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Reduce the range of a Scosch FMT4 FM Transmitter
Wrap tin foil around it? The device: http://www.amazon.com/Scosche-FMT4-tuneIN-Universal-Transmitter/dp/B004NEUK86 -- @~@ Remain silent. Nothing from soldiers and magicians is real! / v \ Simplicity is Beauty! May the Force and farces be with you! /( _ )\ (Fedora 19 i686) Linux 3.11.6-200.fc19.i686 ^ ^ 21:21:02 up 4:34 0 users load average: 0.00 0.01 0.05 不借貸! 不詐騙! 不援交! 不打交! 不打劫! 不自殺! 請考慮綜援 (CSSA): http://www.swd.gov.hk/tc/index/site_...sub_addressesa |
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Reduce the range of a Scosch FMT4 FM Transmitter
Mr. Man-wai Chang wrote:
Wrap tin foil around it? The device: http://www.amazon.com/Scosche-FMT4-tuneIN-Universal-Transmitter/dp/B004NEUK86 The thread here, says "no". They used foil to actually increase the range :-) Hilarious. http://williamgibsonboard.com/eve/fo.../762107918/p/6 ******* Generally, if you "do something stupid" with a radio, the exact opposite of what you wanted to happen, happens. For people with cantennas, their output goes down. For people wrapping stuff with foil, their output goes up. An amateur cannot predict, without testing, exactly what will happen. It's because what an amateur does, is so uncontrolled. They don't really know what they're affecting (coupling, attenuating, leaking, wrong impedance, reflections, you name it). On real transmitters, fooling around can create a high standing wave ratio (SWR). Which is not good for the transmitter (i.e. when the transmitter is dealing with high powers, and the power is reflected back to the transmitter). On the self contained device you have, they've already taken care to match the antenna to the transmitter. ******* You will have to open the unit up, to see what is possible. Simply disconnecting the antenna, would reduce the range. The transmitter then, is looking into a high impedance. The antenna has a characteristic impedance. A way to quiet down a circuit, is to not only disconnect the antenna, but in it's place, use a carbon composition resistor which matches the impedance. Standard impedances used for things like this, are 50 ohms, 75 ohms, (or higher values for various balanced twin conductor setups). The old TV sets for example, used 300 ohm wiring (also used for FM radio). The highest I've heard of for twin lead is 450 ohms for ladder leads. Once the resistor is placed across the two terminals of the RF section, emissions will go way down. A proper termination resistor, a professional one, is surrounded by metal on the ground side (for an unbalanced termination). And that ground would also be joined to chassis ground. Using a resistor in the open, with no metal around it, wouldn't be quite as good a method. But in your case, you still want a little bit of signal. You could also use an attenuator, which would have four terminals on it. Two terminals on input. Two terminals on output. That's another way of reducing power. The intention there, is to continue driving an antenna, just at reduced power levels. On a real radio transmitter, power levels are significant. And such a resistive solution for absorbing power is termed a "dummy load". Your unit is limited by FCC rules for unlicensed FM transmission, so it's going to be a relatively low power unit, and chances are the resistor of your load is not going to fry. The hardest part, will be locating a carbon composition resistor. There are many kinds of resistors, and the carbon composition has the lowest parasitics. An RF guy at work was saying "they're flat to 1GHz", to give some idea, when connected to a network analyzer, how much L or C the resistor has. Which is why it makes a good RF load. It looks like a pure resistance, up to around 1GHz. As long as the leads on the resistor are kept short. They also make film resistors, and if you shop at an arbitrary shop, you're likely to get film resistors instead. Carbon composition is how they "used to make them". This picture is an example of an RF attenuator for a real transmitter. It isn't a dummy load. It's a four terminal device. "Looking into" either end of that thing, you see a standard 50 ohm or 75 ohm or whatever value. Notice the heat sink fins on the outside. As an attenuator for a powerful transmitter, the attenuator gets warm. As less power ends up on the antenna which is connected to one end. The impedance looking into one end, is only correct on that one, if the matched impedance is connected to the other end. Like this. 50 ohm ----- Power ----- Antenna impedance ----- Attenuator ----- Twinax transmitter In your case, with a transmitter at the 100mW level, just about any carbon composition resistor you can find (likely 500mW a.k.a. half watt ones), will not even break a sweat. So look for balanced attenuator designs or dummy loads for RF. Take the cover off the design, and see what's possible. If you add lengths of wire, they'll function as an antenna, so even if you were crafting a dummy load, chances are the transmitter will still be sending a signal. ******* You can also experiment with a Faraday cage, if you want another idea. Use a ferrite bead around the audio signal wire, where it comes out of the Faraday cage. http://demo.physics.uiuc.edu/lectdem...idc?DemoID=169 A Faraday cage is like your "foil" idea, only conditions are more controlled. If you look at that picture, the walls are made from copper wire based hardware cloth. At the corners of the faraday cage, is copper foil flats. The hardware cloth (i.e. screening) is soldered to the copper foil flats. When the door closes (tight), the copper flats touch one another, for a low impedance connection. Since the copper flats are very wide, it's like the contact surface on a microwave door, when the door closes on the chassis. The reason the foil doesn't work, is because there are all sorts of gaps and bad connections. You can't really solder foil, to make it gap free. And the foil could oxidize, making performance unpredictable later. Even ordinary computer equipment, uses caging for portions of the Wifi gear. Engineers use a "can" which fits over something needing protection, but the "can" is soldered down to the PCB. The idea is, when completing the sixth side of the can, to make sure the electrical joints don't have gaps. Soldering the gap ensures a tight fit. (Tin can soldered over MAC chip) http://s.ecrater.com/stores/241982/4...47_241982n.jpg But if you just wrap foil around stuff, and end up touching one of the conductors on the radio, it's just as likely to end up transmitting. If you can place an MP3 player plus the transmitter inside a properly constructed Faraday cage, nothing is going to come out. By opening the door on the cage a tiny bit, you get a tiny bit of signal. Etc. So your foil idea wasn't entirely wrong. You don't want the cage touching metal conductors on the MP3 player or wireless transmitter. You want the seams to be "RF tight". They solder the copper flats to the copper screening, for that reason. Having a door on a Faraday cage is a compromise, and the idea of the flats, is to give the best solution for conduction when the door is closed. On gear we used to make at work, we'd use beryllium copper spring contacts, for things needing RF quality grounding. As to meet FCC on equipment, sometimes springs were needed so removable hardware would remain RF tight. You can see in the following small picture, how springs on the chassis meet the cover, and provide an RF tight box. Beryllium apparently keeps the contact surface "bright" and helps protect the copper. http://www.eclipseshielding.com/imag...etintro_11.gif Paul |
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