LowPowerLab Forum

Hardware support => RF - Range - Antennas - RFM69 library => Topic started by: ChemE on October 11, 2017, 12:30:28 PM

Title: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: ChemE 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
(https://i.imgur.com/1WS1yVP.png)


Close Up Showing Unconnected GND Pin
(https://i.imgur.com/nN6uOgK.jpg)

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
(https://i.imgur.com/DhhMxIw.jpg)
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.
Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: ChemE on October 11, 2017, 12:31:28 PM
Reserved for future summaries.

Ideal Feedline Lengths Additional information from Captcha on this subject (https://lowpowerlab.com/forum/rf-range-antennas-rfm69-library/small-loop-antennas-433-mhz/msg14061/#msg14061)
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
(https://i.imgur.com/qn8znDK.jpg?3)

Total Length of Feedline 11.5cm (center of u.FL connector to left edge of PCB Antenna)
(https://i.imgur.com/ZxgwB6H.jpg?1)
Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: Charly86 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
Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: ChemE 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!
Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: LukaQ 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"
Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: Felix 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? :)

(https://i.imgur.com/VoDbpUs.png)
(https://i.imgur.com/XdwiSSm.png)
Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: ssmall 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.
Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: LukaQ 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  ::)
Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: NicksonYap 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.

Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: ChemE 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.
Title: Ferrite Baluns
Post by: davidbitton on November 07, 2017, 11:21:15 AM
Would the design benefit from a ferrite balun such as this (http://tro.pe/2iCUq5W) based on Martin E. Meserve (K7MEM)'s discussion on Center-Fed Half-Wave Dipoles (http://tro.pe/2iBO3zT)? I'm no expert on the topic so I'm trying to glom together a myriad of resources online. Thanks!
Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: Kilo95 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
Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: davidbitton 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!
Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: LukaQ 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
Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: davidbitton on January 30, 2018, 12:50:31 PM
@LukaQ, can you take a pic of your setup? Thanks!
Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: LukaQ on January 30, 2018, 01:53:13 PM
@LukaQ, can you take a pic of your setup? Thanks!
Here you go
Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: LukaQ on February 02, 2018, 01:10:09 AM
How is it with radiation patent since it has ground plane on other side of antenna. Simple wire dipole would have active upper element and lower ground element. How does this one work?
Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: Felix on February 02, 2018, 09:28:29 AM
LukaQ,
Not meaning to hijack your current quest, but have you considered the velocity factor of your coax? what kind of coax is that?
Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: LukaQ on February 02, 2018, 09:44:24 AM
LukaQ,
Not meaning to hijack your current quest, but have you considered the velocity factor of your coax? what kind of coax is that?
I have not, I went with length that is from this page for the 868. Coax is one from ebay sold as rg316. VF should be probably 0.7?
So what does that mean, that is has to be longer than signal would be in air?
Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: ChemE on February 02, 2018, 04:04:12 PM
I have not, I went with length that is from this page for the 868. Coax is one from ebay sold as rg316. VF should be probably 0.7?
So what does that mean, that is has to be longer than signal would be in air?

For 868MHz, the ideal feedline length accounting for velocity factor was calculated as being 11.40 cm from the ANT pin of the RFM69 to the closest edge of the PCB dipole.  I'm not sure why you are seeing such a strong signal attenuation.  In my own testing with different feedline lengths at 915MHz, going up or down by a few centimeters didn't strongly affect average RSSI.  I use code which reports the average RSSI so it will drift towards the true mean.  I usually collect data from 100 packets sent once per second before recording a result to help account for environmental noise.
Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: LukaQ on February 03, 2018, 02:12:43 AM
For 868MHz, the ideal feedline length accounting for velocity factor was calculated as being 11.40 cm from the ANT pin of the RFM69 to the closest edge of the PCB dipole.  I'm not sure why you are seeing such a strong signal attenuation.  In my own testing with different feedline lengths at 915MHz, going up or down by a few centimeters didn't strongly affect average RSSI.  I use code which reports the average RSSI so it will drift towards the true mean.  I usually collect data from 100 packets sent once per second before recording a result to help account for environmental noise.
What do you mean, strong signal attenuation? That is not attenuation on feedline. Those RSSI were between two nodes some distance apart, no averaging, I saw RSSI to be ~ -56 After adding first feedline on one node, RSSI became much better, adding 2nd one and RSSI went up again. All in all, just by adding feedlines (instead of having antenna soldered to AND pin of RFM), there was 6db or more gain (lower RSSI). One node is pretty much 11.4cm, other one as a bit short at ~11cm for 868MHz. And with that being said, if someone else goes for such approach, you need to add feedline or you'll be pi**ing away sensitivity for nothing
Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: ChemE on February 03, 2018, 09:29:57 AM
Ah ha, my mistake.  I hurriedly read your private message and then the new posts on this thread and completely misread the numbers!  Glad to see that the feedlines did what they were supposed to do and you confirmed as much with RSSI numbers.
Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: tbalon on March 17, 2018, 04:18:45 PM
I had ordered ChemE's version 3 dipole design and have some concerns. It's seems that the
wider trace on this version, or perhaps the board material has a different velocity factor. It
seems its elements (1/4 wave) should be longer (closer to 78mm than 60mm.)

