# Antenna signal amplification

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1. Mar 19, 2015

### synMehdi

I have an Antenna that receives signal coming from a wide band of distances, 10-2000 meters from a 10mW transmitter. I am working with 868 MHz band. What is the cheapest method ( or cheapest circuit ) to amplify this signal with an Automatic Gain Control LNA so that the received signal stays around -20dBm no matter how far the receiver is from the transmitter.
I want to input the amplified signal directly in a phase detector like the AD8302 which has the best performance at around -20dBm.

2. Mar 19, 2015

### BiGyElLoWhAt

maybe use an inverting gain amplifier with a pot that responds to voltage, use that for the variable gain. So look into op amps (if you haven't already) and voltage controlled potentiometers.

The amp should be ~$3-5, the fixed resistor should be about 10 cents, and the pot, I'm not sure, but I would think less than$5.

3. Mar 19, 2015

### Staff: Mentor

868MHz is pretty high for an opamp circuit...

Instead, have a look at the amplifiers available from Mini-Circuits and similar suppliers -- http://www.minicircuits.com/homepage/homepage.html

I don't know if they have AGC-type amps available, though. You might consider using a fixed gain amp with a variable attenuator stage after it...

4. Mar 19, 2015

### BiGyElLoWhAt

Is that beyond the response of op amps? Didn't realize that if that's the case.

5. Mar 19, 2015

### Staff: Mentor

Yeah, standard opamps are usually used in the audio range, and maybe up to 1MHz or so. Especially for low-noise RF gain amps, you usually use specialized amplifier circuits.

6. Mar 19, 2015

### synMehdi

Do Automatic Gain Control attenuators exist? Are they a better solution rather than AGC amplifiers? I thought of a log amp but I'm not sure about it, Do they compress the signal Gain?
I thought that the ISM band would have many relatively cheap components and modules, but I can't find anything suited for me.

7. Mar 19, 2015

### Staff: Mentor

BTW, that is a pretty low power level to be trying to detect at a 2km distance. What is the signal source? What is the modulation?

8. Mar 19, 2015

### synMehdi

My fault, I was not sure about that and I'm new to this. The thing is that I haven't decided the transmitter yet, It will work as a beacon, It wont carry any information so no modulation is applied (?).

9. Mar 19, 2015

### Staff: Mentor

What country are you in? What are the regulations for that 868MHz band? Are you allowed to do a continuous transmission of an unmodulated signal?

BTW, an unmodulated signal will be harder to reliably detect at the receiver. If there is some modulation (even just CW), it makes it easier to pull the signal out of the noise for weak signals...

10. Mar 19, 2015

### synMehdi

I live in France and I'm allowed to use this band and yes it is a Continuous Wave transmission.
Anyways for what I want to use, AM also should work fine because I just want two measure the phase difference ( difference of ToA ) between to antennas.

11. Mar 19, 2015

### Staff: Mentor

12. Mar 20, 2015

### sophiecentaur

If you are looking for low level signals, you would be far better to use some modulation and then synchronously detect that modulation. It will give much better immunity to interference than just plain Carrier Wave.
Can you use a directional antenna for this? Even if you want an omnidirectional horizontal pattern, you can get some useful gain by using suitable a vertical array. Reliable reception at such a distance with such low transmitter power is quite a tall order. You need all the help you can give it.