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 Post subject: TV transmitter ideas
PostPosted: Sep Wed 12, 2018 8:28 pm 
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Joined: Jan Tue 23, 2007 10:23 pm
Posts: 5182
Location: Leon Guanajuato, Mexico
Hello all,

I have been thinking about building a simple TV transmitter for my vintage TVs. Basically what I'm trying to build is just a small amplifier that can take the RF output from a DTV converter and transmit it over the air. Some time ago I experimented with simply adding a wire antenna to a piece of coax with success. With a bit of amplification it should be fairly easy to build something that sends a signal around the house. I imagine you could use something like an antenna or cable amplifier like used in large buildings with many TVs but I would enjoy making something homebrew. I have a couple of VHF RF transistors at my disposal, though of course it should be kept legal.

Anyone have any ideas or schematics?

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 Post subject: Re: TV transmitter ideas
PostPosted: Sep Wed 12, 2018 8:49 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Albuquerque, NM 87123
Why? Just buy a modulator that can be programmed for VHF or UHF or both.


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 Post subject: Re: TV transmitter ideas
PostPosted: Sep Thu 13, 2018 12:59 am 
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Joined: Oct Wed 25, 2017 12:10 am
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Location: Idaho / Colorado
That would be cool if you want to do it the hard way.

The easy way is to get one of those Blonder Tongue agile modulators for $30-$50 on eBay... one that has DIP switches to change the channel. They were intended for closed circuit operation but they work great as low power transmitters if you stick a wire in the RF output jack.

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 Post subject: Re: TV transmitter ideas
PostPosted: Sep Thu 13, 2018 12:08 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 19915
Location: Utica, NY 13502 (USA)
The radiation limits in the US for unlicensed operation in the TV bands are pretty tight. I don't know about Mexico.

Dave


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 Post subject: Re: TV transmitter ideas
PostPosted: Sep Thu 13, 2018 1:55 pm 
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Joined: Apr Thu 12, 2007 3:24 am
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Location: Milwaukee,WI
I don't know if it was FCC rules or just that the tech was cheap and available, but it seems that all the consumer video transmitters I've seen were in the 900mhz band.


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 Post subject: Re: TV transmitter ideas
PostPosted: Sep Thu 13, 2018 2:17 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 118
What's the frequency of operation you are looking at and what RF transistors you've got? I'll cook up a simple linear amplifier schematic for you after you tell me the frequency of operation. I ain't sure of the legality of it but power output can be kept quite low to cover a short distance like 100 meters. 20 mW output should be sufficient.


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 Post subject: Re: TV transmitter ideas
PostPosted: Sep Thu 13, 2018 2:22 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 118
Try this simple linear amplifier. RFC is 1 uH or 50T of thin wire on a 100k resistor. The driver transistor is 2n2369 and final one is 2n3866. It's suitable for 100 - 200 MHz


Attachments:
vhf_amp (3).GIF
vhf_amp (3).GIF [ 3.76 KiB | Viewed 428 times ]


Last edited by Dare4444 on Sep Thu 13, 2018 2:27 pm, edited 1 time in total.
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 Post subject: Re: TV transmitter ideas
PostPosted: Sep Thu 13, 2018 2:24 pm 
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Joined: Aug Thu 06, 2015 2:20 am
Posts: 138
EDIT: Based on the title of this thread, I was going to suggest a separate oscillator circuit with video only input until I read your post more carefully and realized that a linear amplifier would be sufficient.


Last edited by LM386 on Sep Thu 13, 2018 2:46 pm, edited 3 times in total.

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 Post subject: Re: TV transmitter ideas
PostPosted: Sep Thu 13, 2018 2:28 pm 
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Posts: 13046
Years back I fed a microwave channel, converted to Ch 5 into a eight tube Winegard distribution amp. Driving a VHF antenna, neighbors on my side of street(line of sight) could easily receive it four houses down.

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 Post subject: Re: TV transmitter ideas
PostPosted: Sep Thu 13, 2018 2:37 pm 
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Joined: Mar Thu 01, 2018 1:30 am
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https://ibb.co/iruEVU

Simple linear amplifier using one 2n5109 / 2n3866.
Adjust input and output trimmer capacitors for best range. Make sure you get a genuine transistor from mouser electronics, etc. Avoid Ebay. I received fake 2n3866 transistors with only 4 dB gain in the FM band. 2n5109 will give the best gain. It can't get any simpler than this.


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 Post subject: Re: TV transmitter ideas
PostPosted: Sep Thu 13, 2018 5:06 pm 
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Joined: May Sun 16, 2010 3:22 am
Posts: 95
Location: Scotia, NY 12302
I built a low power TV transmitter (video & mono audio) about 15 years ago. I bought it as a pc board
and board mounted parts only kit. Got it from North Country Radio < http://www.northcountryradio.com/ >
but I see they no longer carry it. Works great and tunes to any channel from 2 - 6. I've used it with DVD, VCR,
and video camera. Produces good video and audio and with a short antenna will cover most of my house. No
special parts in it. I'll look for the schematic. The small blue board is an added voltage regulator.

Al

Found the info, here is the schematic and board layout. I'll do a better scan when I have time.


