 MHz, should be .6 m long and vertical, put a proper earth on the kit. (disconnect at L6). You are correct I think the 47/56 PF caps are to stop the thing oscillating. L4 will not reject harmonics unless it is carefully resonated with the 6pF cap. I would use a traditional arrangement whereby L3 is a RF choke, then there is a DC blocking cap, to a series L to the aerial with a cap to earth each side of the series L ( PI matching circuit). Now if your aerial is about .6m long then it will look like 75 ohms, which won't allow your PA transistor to draw enough current unless it is matched a lower impedance (10/20 ohm?), so the capacitors each side of the series L should be in the ratio 2.5 :1 (larger one on the aerial end).
Frank

 

[davenn]

View attachment 82683

https://i45.tinypic.com/2442quu.jpg

Hi,
I'm using this schme for study, it's used to transmitt audio signal to vhf band 120 Mhz FM modulation to receiver (the receiver is SDR radio in the our lab).

it's works very good, stable frequency, good voice quality, and low power consume.

but the transmittion distance is not more than 40 meter in open field. I need to amplify the R.F power output of this transmitter. but i don't know how. could you please help me?

the antenna is the audio cable itself. it's works great. long cable of about 1.5m . I also tried to use standard antenna and no better result.

pleae note that i need to use this layout, i mean i can change and remove component, but i can't change the layout of the scheme because we already maked the PCB.

note: this transmitter used for study purpose only.

Thank you.

HOLY CRAP!!!

Why the heck dont people check frequency allocations before they start playing with transmitters!!

You are lucky its only going ~40metres odd, 120MHz is right in the middle of the world wide aviation band allocation
~ 117 to 125MHz. Upping power or getting a better antenna as several have suggested could lead to severe legal action against you if you transmitter starts intereferring with ground to air/air to ground aircraft communications.

I know I wouldnt wanna be anywhere near you in an aircraft when the poor pilot looses that crucial bit of info from air traffic control that could result a midair collision killing 100's

FIND ANOTHER UNUSED NON-COMMERCIAL FREQUENCY PLEASE

Dave

 

[rohs]

you are missing the point, if you don't know to answer to the question, you don't have to jump and to say " FCC !!! DANGER!!! AIRCRAFT!!! " ....
as i listed, this circuit is for study purpose, it's not transmitt more than 40 meter in good condition.
guys, please stop to write "FCC, KILL AIRCRAFT... " for any thread that talking about transmitters.
instead to help to study this circuit and to get more skills in R.F, you jump and say that my study going to kill 100 people in the aircraft.. you can not be serious.

 

[FvM]

In fact, your original post is complaining the transmittion distance is not more than 40 meter in open field.
How can we know if you are targetting to 100 m or 10 km and if you're aware of possible risks brought up by your "studies"?

 

[volker_muehlhaus]

as i listed, this circuit is for study purpose

Study purpose or other purpose, it is illegal to operate a transmitter in this frequency band.
Why do some people on this forum think they can ignore all frequency regulations and license regulations?

 

[rohs]

o.k it's going to be 108mhz

 

[davenn]

you are missing the point, if you don't know to answer to the question, you don't have to jump and to say " FCC !!! DANGER!!! AIRCRAFT!!! " ....
as i listed, this circuit is for study purpose, it's not transmitt more than 40 meter in good condition.
guys, please stop to write "FCC, KILL AIRCRAFT... " for any thread that talking about transmitters.
instead to help to study this circuit and to get more skills in R.F, you jump and say that my study going to kill 100 people in the aircraft.. you can not be serious.

I didnt miss the point and Im VERY serious!!!
I do know the answers but wasnt going to give them to some one who obviously doesnt know what thay are doing!!
I have been working in the RF game for over 30 years, and altho I will never claim to be an expert in the RF field, I have a very good working knowledge of
building and operating equipment from 1.8MHz through to 47GHz

and its that knowledge that makes me cringe when I see posts like yours
so instead now you will go off to annoy the heck out of people trying to receive their FM broadcasts ??

I dont know what country you are in, but in many of the western world countries there are set aside frequencies for
experimental use... usually labelled ISM (industrial, scientific and medical) Bands ... how about doing some study for
the rules and regs in YOUR country and start doing things legally

Dave