Multiple sdr with bias tee connected to one antenna?

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hruskacha

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I have 4 RTL-SDR.COM V3 SDR dongles that will hopefully go through a Nooelec bias tee powered (or USB powered) +20db gain LNA to one antenna. If i have these 4 dongles antenna cables going into the LNA, should all of the dongles have bias tee enabled or just one?
 

merlin

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You only need one to power the LNA. I would recommend that.
My LNA is 12-18 volt so use a separate power inserter. then use a 2 way splitter to 2 SDRs with bias T off.
 

radiopi

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You might want to use one LNA per SDR.

If you are using a power splitter, there are a few things to be aware of:
1. Not all of them go down to DC
2. There is a finite amount of coupling and that could cause the LO from one SDR to leak into another, DC etc. it could get messy
3. There will be about 7 dB of loss on each port so you will need to account for that.

Here is a post on Bias tees that might be of interest: Software-defined Radios and Bias Tees.
 

hruskacha

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Do you have the dongles connected through a proper splitter? If so, it's not going to pass the bias tee current from the dongles to the LNA.
Not at all, honestly. I have "Y splitter" cables and "T couplers". Im trying to figure out the best configuration here.
 

ka9ucn

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As Merlin had suggested only one bias tee is on "active" the others are passive.
Using the method he uses with a power inserter has the advantage of allowing you to vary the voltage to the amp. Thus controlling the gain.

Joe
 

Ubbe

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I wouldn't suggest changing the operational voltage of an amplifier. It will generate more noise and/or be more susceptible to overload and intermod issues. Let the amplifier run with the voltage it is designed to have and use a variable attenuator in front of the splitter and adjust to the optimum level for the receivers.

/Ubbe
 

ka9ucn

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The LNAs of today have a given voltage range that keeps them within the class of operation design. If you go out of the voltage range. The amplifier will no longer be operating in designed classification. Then you will run into the above mentioned problems.
Running an amp at full output then attenuation is not the way to go. When building amplifier stages such as between the antenna and the first stage or between distribution points no mater if they are between multi distribution runs, filters or different or any other stage. It is best to keep the gain at 0 and never more than 1 or 2 Db.
This rule of thumb is used in all forms of analog signal processing. The only exception might be to add a bit of gain in the final stages to overcome poor ADC sensitivity.

Joe
 
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