Author Topic: Alternative PCB Dipole Designs (433, 868, and 915MHz)  (Read 14593 times)

ChemE

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Alternative PCB Dipole Designs (433, 868, and 915MHz)
« on: October 11, 2017, 12:30:28 PM »
Background Information
Last September captcha designed, tested, and optimized designs for dipole PCB antennas in order to allow anyone to simply order low cost and high performance antennas for their RF projects such as Moteino.  He made these designs available on OSH Park here: https://oshpark.com/profiles/captcha.  Many forums users have used these designs and found them to perform better than the 1/4 monopole antenna which ship with the Moteino giving either a longer maximum range before signal dropout or a stronger signal at the same distance.  Since then, captcha has done even more characterization of various antennas and shared his methodology and results here: https://lowpowerlab.com/2017/08/17/simple-monopole-antenna-testing/.  There is excellent information in both the original PCB antenna thread as well as the antenna shootout post and I do recommend that anyone interested check them out.

Motivation of Current Work
Building on captcha's original work, I wanted to offer smaller and much lower cost versions of the same PCB dipole antennas.  The price from OSH Park probably isn't an issue for those of us who live in rich countries, but not everyone on these forums lives in a rich country.  Plus since captcha's original work, we have become aware of lower cost PCB fabs which produce great work.  At the same time, I shrank everything down as much as possible around the resonators to try to get the antenna to fit within the smallest (and cheapest) board area possible.  There was also some discussion in the original antenna thread about wide resonators and the fact that they allow for wideband antennas and may also allow for a reduction in resonator length (remains to be tested at the time of writing).

An edge-mount RP-SMA connector has five prongs arranged in two rows.  The center prong is the signal from the radio and the other four prongs are GND and are connected to the outside barrel of the connector itself.  To keep things small but with wide resonators, I tried sneaking the resonator connected to ANT underneath one of these GND prongs.  All this required was removing the top pad that this GND pin would connect to and Felix suggested adding some silkscreen to further separate the two.  The final result of the connection from the RP-SMA to the two resonators looks like this:

Schematic of ANT Resonator Sneaking Under a GND Pin



Close Up Showing Unconnected GND Pin


There is no electrical connection between the rightmost prong (GND) and the copper rectangle beneath it (ANT) even though they are only separated by the solder resist and silkscreen layers.  From there, I just had to shrink the board around the resonators right up to the 15-mil offset that PCBs.io requires between copper and the board edge.  The final result of this optimization is shown below for 915MHz.

Compact Design Compared to Baseline Design

Some savings in length but the real board area savings comes from compacting everything into the minimum allowable board width of 0.25". 

Validation of RF Performance
Finally, to ensure that this optimization didn't come at the expense of performance, I tested the average RSSI between a gateway equipped with the PCB antenna under test and a stock Moteino using a 1/4 wave monopole 50' away broadcasting at 300kbps at -18dbm (power level 0 on the RFM69CW) with some walls between them.  Each packet sent out by the node has a packet ID in the payload so that the gateway knows if there was a missed packet and 20 packets were received by the gateway for each test.  The RSSI from these 20 packets was averaged to allow comparison between a known good design and my experimental version.  The results obtained were (larger or less negative values are better):

Captcha's reference standard:  -88.90dbm and 1 missed packet
ChemE's experimental design: -85.45dbm and 0 missed packets

Perhaps it is a reduction in PCB around the resonators or perhaps it is due to the widening of the resonators, but this small test would suggest that the new design is no worse than the standard and perhaps it is more sensitive.

Cost
My designs are shared on PCBs.io and the direct links and costs are below.

915MHz - https://PCBs.io/share/4XRDg - $1.20 each vs. $4.37 at OSH Park
868MHz - https://PCBs.io/share/46L3w - $1.28 each vs. $4.65 at OSH Park
433MHz - https://PCBs.io/share/rJ36o - $2.56 each vs. $9.02 at OSH Park

A cost reduction of around 72% without any apparent sacrifice in performance.  I will attach the eagle files to this post in case anyone would like to modify my designs.  PCBs.io ships free to any country in the world (how is this possible?) so these should be usable and affordable to just about everyone.
« Last Edit: October 11, 2017, 03:46:36 PM by Felix »

ChemE

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Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
« Reply #1 on: October 11, 2017, 12:31:28 PM »
Reserved for future summaries.

Ideal Feedline Lengths Additional information from Captcha on this subject
To get the strongest signal with the lowest noise, the feedline length should be λ/2.  This length is technically from the ANT pin of the RFM69 to the point where the two resonators diverge away from each other on the PCB dipole.  Corrected for typical velocity factors, the ideal feedline lengths are as follows:

433MHz = 22.86cm implies a 21.26cm pigtail
868MHz = 11.40cm implies a 9.8cm pigtail
915MHz = 10.80cm implies a 9.2cm pigtail

Note the the connection between the end of the pigtail to the male RP-SMA jack is around 1.6cm in length so you need to subtract this distance to get your nominal pigtail length.


Connectors, Jacks, and Pigtails
u.FL connector for Moteino: https://lowpowerlab.com/shop/product/153?search=u.fl
RP-SMA edge connector for PCB Dipole: https://lowpowerlab.com/shop/product/142 (Note: I used a male connector obtained elsewhere)

For 915MHz or 868MHz:
10cm u.FL to RP-SMA female pigtail: https://www.amazon.com/RP-SMA-Female-Pigtail-Antenna-Coaxial/dp/B072VWKXCR/ (requires a male RP-SMA connector on the antenna)

For 433MHz:
20cm u.FL to RP-SMA female pigtail: https://www.amazon.com/OdiySurveil-pigtail-Female-Bulkhead-Adapter/dp/B01E5PU8DY/ (requires a male RP-SMA connector on the antenna)

Male RP-SMA connectors for 1.6mm thick PCBs: http://www.ebay.com/itm/1Pcs-RP-SMA-Male-Jack-Pin-Solder-Edge-1-6mm-PCB-Clip-Mount-5-08mm-RF-Connector-/272619688310?

