The signal strength may destroy the TV Tuner or you will not see anything under this signal level.Hey
This is a very beginner question, but...
I am making this transmitter:
http://www.circuitdiagram.org/images/tv-vhf-video-transmitter.gif
View attachment 139439
and I want to test it by ATTACHING IT BY WIRE to TV...
Tv antenna output has two terminals - inner and outer.
How do I connect it?
The outer terminal is the GND?
Thanks in advance!
for start I want to at least receive something directly by cable or from 10cm....It isn't a particularly good design so don't expect perfect results.
Note that it will probably operate at a frequency lower than about 200MHz so on a UHF TV you will have to rely on picking up harmonics.
Brian.
Here's a simple schematic of a TV transmitter circuit or video transmitter circuit which is able to broadcast on VHF between 60 to 200 MHz. The input video can be from any CCD camera and VCR. The output power of this VHF transmitter circuit is 80mW and by using telescopic antenna this
circuit will transmit at a distance of 100 meter.
The circuit is using only one transistor that can be a BC337, 2N2222, BC546 or BC108. For L1 wound 6 turns of #24 enameled wire on a 10mm air former for frequency 60 - 80 MHz.
For 150 - 180 MHz wound 4 turns and for 180 - 200MHz wound 2 turns.
If you want to transmit sound then make a FM transmitter and tune it on audio channel.
This circuit is for educational purpose only.
That transistor is completely unsuitable, the other may work but you really should be using a transistor designed for high frequency operation. The type isn't critical but use one with an fT rating of at least 300MHz for best results. (the BC337 fT is 100MHz!)I have assembled it with BC337.
Historically, TV has been used, and is still used in some places, on the VHF bands using frequencies in the range 50 - 70MHz and around 150 to 200MHz. the exact frequencies varies from one country to another. Almost all TVs made recently are designed to receive in the UHF band which covers around 470 - 800MHz but again the actual band edges and occupancy varies between regions.Can you explain? As far as I know, the analogue channels in my country were around 500MHz and this transmitter in descriptions has sometihng about 50-200MHz. Why are those frequencies different?
Quality problems are most likely due to over-modulation. Did you try the video level adjustment (470R pot) at the modulator?
I am choosing a new transistor now. I have looked at Bandwidth Product of datasheets of transistor I have and I have selected those:That transistor is completely unsuitable, the other may work but you really should be using a transistor designed for high frequency operation. The type isn't critical but use one with an fT rating of at least 300MHz for best results. (the BC337 fT is 100MHz!)
Thanks in advance!For L1 wound 6 turns of #24 enameled wire on a 10mm air former for frequency 60 - 80 MHz.
For 150 - 180 MHz wound 4 turns and for 180 - 200MHz wound 2 turns.
2N918/PN918 sounds good but I searched for online offers and it looks like it's no longer in the production, and the only offer is 10 pieces for 15$.... are there any newer/cheaper high frequency RF transistors?
Would making this circuit in SMD improve the quality?
If I were to use small electrolytic 4.7uF capacitor there, how would I place it + sign? At the side of the "Video In" or at the side of the Diode?
I hope it won't be treated as advertising, but just to answer your question, betwixt...As for transistors, we have no idea what may be available in your locality but look out for transistors intended to be RF amplifiers or oscillators rather than AF or general purpose types.
It won't be a problem? I've read that electrolitycs are bad at higher frequencies...The + side of the capacitor goes to the diode.
BFG21W is good BJT for OSC applications
It's true their impedance increases with frequency but in this application that isn't too important. It only carries modulation frequency (the video) so at most it will have to carry about 5MHz.It won't be a problem? I've read that electrolitycs are bad at higher frequencies...
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