Is antenna matching important for receiver with LNA? - Page 1
Forum section: electronics / rf-microwave

author: Mad ID
author_rank: Regular Contributor
timestamp: May 28, 2016, 08:46:57 am
content: Hi,I'm trying to improve the performance of my GPS receiver based on Quectel L70-R. I have included an external LNA as the datasheet recommends to boost sensitivity by 3dB.Since my board is really small I have metal parts near the antenna which detune it. A matching network is present on the PCB to match the antenna.My S11 was around -2dB at the GPS frequency, and after matching its now -10dB. The problem is that when measuring satelite CN0 (carrier-to-noise) with two receiver simultaneously (with and without matching) I don't see any conclusive performance benefits.I have found in the u-blox application note that LNA after the antenna makes the matching loss less significant.An LNA placed very close to the antenna can help to relieve the matching requirements. If the interconnectlength between antenna and LNA is much shorter than the wavelength (9.5 cm on FR-4), the matching lossesbecome less important. Under these conditions the matching of the input to the LNA becomes more important.Within a reasonable mismatch range, integrated LNAs can show a gain decrease in the order of a few dBs versusan increase of noise figure in the order of several tenths of a dB. If your application requires a very smallantenna, an LNA can help to match the impedance of the antenna to a 50 Ohms cable. This effect is indeedbeneficial if the antenna cable between the antenna and the receiver is only short. In this case, there’s no needfor the gain of the LNA to exceed 10-15 dB. In this environment the sole purpose of the LNA is to provideimpedance matching and not signal amplification.My question is, does this mean that I don't have to bother with tuning the antenna at all? What the theory behind it? Can you give me an explanation or point me to some literature?Thank you very much.

author: Earendil
author_rank: Regular Contributor
timestamp: May 28, 2016, 10:32:29 am
content: I'm sorry, but I really don't understand how you can talk about S parameters and impedance matching and then ask a question like this.The quoted appnote explains it quite well. There are two pieces of background information you should probably know:- At 1/10 of wavelength or less you don't need to worry about transmission line effects.- Amplifiers provide large reverse isolation.You can find these basically in any of your electronics textbooks.Obviously this is only enough for an intuitive understanding of what's going on. For properly quantifying the effect you need to do EM simulations or make some measurements. I mean if you can measure the S11 of your antenna then why you can't measure LNA+antenna?

author: hamdi.tn
author_rank: Frequent Contributor
timestamp: May 28, 2016, 01:27:31 pm
content: Quotesorry for the attitude, he's just asking a question for the things he don't understand.Quoteso everything else ... suddenly no need for this forum anymore

author: KJDS
author_rank: Super Contributor
timestamp: May 28, 2016, 02:10:31 pm
content: If you match an RF FET, or pHEMT for best match then you'll usually miss out on best noise figure. Most RF transistors S parameters also include noise parameters and usually a compromise between best noise figure and best match will be used. If you can get the antenna within a fraction of a wavelength all it means is that you've got less rotation round the Smith chart to account for in the matching. The antenna won't be a perfect 50 ohms and you don't want the transistor to see that either.

author: Earendil
author_rank: Regular Contributor
timestamp: May 28, 2016, 02:33:28 pm
content: Quote from: hamdi.tn on May 28, 2016, 01:27:31 pmYeah, I'm sorry. But the question really seemed weird to me.

author: Earendil
author_rank: Regular Contributor
timestamp: May 28, 2016, 02:56:49 pm
content: Quote from: KJDS on May 28, 2016, 02:10:31 pmIn my interpretation of the appnote the antenna is also very small (not just very close).E.g.:http://www.antenna-theory.com/antennas/shortdipole.phpAre you sure that the only effect of this is that you get less rotation?

author: KJDS
author_rank: Super Contributor
timestamp: May 28, 2016, 05:55:01 pm
content: Quote from: Earendil on May 28, 2016, 02:56:49 pmIf tyhe line is lossy then it makes a difference, but you're not going to be using a very lossy line, so it will just rotate round the smith chart.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 28, 2016, 08:05:16 pm
content: The quote is talking about matching after the LNA, not before.In fact, the part where they say you must match between antenna and LNA rather gives it away...Tim