I can see that capcha's design v1 pcb uses a narrow trace element. I was confused by the
60mm length as a 1/4 wave 915 Mhz element should be 78mm (about 3"). His tuning was
done presumably to deal with the velocity factor of the copper trace track with 1mm on FR4 of 0.77
which yields 78mm * 0.77 or 60mm which is what he made the trace length on his v1.0 915Mhz
PCB antenna.

I tested the Captcha v1.0 version and used it with a LORA mote. It worked very well. It worked even
better when I optimized the feed line length as discussed here to make my coax the optimum
length. Attaching Captchs dipole to my VNA I got a VSWR of about 1.6 with an impedance of 52 ohms
at 915 Mhz.  (This was using a 15cm feed line I had for testing so it was not ideal. )

Using ChemE's version 3.0 with the same feedline I got a VSWR of 5.1 with an impedance of only about
15 ohms. at 915 Mhz. However, at 1100 Mhz, the VSWR drops below 2 and the antenna's impedance is about 50
ohms. It took some time for me to believe this so I double checked the antenna connections and tried again. The
VNA showed ChemE's antenna had a nice wide bandwidth that was shifted more toward 1.1 Ghz.

So, ChemE's version 3.0 appears to be a nice antenna, but it's tuned for 1.1 Ghz not 915 Mhz. I believe this is
due to the shortness of the elements which were calculated by Captcha for 1 mil copper trace by applying the
velocity factor for that particular trace on FR4. I believe the easiest way to address this would be to increase
the length of the elements so they are closer to 1/4 wave (i.e. 78mm ).

Here are some photo's of the tests. https://photos.app.goo.gl/JmiqdGHLOQTF2m6m1 (https://photos.app.goo.gl/JmiqdGHLOQTF2m6m1)


Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: ChemE on March 18, 2018, 09:44:11 AM
Wow tbalon that is one heck of a first post!  Welcome to the forums!  That is awesome that you tested my adaption of captcha's work, thanks for the data and write up.  I'm all too happy to increase the length of the resonators (even though the boards will be a little more expensive) if it means we all get better performance.  As I said when I set out on my adaption, I don't have a VNA and have no plans to buy one and learn how to use it, so hopefully captcha's work transfers directly.  It would seem that perhaps it doesn't and the design can be further optimized.  I happen to have the 868 and 433 MHz versions of my antenna here so I can do some RSSI measurements using those longer resonators and see if they do indeed improve upon the RSSI at 915MHz.  Longer term, I can post revision 4.0 designs which have longer resonators and hopefully you'll be willing to test them as well.  If you prefer not to buy copies of those boards, I can and ship them to you if you are in the US.  I'm curious to see what others think since RF is most certainly not my forte.
Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: tbalon on March 18, 2018, 10:51:38 AM
Wow tbalon that is one heck of a first post!  Welcome to the forums!  That is awesome that you tested my adaption of captcha's work, thanks for the data and write up.  I'm all too happy to increase the length of the resonators (even though the boards will be a little more expensive) if it means we all get better performance.  As I said when I set out on my adaption, I don't have a VNA and have no plans to buy one and learn how to use it, so hopefully captcha's work transfers directly.  It would seem that perhaps it doesn't and the design can be further optimized.  I happen to have the 868 and 433 MHz versions of my antenna here so I can do some RSSI measurements using those longer resonators and see if they do indeed improve upon the RSSI at 915MHz.  Longer term, I can post revision 4.0 designs which have longer resonators and hopefully you'll be willing to test them as well.  If you prefer not to buy copies of those boards, I can and ship them to you if you are in the US.  I'm curious to see what others think since RF is most certainly not my forte.

