FM RF amplifier for VHF TV?

[bghartono]

Hi,

I built a FM (88-108 Mhz) transmitter a couple years ago.
At that time, this FM transmitter is strong enough since it use 2SC2053-->2SC1970-->2SC1971 for PA.

Now, I'm interested to build VHF TV transmitter.
Is that possible to use same transistors (2SC2053-->2SC1970-->2SC1971) as PA for VHF TV application?

I need 5-15 watts output, any idea?

 

[goldsmith]

Dear bghartono
Hi
The main important think that you should pay more attention to that , is , current gain bandwidth product for your transistors . and internal capacitors .
Best Wishes
Goldsmith

 

[chuckey]

How are you going to modulate the TV transmitter - low level mod or high level? if you use low level mod, then the amplifiers after the modulator must be very linear (unlike FM amplifiers) so to get your output power I think you will need a push-pull O/P stage (2 X 2SC1971) with another one driving them.
Frank

 

[bghartono]

Dear bghartono
Hi
The main important think that you should pay more attention to that , is , current gain bandwidth product for your transistors . and internal capacitors .
Best Wishes
Goldsmith

Hi Goldsmith,
Thank you for quick response.

I found VHF TV transmitter schematic (basically an old SEGA/Nintendo RF modulator + VHF booster):



According to its designer, it could produce 500 mW output.
Since it use 2SC2053 & 2SC1970 (same with my FM transmitter) I think I can add 2SC1971 for greater output.

I'm just beginner, please help me.

 

[ALERTLINKS]

2N3866 is good for driver stage. If connected in parallel, give more power in output stage.
I have seen exactly this type of transmitter built by one of my fellow using 2N3866 in output stage. RF modulator was from a commonly available low power (50mW) transmitter for home use. VHF booster was built. A few such devices were built and used to re-transmit satellite channels in cantonment residential area covering 2-3 miles. Half of city could catch this transmissions. It was some fifteen years back. This arrangement served about three years. Then it was replaced by cable network due to complaints about illegal operation.

 

[bghartono]

2N3866 is good for driver stage. If connected in parallel, give more power in output stage.
I have seen exactly this type of transmitter built by one of my fellow using 2N3866 in output stage. RF modulator was from a commonly available low power (50mW) transmitter for home use. VHF booster was built. A few such devices were built and used to re-transmit satellite channels in cantonment residential area covering 2-3 miles. Half of city could catch this transmissions. It was some fifteen years back. This arrangement served about three years. Then it was replaced by cable network due to complaints about illegal operation.

Hi ALERTLINKS,

Do you have the schematic? I really need to built that kind of booster.

Best regards,
bghartono

 

[ALERTLINKS]

I don't have it but i recall it was very similar to the schematics you have posted. Layout is more critical as all stages operate on same frequency. It is difficult to avoid self oscillation and instability if layout is not right.

 

[bghartono]

For VHF TV apllication, which one is better..2SC1970/2SC1971 or 2N3866/2N3553?

Regards,
bghartono

 

[ALERTLINKS]

I found 2n3866 operate more stable. It has 800 MHz Current-Gain Bandwidth Product.

 

[bghartono]

How are you going to modulate the TV transmitter - low level mod or high level? if you use low level mod, then the amplifiers after the modulator must be very linear (unlike FM amplifiers) so to get your output power I think you will need a push-pull O/P stage (2 X 2SC1971) with another one driving them.
Frank

Hi Frank,

Would you show me a simple push-pull RF PA schematic which use 2SC1971 transistors?

Regards,
bghartono

 

[goldsmith]

Dear bghartono
Hi
Why push pull ? however it can be a good choice . but i suggest you , a simple class E or class F amplifier , and a class C as preamplifier . or perhaps a simple Gali-5
Good luck
Goldsmith

 

[bghartono]

Dear bghartono
Hi
Why push pull ? however it can be a good choice . but i suggest you , a simple class E or class F amplifier , and a class C as preamplifier . or perhaps a simple Gali-5
Good luck
Goldsmith

Hi Goldsmith,

Thank you for your suggestion.


Do you think this schematic will work?

 

[goldsmith]

Hi again
Of course it can't give you your desired power ! just consider that you have 10 volts across the 50 ohms antenna . what will happen ?

 

[gurudattapanda]

Need Help:
How to check 2SC1971 is burned or not (in circuit) using multimeter only. (I dont have watt meter) The reason for this check is that, I am not getting more than 200 meter range using the 2SC1971 at 99 Mhz in FM band. My GP antenna is 30 feet above the Ground in a semi urban area connected to the Transmitter.
Please help.

 

[chuckey]

Build your self a RF probe. 100 pF cap (one end is input) then a RF diode to earth. Connect a 10k resistor to the cap/diode junction, the other end is the DC out. Build it small with short leads and use your meter on volts to measure peak RF volts, make sure every stage has got some gain. Importantly, put it on the output connector and peak each circuit. For the output stage, connect a 50 ohm resistor as a load and measure the volts across it while peaking the output caps. They might need "walking", i.e. try putting more on one cap and try peaking with the other - more output, do it again (and again) until the output falls. If putting more capacity on results in a lower output after peaking, try less capacity and re-peak.
Frank