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Antenna feed return loss

Melvyn Jones All Messages By This Member #1455 A question for the RF engineers amongst us. How important the output return loss of the antenna feed. To my mind if you have an LNA of 75K input noise temperature, as well as adding that noise to the receive signal it will also transmit that noise back towards the antenna where it is radiated out to space. But if the feed has a poor return loss, say 6dB, one quarter of that noise (19K) will be reflected back towards the LNA together with the received signal. So instead of a nominal system noise of 75K it is actually nearer 94K. Is this true?? A second point is the LNA actually has a physical temperature nearer 290K in practice. Is it the 300K or 75K that is radiated outwards to space? Melvyn
pwe237 All Messages By This Member #1457 It is not easy to give you a satisfactory answer Melvyn. If you connect your antenna directly to the LNA, then you can almost use ohms law to work out the power loss plus the equivalent loss added noise temperature. Certainly if both the antenna and LNA impedances are purely resistive. But, the manufacturer's LNA noise figure is measured with a 50 ohm resistive source which will change as will its equivalent temperature if the antenna resistance differs from 50 ohms. It gets a lot more complicated in? real-life cases when both the antenna and LNA impedances contain reactive components and even more complicated if you add a length of 50 ohm transmission line between the antenna and LNA which acts as a length-variable complex impedance transformer. Reflections only appear on transmission lines because of their variable length-dependent input impedance property when mismatched. Re your second point - noise figure is measured at nominally 290 deg K and is a measure of the component internally added noise. Peter Peter W East Get On 27 Jan 2024, at 14:29, "melvyn.jones via " <gmx.co.uk@groups.io target=_blank>[email protected]> wrote: toggle quoted message Show quoted text A question for the RF engineers amongst us. How important the output return loss of the antenna feed. To my mind if you have an LNA of 75K input noise temperature, as well as adding that noise to the receive signal it will also transmit that noise back towards the antenna where it is radiated out to space. But if the feed has a poor return loss, say 6dB, one quarter of that noise (19K) will be reflected back towards the LNA together with the received signal. So instead of a nominal system noise of 75K it is actually nearer 94K. Is this true?? A second point is the LNA actually has a physical temperature nearer 290K in practice. Is it the 300K or 75K that is radiated outwards to space? Melvyn
Melvyn Jones All Messages By This Member #1460 Thanks Peter, I was trying to keep things simple and not have a feed cable (although I accept even a connector is a short length of 50ohm coax) but had forgotten I had measured the antenna into 50 ohms as would the LNA have been. I think the simplest thing is to accept the hot/ cold system test results and not to try to over analyse the system. The results are better than I expected anyway. Melvyn

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