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RF transmitters and Shielding

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pike

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Heya guys,

Finally setup my FM transmitter after that interesting thread, but I hit a small problem.

As the topic suggests, I want to shield my FM transmitter from stray capacitance and external noise. This is because everytime I hold the transmitter or move it around it becomes distorted. I suspect shilding would help solve this issue, but should i connect the negative terminal to the aluminum casing??? Or should i just leave it as is??

When I go near the antenna (3cm or less) the signal starts to null off, there isn't even any white noise.

Should I expect better range and performance from this shielding??

edit: this is the kit I bought just down the road, it's cheap, it works and it has a 3 stage system!!

**broken link removed**
 
The shield should make contact with the -ve line on the circuit board using as short a connection a reasonably possible.

I am not sure what you mean by the signal "nulling off" when you approach the antenna, can you explain a bit more please?

JimB
 
There is a limit as to what you can expect from a relatively simple circuit but l'd try to get the most out of it by doing the simple things first. Install the board in a metal enclosure with the ground of the circuit board connected to the metal enclosure. That may solve some of the problems.

Do what you can to get the antenna into a fixed position. The antenna is the load that accepts the power from the transmitter. At this frequency even subtle changes in surroundings change the way the antenna looks to the transmitter and those changes are likely seen in pulling the transmitter off frequency or causing the RF stage to operate at reduced power. You might take a short length of coaxial cable and run the output to a fixed antenna that's up and out of the way. You might then optimize the output by matching the transmitter output to the antenna and cable - that's something amateur radio operators (and others) do. The most significant result of this is to stabilize what the transmitter sees.

The transmitter frequency is controlled by a coil as shown in the picture. Any mechanical movement or vibration is likely to shift the transmit frequency to a point where you'll "hear" noises that you might otherwise think are coming from the microphone. The circuit is also likely to change as temperature changes too. There are some things you can do but at some point you might as well upgrade to a better transmitter.

Personally, I think it would be fun to see just how far you can push a simple transmitter like this in terms of stability and reliability. Mounting the transmitter to a heavy, stable base and doing a good job with a simple antenna can do a lot. Amateur radio folks have been successful with homebrew projects. Take a look for 6 meter (50 mHz) and 2 meter (144 mHz) projects for ideas.
 
G'day Pike,
Please post the schematic with parts values for your transmitter.
 
I suspect that two stages are audio and the oscillator is directly connected to the antenna. The antenna is part of the tuned circuit and causes the transmitter to change frequency when you move it or move around it.
 
When i say null off i mean the sound, drops off but there is no white noise. This means that the oscillator still works but it has zero modulation signal.

here is the shematic as wanted:
 

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This is an MOPA (master oscillator, power amp) design, much better than I expected. A shielded box is all you should need, connect the common of the circuit to the box. You should not expect to be within 3 cm of the antenna without absorbing most of the signal.
 
Its antenna is oddly connected between two high impedance tuned circuits. Even though they are staggar-tuned to try to cover the entire FM band, they make a poor match to the low impedance of the antenna.

Its electret mic is powered by a 47K resistor. I use 10K in my circuits and I've seen some with only 2.2K. With 47K the voltage across the mic is close to nothing.

The supply bypass cap for the preamp and mic powering resistor allows gain-reducing negative feedback at audio frequencies. The circuit needs an additional 22uF or 100uF cap there.

Maybe the radio mutes its output when it is close to the transmitter or when your hand is near the transmitter, boosting its output. Cheap FM radios don't have tuned circuits at their input, allowing strong RF at any frequency to overload their input transistor or mixer.
 
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