Author Topic: AM transmitter (Read 32642 times)

interoth · « **on:** July 05, 2012, 10:10:59 pm »

Hey, I am creating a simple AM transmitter consisting of just the carrier wave.

I have attached a schematic of the 1MHz oscillator. I have made this design from other designs so I am not sure if it will work.

Am I right in thinking that the output is a 1MHz Square wave at +9v?
If so how would I attach an antenna?
I have seen designs that include a inductor to increase the range/power and I am not sure as to how to how to use an antenna.

Thanks for any help

RCMR · « **Reply #1 on:** July 06, 2012, 12:37:18 am »

You don't want a square wave -- that will produce a whole heap of harmonics.

What's needed to create a decent 1MHz RF carrier is a nice LC oscillator that produces a sine-wave output (low in harmonics).

There are plenty of examples out there -- go look up "colpitts oscillator" for some suggestions.

vk6zgo · « **Reply #2 on:** July 06, 2012, 01:17:15 am »

Don't attach an antenna to that thing,unless you want to incur the wrath of your neighbours!

Take RCMR's advice,as with your square wave,you will interfere with people's radios,& they will become quite upset,plus you may attract the attention of the licencing authority--& believe me,you don't want that!!

Even with a nice 1MHz sine wave,I would be very wary about connecting an antenna,for the same reasons.
Many Administrations let you play about with very low power,& minimal antennas on the Broadcast Band--some others forbid it !

interoth · « **Reply #3 on:** July 06, 2012, 06:53:57 am »

Ha ha, thanks for the advice. It was meant to only be on for up to 5 sec as a sort of remote control.

Simon · « **Reply #4 on:** July 06, 2012, 10:40:37 am »

you need I believe a colpits oscilator, this will have a "tank circuit" in it that is the Lc part and will produce a sine wave. Use only as much power as you need. I'd suggest one of the frequencies used by AM radio as they are seldom used and just enough power to transmit as far as you need.

vk6zgo · « **Reply #5 on:** July 06, 2012, 11:24:16 am »

Quote from: Simon on July 06, 2012, 10:40:37 am

you need I believe a colpits oscilator, this will have a "tank circuit" in it that is the Lc part and will produce a sine wave. Use only as much power as you need. I'd suggest one of the frequencies used by AM radio as they are seldom used and just enough power to transmit as far as you need.

They may be seldom used in the UK,but the MF Broadcast band is full of Radio stations in Australia!

Simon · « **Reply #6 on:** July 06, 2012, 11:43:16 am »

well due the the not excellent coverage in the UK most are forced to listen to the radio via the internet. I guess that is why we are having to scale up our internet network - just so we can listen to the godamn radio

My paramount suggestion is keep the power as low as possible, if it is just a carrier it should not upset reception to other devices too much unless they are the same distance as the intended receiver (5 meters ?)

vk6zgo · « **Reply #7 on:** July 06, 2012, 12:24:03 pm »

Remember real radios are likely to be considerably more sensitive than the probable receiver!

Simon · « **Reply #8 on:** July 06, 2012, 12:29:31 pm »

True or the receiver needs to be very sensitive

G7PSK · « **Reply #9 on:** July 06, 2012, 03:08:51 pm »

I would go for a clapp oscillator as they are a bit more stable than the colpitts but if you only want a 1khz signal a wein bridge circuit would be better as no inductance is required.

interoth · « **Reply #10 on:** July 06, 2012, 06:26:39 pm »

After the good advice of not connecting an antenna to my 1MHz Square wave, I have made a 50MHz Sine wave Colpitts oscillator.

I am hopeful this is correct, if you can see any problems please tell me.
I want to know what's the best way to attach an antenna is. Do I need an inductor, capacitor or what?

Thanks for your patience, I am quite new to electronics.

Simon · « **Reply #11 on:** July 06, 2012, 07:14:47 pm »

I am interested, how does one select the capacitor values ? I have need for an oscillator around 100 MHz

interoth · « **Reply #12 on:** July 06, 2012, 07:22:13 pm »

I used a already existing example, but this page goes into detail on how to calculate the values.

vk6zgo · « **Reply #13 on:** July 07, 2012, 04:51:58 am »

Quote from: interoth on July 06, 2012, 06:26:39 pm

After the good advice of not connecting an antenna to my 1MHz Square wave, I have made a 50MHz Sine wave Colpitts oscillator.

