I'm gonna build some kind of RF-remote control for my PC, but I need one particular signal to not travel far (2m or so), which is easily accomplished by just not adding an antenna, but the problem is that the other signals (other buttons pressed) need to go as far as they can, so they will need an antenna. So how do I best accomplish this? And won't using something like a relay, alter the signal?
Hmm, so you want some buttons to travel a short distance, but other to travel farther, right? You could wire up a switched resistor to the antenna, Very large resistor should do it, antenna to ground connection With the resistor in place the antenna would not transmit very far, breaking the resistor's connection to ground with a switch would allow for farther tranmission. But RF is a fragile thing, is there any IC you can hack? (Op-amp or Transmitter IC?) I know your projects are pretty secret right now, but what is the application?
Assuming this is all being driver from a single transmitter, you may have to consider the 'finals' in the transmitter, if you omit or alter the antenna length you will throw the SWR right through the roof and possibly burn the final driver up. You may want to look at ways to the the antenna alone and look for a way to run the transmitter at normal/full power for certain events and attenuate it for others.
I'm gonna use one of them Rx and Tx modules at which you just have to apply a voltage, ground, an antenna wire and an input (or output, in case of an Rx)...and as I really don't know what an Op-Amp is (what is it? )... Okay, and what kind of "section" of resistors are we in? 1Ohm? 10K Ohm? edit: Dakar, how would you do it then?
Ah! I am the project leader in my Data Aquasition class, we are using serial transmitter/receiver modules. Mabye you are using the same, or similar to me? http://www.linxtechnologies.com is the module we are using. (HP Series II) They have tons of info on antennas Here is a good read on antennas from their site: http://www.linxtechnologies.com/ldocs/pdfs/AN00500.pdf There are couple different types of Antennas you can use, mabye you could use a switched antenna? Make two, a short haul, and long range antenna? (BTW a Op-Amp is a Opterational Amplifier, one of the most widely used components in electronics. It basicly amplifies nearly any signal)
Not too sure about the unit you are working with, however they should all be basicaly the same to some degree... but i'd start by reducing voltage/current to the 'final' transistor. Obviously we are not talking about any serious RF power, so i'd start with by pulling very small amounts of voltage and/or current away from the input of the final and tanking it to ground, basically underpowering the final amplification of the signal. This way the physical length of the antenna is still matched to the frequency of the transmitter just running at reduced power. Can you get your hands on a SWR bridge/meter near the frequency range your xmitter/rcvr are operating on? Most have a field strength meter built in that would give you a relalive idea of how well you are attenuating the signal. BTW on what frequency are the units operating? and can you post a link to a schematic/spec sheet for the radios you are using? I'm pulling all this from memory buried deep behind a lot of cobwebs, studied for and passed all the ham radio tests (extra class) like 10 years ago, and have promptly forgotten the majority of everything but antenna characteristics/building etc... So if i misstate something everyone can feel free to jump on my head and correct me.
Here're some specs (of the transmitter): Voltage: 3 - 12 V Power: 2 - 10 mA Output less than 10 Frequency: 433,95 MHz AM HF-modulator Bandwidth: 2 kHz Temp: 20 tot 70 °C link to some datasheets: http://www.produktinfo.conrad.com/cgi-bin/dlc/dlc.cgi?art=130428&lang=nl (just click on the link next to the flag of your choice )
