Increased db gain

1

Hi all,

Having just separated the LNA from the SDR with a small 3" patch of coax, I seem to have stumbled on a reduction in noise in the signal noise ratio.

My setup is now +1 db over a direct connection.

Would those that are able see if they can reproduce the finding please?

2

I’ve heard of that before. Connector to connector sometimes does not achieve the best match.The 50 ohm output wants to see a smooth transition to another 50 ohms. I think a short cable provides a better transition than the abrupt connector to connector connection with no transmission line in between. Only my opinion.

3

What does the voltage measure at the LNA?

See: http://datasheets.maximintegrated.com/en/ds/MAX12000.pdf, p.3. As voltage decreases from 3.5V down, you get a little bit of bump on gain on the second amplifier stage.
Maybe your cable introduced a slight voltage drop.

4

AFAIK, none of the RTL type dongles have an actual 50 ohm antenna port. Remember, these were originally designed as TV tuners using RG6 or equivalent. It is a 50 ohm SMA jack connected to a possibly 50 ohm LNA interface, with output going into a 75 ohm tuner. There are possibly two thing in play here with the increased SNR:

  1. moving receiver slightly farther away from the antenna, electrically, may reduce the amount of spurious signals being received by the antenna and pumped right back into the front end of the RTL

  2. Impedance changed for the better

I have seen the same behavior when connecting a RTL directly to the feedpoint of my homebrew helical, as opposed to putting in a 10" jumper. SNR is much better with the jumper in. AND< I noticed the farther away I place the dongle, off axis of the antenna pattern, the better the SNR.

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