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I have designed a receiver at 440 MHz with a 10 MHz signal bandwidth. I have measured the sensitivity of the receiver in a cabled setup (coax cable between transmit and receive) and that is about -93 dBm at the moment.

If I connect an antenna at both ends in line of sight, my receiver sensitivity becomes -83 dBm (a loss of about 10 dB in over-the-air setup vs cabled). Is that expected? I can see that as soon as I connect the antenna to my receiver, the noise power increases by about 10 dB (although there is no other signal at 440 MHz at the site where I am testing). That increased noise power is contributing to the link failure at -83 dBm instead of -93.

My question: is that why the noise floor goes up by 10 dB? Is that expected?

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  • $\begingroup$ have you measured the noise floor you get out of your antenna? $\endgroup$
    – Marcus Müller
    Commented Feb 17, 2023 at 14:07
  • $\begingroup$ I connected the antenna to the spectrum analyzer and the noise floor goes up by more than 10 dB. It's the same behavior as I connect it to my receiver. I am not sure what is the source of this added noise. I have seen most of the calculation of link budget only take thermal noise into account. This additional noise if throwing off the calculations of link budget. $\endgroup$
    – Malik
    Commented Feb 20, 2023 at 4:29
  • $\begingroup$ soooo, that means your question is more about the antenna's noise floor than about your amplifier, and that's good! (Noise can easily saturate a insufficiently prefiltered/bandwidth-limited amplifier, thus intermodulating to frequencies that "looked" clean.) But: if your antenna picks up signal so well, that's good, isn't it? $\endgroup$
    – Marcus Müller
    Commented Feb 20, 2023 at 14:35
  • $\begingroup$ I am not sure how. You mean there is actually some other signal over-the-air that the antenna is picking up? But to me, it appears that the whole 10 MHz spectrum just moves up when I connect the antenna and the SNR degrades. If I input a signal of a certain power to the receiver through a cable connection I get a certain SNR but when I input a signal of same power through an antenna, the SNR is 10 dB lower. $\endgroup$
    – Malik
    Commented Feb 22, 2023 at 5:05
  • $\begingroup$ Well the other explanation would be that your antenna itself produces noise - but it's a price device at room temperature whose resistance should be benign! $\endgroup$
    – Marcus Müller
    Commented Feb 22, 2023 at 8:57

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The cabled connection is mostly, 80% to 99% depending on the quality of the cables, shielded from outside interference, and is the best case scenario.

I connected the antenna to the spectrum analyzer and the noise floor goes up by more than 10 dB. It's the same behavior as I connect it to my receiver.

The noise increase is most likely QRM, man-made, and a small amount of QRN, natural noise.

I have seen most of the calculation of link budget only take thermal noise into account.

Thermal noise is just the absolute minimum noise at a given temperature and bandwidth. A link budget should add the maximum expected value, but unless you are designing a satellite (high temperature) or high bandwidth, this value should be one of the smallest in the link budget.

The greatest value in the link budget is distance at one over distance squared (1/(d^2)) in a vacuum. Adding air to the equation makes the loss greater at higher frequencies.

If your link budget does not include the distance factor + noise, your receiver will not work very well.

Antennas with a gain > 1 db will only amplify the noise problem, unless focused away from the noise source(s).

Is that why the noise floor goes up by 10 dB? Is that expected?

In the modern electrical age with noisy unshielded switching power supplies, power transformers, power lines everywhere: Yes, this can be expected as the new noise floor for your location.

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