FlightAware Discussions

LNA with bias tee at 5v or 12v: Does it matter?

I have an LNA up the mast which handle can both 12v and 5v, selected by a jumper setting. Currently it is receiving 12v. The question is: What is the benefit, if any, to setting voltage to 12v over 5v, or is there no real benefit? The length of the LMR240 between the LNA and the bias-tee is 15-ish meters. Is the purpose of the settings to bridge longer cabling? May I expect to use 5v without loss of AC and message rate?

The reason it matters is that most LNA use 5v. It easier to just change that LNA rather than also change the bias-tee voltage (which is hard to get to though not impossible).

I have an LNA that takes 5-20 volts using the bias-t, but must use 5 volts if it’s powered by the header. Google it and see if you can find the specs.

This will just be a convenience to allow a broader range of input options.
It may be that the chip runs at 5V and the 12V option adds a voltage regulator.
More likely, the chip runs at 3.3V and jumper alters the input to make the best of the voltage supplied.

Do you have a model (or pics)?

I had a 5V LNA and I modded it to accept higher voltages (in my case some 7.5V), by adding a voltage stabilizer chip inside the LNA case. It does generate a bit more heat inside, but all this summer it was fine.
This is needed because of voltage drop on longer cables. The LNA chips are working better between 4.5 and 5.5V. You can’t just randomly increase the injected voltage without a stabilizer at the other end.
If the cable is shorter, there is no need for that.

thanks for all the responses. This helps. It would be nice if I can exchange LNA at the mast so as to try several different LNA. Hence the desire for 5v. Sounds like this works.

PGA-103+ is sitting behind a cavity filter.

Ok, makes sense.
Odd to use exactly the same part number as the chip, but makes it easy too.

The chip its self wants 5V, so you can either feed 5V up the coax or in on J3.

You can also feed it with 8~16V either up the coax or via J4. When you set the jumper to ‘12V’, the supply then (and only then) goes through the VReg.

To answer your question - yes it does matter, but only at the DC supply side. It makes no difference to the RF side.

The PG-103+ is a very broad-band amp (50~4000MHz), so matching it with a cavity is a wise move.

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@geckoVN thank you for your response. Appreciated.