I'm in Toronto Ontario and I would be happy to test your v4 board.  I did another test where I scraped off some of the
laminate to expose the copper at each end of the dipole. I then attached copper foil and extended the length to about
78mm and tested again. The results looked very good .. VSWR dropped to 1.2 at 915 Mhz.

This was only a quick test, but I think its fair to say the longer elements did help. I was trying to find out the influence of
the copper trace width .. but it could also be attributable to the substrate / board material.

I've added a few pics of the antenna with copper foil soldered to the ends to extend the elements to get a better match
at 915 Mhz.  https://photos.app.goo.gl/JmiqdGHLOQTF2m6m1  What I like about your version is the increased bandwidth.
LORA transmits in the 900-905 Mhz range (uplink) the downlink is higher, up to 927 Mhz I believe. The wider bandwidth
will perform better in this case.

Happy to help...
Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: ChemE on March 18, 2018, 12:22:05 PM
Wow a VSWR of 1.158 means that about 99.5% of the RF power is being radiated to air as far as I can tell.  I believe it was Joelucid that posted a link to a paper which claimed wider resonators are more wide band and perhaps change the optimal length shorter.  Glad to see at least one of those claims is true!  I'll post a link to rev4 later today with slightly over-sized resonators that can be trimmed at your test bench to confirm that indeed on FR4 78mm is optimal.  I'm thinking to make the resonators 82mm would you suggest a different starting point?  It is nice to see data each mm and confirm indeed that 78mm is better than 77mm and 79mm.
Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: ChemE on March 18, 2018, 12:56:21 PM
Here is the link to the refined 915MHz design: https://PCBs.io/share/zOdQn.  I went for 80mm resonators since the math is very easy that way.  It is interesting that these may work particularly well for LoRa.  I haven't yet fooled with LoRa but it is on my list and now that the SX1261 has been announced: https://www.semtech.com/products/wireless-rf/lora-transceivers/sx1261 with its 4.2mA receive current, it has moved higher on the list.  Hopefully Hope has some transceivers soon that I can play with.
Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: perky on March 18, 2018, 01:11:15 PM
Careful, the extensions have no dielectric under them so the velocity factor for that part is about 0.98 while the rest is lower (it seems greater than 0.77, but in theory you should be able to work it out from the data you now have), so the PCB trace should be shorted a little to take that into account.

Mark.
Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: tbalon on March 19, 2018, 12:16:27 PM
Careful, the extensions have no dielectric under them so the velocity factor for that part is about 0.98 while the rest is lower (it seems greater than 0.77, but in theory you should be able to work it out from the data you now have), so the PCB trace should be shorted a little to take that into account.

Mark.

Yes indeed. As I stated this was a quick test to illustrate my belief that the elements should be closer to the 78mm than 60mm that
the v3.0 boards were built. The photo shows that adding length made the antenna work better for the freq of interest ( 915 Mhz )

Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: tbalon on March 19, 2018, 12:46:38 PM
Here is the link to the refined 915MHz design: https://PCBs.io/share/zOdQn.  I went for 80mm resonators since the math is very easy that way.  It is interesting that these may work particularly well for LoRa.  I haven't yet fooled with LoRa but it is on my list and now that the SX1261 has been announced: https://www.semtech.com/products/wireless-rf/lora-transceivers/sx1261 with its 4.2mA receive current, it has moved higher on the list.  Hopefully Hope has some transceivers soon that I can play with.

I believe the VF of FR4 should be in the range of 0,84 - 0,86 so a length between say 65.5 mm and 67 mm would be my guess.

