LAA+ FM broadcast interference using SDRplay Bias-T but goes away using 12v Bias-T power inserter?

Eric Inloes

So it seems that there is FM broadcast interference using the SDRplay Bias-T throughout the HF band but it goes away when i use the 12V Bias-T power inserter you include as an option. 

Any ideas as to what's going on here? 

Chris Moulding

Hi Eric,

For the best dynamic range from the head amplifier it is best to feed the amplifier with 12V via an unamplified Bias T. The base unit running off 5 V USB also has an additional amplifier so this may be over-driving your receiver.

If you are using a 1m diameter loop it will be resonant as a full wave quad loop in the VHF FM band. Make the loop larger or smaller to move the resonant frequency.



Eric Inloes

Hi Chris,

In this case it's just 5V form the SDRplay Bias-T and it's not anything else. Is there perhaps something more going on here? 

I am using a 1m diameter loop of 15.8mm OD copper pipe.




Chris answered your question.

To help stop intermod from strong stations the amp likes 12v rather than 5v.

As you have noticed it goes away when using may i suggest you use the 12v tee rather than the 5v from your sdr.

This is not a fault with the ccw amp but is down to how the amp is the same with many high end loop amps..they need full working voltage to “ reject” interference from unwanted signals..all my loop amps run at 12v..

Some people can get away with using 5v. Some can not..

Regards Simon g0zen

Eric Inloes

Hi Simon,

Ah, I figured more voltage would make IMD worse as it would result in higher gain. I guess that's not the case here.

Thanks clarifying that. 



No exactly the opposite..Ie more volts the better..( up to a point!)
No more than 13.8v though as more than that and I suspect goodbye transistor..

The gain may increase alittle from 5v to 12v BUT the strong signal handling capability goes up much more in relation to gain.

Do not use ccw 5v amplified tee though ( but at 12v.) a standard tee is all that’s needed.

I run mine at 13.8v from the psu ( fused and common mode choked.)