# Crude 5.8GHz RSSI Circuit

Thread Starter

#### Jackpot17

Joined Jan 29, 2017
8
I have taken on a project (involving mini quads if you know anything about them) and I have come up with a half baked solution that would involve measuring the RSSI of a signal of a giving frequency. These frequencies would be in the 5.8GHz range. I've read it may be possible to just use a piece of wire as an antenna etc however I don't know of a way of tuning the circuit to the given frequencies. Maybe using a transistors to connect some capacitors of different values? I don't have a clue.

Thank you

Thread Starter

#### Jackpot17

Joined Jan 29, 2017
8
Hello,

I assume that you are using the therm RSSI for Received Signal Strenght Indication.
http://encyclopedia.thefreedictionary.com/Received+Signal+Strength+Indication
For 5.8Ghz, the circuit will be quite a challenge to make.
Also how will you make the dBm scale, that is used for RSSI.

Bertus
Yes I am, I'm not interested in giving the signal strength a unit. I'm just interested in relative signal strength essentially.
So why would that be a challenge to make? I assume it's easier to build circuits for a lower frequency.

Jack

#### Papabravo

Joined Feb 24, 2006
13,469
Yes I am, I'm not interested in giving the signal strength a unit. I'm just interested in relative signal strength essentially.
So why would that be a challenge to make? I assume it's easier to build circuits for a lower frequency.

Jack
It is difficult because the wavelength of the signals is the same order of magnitude as the dimensions of the circuit. This means that ordinary lumped circuit components are no longer what you think they are. Inductors turn into distributed capacitors, capacitors turn into distributed inductors and active devices are unable to provide much in the way of gain. You can't design with pencil and paper and you can't debug without instruments that cost more than a house in a middle class neighborhood. So knock yourself out.

Last edited:

#### SgtWookie

Joined Jul 17, 2007
22,210
PapaBravo gave you the straight scoop.

When you are working in the GHz range, parasitic capacitance in inductors and particularly parasitic inductance in capacitors can make your life difficult.

For example, in the 5GHz range, small 0402 size MLC's (multilayer ceramic capacitors, about the size of a lowercase "o") have too much inductance due to how the conducting layers inside the cap meander around; you have to use SLCs (single layer ceramic capacitors) and custom trim them using a $30,000 material analyzer to get the capacitance you need. The inductors would look about like "_n_", and you would need a$40,000 network analyzer to ensure you had it properly tuned.

#### BR-549

Joined Sep 22, 2013
4,938
Is a mini quad a R/C craft? Does it currently use a 5.8 GHz link?

Thread Starter

#### Jackpot17

Joined Jan 29, 2017
8
Is a mini quad a R/C craft? Does it currently use a 5.8 GHz link?
Yes

Thread Starter

#### Jackpot17

Joined Jan 29, 2017
8
It is difficult because the wavelength of the signals is the same order of magnitude as the dimensions of the circuit. This means that ordinary lumped circuit components are no longer what you think they are. Inductors turn into distributed capacitors, capacitors turn into distributed inductors and active devices are unable to provide much in the way of gain. You can't design with pencil and paper and you can't debug without instruments that cost more than a house in a middle class neighborhood. So knock yourself out.
Would it possible to use an op amp to amplify the signal a little. Then I could compare the input signal to the op amp and its output signal's voltage to determine the gain. Then could I determine the frequency right?

#### Papabravo

Joined Feb 24, 2006
13,469
Would it possible to use an op amp to amplify the signal a little. Then I could compare the input signal to the op amp and its output signal's voltage to determine the gain. Then could I determine the frequency right?
Take a typical opamp, look at the datsheet, find a parameter called GBW (Gain Bandwidth Product). If you can find an opamp that will produce a gain of greater than 1 at 5.8 GHz then be my guest at creating a circuit. All the stuff I mentioned before still applies.

#### SgtWookie

Joined Jul 17, 2007
22,210
If you are determined to pursue using an amplifier, I recommend you look at Mini-circuits offerrings:
https://www.minicircuits.com

Their SMA modules make it simple and remarkably affordable to prototype a system.

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