Tom
Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: perky on March 19, 2018, 08:29:10 PM
Yes indeed. As I stated this was a quick test to illustrate my belief that the elements should be closer to the 78mm than 60mm that
the v3.0 boards were built. The photo shows that adding length made the antenna work better for the freq of interest ( 915 Mhz )

OK, I assume that the total length of the elements with the extensions was actually greater than 78mm, and you've done some calculations as to what the real velocity factor appears to be for the dielectric part using a combination of lower velocity rate for the part with the dielectric and 0.98 for the extensions, then calculated 78mm for elements from that assuming the elements have dielectric beneath them all the way along. If 78mm is the total measured with the extensions the length would need to be reduced. Just asking to be sure ;)

BTW here's a document that appears to show the empirically derived velocity factor of 0.82 for FR4 (page 7), and is inbetween the dielectric alone and air values due to the fields partially travelling through the dielectric and partially through the air:

https://www.silabs.com/documents/public/application-notes/AN639.pdf

Mark.
Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: luisgcu on March 29, 2018, 07:58:14 PM
Wow tbalon that is one heck of a first post!  Welcome to the forums!  That is awesome that you tested my adaption of captcha's work, thanks for the data and write up.  I'm all too happy to increase the length of the resonators (even though the boards will be a little more expensive) if it means we all get better performance.  As I said when I set out on my adaption, I don't have a VNA and have no plans to buy one and learn how to use it, so hopefully captcha's work transfers directly.  It would seem that perhaps it doesn't and the design can be further optimized.  I happen to have the 868 and 433 MHz versions of my antenna here so I can do some RSSI measurements using those longer resonators and see if they do indeed improve upon the RSSI at 915MHz.  Longer term, I can post revision 4.0 designs which have longer resonators and hopefully you'll be willing to test them as well.  If you prefer not to buy copies of those boards, I can and ship them to you if you are in the US.  I'm curious to see what others think since RF is most certainly not my forte.
I recommend this vector antenna analyzer https://www.ebay.com/itm/N1201SA-UV-RF-Vector-Impedance-ANT-SWR-Antenna-Analyzer-Meter-140MHz-2-7GHZ/182592795913?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649 (https://www.ebay.com/itm/N1201SA-UV-RF-Vector-Impedance-ANT-SWR-Antenna-Analyzer-Meter-140MHz-2-7GHZ/182592795913?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649) , I tested your antenna and I get pretty much the same result as @tbalon.
Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: nick-crow on May 15, 2018, 04:00:31 AM
Hi I'm using the 868mhz version for my flight instrument (paragliding) we use a lora system to allow us to see each over and be seen by ground stations which forwards to the web I'm interested in the radiation pattern of these pcb antennas to check if I have used it in the optimal position. 

https://imgur.com/a/X7oBwTE     

http://www.air-ware.co.uk/

Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: Felix on May 22, 2018, 09:21:04 AM
ChemE,

PCBs.io does not seem to allow download of your shared project, as opposed to OSHPark where you can download the files.
Can you post your latest design here in a ZIP or post it on OSHPark?
Or better yet post the Eagle files like you did for the first version?

Thanks!

FWIW Black is one of the worst possible silkscreen colors for a PCB especially for prototyping. Anything that completely masks the copper is a bad idea.
You can't see the copper and if you need to fiddle with the copper layer - like in this case cut the trace, you have to first scrape off the black paint to see what you're doing).
Purple is still not great either but at least the copper is visible. I will probably never use black again in any of my designs or prototypes.
Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: TomWS on May 22, 2018, 10:34:17 AM
Purple is still not great either but at least the copper is visible. I will probably never use black again in any of my designs or prototypes.
Black is the only choice on PCBs.io.  The good news is that you'll probably never receive your boards from PCBs.io, so it won't matter what color the mask is!  ;)

Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: Felix on May 22, 2018, 11:35:21 AM
Black is the only choice on PCBs.io.
Yeah too bad :(
If PCBs.io reads this, they should consider changes.

The good news is that you'll probably never receive your boards from PCBs.io, so it won't matter what color the mask is!  ;)
Did you not get yours lately?

