Low noise amplifier (LNA)

jgreentnjgreentn
in Configuration and Operation

Any sense in adding an LNA to my setup? If so, does anyone have a recommendation? Thanks


Jerry

Comments

  • G8JNJG8JNJ

    It depends, but generally I'd say no, you will most likely only boost both the wanted and unwanted signals, so the overall Signal to Noise ratio will remain the same.

    However, you will eat away at the KiWi's available dynamic range, and most likely introduce additional unwanted IMD products. Both of which are undesirable.

    What are you hoping to achieve, and what antenna are you currently using ?

    Regards,

    Martin

  • jgreentnjgreentn

    Thanks Martin. I'm using a T2FD (folded dipole), but it's not high off the ground, maybe 2 m at most. I've heard getting the T2FD up higher improves reception.

  • smgsmg

    @jgreentn - There is a rabbit hole you are probably looking down right now. Uplifting that dipole would likely help, however, what is your noise floor like? i.e, what's the S Meter showing when not "tuned in" to any signals?

    example:

    image.png

    So for me, on 20mtrs, the noise floor is just above S3 and I live in a built up urban area. Where my Xiegu G90 connected to an EFHW gets an S5-6 noise floor.

    The difference being my KiwiSDR is using a magnetic loop

    I've tried different LNAs on passive antenna, and as Martin said, you just raise the noise floor and lose even more weak signals.

  • TremolatTremolat
    edited December 13

    Hello everyone. It's a very good question whether or not to use an LNA. I use one primarily to compensate for the length of my antenna cable, which is 250 meters long. Secondly, I slightly increase the sensitivity of the KiwiSDR.

    The LNA needs to have very low internal noise. I discussed this with Martin, and his advice was very helpful. I thank him for it.

    I think the first task is to eliminate all sources of background noise, noise caused by the installation and also noise in the immediate vicinity of the antenna.

    It's not simple and we have to be very methodical.

    My antenna is currently a vertical triangular mast. It's working correctly. But I'd like to build one (or more) magnetic loops in the coming months. Loops with a diameter of 2 to 3 meters. I need to find the time to do it.

    Best regards.

  • G8JNJG8JNJ
    edited December 14

    If you are using a T2FD at low height, you may possibly benefit from an LNA.

    A lot depends on your local circumstances and level of background noise and interference.

    The KiWi has a noise figure of around 14dB (from memory), which by itself is just about good enough for reception on the upper HF bands, providing a reasonable antenna is connected. This noise figure is more than adequate on the lower frequency bands, as the natural noise floor is much higher, and the receiver noise contribution becomes much less significant.

    One test you can perform is to listen on a quiet HF frequency, typically between 20 and 30MHz, then make a note of the receiver noise floor, with the antenna connected, and then disconnected.

    If there is an increase of 6dB, or more, with the antenna connected, then adding an LNA will not make any difference.

    This is most likely because you have a high level of noise and interference, especially in an urban area, and your efforts would be better served by trying to locate and remote the sources of the noise, if at all possible. From experience, most high levels of noise, usually originate in your own home, or that of near neighbours, so you may stand a chance of improving things.

    If the noise level doesn't increase by 6dB, then your antenna in not big or high enough, and adding an LNA may help. As is the case with "Loop on the Ground" or "Wire on the Ground" antennas, which by themselves have very low gain.

    If you are using an LNA, then the test should be conducted by removing the actual antenna (or loop), and not the LNA output to the receiver, as it needs to include the additional LNA gain and noise contribution.

    If you don't have an LNA connected, and have massively more than 10dB of increase in noise with the antenna connected, then you have serious noise problems, and these definitely need resolving.

    If you do have an LNA connected, then it would be better to apply some attenuation, so that the noise increase is around 6dB, as this is the "sweet spot" in terms of system sensitivity vs. Dynamic range and IMD generation.

    I hope this helps,

    Martin

    studentkra
  • n6gnn6gn
    edited 3:25AM

    To the degree that NF is a good description, it's more like 20 dB. Noise floor in the absence of large signal is around -155 dBm/Hz. That's not quite good enough to meet ITU Quiet Rural with a perfectly matched 30 MHz antenna. But a preamp won't help unless you can put it in front of the mechanisms causing mismatch and noise ingress.

    If noise floor comes up with 'antenna' connected then a preamp likely won't help but fixing the antenna system might.

    See discussion related to the SAS on OSHW pages. not least the antenna factor and deployment pages.

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