Author Topic: easy PCB dipoles for 433, 868 and 915MHz  (Read 28399 times)

ChemE

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #90 on: August 21, 2017, 08:53:46 AM »
With regards to the pads holding the mounting pins for the SMA connector, make sure that the right-most of the three pads (seen as an unconnected island) is not too close to the radiating element. There is a ratio to maintain 50 ohms impedance on PCB tracks. This also applies to the spacing and pcb track with of the center conductor in relation to the adjacent GND pads.

Okay from what I can Google up the spacing for a coplanar waveguide on FR4 is a 40 mil wide conductor with ground planes 19 mils away from each side and a ground plane beneath. Ideally the side ground planes are stiched to the bottom ground plane with lots of vias.

If I am extending these concepts correctly, I want to make sure the end of the rightmost SMD is exactly 19 mils away from the polygon coming off the feedline (center pin).  And I should connect the bottom SMDs together and stitch them to the top outer SMDs so that this whole area becomes a very short 50ohm coplanar waveguide.

Is that what you meant?

PCBs.io

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #91 on: August 23, 2017, 02:17:29 PM »
Thanks for the feedback captcha and good video showing the sticks and range testing.  I'll be doing something similar in East Park once my boards arrive!  I am adding scale marks every 25 mils on rev2 which I'll be ordering this weekend (for $5 and less than 2 weeks there is little penalty to fooling around).  @PCBs.io is on these forums so we can easily find out what board material they are using.

I'll have to look into the needed spacing more to keep 50 ohms and rev2 was going to use elements the full board width to try to get better bandwidth and perhaps be shorter because of their wide width.  I guess I'll just Dremel off that pin rather than rely on the soldermask.  I'll try it both ways since all the pieces parts are so cheap.  I was wondering if there is any benefit to placing a larger pad beneath the feedpoint to connect those two ground pins and stitching it to the top pad as you did in your design.  This way there is a ground plane beneath the feedline up until it enter the element.

Sorry I missed this post. Our PCB material is FR4 with Er value normally between 4.3 to 4.9.

If there is sufficient demand/interest, we are not opposed making available a PCB antenna with edge connector kit.

Thank you,

Leo Wolfe
PCBs.io

PCBs.io

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #92 on: August 29, 2017, 11:52:50 PM »
Okay, I got impatient waiting for captcha's revised design so I decided to take his initial measurement of 6cm for trace lengths and design from that detail alone.



I shared my design on PCBs.io since they are so impressively cheap and ship free all over the world: https://PCBs.io/share/zd3Ya
And the edge-mount connector I designed with: http://www.mouser.com/ds/2/238/consma003.062-273742.pdf
They sell 4 copies making them $1.17/each after your area discount but my last two orders came with 5 PCBs each making these $0.94 each if they keep up that freebie.

Five boards shipped Tuesday!  8)

ChemE

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #93 on: August 30, 2017, 03:26:34 AM »
Thanks Leo!  And I've got 8 or 10 boards on the next panel too (with smarter silkscreening for tuning the traces).  Gonna have us an antenna shootout once those ship!

ChemE

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #94 on: October 09, 2017, 11:37:49 AM »
ADMIN UPDATE: The following posts and dipole work by ChemE are kept for reference. But his latest dipole updates and design files are at this new thread.

First things first; major credit to captcha for putting out enough excellent information that I could get going on modifying his design!  I very much appreciate the foundation you created since my RF skills are non-existent.

Ok, I still have some tuning and experimenting left to do on this project and Luka has asked for a 433MHz version, but I'm going to call my smaller cheaper PCBs.io antennas an absolute unqualified success.  My testing method is to place a node sending a packet every 8s at 300kbps about 50' away broadcasting at -18dbm (power level 0) with a few walls in the way and then listened for 20 packets.  I sum the RSSI of those 20 packets and base my results on the average of those 20 RSSIs.  Captcha has already done quite extensive work to show how his PCB antenna compares to 1/4 wave monopoles, eBay purpose built antennas, etc, so I'm being lazy and just comparing mine to his. 

Captcha's average RSSI =         -91.65dbm
ChemE rev2 trimmed (too far):  -88.90dbm
ChemE rev2 untrimmed:           -84.5dbm

I don't have a VNA nor do I plan to acquire one so based on what I can test, these little babies are more sensitive than the baseline design (no idea why that would be the case) and importantly they are dramatically cheaper which was my goal.