Attachments:
TV Xmitter.jpg
TV Xmitter.jpg [ 165.93 KiB | Viewed 410 times ]
TV Xmit Open.jpg
TV Xmit Open.jpg [ 205.85 KiB | Viewed 410 times ]
TV-Schematic.jpg
TV-Schematic.jpg [ 152.67 KiB | Viewed 332 times ]
TV_Layout.jpg
TV_Layout.jpg [ 221.9 KiB | Viewed 332 times ]


Last edited by Als_Basement on Sep Fri 14, 2018 3:45 am, edited 1 time in total.
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 Post subject: Re: TV transmitter ideas
PostPosted: Sep Thu 13, 2018 10:21 pm 
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Joined: Jan Tue 23, 2007 10:23 pm
Posts: 5182
Location: Leon Guanajuato, Mexico
Thanks a lot for the info guys, I want to build something for the sake of having a project and to save some money since I figure I can build something out of scrap.

Dare4444 wrote:
https://ibb.co/iruEVU

Simple linear amplifier using one 2n5109 / 2n3866.
Adjust input and output trimmer capacitors for best range. Make sure you get a genuine transistor from mouser electronics, etc. Avoid Ebay. I received fake 2n3866 transistors with only 4 dB gain in the FM band. 2n5109 will give the best gain. It can't get any simpler than this.


Perfect! I have a 2n3866 right here. :D I also have a 2N4427 both ordered from mouser. I hope these dont get discontinued soon since they seem good for a lot of purposes. The frequency I'll be using it on will be either channel 3 or 4. Seems that's around the 65mhz range according to google. Is this the frequency you designed this for? Since you used a resistor for the former, did you use a 1/4 watt or 1/2 watt resistor? Approximately what gauge wire did you use?


Also hope you can find that schematic, Al!

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 Post subject: Re: TV transmitter ideas
PostPosted: Sep Thu 13, 2018 11:20 pm 
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Joined: Mar Thu 01, 2018 1:30 am
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65 MHz? Use the 2n3866! Let me know the range you get. I use 70T of 37 AWG wire on a 100k - 1M resistor as my RFC in VHF circuits. You can use a ready-made moulded choke if you have one.


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 Post subject: Re: TV transmitter ideas
PostPosted: Sep Thu 13, 2018 11:24 pm 
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Joined: Jan Tue 23, 2007 10:23 pm
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Location: Leon Guanajuato, Mexico
Thanks!

is that a 1/4 quarter watt or 1/2 watt size resistor, just so I know the size for the former. If I use a ready made choke, what value would that be? I imagine the resistor is still necessary right? Sorry if that's a dumb question. lol

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 Post subject: Re: TV transmitter ideas
PostPosted: Sep Thu 13, 2018 11:37 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 118
Nope the resistor is not necessary. Winding 70T on 1/2 W resistor will give you more inductance. On 1/4 w resistor it gives me 1.5 uH. Use a 10uH choke. Got one in your junkbox?


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 Post subject: Re: TV transmitter ideas
PostPosted: Sep Thu 13, 2018 11:40 pm 
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Joined: Jan Tue 23, 2007 10:23 pm
Posts: 5182
Location: Leon Guanajuato, Mexico
There should be one around here somewhere. Let me check, if not I'll use the resistor. I think most of what I have are classified as inductors, they look like this. https://tmgeletronica.com.br/Eshop.Admi ... uctors.jpg

I'm not sure if that would work. Besides that most of what I have is on ferrite.

If I go with the resistor method do I go with the 1.5uh or do I need the higher inductance from the 1/2 watt resistor?

EDIT: Would it be a good idea to use an adjustable coil for this in order to obtain maximum output? I have lots of 8mm formers with a slug in the center like those found in the output stages of CB radios. What would be a good amount of turns and wire gauge to use on this for the purpose should this be a good alternative?

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 Post subject: Re: TV transmitter ideas
PostPosted: Sep Fri 14, 2018 1:18 am 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 118
Use a big inductor as frequency is low. Inductive reactance of 1.5 uH is only 600 ohm or so at 65 MHz. Find a ready-made moulded inductance of 10 uH or more and you would be fine. No need to use a variable inductor unless the collector is tuned to 65 MHz. Using a RFC will give broadband characteristic. The idea is to have enough inductance so it offere high resistance to 65 MHz RF while letting DC to pass through. 10 uH or more is okay.

Use this at the output and adjust trimmer capacitors for highest voltage reading

https://ibb.co/cmPmvU

Then connect a 1/4 wave wire antenna and ur all set


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 Post subject: Re: TV transmitter ideas
PostPosted: Sep Fri 14, 2018 2:53 am 
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Location: Leon Guanajuato, Mexico
Thanks. Closest I could find at the moment is 20uH. I guess in any case I could take off some turns.

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 Post subject: Re: TV transmitter ideas
PostPosted: Sep Fri 14, 2018 4:58 am 
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Posts: 118
20 uH is good! I hope the drive level is enough to give usable gain from the 2n3866. You should get a 50 meters range.


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 Post subject: Re: TV transmitter ideas
PostPosted: Sep Fri 14, 2018 5:42 am 
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Joined: Sep Thu 23, 2010 6:37 am
Posts: 10240
Location: Powell River BC Canada
Many years ago I set up radiation type TV signal boosters for stores
the firm operated. The amplifier was a 30dB booster made by Delco
for networks of 75 ohm splitters.

The signal feeding the amp was about -6 dBj.

The output fed a VHF Yagi type antenna placed in the false ceiling
of the store.

The store display was interesting, at the back of the store, wall to wall
TV sets three high on shelves made for the purpose. I recall we were
hawking Westinghouse portable BW sets 1966ish.

If you walked by not paying attention, and the image on the TVs
shifted right or left, it was like sensing a train pulling out. You might
lose your balance. I think that was exactly what our store designer
wanted as an effect.

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