Low power Band I transmitter design?

[Ian - G4JQT]

Can anyone point me in the direction of a low power (say about 1 watt) Band I, Ch 1, TV transmitter design? Perhaps something along the lines that some of us use for unofficial, domestic, re-broadcasting of FM/CDs etc. onto medium wave.

Anyway, it would be nice to actually transmit the TV sound and vision signals rather than connect directly to the aerial socket via a converter. It could be received by more than one TV at once – in an exhibition for example. Providing the signal only reaches a few tens of yards, I doubt it should attract any undue attention!

I’m not sure if reflections from within a building would give significant ghosting, but if it's strong enough from just across a room, just a bit of wire from the TV aerial input should give a good picture – shouldn’t it?

I did think of designing a linear driven from the RF output of the Domino standards converter, but I’m not up to designing something with that much gain that MUST be really linear.

The Domino (and I guess other converters) has base-band 405-line output so the transmitter can be driven from this.

I’ve Googled the web to death and also searched this forum, but not quite found what I’m after. Any circuit ideas?

Thanks,

Ian

[ppppenguin]

Use a CATV launch amplifier. You'll need to find one that goes down to Ch1. You may not a get a full watt but it will be a useful amount of power. Actually a Domino has a pretty high output even without an amp.

PS: To anyone who's thinking of doing this, please remember that tranmitting without a licence is illegal.

[Paul Stenning]

I think 1W is way too much for the intended purpose and with the right antenna could cover several miles. This self-help UHF relay http://tx.mb21.co.uk/gallery/kinlochewe.shtml uses a distribution amp giving an output of approx 1W to cover a village with four UHF channels.

I think you should be looking at nearer 100mW (which on MW radio is enough to cover an average home with an unmatched wire aerial). You may well have enough output from the Domino to do that directly into an antenna. Otherwise you may be able to use a distribution amp, although a modern FM/UHF one may not do much at band 1. I don't think you need much more than that, really.

[Sean Williams]

Hi Ian,

Please be aware that the frequencies that were used for band 1 TV are currently allocated to the MOD.

I would not advocate connecting Domino (or any other rf modulator) directly to an amplifier - the harmonic content is way too high - I will post a Screenshot of my Aurora on an analyser.

Whilst the idea in principle is possible, the amount of filtering and design to make a clean linear transmission, would in my opinion far outweigh the benefit of such a device.

Yep, Baseband video is available from the converters mentioned, but you would then really need to design an effective modulator, that was low in harmonic content, and had filtering very similar to Darius (oldeurope) designs.

Lots of work, and sadly not legal either.

Better to go for a cabled distribution system really.

Cheers
Sean M1ECY

[Paul Stenning]

Cabled distribution certainly worked very well with the vintage TV display at the NVCF last year. A couple of Dominos and distribution amps fed about 30 sets with good clean signals. Photos (including the distribution lash-up) are here http://www.domino405.co.uk/nvcf_may_2006.html

[evingar]

(including the distribution lash-up)

The reason why people like the idea of wireless ?
Could always line your house with lead - conduit anyone ?

[Steve_P]

Chas E. Milller designed one using valves - in the 80's. If you want me to dig this out then it'll have to wait until tomorrow! It's somewhere in the garage and it's late.

1 Watt is plenty if it's used right, but in the situation Pual gives, the power is in the Aerials, not the transmitter.

Firstly, it recieves in one polarisation and radiates in another. Does not matter which way round, but the difference between H and V Polarisation is about 30dB.

Also, the transmitting aerial, a yagi, goes only in one direction. You see, an Isotrophic Radiator goes in all directions. A dipole goes outward, so it saves power by not going up or down. A yagi mainly radiates in one way, and the more elements it has, the more directional it is.

Look at it this way - A lightbulb radiates 60w (or whatever rating it is) in all directions. Wherever you are in the room, you see a 60w bulb. But put a mirror next to it, and in one way you will see 120w, but behind the mirror you will see nothing.