I am hopeful this is correct, if you can see any problems please tell me.
I want to know what's the best way to attach an antenna is. Do I need an inductor, capacitor or what?

Thanks for your patience, I am quite new to electronics.

Now you will have upset all the Hams ,who do "weak signal communications" at frequencies just above 50MHz!
There are reasons why people don't just choose frequencies at random to do stuff,& that is because the Radio Spectrum is already allocated to various services,who don't want your little "drifty" oscillator to interfere with them!

OK,if it is just for fun,nobody is going to be very worried,but you seem to want to regularly use this thing as a remote control.
They won't care,if it is once or twice,but if it becomes a regular thing,you will annoy someone!

As Auric Goldfinger said: "Once is happenstance,twice is coincidence,but three times............".

SeanB · « **Reply #14 on:** July 07, 2012, 05:44:28 am »

Remember that Hams are pretty good at tracking transmitters, as they do it for fun. Cheese those in the neighbourhood off enough and they will come and find you, then the regulatory authorities will come knocking on your door and they have the authority to confiscate as well as fine you.

dda · « **Reply #15 on:** July 07, 2012, 06:13:30 am »

Crappy RC toys use 27MHz. I was thinking of using it in a similar way to the OP. For freq where no licence is required can you just blast out RF signals?.

vk6zgo · « **Reply #16 on:** July 07, 2012, 06:25:34 am »

No,you are severely limited in power.
And not just anywhere on 27MHz-----AFAIK the toys are crystal controlled,so as not to interfere with CB & Marine channels on & around 27MHz.(In Oz)
The receivers are often pretty grotty--superregenerative Rx were common a few years back.
It you just steal a toy setup from the toy,nobody will know any difference.

dda · « **Reply #17 on:** July 07, 2012, 10:34:26 am »

I was basically thinking of cannibilising an rc toy / duplicating the radio circuit and using it to send data over short distances.

Is there a guide on what is good / legal RF practice?

Simon · « **Reply #18 on:** July 07, 2012, 10:36:18 am »

you should be ok with radio control frequencies and keep the power LOW. There are regulations on frequencies AND power. For example in Italy we could use the FM band providing we were not transmitting over a few metres.

interoth · « **Reply #19 on:** July 07, 2012, 02:01:02 pm »

So what frequency should I use?
I am only interested in broadcasting it over a few meters such as across a room.

vk6zgo · « **Reply #20 on:** July 07, 2012, 02:17:24 pm »

I think you would get in the least amount of trouble with dda's idea of using the Transmitter (& probably Receiver),out of a toy!
Have you thought about what I suggested in the other thread,using Ultrasonics?

interoth · « **Reply #21 on:** July 07, 2012, 03:16:42 pm »

I am currently making this to learn electronics, so using one out of a toy would not help me too much.

vk6zgo · « **Reply #22 on:** July 07, 2012, 03:49:31 pm »

You don't have to build every circuit in the world to learn Electronics.
You can learn a lot from playing with stuff other people have made,as in adapting the 27MHz RC stuff.

By the same token, not everything you do build has to have to have an application,
Build your oscillators,find out their characteristics,then pull them apart & build the next one!

I learned a lot about astable multivibrators on midnight shift at my then job,using spare twin triode tubes from their store ,caps & resistors from my home "junkbox",& an old dead mains radio chassis to supply power,plus the firm's Telequipment "Serviscope" to look at the waveforms.

Much more fun at 2am than reading old magazines,the stuff on the noticeboard,etc.
My M/Vs were like Cinderella,though,the 12AT7s had to go back into the store before the shift ended,& the old radio became just an old radio again.

interoth · « **Reply #23 on:** July 07, 2012, 05:10:44 pm »

I would still rather make my own, I find this subject interesting and want to know how to make one. What I am having issues with is finding a good frequency to use.

Simon · « **Reply #24 on:** July 07, 2012, 05:18:07 pm »

well for starters what country are you in ?


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Author Topic: AM transmitter (Read 32642 times)

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