I need proto boards FAST, I usually use OSHPark RUSH service if it's urgent, sometimes I get automatically upgraded for free :)
They arrive within 8-10 days from ordering. 3 boards is enough for prototyping.
Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: TomWS on May 22, 2018, 12:09:52 PM
Did you not get yours lately?
The last time I ordered boards from them (and I DO MEAN the LAST time), it took 39 days to get the boards with 2 day shipping...
This was in March/April.

Their online 'status' information is meaningless and they provide no feedback or response to queries.  Net:  Too long and too much uncertainty.

OshPark is my goto...
Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: ChemE on May 22, 2018, 12:11:57 PM
ChemE,

PCBs.io does not seem to allow download of your shared project, as opposed to OSHPark where you can download the files.
Can you post your latest design here in a ZIP or post it on OSHPark?
Or better yet post the Eagle files like you did for the first version?

Thanks!

Sure Felix, here they are.  I should note that this newer design has not yet been verified to be centered at 915MHz and I left it a little long in case some trimming was needed.  Life has been busy and I've put this project down for the moment.
Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: Felix on May 22, 2018, 12:16:36 PM
Sure Felix, here they are.  I should note that this newer design has not yet been verified to be centered at 915MHz and I left it a little long in case some trimming was needed.  Life has been busy and I've put this project down for the moment.
ChemE,
Thanks!
I got the N1201SA Analyzer which has confirmed the findings reported by tbalon (https://lowpowerlab.com/forum/rf-range-antennas-rfm69-library/alternative-pcb-dipole-designs-(433-868-and-915mhz)/msg22717/#msg22717).
I can try the new design when I get some time, thanks for sharing the new design!
I see you kept the radiators width the same, any thoughts on this and the supposed wideband they bring with this design?

Tom,
RE PCBs.io - I reckon they are in a hard market. OSHPark is probably very established. And with such delays being self repelling, any OSHPark competitors cannot really survive, UNLESS they come up with an edge product or service, that's the only way.
Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: ChemE on May 22, 2018, 01:00:12 PM
Yes, the feedback that I've gotten so far is the LoRa community crave wideband antennas because of the way it uses spread-spectrum and the wider radiators do indeed result in an impressively wideband dipole.  I would think given this, there is no reason to switch back to narrow radiators.

Leo did tell me that they switched board houses for a few batches and this created a long delay that was unexpected but also not likely to recur.  I am still happy to use PCBs.io but I understand why others have moved on.
Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: mgrooms on May 29, 2018, 12:46:06 PM
Felix,

I don't think I ever got the antennas I ordered from PCBios either. It took so long I forgot about them. Will have to check when I get home.

I see you received your N1201SA. Do you mind sharing your ordering experience / who you bought from? I'm ready to pull the trigger on one, but don't want to over pay, nor wait 45 days for it to arrive.

Thanks,
- Matt

Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: Felix on May 29, 2018, 02:54:56 PM
I bought it from an US ebay seller, if you search for it there should only be a few US sellers. I didnt want to wait so long for something like that either. I paid under $200 i think.
Also I got it from here so if something is wrong with such an instrument I can return and get my cash back quickly.

So far so good, I havent played with it a whole lot, just tested a few things and learned how to get around through the UI. Pretty cool toy, I dont really have a way to benchmark it but the results are pretty convincing and within expectations, plus scores of online reviews seem to back that these are legit analyzers.

UDPATE: this is the item I got from ebay:
https://www.ebay.com/itm/N1201SA-UV-RF-Vector-Impedance-ANT-SWR-Antenna-Analyzer-Meter-140MHz-2-7GHZ/182592795913?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649
Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: mgrooms on May 30, 2018, 08:35:37 PM
Felix,

Thanks for the info on the eBay seller you used to purchase your N1201SA antenna analyzer. I just placed my order with the same eBay seller, with expected delivery to Texas on 4/5 June.

On the PCBs.io circuit board fab-house non-delivery of my order of Chem-E's 915MHz v2 dipole antennas. . .  I checked my stack of recently received goodies and didn't find a package from them. Looks like another strike against them for non-delivery. If I do find them lying around I'll post a correction here.

