Author Topic: Small loop antennas @ 433 Mhz  (Read 44023 times)

WhiteHare

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Re: Small loop antennas @ 433 Mhz
« Reply #60 on: October 16, 2016, 12:34:53 PM »
As to the board layout for the micro Splatch, the datasheet refers to a "50 ohm microstrip (0.21" minimum)" which appears to be connected to the antenna pad of the micro Splatch.  Can anyone here shed light on what such a microstrip is?  Is it a trace of some kind, or a distinct discrete component, or....?

perky

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Re: Small loop antennas @ 433 Mhz
« Reply #61 on: October 16, 2016, 01:43:22 PM »
It's a trace on a PCB with a ground plane beneath forming a transmission line. The trace and the ground plane means the trace has a capacitance per unit length with the ground plane and inductance per unit length, the very characteristics of a transmission line. The ratio of the width of the trace to the distance between the trace and the ground plane defines its characteristic impedance. For a two layer FR4 type PCB and 1.6mm thick the width of the trace needs to be about 3mm (i.e. roughly 2:1 ratio) to get 50 ohms.
« Last Edit: October 16, 2016, 01:52:29 PM by perky »

joelucid

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Re: Small loop antennas @ 433 Mhz
« Reply #62 on: October 16, 2016, 01:44:05 PM »
Can anyone here shed light on what such a microstrip is?  Is it a trace of some kind, or a distinct discrete component, or....?

It's a trace of a certain width on top of a ground plane. Think of it as a pcb implementation of a 50 ohm coax cable. The width depends on the pcb parameters like thickness, dielectric constant etc. Google and you'll find plenty of calculators which will give you the width based on those parameters.

Now practically speaking you'll want the antenna directly on the ant contact of the radio. So you shouldn't need to worry about a 50 ohm micro strip from module to splatch.

Joe

WhiteHare

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Re: Small loop antennas @ 433 Mhz
« Reply #63 on: October 16, 2016, 02:10:47 PM »
Here's the top layer (see attached) from another version of experimental node that I sent into the fab last night.  I made wider both the antenna and ground traces that connect the RFM69 to the splatch, which I positioned to be very near the pins of the RFM69.
« Last Edit: October 17, 2016, 03:29:47 AM by WhiteHare »

WhiteHare

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Re: Small loop antennas @ 433 Mhz
« Reply #64 on: October 16, 2016, 02:49:53 PM »
I did it a little differently for the top layer on the splatch version of the bigger experimental sensor (shown in the photo above).  I include that here, since it might be easier to understand how it relates (see attached).  In total I now have sent three different experimental nodes (small, medium, and large) to that fab that have been revised to include PCB traces and pads for connecting up a micro splatch.  On the big experimental node (shown in the attachment to this post), you can see them at the very bottom.  The datasheet doesn't mention anything about the minimum width for the ground traces, but I beefed some of them up a bit.  I didn't use a copper pour on the earlier experimental nodes, and even the small one seems to be working just fine without it.
« Last Edit: October 16, 2016, 03:11:57 PM by WhiteHare »

WhiteHare

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Re: Small loop antennas @ 433 Mhz
« Reply #65 on: October 16, 2016, 03:54:23 PM »
For the small node, I only beefed up the antenna trace and left the ground traces at normal thickness.  So, this way, perhaps I'll see whether the ground trace thickness makes any difference to the splatch.  Not sure whether the right angle on the ground trace will make any difference or not (?).  In general, I've tried to avoid 90 degree trace angles by using a couple 45's instead of a 90.  I didn't catch this 90 before shipping it to the fab, but with more carefully considered routing the trace angle could have been handled with a single 45 instead of the 90.
« Last Edit: October 16, 2016, 04:02:47 PM by WhiteHare »

perky

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Re: Small loop antennas @ 433 Mhz
« Reply #66 on: October 16, 2016, 05:07:45 PM »
The 90 degree thing will make very little difference, only up in the several GHz region which your radio should not be outputting as this would be spurious. I wouldn't worry about it.

WhiteHare

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Re: Small loop antennas @ 433 Mhz
« Reply #67 on: October 16, 2016, 07:19:20 PM »
@perky :  Thanks.  Good to know!

@JoeLucid: I'm forgetting now, but have you found the larger splatch to perform a lot better than the micro splatch?  I'm assuming it does, because otherwise why would Linx make it or people buy it?  Presently I have 3 of the microsplatches and 3 of the larger splatches that I got from Digikey after you first started posting about them.  I'm debating whether to cook up a batch of experimental nodes with pads for connecting to the larger splatch, but thought I'd touch base with you first to see which of the two you now prefer most based on whatever testing you've performed to date.

WhiteHare

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Re: Small loop antennas @ 433 Mhz
« Reply #68 on: October 18, 2016, 02:55:54 PM »
See attached layout.  I refined the layout to do two things:
1.  Give it a ground plane, and
2.  Have one design which equally accommodates either the Splatch or the uSplatch.

