Hi

I have a real urgent project that needs completing and I was hoping you gous can maybe help me
with some aspects.
I have electronic knowledge;I know the most of the fundementals but not full enough knowledge to
complete my project.

I am embarking on a radio/data-transmition project.

OVERVIEW:
I have 4 transmitters and one(ore more) reciever(s) wich are x distance appart.
When a button is pressed on the tx then the rx activates a digital 1 at a certain output(from there I know what to do).
Every tx,when transmitting(switch is closed), activates a diffrent output on the rx(parralel outputs).

I have no knowledge of radio(!!!), only bits and bieces.

My questions:

1))) Can I use the morse-code(CW) concept with this? I figured if I give each tx a unique
pulsed frequency(a amplifiede 555-timer), and give the rx corresponding filters(normal active rc-filter then connected to a "detector"),
then I have precisely what I want, but is it practical and will it work....??
example:
The 4 transmitters are tuned to 1Mhz, 2Mhz, 3Mhz, 4Mhz pulsed(non-ideal sine) respectively.
The 1 reciever has 4 parralel,highly selective, filters: 1Mhz, 2Mhz 3Mhz 4Mhz respectively.
When the 3Mhz-transmitter is activated, the 3Mhz-filter(or any other) is "activated" while the rest of the
filters are not(until the apropriate tx is transmitting).

2))) What antenna should I use??(please dont say dipole or unipole ..or any-pole.. )
I want as small as possible antenna.
I saw some radios with only a small wire,instead of a big "bunny-eared", for antennas, how can I implement
this??


Please please please help me, you guys are the only resources that I have for this project(and some out-dated books).
This is an educational and a much needed project, it's not for a school or varsity but for enhancing my
parents' home.

CHEERS
*******

__________________
Beware, you who seek first and final principles, for you are
trampling the garden of an angry God and he awaits you just beyond the last theorem.

 

I'm not quite sure what your trying to design.......but,
I can answer at least a few of your questions.

First you dont want to try use different frequencies of the transmitters, the easyest path is to have 1 freq and transmit a tone or code ( like a garage opener) and discriminate the different transmitters by the tone or code at the reciever. Clear?

Don't worry about CW-AM-FM what ever there are pretuned transmitters mostly UHF-FM that are off the shelf available for cheaper then you could build them for. as an example : http://www.parallax.com/detail.asp?product_id=27986 , http://www.parallax.com/detail.asp?product_id=27987

There are places that sell them much cheaper and I guarentee that it is prefreble to homebrew coil winding and filter maths.

The last thing is the antenna. The size of a transmitter ( and ideal reciever) antenna is dependant on the transmitters freq. Higher the freq the smaller the transmitting element. The two examples I gave you use a piece of copper on the pc board about the size of a postage stamp. Hopes this helps a bit. good luck.

 

Frequencies above 100kHz are too high for active RC filters, but LC filters will work for your frequency range. If you want a small antenna, the frequency should be high. Yageo makes a small ( several centimeters) antenna for 433 mHz which works well. If there is only one transmitter on at a time, you can code each one with the 555 oscillator and use an RC active filter in the receiver. But, if more than one transmitter will be on at the same time, you probably need 4 transmitters and 4 receivers, each on a different RF frequency. The 27 mHz citizens band might be good for you because you can pick up used transmitters cheap and they have enough power that you don't need an efficient antenna.

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