In a house, unless you want to create trouble for the neighbours (Like I do!) you use a small multi-directional antenna or a leaky feeder system. A watt is fine here, although you may well get away with less.

Oh, and you won't cause much interference on Band 1 anyway. Apart from some geezer's baby alarm. And they often have 2 channels anyway.

So don't worry about it. I never have....

Cheers,

Steve P

[Ian - G4JQT]

Yes, 1 watt would probably be too much in retrospect. Transmitter-wise I'm used to working on MF with RSL stations running VERY inefficient antennas!

On 45MHz I was not planning to use more than a very short (say 1 to 2ft) loaded vertical whip - still not particularly efficient.

The Yagis used in the self-help sites look like they have a gain of 10 to 12dB, with similar gains likely at the receiving end.

My vertical will probably have a loss of at least 8dB, and even more when you include building loss - it'll be used indoors. A “token” wire antenna was all I was thinking of for the TV receiver.

I had considered plugging the Domino into a vertical dipole to see how effective it would be, but the antenna would be a bit unwieldy at 3m long.

I asked Malcolm if he padded down the Domino’s output. Yes, he said, to enable the two carriers to be combined and he didn’t recommend it be changed.

I must admit, I never thought of a CCTV distribution amp – that’s given me another avenue to explore.

Although the MOD have been allocated Band I, but I have never heard anything on it. Despite that, a visit from Ofcom would be most unwelcome.

So leaky feeder is another option…

Thanks,

Ian

[Danny]

hi ive pluged rabbits ears in to my modulator output and have had sets all with rabbit ears on and got perfect results. i do know of someone hew may have built an output stage to hang on a modulator giving aprox 4watts and it was watchable 20miles away approx this would have been vision only with a testcard playing and viewed on a tmb272 so yes one watt could do 5 milesand cause havoc with the harmonics produced. Danny

[Panrock]

There is a legend that many, many years ago, someone living in a low lying rural area built and connected a 100-watt linear amplifier to the output of a Band I modulator and radiated Channel 1 from the 'H' on his chimney.

He then went off in his car accompanied by a small portable receiver. Apparently he received a strong signal for 2 or 3 miles then it completely disappeared. This must show that power isn't everything but antenna throw-height matters too.

Steve

[Andy Green]

Originally Posted by Panrock

There is a legend that many, many years ago, someone living in a low lying rural area built and connected a 100-watt linear amplifier to the output of a Band I modulator and radiated Channel 1 from the 'H' on his chimney.

He then went off in his car accompanied by a small portable receiver. Apparently he received a strong signal for 2 or 3 miles then it completely disappeared. This must show that power isn't everything but antenna throw-height matters too.

Steve

Hmmmm;-) Many of us have been there, done that etc many years ago!
When I was 17, and living in Sussex, I had a clear view to the South Downs about 20 miles distant. One day, I put on a superbug type FM transmitter (3 transistors, about 1-2 watts out) into a dipole stuck out my bedroom windowand drove off to see how it was getting out - needless to say, I nearly got all the way there, before bottling out and rushing back to turn it off before I was 'raided' by the GPO!

In my day job as a broadcast engineer, due to the mountainous topography of much of Ireland, I have to regularly deal with a large number of local radio VHF FM transmitters, which use anything from around 5 watts e.r.p up to a few 10s of kw, and I can tell you that 25 watts from a good high site will regularly out perform a couple of kw from a low site - the best site for low power appears to be on the side of a hill, rather than the top

[Hybrid tellies]

For a sender I use my vcr uhf rf output which feeds into an aerial distribution amp, the high output of this distribution amp drives a set top aerial. Using a clear channel I can pick up clear pictures on my handheld lcd tv right around the house and out into the garden where I have to go and smoke these days.
I did a range check the other day and went for a walk around the neighborhood and was very surprised at how far the signal travelled with good colour pictures and sound being received nearly half way down our long street. I might have to find a way to reduce it's power.
This might be an answer as some vcr's (sold in the UK) did have modulators which covered the vhf tv bands. The trouble is they would only cover 47-68Mhz in band 1 so you would have to forget ch1 but it would be good for ch's 2-5.

