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    TV transmitter connection with antena....

    Hey
    This is a very beginner question, but...
    I am making this transmitter:
    http://www.circuitdiagram.org/images...ransmitter.gif
    Click image for larger version. 

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    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!

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    Re: TV transmitter connection with antena....

    Quote Originally Posted by Garyl View Post
    Hey
    This is a very beginner question, but...
    I am making this transmitter:
    http://www.circuitdiagram.org/images...ransmitter.gif
    Click image for larger version. 

Name:	tv-vhf-video-transmitter.gif 
Views:	19 
Size:	7.4 KB 
ID:	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!
    The signal strength may destroy the TV Tuner or you will not see anything under this signal level.


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    Re: TV transmitter connection with antena....

    It isn't a particularly good design so don't expect perfect results.

    If you want to directly couple to the antenna socket, use co-axial cable and wire the shield (outer wire) to the negative side of the battery and the inner to the antenna connection as shown. I strongly advise you remove the 47pF capacitor and replace it with a much smaller value, no more than 2.2pF. The easiest way to do that is forget the physical capacitor completely and make one yourself from two short insulated wires (10mm each) twisted around each other twice.

    If you build this, it is vitally important that you keep all the wiring lengths, including component legs as short as possible. 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.
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    It's better to share your questions and answers on Edaboard so we can all benefit from each others experiences.


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    Re: TV transmitter connection with antena....

    I have already tried connecting it directly to tuner but I didn't get any results, I hope it's not damaged.

    I have assembled it with BC337.
    I have used 40pF trimmer instead od 22pF one.

    The input PAL signal was recognized directly by TV, but it couldn't get transmitted.

    All I can receive is two white horizontal white bars, but the signal is transmitting black-white TEXT and not bars so something is wrong.

    I have tried:
    - searching frequency all the time
    - changing coil from 2 turns to 6 turns
    - changed 40pF trimmer to 10pF capcaitor
    - changing 47pF capacitor on antenna to 10pF capacitor
    Everything is soldered on the prototyping board....

    Quote Originally Posted by betwixt View Post
    It isn't a particularly good design so don't expect perfect results.
    for start I want to at least receive something directly by cable or from 10cm....

    Quote Originally Posted by betwixt View Post
    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.
    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?


    the circuit description is:
    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.



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    Re: TV transmitter connection with antena....

    I have assembled it with BC337.
    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!)
    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?
    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.
    An absolutely clean signal will only transmit on one frequency but your schematic will produce quite a 'dirty' signal which contains harmonics. Think of these as 'ghost' signals that you normally don't want because they are wasting power by transmitting it where you don't want it to be. It can also interfere with other users. The harmonics will be at multiples of the frequency your transmitter is tuned to. For example if you are tuned to 150MHz there will be harmonics at 300MHz, 450MHz, 600MHz and so on. With a circuit like that the harmonics could stretch to quite high frequencies. You can receive the harmonics on your TV by tuning it into them (or tuning the transmitter so the harmonic lines up with the TV channel) but there is problem: the deficiencies in the original signal are also multiplied along with the harmonic. So a poor picture at say 150MHz will be significantly worse at 300MHZ and even poorer at 450MHz and so on.

    What is critical is the construction of the circuit. You MUST use short wires on all components and it must be mechanically stable as it has nothing to prevent it's frequency drifting with temperature changes or being susceptible to vibration and proximity. Can you post a picture of it so I can see how it is built.

    Brian.
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    •   Alt17th June 2017, 09:13

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    Re: TV transmitter connection with antena....

    I have a progress!

    I have tried with different CRT TV, which I've found in the garbage but it apparently works and I can tune it without the remote (it has buttons).

    I am transmitting a white-black PAL signal from Arduino tvOUT library with white text on black screen.

    I have used circuit powered by 12V, the antenna capacitor was 10pF. First the trimmer cap was replaced with 10pF as well, but later I insreted some unknown trimmer (prolly 20pF or so) but no change.
    The transistor is BC337 from the start but I am going to change it soon, as betwixt said.

    When transmitted wirelessly (10 cm wire as antenna or no antenna, no antenna or TV or coaxial cable) I get:
    - TV can't find automatically channel
    - I can find channel myself
    - the screen is scrolling randomly and ASCII text is not readable, but that's it. It changes frequency when I squize the coil..

    When connected to TV by coaxial cable, both ground and antenna signal:
    - TV is able to find automatically the channel
    - ASCII text is readable but disoreted
    - screen is sometimes scrolling/moved a little

    This is indeed a progress, but the quality of both cable and wireless transfer is not acceptable...
    I want, well, 50cm wirelessly with acceptable quality for the start.

    Okay, as requested, here are the photos of the setup:


    More photos without description:


    Solder side (it's now pretty messed up because I already replaced the variable capacitor twice, the BC337, the coil, etc)


    CABLE TO THE TV:



    Can you give me more hints how to improve the transmission? I can make the PCB or even etch it if it's needed...
    Also, how do I tell which frequency range my TV covers? It only shows "Channel numbers" and there is no MHz on the screen...



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    Re: TV transmitter connection with antena....

    Quality problems are most likely due to over-modulation. Did you try the video level adjustment (470R pot) at the modulator?



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    Re: TV transmitter connection with antena....