I looked again and found the delivered order from PCBs.io. The post mark date on the package matched the promised 'delivery' date [4 April] provided in the order history. USPS made the physical delivery to my door some days after that. To PCBs.io's credit they included an extra board in my order. [ordered 4 / received 5]. I just now placed an order for eight more.

Could someone make them available on OSHPark? [not familiar with the process...]. Would gladly pay more for them there, in return for more rapid, reliable delivery.

-Matt, San Antonio, TX.
Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: LukaQ on June 04, 2018, 03:43:30 AM
you guys are spoiled  ;D
Many places in the world have to wait weeks (for free shipping postage) for delivery, in EU from China and even US takes time to get something delivered. Free is not free but = more time, almost everything is not free, you do pay in some way or the other. Take that into calculation ;)
Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: Sergegsx on June 16, 2018, 03:48:28 AM
hello, do I need to make any distance cut in the pcb for a 915mhz?

I seem to be getting same/little worst results than with a simple wire.

thank you.
Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: LukaQ on June 16, 2018, 05:01:55 AM
coax needs to be long for 915 and antenna needs to be for 915
Btw, are you in EU?
Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: mgrooms on June 17, 2018, 02:42:43 PM
hello, do I need to make any distance cut in the pcb for a 915mhz?

I seem to be getting same/little worst results than with a simple wire.

thank you.

Yes. The consensus view was that the elements of Chem E's v.3 antenna are a little short for 915 MHz; so he intentionally made the elements on the v.4 antenna too long. They must be trimmed to a shorter length for operation on 915 MHz. The question is HOW MUCH they need to be trimmed.

Has anyone [with an antenna analyzer] determined the best trim-length for operation on the 915 MHz ISM band? [for the Chem E v.4 antenna]?
Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: ChemE on June 17, 2018, 04:20:27 PM
To my knowledge, no one yet has measured the newest design to determine its frequency or how much the elements need to be trimmed.  I know Felix has both the newest PCB antennas and the SWR analyzer.  I am very interested to see how the latest design turned out though.
Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: Sergegsx on June 17, 2018, 05:02:03 PM
coax needs to be long for 915 and antenna needs to be for 915
Btw, are you in EU?

I am in the EU yes.
Why?
Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: LukaQ on June 18, 2018, 12:08:20 AM
Just asking, because ISM band in EU is 868MHz or 433MHz, you want to use 915MHz, which is probably taken by some mobile phone operator in your region. It's ok, if your far away (>200m) from anyone or anything that operates in 915MHz region.
Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: Felix on June 18, 2018, 11:59:03 AM
To my knowledge, no one yet has measured the newest design to determine its frequency or how much the elements need to be trimmed.  I know Felix has both the newest PCB antennas and the SWR analyzer.  I am very interested to see how the latest design turned out though.
New dipoles will be available in the shop in a few days.
I will post analyzer results with at mm difference from 80mm to 72mm. The 915mhz will be resonant around 72.5-73 mm with a VSWR of < 1.1
Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: kni on July 05, 2018, 02:30:56 PM
Help. My brain is stuck in "does not compute".

I am looking to confirm the optimal 915MHz distance from the moteino antenna pad to the point on the antenna where the poles diverge.

Here is what I have:
- Previous generation of dipole antenna from the moteino store (black pcb)
- Male pcb SMA soldered to the antenna
- 15cm rg316 male sma to female sma jumper cable:
https://www.ebay.com/itm/5PCS-15cm-RG316-Cable-SMA-Male-To-SMA-Female-Jack-Jump-Pigtail-6in-BS1/263266232496?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649

The plan is to cut off the male sma from the jumper cable and solder that end directly to the moteino's antenna pad. I would then attach the male sma connector on the antenna to the female sma connector on the jumper cable.

I'm getting hung up on the optimal length from point A to point B.

The first page of this thread says:
Quote
915MHz = 10.80cm implies a 9.2cm pigtail

From the table for insulated copper wire in the Antenna Theory blog, the length reads:
Quote
915MHz   16.4cm   16.4 x 0.95 x 0.95 = 14.8cm

Should the distance be 10.80cm, 14.8cm, or some other distance? I am certainly misunderstanding something and could use a bump to get me back on track.

Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: MonZon on September 06, 2018, 03:55:09 AM
Hi all!

I've ordered 433MHz antennas from pcb.io and it appears that it is shorter than expected. It measures 267mm long whether my 435MHz dipoles are 305mm long. Also, my antenna analyzer says that it is tuned to 545MHz, so cutting it is not going to help. Is it pcb.io mistake, wrong link or something? I'll have to cut all of them to 868 or 915 and sell them.
Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: ChemE on September 07, 2018, 07:40:11 PM
Not the wrong link or PCBs.io's fault; rather it is my fault.  I don't have a SWR to test these, so I just copied captcha's designs as far as radiator length goes.  Others have found that this combination of PCBs.io's fiberglass and captcha's lengths is tuned higher than desired.  I had posted a stop-gap version with slightly long resonators to cover the 915 and 868 users but the old 433 design wasn't updated so it is too high unfortunately.  Since you are trimming it anyway, would you be able to confirm the lengths which resonate at 868 and 915?  I do need to get an SWR and finish tweaking this project.
Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: MonZon on September 12, 2018, 11:12:48 AM
Okay then, I get it now.

I've cut one 433MHz antenna to 868MHz - it turned out 164mm long 1.2 SWR  8) (overall length, both sides together). As for 915 - I don't have anything with that frequency. If you really need me to cut it - I will, for the sake of this project  ??? But I don't really think it is necessary. It would be better to make those pcb's a tad longer, so anyone can trim them.

Do you mind sharing 433 and 868 dip trace files? I'm thinking to lengthen 433 version and order it again.
Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: MonZon on November 24, 2018, 06:56:31 AM
Not the wrong link ...

Any updates? Can I have the files?
Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: Slople on April 05, 2019, 05:46:14 AM
Hey everyone

I hope this thread is still active?

I ordered some of ChemE's PCB-Antenna [868MHz], Revision 2 and would like to use them for my paragliding-FLARM-equipment (collision warning). FLARM works between 868.0 and 868.6MHz.
I will solder some coax-wire directly to the board and connect it via u.FL to the FLARM-board.

Could someone with more knowledge than me give me a hint:

1. do I need to shorten the original boards? If so, to what length?
2. which length should the coax be?

Thanks so much,
Raphael
Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: Felix on April 08, 2019, 09:32:02 AM
Answered here (https://lowpowerlab.com/forum/rf-range-antennas-rfm69-library/using-the-pcb-dipole-on-915mhz-3966/msg25967/#msg25967).
Please take a moment and do a basic search before posting a repeat question.
Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: Slople on April 08, 2019, 10:44:03 AM
Thank you Felix and sorry for not reading this before.

I was hoping, someone else already did what I have in mind, as my most sophisticated electronical measuring instrument is a multimeter ;-)

Regarding the coax-length, my first Google-result returned this: http://www.bellscb.com/cb_radio_hobby/how_long_should_my_coax_cable_be.htm - stating "For most cases when using a single antenna system with a 50 ohm coax feed ANY coaxial length will work providing the antenna is tuned properly at the feed-point of the antenna (where the coax attaches)."

The 2nd result is http://www.m5bxb.com/coaxlengthcalc.htm, which "recommends" "tuning" the coax-cable to 50 Ohm impedance and correct length. I fear this is beyond my knowledge :-(
Title: Re: Alternative PCB Dipole Designs (433, 868, and 915MHz)
Post by: LukaQ on April 28, 2019, 03:37:34 AM
Just to let everyone know:

the 868MHz dipole antenna here has active side 64mm long, which corresponds to 1060MHz tuning. If you select VF on site below to 90% (insulated wire AND end effects dipole) and enter 1060, you will get 64mm. Which agrees with what is measure by n1201sa. This means that this PCB design is too short! For 868MHz, each pole would need to be 78mm. So if you get the 433MHz version, and make it shorter.

One thing to note also. If you use coax to extend the antenna away from board and you add 11.4cm from exposed to exposed active wire, tuning freq. moves down to about 1000MHz, still far above 868M. VSWR is at that point 4.2, which is bad. Without coax is even worse at 4.8
So this design need to have longer poles


http://wxtofly.net/wavecalc.htm