Author Topic: easy PCB dipoles for 433, 868 and 915MHz  (Read 166907 times)

ChemE

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #75 on: April 16, 2017, 02:11:04 AM »
From the looks of it, it won't be too difficult to slide the holes up closer to the traces and shrink the height to 0.25" or pretty darn close.  Perhaps shrink the diameter of the holes if the board is still taller than 0.25" even after things are positioned more tightly.  I'm also thinking of trying these out with https://www.pcbs.io/ .  Four 915MHz dipoles for $5 shipped sounds pretty amazing to me.

EDIT: In looking again at your PCB on OSH Park, it looks like you could get pretty close to 4.5"x0.25" with the 915MHz u.fl version which would be $4.50 shipped for four!
« Last Edit: April 16, 2017, 02:21:32 AM by ChemE »

joelucid

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #76 on: April 16, 2017, 02:25:49 AM »
Btw if you make the dipole wider it can be shorter and will be more wide-band. See http://www.antenna-theory.com/m/antennas/broaddipole.php. For my gw I've got a 9mm wide dipole in the works which might cover the whole 868-925 MHz range. Which would be nice.

ChemE

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #77 on: April 16, 2017, 02:51:51 AM »
Very interesting page.  Obviously the traces could be a good deal wider and the board still come in at 0.25" tall.  If this resulted in a still shorter PCB with better bandwidth so much the better!

captcha

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #78 on: April 16, 2017, 07:17:45 AM »
I've also been wondering whether to go for 2 ounce copper (resulting in a pcb trace of 70um [2.75 thou] tickness). The skin effect at 433MHz is just a mere 3.1um and only gets thinner as the frequency increases. It won't help broadbandedness so much but it does reduce ohmic resistance and may help make it a bit more rugged.


captcha

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #80 on: June 25, 2017, 01:24:49 AM »
Just to keep everyone in the loop, I have just sent off a Rev2 of the 433MHz SMA dipole and a 915/868 U.FL dipole to OSH Park for manufacturing.

433MHz SMA dipole changes:
- increased the dipole element diameter from 1mm to 1.23mm
- pcb traces from SMA connector to feedpoint of the dipole has been replaced with curved traces with a bend radius of 3x the trace thickness
- modified the footprint of the SMA connector pads so they are now 50 Ohms Z.
- changed the center feedpoint of the dipole to be the middle of the board (not the SMA connector)

915/868 U.FL dipole:
- U.FL connector pads are 50 ohms Z
- trace thickness 1.52mm

These versions are deliberately 'too long' so I can hook them up to a VNA and cut them to resonance. Once that's done I will adjust the design for the correct lengths and post the link to the final versions on OSH Park so anyone can order them.


captcha

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #81 on: June 25, 2017, 01:27:15 AM »
Sorry for the late reply:

Quote
Will these 433Mhz antennas work just as well with LoRa spread spectrum?


I believe so. Spread spectrum centers around the middle frequency that these boards are designed for.

ChemE

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #82 on: June 26, 2017, 08:05:01 PM »
Thanks for your efforts captcha.  I've been holding out ordering my dipole until this improved design is ready for action so I'm glad to hear that it is mere weeks away now.

captcha

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #83 on: June 26, 2017, 10:40:21 PM »
Thanks for your patience.

I had quite a few distractions and even lost interest when I wasn't getting anywhere with some of my other projects. Anyway, we're back on deck! :)

Yeah, it would be nice to have a complete set of dipoles for the three ISM bands with some common connection options (direct-solder-on, SMA and U.FL).

For now I haven't designed the direct-solder-on boards as I'm still contemplating whether or not to just use the SMA board for direct-solder-on; it already has large-ish exposed pads and it shouldn't be too difficult to solder the 3mm dia RG-173 onto it. Initially, with the solder connections I was thinking of re-using the U.FL board and putting in two through-holes instead of the U.FL SMD pads. That would make the board the same size but I'd still have to do the whole cut-to-size thing again because there will be slight differences in length from the connector pins to the dipole elements.

I think if most are happy with the SMA design we can safely say that this will behave fine with a direct-soldered-on bit of coax.

Felix

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #84 on: June 27, 2017, 07:59:35 AM »
captcha,
I think the u.FL makes no sense on the dipole, SMA is more than appropriate.
IMO the u.FL would be more appropriate where space is very limited, and only from a PCB to panel inside an enclosure.

ChemE

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #85 on: August 06, 2017, 09:43:09 PM »
Rather than the usual OSH Park, if you also upload your new design to PCBs.io and share it you get a portion of our purchase as a product credit for yourself.  Seems fitting that we could give a little back to you in this way.

ChemE

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #86 on: August 18, 2017, 07:44:19 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.

I had to get a little sneaky with the SMDs to get a standard RP-SMA connector to mount on the board and still have room to squeeze the dipoles into a 0.25" board width and keep all traces 40 mils away from the board edge.  I have a screen grab from Eagle below showing the pins and where they should land.


Going up from 0.25" drives the cost up rapidly so I wanted to avoid that if possible because I really want these to be super dirt cheap.  I went ahead and ordered a set of four and I'll be able to tell if my RSSI from nodes goes up using this as opposed to a 1/4 wavelength monopole.  I don't have a vector analyzer to be able to optimize the dipole lengths but if anyone in the US does, I can send you a spare copy for testing.

A question for the RF blackbelts: Are there any dire consequences to the choices I made in my design to squeeze this down to 0.25" board width?  I'm by no means an RF noob much less guru.
« Last Edit: August 18, 2017, 07:58:04 PM by ChemE »

captcha

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #87 on: August 19, 2017, 11:01:34 PM »
Great to see you're having a crack at it. You'll find it's not rocket surgery and once you have some working prototypes it's a super feeling that you're getting these awesome results with something you designed yourself.

I am in the process of making a video where I have done a shootout of several omnidirectional antenna designs and the clear winner is the dipole. I think this is purely because it has balanced radiating elements.

Thanks for your earlier post for mentioning to share the designs on pcbs.io and get a financial kickback. My main reluctance is that it's a different PCB manufacturer and I cannot assume that they will be using the same pcb material and manufacturing process as OSH Park. As you may know, the dielectric constant of the FR4 greatly affects the length of the copper tracks making up the elements and I would have to do another run of prototyping and testing before I would be confident sharing it with the rest of you.

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. Other than that, it looks fine to me. You've got a revision number printed as well, which helps you identify which board is which. I wouldn't worry about right-angled tracks either. I think you're on the right track (pun intended) to make a very useful and small antenna. Good on ya.

One thing I would include on your prototype versions, is to put a scale (like a small ruler) near the end of each track. That way, when you trim the ends for highest RSSI (or lowest SWR if you have the equipment) it's much easier to make both elements the same length.

Looks like you're not going u.fl after all??

ChemE

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #88 on: August 20, 2017, 08:44:40 AM »
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.

ChemE

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #89 on: August 20, 2017, 12:32:28 PM »
Looks like you're not going u.fl after all??

I think Felix is right and on a PCB dipole we don't need such a small delicate connector.  I did get some to solder onto my Motes and down the line a custom little Mote-clone.  They certainly make sense there where we want to keep our projects small but not lose the ability to go with a superior antenna if needed.  I initially wanted to get away from the SMA because I thought it was going to cause the board to be wider than 0.25", but if I can keep the robust connector on a tiny dipole then perfect.  I'm planning on also getting your design from OSH Park since they are the benchmark.  My own results won't be terribly useful to the wider community unless they are compared to the standard.