    Quote Originally Posted by FvM View Post
    Quality problems are most likely due to over-modulation. Did you try the video level adjustment (470R pot) at the modulator?
    Yes, I have tried it already. This pot seems to help just a bit, it increases the sharpness of the video and makes the font a little more readable.



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    Re: TV transmitter connection with antena....

    That it has worked is a great success.

    Modern tuners can lock onto the carrier on small excursions (of frequency) but your transmitter can drift at will. You certainly need a better transistor.

    The carrier encodes everything: vsync, hsync and the z modulation. All these come from the software. Please use a thicker and stiffer wire for the inductor to start with.



    •   Alt17th June 2017, 14:37

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    Re: TV transmitter connection with antena....

    I certainly agree about the inductor, it needs to be much thicker wire so it stays rigid or it will change frequency every time you touch it.

    Something else that doesn't look quite right is the input capacitor. It should be 4.7uF but it looks like you have used 4.7nF instead. If I'm right, the effect would be to reduce the size of the sync pulses and that would make the picture roll and tear, exactly the problem you have noted.

    Can you confirm that if you turn the potentiometer to minimum level, you still get a signal on the TV but the text is missing, just a blank screen.

    Brian.
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    •   Alt17th June 2017, 20:04

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    Re: TV transmitter connection with antena....

    The output level probably shifts the black level of the TV RGB processor.
    Try not connect directly to the antenna output but keep it close to TV antenna input.


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    Re: TV transmitter connection with antena....

    The 4.7uF capacitor and diode form a crude black level clamp already so as long as the modulation is approximately right it should be OK later down the receiver chain.

    If the capacitor value is wrong though, it will make the RC time constant too short to clamp throughout a full sync cycle and the video will 'tilt' or even worse, spike on the sync edges.

    The circuit is far from good in many respects but it should still be capable of displaying a stable picture.

    Brian.
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    Re: TV transmitter connection with antena....

    I wanted to desolder that capacitor and check it with Capacitance Meter but unfortunatelly it leg has broken of, so I decided that I will assemble everything again on better board.

    Here is the project:
    Click image for larger version. 

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    Click image for larger version. 

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    I am going to cut the PCB copper with dremel so I can make separate solder spots for parts.
    Will such PCB be better?

    Quote Originally Posted by betwixt View Post
    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 am choosing a new transistor now. I have looked at Bandwidth Product of datasheets of transistor I have and I have selected those:
    - 2N3904
    - C3953
    - BC547
    - BC239
    - C945
    - BC307
    - 2N2222
    - 2N5551
    - BF245A
    - BC549
    which one of those would be best for higher frequencies in this circuit? Or maybe you can recommend a specific one to look for?

    Also, what else can I modify in this circuit so I can get broadcast on 400-800MHz?
    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.
    Thanks in advance!



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    Re: TV transmitter connection with antena....

    4.7uF will probably have to be a small aluminum electrolytic or tantalum type.

    All those transistors are general purpose types, you really want one designed for high frequency, for example the 2N918/PN918.
    The BF245A is a JFET so that one definitely won't work, at least in that circuit.

    To make it work at higher frequencies you need to reduce the capacitance at the tuned circuit and lower the inductance of the coil. Try maybe 3 turns instead of 6 and drop the feedback capacitor from 10pF to 4.7pF and use 2.2pF for the antenna coupling capacitor.. The coil MUST be rigid so use fairly thick copper wire as it is very susceptible to vibration. As I stated before, the circuit has many flaws so although you may get acceptable results they will be far from perfect. A good AM modulator will have an oscillator stage, a buffer and then an amplitude modulator. As it stands you circuit will not be linear and will produce as much FM as AM. It will also change frequency as the antenna or objects nearby are moved.

    Brian.
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    Re: TV transmitter connection with antena....

    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?



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    Re: TV transmitter connection with antena....

    Quote Originally Posted by Garyl View Post
    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?
    If you chose a SMD transistor ( and also other components) this will improve the performance due to lesser unwanted elements.
    BFG21W is good BJT for OSC applications.Coilcraft has very nice air cored inductors, I recommend you to use them for less headache.



    •   Alt19th June 2017, 06:12

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    Re: TV transmitter connection with antena....

    SMD will remove the variation due to component leg lengths but it won't make the quality any better.
    The + side of the capacitor goes to the diode.

    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. RF transistors are designed with low inductance packages and both package and silicon junction capacitance is optimized. It helps them work effectively at higher frequencies.

    Brian.
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    Re: TV transmitter connection with antena....

    Quote Originally Posted by betwixt View Post
    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.
    I hope it won't be treated as advertising, but just to answer your question, betwixt...
    I get my stuff here: https://www.aliexpress.com it's just that components there are sometimes 5x or even 10x cheaper than in the local shops in my country.


    Quote Originally Posted by betwixt View Post
    The + side of the capacitor goes to the diode.
    It won't be a problem? I've read that electrolitycs are bad at higher frequencies...

    - - - Updated - - -

    Quote Originally Posted by BigBoss View Post
    BFG21W is good BJT for OSC applications
    Seems like a nice and available cheap one, I will order 10 of them ASAP.



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    Re: TV transmitter connection with antena....

    It won't be a problem? I've read that electrolitycs are bad at higher frequencies...
    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.

    Brian.
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    Re: TV transmitter connection with antena....

    I have designed a VCO with avaricap for tuning from 175MHz-203MHz.It's quite low noise oscillator and stable in simulation.
    If you would , I can post schematic here.It would be a practical experience for you.



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