Placement and Layout of Antenna Tuning Passives - Page 1
Forum section: electronics / rf-microwave

author: E.Buer
author_rank: Contributor
timestamp: July 16, 2020, 07:20:57 am
content: Hi,I'm working on a board with a set of antennas that will need lumped component matching.Initially, I am laying out dummy footprints for passive matching/tuning. Two components in series, and two in shunt. This got me wondering if there is an optimal way of placing these.It looks like there are two possible approaches:1) Laying out the passives with a (short) transmission line in between.2) Squeezing the components in as close as possible to the antenna, neglecting trace impedance. The short traces will be way shorter than a fraction of the wavelength.Is there a "best practice" I should be aware of? Any literature that covers this?Thanks!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 16, 2020, 08:06:57 am
content: I take it, this will be near GHz or something like that? RF modules, multilayer board, planar antennas?Presumably, the radios will be expecting 50 ohm loads, so making the connection with a 50 ohm trace (microstrip or stripline) and a modest peppering of vias (particularly around the radio's grounds, any passives, the antenna, and periodically around the trace) will do there. Then the matching network can be any length from the radio.That leaves the distances between passives and passives, and the antenna.If you're solving the impedance iteratively anyway, I suppose it won't much matter where the passives are, as long as you have an L network at each point. Because, the last thing you would want to have, is an antenna whose impedance is, say, higher than Zo, so it needs a parallel impedance to bring it down; but if the nearest shunt element happens to be (2n+1)/4 wavelength away, that's going to do precisely the opposite of what's needed. And if the nearest series element is n/2 wavelengths away, that won't do anything either. (The opposite can work, however: i.e., series elements at (2n+1)/4 and/or parallel at n/2 wavelengths).Best idea is to position some kind of test connector inline, right at the antenna, and a matching network (three elements in a pi or tee is minimal, but sure, four or more affords more filtering options) directly after it. DNP the adjacent network components, and use a VNA to measure the antenna's response (and adjust it if applicable). Put the measured impedance into a simulator and tune the network for best results. Place components, DNP the antenna test connector, and position another test connector close to the radio module -- DNP the radio. Measure again; if you've done your job correctly, it should measure a solid 50 ohms or whatever in the bandwidth of interest.Typical test connector would be a U.FL type. Low profile, cheap, common. The footprint may not exactly permit placement over a trace without creating a stub length, I'm not sure. Ideally, the signal would pass through, in one side, out the other, so when it's DNP, it doesn't affect signal quality at all.Think the last one I saw, had something of a four-sided-square footprint, with ground pads on three sides and signal on the last? The opposite side could maybe be left open, to allow the signal trace to pass under the ground tab, at minor expense to CMRR.Or you could go with just the test connector footprint at the radio end. Place 0-ohm jumpers for series elements and DNP the parallel elements. Difference is, whatever impedance you measure, it must be de-embedded from the trace length between connector, matching network and antenna. Also, if the trace itself isn't well matched (Zo within 10% is considered precise, for FR-4 PCBs..), you might want to perform the de-embedding calibration right there (i.e., making the first shunt element a short or open or terminator, leaving the rest of the network DNP).Oh, and not to forget the cable itself. That needs to be de-embedded; it's probably not the greatest transmission line (or whatever adapters go between it and the VNA -- which, I'm guessing U.FL to SMA cables are cheap and plentiful, so that should be easy enough). Easy enough to do with U.FL connectors on a test card, with the requisite impedances.Tim

author: E.Buer
author_rank: Contributor
timestamp: July 16, 2020, 08:26:32 am
content: Thanks for the thorough response. It might be a mixture of chip and planar antennas. It is not decided at this time.The matching network will be placed right next to the antenna. I was thinking more about how the passives are placed relative to each other.I'm going to mill out a board with them squeezed together, and measure out what I get.The U.FL on the radio side is a neat Idea! I'll definitely try that. For now, I'm going to solder down a coaxial stub with an SMA.- Erik