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help-coupling fm signal into mains

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bond000

New Member
hello

i want to build a fm transmitter which transmits signal via 220v mains lines. i have the transmitter ready.but,how do i connect the fm output to the mains?i.e i want to use the power lines as the transmission lines.also i want to derive the 9v power supply required for the transmitter,from the mains itself without using a transformer. please help.
 
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MikeMl

Well-Known Member
Most Helpful Member
What freq is the transmitter on?
 

Hero999

Banned
Sorry but it's a silly idea.

The mains wiring won't take 100Mhz very far and you will get into trouble with the electricity company.
 

bond000

New Member
yes. i know. i want to get a maximum range of 50 to 100feet. if i use AM band it will go far.but,AM is noisy. so , give some circuit ideas.
 

Hero999

Banned
Why not send the signal through the air?

I don't see the point in using the mains unless the transmitter and receiver are both enclosed in separate Faraday cages.
 

Sceadwian

Banned
AM isnt' so bad if your transmission is digital, do that instead.
 

ronsimpson

Well-Known Member
Most Helpful Member
X-10 sends information on 100khz.
There is a second format for 200-300khz.
National semiconductor has an application note for sending FM down the power line.
The power company sends data down the high voltage lines. I found many transmitters at 5mhz through 10mhz.
 

RadioRon

Well-Known Member
hello

i want to build a fm transmitter which transmits signal via 220v mains lines. i have the transmitter ready.but,how do i connect the fm output to the mains?i.e i want to use the power lines as the transmission lines.also i want to derive the 9v power supply required for the transmitter,from the mains itself without using a transformer. please help.
Mains wiring in a residence or commercial building might serve as a transmission line but it is a poor one. The reason is that while the cable appears to be a reasonable two-wire balanced line, the common practice in wiring a building is to daisy-chain the cable from one load point to the next until it terminates at the breaker panel, for example. So every few meters or so, the parallel orientation of the conductors is disturbed as the wires go into each electrical box and connect up to a load, a switch, or some other wires. Then, at the end of the line, in the breaker panel, the wires are separated and one goes to one of the incoming power wires, while the other one goes to ground.

At high frequencies, like FM broadcast band for example, these disturbances in the geometry of the conductors will be quite serious and much power will be radiated or reflected back to the source.

So there is no point in approaching this idea with great precision. You might as well just go ahead and try it. The way to couple transmitter energy into the house wiring is to configure your transmitter to a balanced output configuration (perhaps using an RF transformer), and then connect each of the two wires to one side of the mains through two small ceramic capacitors that have a high voltage rating. I suggest a value of 1000pF with a 500Volt rating. Safety rated capacitors such as these might be appropriate for this:
http://www.vishay.com/docs/28537/28537.pdf
 

MikeMl

Well-Known Member
Most Helpful Member
At 100MHz, I'd be more inclined to feed RF between Neutral(white wire) and GND (green wire) instead of L1 (black wire) to Neutral. Lots of appliances have internal filter capacitors between L1 and Neutral, as well resistive loads, which would tend to short the 100MHz signal, maybe even plugged into the same duplex outlet only inches away.

Between Neutral and GND, there is also a short circuit, but it is back at the main distribution panel, many wavelengths (at 100MHz) away from where the transmitter is likely to be plugged in... Also, there is no voltage between Neutral and GND.
 
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RadioRon

Well-Known Member
At 100MHz, I'd be more inclined to feed RF between Neutral(white wire) and GND (green wire) instead of L1 (black wire) to Neutral. Lots of appliances have internal filter capacitors between L1 and Neutral, as well resistive loads, which would tend to short the 100MHz signal, maybe even plugged into the same duplex outlet only inches away.

Between Neutral and GND, there is also a short circuit, but it is back at the main distribution panel, many wavelengths (at 100MHz) away from where the transmitter is likely to be plugged in... Also, there is no voltage between Neutral and GND.
Very good idea. I agree that this would be better.
 

bond000

New Member
thank you very much for your help gentlemen. if i use the circuit given below, which is transformer less power supply. will the RF get coupled into the mains? i mean without feeding the rf output via a cap. into the mains?i think this way,because there is no choke used to stop the rf from getting coupled into mains. i will use a cap to avoid the 60hz hum. will it work?
or will i have to feed the rf from the output to the mains through a high voltage capacitor? please help.
 

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ronsimpson

Well-Known Member
Most Helpful Member
I like Mike's idea of driving RF from Neutral to GND.
So why none of the commercial unites do that?
One of the points if the transmitter is to not send RF into the air. Only have it differently on the power line. I believe Line and Neutral balance out well when differently driven and thus do not radiate much RF.
 

Sceadwian

Banned
Because neutral and ground are tied to the same point at the junction box in many situations. Not sure if it'd work.
 

MrAl

Well-Known Member
Most Helpful Member
Hello,

This question came up several months ago and we talked about it in great
detail. It someone can find that original thread you will gain more info.

BTW, the frequency is under 1MHz in National's app notes.
 

rmn_tech

Member
... Also, there is no voltage between Neutral and GND.
In the U.K. there is approximately 0.4 - 1v @ 50Hz between neutral and ground. Due to the earth and neutral being connected at the distribution transformer and this can be some distance away from the consumer
 

Hero999

Banned
In my house the neutral and earth is connected at the entrance of the building.

Either way it doesn't matter as a simple low pass filter can be used to get rid of the 50Hz.
 
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