There are some copper islands left over after the ground pour.  Is it best practice to leave them, remove them, or it doesn't matter?  Anyone know?
« Last Edit: October 18, 2016, 03:07:19 PM by WhiteHare »

joelucid

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Re: Small loop antennas @ 433 Mhz
« Reply #69 on: October 20, 2016, 07:43:16 AM »
Got my first test board today.

Here's the design:




Turns out I could get a perfect match with 2.8 pF implemented using 1x 1 pF + 1x 1.8 pF. Note that the small loop could be a little smaller to get even closer to 50 Ohm.




Performance is very similar to the l/4 monopole.

Joe

WhiteHare

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Re: Small loop antennas @ 433 Mhz
« Reply #70 on: October 20, 2016, 09:29:55 AM »
@JoeLucid
That's truly very beautiful to be behold.   :)  Sorry if my x-ray vision isn't as good as it used to be, but looking at the images you posted it appears that the small round loop is a pathway whereby ANT connects to GND, but with a capacitor in parallel, bridging the open gap in the loop.  The large square loop is completely standalone (not connected to anything else by traces), with  2 capacitors plus something else( ?) closing what would otherwise be an open loop.  Is that what's going on?

joelucid

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Re: Small loop antennas @ 433 Mhz
« Reply #71 on: October 20, 2016, 09:58:46 AM »
Quote
Sorry if my x-ray vision isn't as good as it used to be, but looking at the images you posted it appears that the small round loop is a pathway whereby ANT connects to GND, but with a capacitor in parallel, bridging the open gap in the loop.  The large square loop is completely standalone (not connected to anything else by traces), with  2 capacitors plus something else( ?) closing what would otherwise be an open loop.  Is that what's going on?

The small loop actually doesn't have a cap in parallel. I only created those two pads when I created the antenna as component in eagle so I could connect the radio to it. Ideally they would not be blank.

The large loop is completely standalone, correct, only inductively coupled to the small one. By choosing the relative sizes you can match the real impedance once the large loop is tuned for resonance. The large loop has 3 components. Two are for capacitors allowing finer tuning than possible with standard sizes. One is for a resistor to broaden bandwidth where needed (e.g. if you can't get a good enough match or if you need to do frequency hopping).

I have two more designs coming: one exactly like this but with a large loop that's duplicated on the other side of the board for less resistive loss. The other one has a round loop around the coin cell instead of to the side, allowing for a larger circumference with hopefully larger radiation loss and more aesthetic form factor.

Additionally I think the smaller loop should rather be flatter and closer to the large loop for better radiation since that will induce a larger current in the large loop. I expect to keep improving this design with the unfortunately large turnaround times caused by pcb manufacturing.

Joe
« Last Edit: October 20, 2016, 10:01:43 AM by joelucid »

joelucid

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Re: Small loop antennas @ 433 Mhz
« Reply #72 on: October 20, 2016, 10:25:29 AM »
Here are the other two designs:




and




Joe

WhiteHare

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Re: Small loop antennas @ 433 Mhz
« Reply #73 on: October 20, 2016, 12:24:14 PM »
A couple of things I'm wondering:

1.

Note that the small loop could be a little smaller to get even closer to 50 Ohm.


Are you saying that the small loop is (or could be made to be) 50 Ohm?  i.e. in a sense, the small loop is acting like the dummy load resistor antenna on the other thread?  If so, that would be a lucky find for me, as it would probably serve that purpose far better than I could approximate with an SMD and just a guess as to lead length (re: https://lowpowerlab.com/forum/rf-range-antennas-rfm69-library/inverted-f-antenna/msg15865/#msg15865)

2.  Since the overall footprint is bigger than what you already had previoiusly with a splatch antenna, is the motivation for this approach primarily better omnidirectionality?
« Last Edit: October 20, 2016, 12:29:07 PM by WhiteHare »

joelucid

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Re: Small loop antennas @ 433 Mhz
« Reply #74 on: October 20, 2016, 12:48:49 PM »
Quote
Are you saying that the small loop is (or could be made to be) 50 Ohm?

Only together with the large loop. This is like a transformer which transforms the tiny impedance of the large loop up to 50 ohm.

Quote
If so, that would be a lucky find for me, as it would probably serve that purpose far better than I could approximate with an SMD and just a guess as to lead length (re: https://lowpowerlab.com/forum/rf-range-antennas-rfm69-library/inverted-f-antenna/msg15865/#msg15865)

Why anyone would want to build an antenna that consists exclusively of a dissipative element is frankly beyond me. I think one would want to match impedance without resistive elements.

Quote
2.  Since the overall footprint is bigger than what you already had previoiusly with a splatch antenna, is the motivation for this approach primarily better omnidirectionality?

I think the small loop can perform better than a splatch with a super tiny ground plane. I'm particularly hopeful for the round design. Also as you say the loop is very omnidirectional - which the splatch is probably not unless you use it vertically oriented.

The current design is about as large as a Moteino - which is not bigger than a Moteino with splatch.

Joe