You can order my 915MHz version here ($4.79 for 4 copies shipped anywhere in the world): https://PCBs.io/share/4XRDg

EDIT: Added the eagle files which confusingly are named rev3 but they are the correct ones which correspond to the board I tested and you can order from PCBs.io.
« Last Edit: October 11, 2017, 04:01:50 PM by Felix »

Felix

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #95 on: October 09, 2017, 11:42:35 AM »
Awesome, just ordered some  ;)

ChemE

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #96 on: October 09, 2017, 12:21:42 PM »
Cool, glad to hear that Felix!  A note to anyone trimming these, the two radiating elements are the full width of the board (less the edge offset, so 220 mils) so a small scribe mark with a razor blade might not be sufficient.  I had best results getting a different RSSI by trimming the antennas with some heavy duty diagonal cutters that could clip all the way through the PCB and just physically shorten it.  At first I scored all the way down to the 20th silkscreen mark and wasn't seeing any real changes in RSSI before I busted out the dikes and chopped them and the RSSI dropped like a stone.  Luckily I have 5 copies of the board to make stupid mistakes on!

captcha

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #97 on: October 09, 2017, 05:31:11 PM »
Excellent work Mr ChemE !  :)

I think you did really well and have the results to show for it. I can very much relate the to the feeling of accomplishment when you've designed something from scratch and end up with a solution that is working the way you intended it.

Thanks for the kind words, but remember, I'm also standing on the shoulders of giants, haha..

As you mentioned, these PCB dipoles are just so much better than most cheapo vertical antennas you can get on eBay because there is a purposely designed symmetry between the two elements that make these antennas work very well. Which reminds me.. I still gotta post my antenna shootout video on youtube..


Kylix

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #98 on: October 10, 2017, 05:48:54 AM »
Any chance to have a 433Mhz version of the antenna?  :)

ChemE

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #99 on: October 10, 2017, 12:25:34 PM »
Awesome thanks Felix! I spaced the trim marks 50 mils apart from one another and the first mark is 25 mils inside of the edge of the resonator (true on both sides).  I used captcha's lengths of 6cm for the 915MHz antenna and 13.3cm for the 433MHz antenna.  From my R6 I soldered a u.fl connector and then used a 10cm pigtail to be between the u.fl connector and the RP-SMA edge-mount connector soldered onto the PCB antenna.  To keep things as tiny as possible, one of the ground prongs of the RP-SMA connector is isolated from the ANT resonator by just the solder mask.  There were some questions as to whether or not that was too close to cause problems but my results would indicate that this concern is more academic than real.  This all means the edge mount connector isn't quite in the center of the board; it is shifted right by 50 mils.  The real mirror plane of symmetry is centered between the two resonators.  Hopefully that give you enough to go on but if there are any any vagaries, let me know.

EDIT: Attached are my 433MHz Eagle files.  I got the board lengthened, the resonators stretched and the silkscreen shifted correctly.  The only thing left to do is place the edge mount connector in the right spot and then tune the gap between the two resonators; they look too close together.
« Last Edit: October 10, 2017, 12:27:39 PM by ChemE »

SabineT

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #100 on: October 10, 2017, 12:27:22 PM »
A version for 868MHz would be fine as well.
I'm in Europe, so I have to use 868MHz ;-)
« Last Edit: October 10, 2017, 01:02:56 PM by Felix »

ChemE

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #101 on: October 10, 2017, 12:58:18 PM »
SabineT: No problem, I'll design one this afternoon.  It is entirely possible that the 915MHz antenna will work excellently with a 868MHz radio.  Using wide resonators is purported to make the antenna wideband; perhaps enough so that one design will cover both bands.  It is a cheap experiment to test the 915MHz antenna vs. the soon-to-be-designed 868MHz antenna if you have the right radio.  All my radios are 915MHz.

I'm in Europe, so I have to use 868MHz ;-)

Transmit frequency yes.  But the 915MHz antenna might be wideband enough to still get great reception at 868MHz even though its length was tuned to 915MHz.  Regardless, making a 868MHz design was trivial: https://PCBs.io/share/46L3w.  This will cost $5.11 for four copies shipped anywhere in the world.  Please let us know how these work.  I'm going to order a set and test this antenna with my 915MHz radios to see how it compares to the 915MHz antennas.
« Last Edit: October 10, 2017, 01:27:16 PM by Felix »

SabineT

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #102 on: October 10, 2017, 01:41:25 PM »
Regardless, making a 868MHz design was trivial: https://PCBs.io/share/46L3w.  This will cost $5.11 for four copies shipped anywhere in the world.  Please let us know how these work.  I'm going to order a set and test this antenna with my 915MHz radios to see how it compares to the 915MHz antennas.
I ordered 4 copies right now. Thank you!

Regards, Sabine

Felix

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #103 on: October 11, 2017, 04:02:48 PM »
ADMIN UPDATE: The dipole research by ChemE and related posts above are kept for reference. But his latest dipole updates and design files are at this new thread.
Thanks ChemE for all the effort, and of course to captcha for his research and valuable input!