Voltage check of LAA+ bias-t

Rob Q

Hi, I am unable to catch anything on longwave and I am using a 12V power adapter. Is there a way that I can see what the voltage is coming out of the coax and in to the amplifier?

Chris Moulding

Use a volt meter to measure the voltage coming out of the Bias Tee unit.

This should be around 0.7 V below the input voltage due to the reverse polarity protection diode.

You can check the voltage at the amplifier end of the coax. With the amplifier disconnected this should read the same voltage as that coming out of the Bias Tee unit.

If the LAA++ is working OK on HF frequencies then it is working.

I don't know your location but many areas of the world don't have long wave broadcast stations.




A BNC T-piece along with aBNC to terminal adapter is useful for measuring the voltage at the LAA.

I had a situation where I was getting unusual instability effects in the receiver which appeared to be coming from the LAA. I had checked the no-load voltage at the LAA end of the co-ax as being OK, but when I checked it on-load (connected to the LAA via the T-piece) the voltage was somewhat reduced. The centre pin of the BNC on the co-ax was poorly soldered to the core, creating a high resistance joint.


Rob Q


Like these? How do you check the voltage with that?

See the source imageSee the source image

Rob Q

Chris, we have mostly NDB's and DGPS. I should be getting the locals such as SN, HM, etc. but it's deaf below 530 kHz.
My location is Ontario, Canada. Grid FN03. 
HF doesn't seem to be affected though. Plenty of signals there.
I've just switched the loop over to a 60cm diameter hula hoop with a solid copper wire running through. It's not the thickest though.


60cm is rather small for even mw, especially lw and below..
Fine for hf..

Rig up a 1m diameter loop ( lash it up) and see if any better..one would expect it to be..

To check if voltage is at end of cable ( amp) volt meter from centre pin to outer of coax..expect circa 11-13v depending upon cable length and V from psu..

As Chris said..if working on hf then amp works..i bet loop too small.( ie i use crossed parallel loops of total dimensions 2 x 2 m for low bands..) 60cms is good for ( CIRCA) 10mhz above.

Simon g0zen


On 12/20/2022 5:22 PM, Rob Q wrote:


Like these? How do you check the voltage with that? I was thinking of a multi-meter. Stick the red lead on the center pin and the black on the shield. Well, something like that anyway.

See the source imageSee the source image


I love those "green" adapters!   So useful for temporary and experimental hookups when you don't want to solder up cable assemblies.  I've got a pile of them with male and female ends for RCA jacks,  male and female ends for BNC connectors,  2 and 3-cond 3.5mm plugs and jacks,  1/" plugs and jacks,  6-cond RJ-11/13 phone plugs and jacks,   8-cond RJ-45 "Ethernet" plugs and jacks,  and 6-pin mini-DIN "PS/2" plugs.  I just keep finding more types of connectors packaged this way on Amazon! 

I use the RJ-45 male plugs to break out RJ-45 mic jacks on radios, when experimenting with sound card interfaces.  I make temporary attenuators for both audio and RF by screwing down 1/8th watt resistors between male / female RCA or BNC pairs.  I have used a pair with male/female XLR connectors, with resistors in-between, to create one-of-a-kind attenuators for my audio mixer.  

Just so convenient to be able to whip up these setups in the field,  without soldering.

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Rob Q

Hi Chris, how do you test it with an analog multi meter? I don't want to accidently short it or get a nice zap. I'm guessing the red probe in the center and black on the outer circle?

Rob Q

I find it kicks butt at MW at 60cm. You mean it can do better??

Rob Q

I've now have it at a 1m diameter. Only thing I've notice now that is the 40m band really came alive. My wire is 14 guage solid copper electrical wire, I don't know if that's good enough. Just a quick recap, this is the power supply I'm using. 
DCU120020-G2421: Jameco ReliaPro : 2.4 Watt 12 VDC 200mA Regulated Linear Wall Adapter 2.1mm Center Positive : Power Supplies & Wall Adapters
Chris did mention that with the voltage coming from the bias-tee of the SDRPlay devices that the LAA++ is limited to a minimum of 500 kHz.

Rob Q

Problem solved. I've learned that the power adapter is outputting noise at LF, so if I want to explore below 530 kHz, I'm now using a battery pack with 8x 1.5v Alkaline batteries. Nice clean signals!


For a cheap clean psu..if you can find? an old school cb power supply works well..( circa £15 in uk on ebay.)
It will be about size of half a shoe box if old school..if old school it is a linear power supply..ie not switched mode..= no rf noise.

As will be second hand just get it checked over before using.. ( unless competent to do so yourself.)