Fm transmitter and antennas  - Page 1
Forum section: electronics / projects

author: Asim
author_rank: Regular Contributor
timestamp: August 08, 2013, 04:33:38 am
content: Hi guys,i built a simple 3 stage transmitter yesterday, and i used a 160 cm wire as an antenna, it worked from the first try I was able to detect the signalfrom a Fm receiver but i want to improve the quality of the signal coming out of the antenna.my questions are:1- Is the diameter and type (multi-core or single core) of the wire changes the quality?2- Should the wire be arranged in a specific way (twisted or something ) ?i am open to other suggestions.thanks in advance

author: Kremmen
author_rank: Super Contributor
timestamp: August 08, 2013, 06:49:31 am
content: Check this:http://www.apparentlyapparel.com/uploads/5/3/5/6/5356442/_____practical_antenna_handbook_fourth_edition_carr.pdf. Probably uploading that one to the net was a copyright violation but there it is anyway, for anyone to read. (Not uploaded by me, i wish to point out...)

author: vk3yedotcom
author_rank: Frequent Contributor
timestamp: August 08, 2013, 09:43:27 am
content: Quote from: Asim on August 08, 2013, 04:33:38 am1. shouldn't have any effect. 2. might have an effect but there are more important considerations.The transmitter and antenna needs to be considered as a system. There are many reasons why range isn't as much as it could be. Also note that published designs may not be optimised for long range due to risk of interference in the wrong hands.Important points to consider are:1. Sufficiency of drive into final RF transistor amplifier stage2. Efficiency of final amplifier stage3. Impedence matching of final amplifier stage to antenna4. Length of antenna5. Radiation pattern of antenna6. Height of antenna1 & 2 require optimising and measurements. An untuned field strength meter may not necessarily be enough as you could be getting higher measured output but it might just be useless spurii or transistor self-oscillation. But a narrow band field strength meter (eg wavemeter) or receiver with a signal strength meter may help.3. is absolutely critical. Toy FM bug circuits often don't get this right. Just tapping an antenna wire onto the final collector might load it down and detune the output stage, reducing efficiency. You may need additional circuitry eg a low pass filter to clean up the output and some form of matching network (often just a coil and capacitor).4. Also critical. Changing the antenna length changes the load it presents to the transmitter and power transfer. A quarter wavelength (75cm) presents a low impedence to the final stage. Whereas a half wavelength (150cm) presents a very high impedence. Matching these to the transistor's final impedence requires different types of inductor and capacitor circuits.Especially with a quarter wavelength antenna you need a ground counterpoise connected to earth - also 75cm. This makes up a dipole and it's best to have the two wires 180 degrees apart.5. Very long antennas of several metres can work but may present odd impedences and have odd radiation patterns. If you want gain and a clean radiation pattern (though concentrated in a single direction) consider a small beam. But you don't want to be doing this until you've sorted out the other stuff with basic antennas.6. Height is might. You can get good range if you mount the whole transmitter and antenna at the top of a mast and have power and audio cables drooping down.Rules vary country to country as to what you can do but in any event pick a clear frequency away from a broadcasting station.I suggest going to talkingelectronics.com and reading every article there about FM bugs.

author: Asim
author_rank: Regular Contributor
timestamp: August 08, 2013, 12:37:34 pm
content: Quote from: Kremmen on August 08, 2013, 06:49:31 amthanks for the file, very interesting one!Quote from: vk3yedotcom on August 08, 2013, 09:43:27 ami wanted to put a matching network, but i don't know how YET. i guess i will look into it.thank you very much.