[Steve_P]

Here's a story from my mis-spent youth that proves a point. Places are kept quiet here, for obvious reasons. I worked for a time for a Transmitter Dept. of a certain large broadcasting organisation.

Anyway, after a night out, we got back in at about 11.30 and, a little inebriated, we decided to put a film on the video and, rather than pull the TV out and make a connection, we used a TV aerial - set top kind - to pipe the video into the TV.

While I mucked about, not entirely knowing what I was doing, I found that the Video and Sender could capture the TV from BBC1. Suddenly the lads were awake and one of them had a Large Yagi Aerial, which he was going to put on his house at some point.

Well, a bit of soldering later, and a certain type of film in the video, I leaned out the window with the Yagi, pointing it at people's chimneys.

A few minutes later, a bloke came out of his house and looked at his TV Aerial.
'Good Evening Sir', I said, managing somehow not to sound as pie-eyed as we all were.
'Hi There', he said back as he looked at his aerial again and walked back inside. By now, the other lads were all sniggering behind me. With a Yagi in one hand, pointing at his aerial, he didn't guess where the pirated signal came from.

Some people really are daft. And some people (us!) found it very funny....

Cheers,

Steve P

[arjoll]

Originally Posted by Hybrid tellies

This might be an answer as some vcr's (sold in the UK) did have modulators which covered the vhf tv bands.

Kat Manton's already done some experimentation with some system B VHF modulators - the thread is here.

[BGmidsUK]

I used to relay the neighbouring regional ITV channels (picked up with a high-gain antenna) using a couple of VHF modulators coupled to a high gain aerial amplifier and a simple vertical rod cut to approx ¼-wave, positioned in the loft, using Chs E2 and E4. This setup was on 24/7 for some time (several years) and using my new pocket TV set (Citizen TC53, one of the first colour pocket TVs) could get a good colour picture with clear sound for a couple of hundred yards or so in any direction this was handy for viewing those channels in my workshop as the local station (Central) was rubbish at night. These modulators provided a surprisingly clean output! Using a low-pass filter eliminated any noticeable interference.

I also used to experiment with modified ASTEC UM1286 modulators and had these running in Band 3 and depending on how I wound the coil I could cover chs E5-E12. Unfortunately the sound carrier was weak on those modulators.

I still have the original kit I built in a cupboard and have been tempted to retrieve it and have another dabble will have to see what the modulators were just in case more can be found .. I got them from an electronics "alladin's cave" type shop in the late 1980s


Love this stuff, wish I'd found this forum a long time ago time to start experimenting with 405-line stuff


BG

[murphyv310]

Hello.
I remember posting a discussion before about transmitting on CH1 and the post was removed!
Why are we now allowing posts on a potentially illegal subject, there can't be rules for some and not others.
Yours
Trevor

[Sean Williams]

Trevor,

This is in reference to a low power design, not along the idea that you were proposing, thus we will allow discussion in this case.

Sean

[BGmidsUK]

I think what I was doing was illegal .. but it was never picked up on, if you'll mind the pun



BG

[Hybrid tellies]

I use a little Citizen handheld for when I go outside for a coffee and smoke. As the main signal from Mendip is not strong enough on my front doorstep, I use the following setup as a low powered relay.
I connect the output from my VCR to a settop aerial via an amp. I watch via the VCR output channel with its modulator set to a clear UHF channel. It is low power but very effective. I would assume a VCR with a VHF modulator feeding a modest set top aerial via an amp would also be just as effective on bands 1 or 3..

[murphyv310]

Originally Posted by Sean Williams

Trevor,

This is in reference to a low power design.

So Where do you draw the line on power and range?

Trevor