Connection to R6 Motenio


Total Length of Feedline 11.5cm (center of u.FL connector to left edge of PCB Antenna)
« Last Edit: October 11, 2017, 03:47:04 PM by Felix »

Charly86

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Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
« Reply #2 on: October 12, 2017, 09:01:19 AM »
Very nice, thanks for sharing

Any objection using this IPX (on moteino side) and solder the other side on PCB Antenna instead using SMA connectors ?

https://www.ebay.com/itm/282550843299
« Last Edit: October 12, 2017, 09:27:54 AM by Charly86 »

ChemE

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Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
« Reply #3 on: October 12, 2017, 09:10:48 AM »
Very nice, thank for sharing

Any objection using this IPX (on moteino side) and solder the other side on PCB Antenna instead using SMA connectors ?

https://www.ebay.com/itm/282550843299

As far as I know, that would work as well or better since you don't have the loss associated with the connector and you can ensure that your length from the ANT pin of the RFM69 to the nearest edge of the dipole is exactly your ideal feedline length.  And it makes the cost substantially less!  The one downside is that the u.FL connectors can only handle 30ish plug/unplug cycles so if you are wanting to test different dipoles (like me) you might eventually fry your u.FL jack on your Mote.  I think that is the only negative and there are a few positives.  I might grab 10 to fool around with myself; thanks!

LukaQ

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Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
« Reply #4 on: October 24, 2017, 02:15:29 AM »
What about not using coax at all?
meaning antenna would be almost next to output of RFM69, maybe 1cm away, 1/2"

Felix

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Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
« Reply #5 on: October 27, 2017, 05:17:34 PM »
Finally got mine from PCBs.io They are great!
Haven't done any testing but below are some shots of the 5 PCBs I got.

RE PCBs.io - It took 18 days to receive the boards, similar to my previous order (with free shipping). There is no tracking on the packages with the free shipping. There is no rush service like OSHPark. The quality is very good. As you can see the silkscreen is very crisp.
For my own preference I always disliked the black soldermask, you can't see traces, it makes debugging PCBs very hard. Not sure why they chose this color, I would suggest them to change it to red or some other color. But for dipoles nothing can really go wrong and they look good.

Would anyone like to see dipoles in the shop? :)



ssmall

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Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
« Reply #6 on: October 27, 2017, 07:10:10 PM »
I ordered some antennas and they came today.  Just wanted to thank you for posting your work.  You laid things out in an understandable manner so that I think I can get this to work for my setup.

LukaQ

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Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
« Reply #7 on: October 28, 2017, 08:29:44 AM »
My sould be on the way also. I might want to switch to 868MHz also, here in EU, 915 is taken, upss  ::)

NicksonYap

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Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
« Reply #8 on: October 28, 2017, 09:17:44 AM »
Hi ChemE,

Great work here!
I'm working on a LoRa project and this helps a lot!

About the feed line,
Does it also make sense to make i as short as possible?
Say I place the radio module output as close to the antenna input.


ChemE

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Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
« Reply #9 on: October 31, 2017, 04:08:09 AM »
Hi ChemE,

Great work here!
I'm working on a LoRa project and this helps a lot!

About the feed line,
Does it also make sense to make i as short as possible?
Say I place the radio module output as close to the antenna input.

Thank you NicksonYap.  This idea of a zero-length-feedline has been both discussed and tested/troubleshot elsewhere on these forums and the short answer is this is a bad idea.  There is a large amount of noise directly at the ANT output of the radio and RSSI will suffer very significantly.  I believe joelucid was able to get this working by using a balun but the details escape me.  It was buried in the original PCB dipole thread if you are interested.

davidbitton

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Ferrite Baluns
« Reply #10 on: November 07, 2017, 11:21:15 AM »
Would the design benefit from a ferrite balun such as this based on Martin E. Meserve (K7MEM)'s discussion on Center-Fed Half-Wave Dipoles? I'm no expert on the topic so I'm trying to glom together a myriad of resources online. Thanks!

Kilo95

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Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
« Reply #11 on: November 07, 2017, 10:22:42 PM »
A current balun shouldn’t be necessary if you have a feedline that is the correct length. That said, it shouldn’t hurt if you feel bound and determined to make one. It’d be of more use if the length of the feedline is in correct

davidbitton

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Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
« Reply #12 on: November 08, 2017, 10:06:57 AM »
A current balun shouldn’t be necessary if you have a feedline that is the correct length. That said, it shouldn’t hurt if you feel bound and determined to make one. It’d be of more use if the length of the feedline is in correct

I need to read up on the feedline thing. Thanks!

LukaQ

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Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
« Reply #13 on: January 30, 2018, 01:55:56 AM »
Feedline (coax) is VERY important! In my test, each side (node) feedline give ~3dB better RSSI, so just two ~11.4cm feedlines reduced RRSI from -56 to -50(even more often -49 and even -48) and I'm sure one of them is a bit longer, ~5mm (first time ever doing coax). Now this 11.4cm is from end of module (not IC BOOST pin) to edge of PCB antenna, no connectors
« Last Edit: January 30, 2018, 02:08:53 AM by LukaQ »

davidbitton

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Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
« Reply #14 on: January 30, 2018, 12:50:31 PM »
@LukaQ, can you take a pic of your setup? Thanks!