Author Topic: easy PCB dipoles for 433, 868 and 915MHz  (Read 30692 times)

john4444

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #30 on: October 05, 2016, 06:50:53 PM »
Hi Joe,

It is not really surprising that you are seeing somewhat different values for the antennas.
There are a lot of variables affecting antennas that are difficult to control.
The environmental factors are especially difficult.

Just to complicate a difficult subject even more, sometimes, perfect impedance-matching
in the transmitter antenna does not always result in the best received signal level in the receiver.

It is very difficult to quantitatively evaluate different antennas.
I would urge you to compare (qualitatively) the antennas by the received signal strength level
(RSSI) or maybe better by the number of missed packets per 100 or 1,000 total packets.
It will probably be necessary to operate at low transmit power levels to get reasonable
numbers of missed packets but that shouldn't be much of a problem with RFM69's.

Maybe perfect impedance-matching will result in the best RSSI or fewest missed packets,
but it is more likely that some other factor is more significant.

Captcha,
Luckily, I got a notice of your comment as I was responding to Joe.
Nice job on the antennas.
I am speculating but, I believe that you are probably 95% as good as you can get.
Any significant improvements are unlikely.
However, most users will be able to compare for themselves the antenna performance
as I outlined above without any specialized equipment.
Keep up the good work.

Good Luck, John
John AE5HQ

captcha

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #31 on: October 07, 2016, 06:25:16 PM »
Thanks all for your feedback so far.

@ raggedyanne
Quote
oscillator is of low quality

Of the VNAs? I very much doubt that. These devices are made specifically to be spot-on when it comes to frequency measurements. Remember, these tests are done without the RFMxxx board attached, it's purely the antenna system's characteristics we're measuring.


@john4444
Quote
I believe that you are probably 95% as good as you can get.

Ooh.. I like the sound of that! :-)

The original idea was to come up with a design for a low-cost, well-performing antenna within the constraints of a) my access to measuring equipment, b) catering for many different end-user environment scenarios and c) a super-simple design that could be easily and accurately replicated (thanks to cheap pcb manufacturers like OSH Park).

I like to think that version v1.0 of these antennas is already a huge improvement over the 'monopole' wire that has been suggested in other forum posts. Yes, this is a design that requires a bit more care to set up properly but I believe it is a step in the right direction to properly satisfy the radio's swr and impedance needs. Once you have that you're almost guaranteed to have good antenna.

Thanks for the extra eyes looking at these designs, I will try to stay in touch to find out what's going on and why Joe's measurements were slightly off.


captcha

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #32 on: October 08, 2016, 05:28:21 AM »
Joe mentioned that when he hooked up these dipoles to his VNA (Vector Network Analyzer) he noticed that they were resonant at frequencies slightly above the desired frequencies. He did not obtain these results using the radio modules whatsoever.

Hope this clears things up.
« Last Edit: October 08, 2016, 12:31:13 PM by Felix »

WhiteHare

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #33 on: February 28, 2017, 08:28:56 PM »

Now that I have accurate measurements of how long each leg needs to be I have updated my PCB designs and uploaded them to OSH Park for the whole world to grab and reproduce: https://oshpark.com/profiles/captcha. Just order the ones you need and OSH Park will take care of the rest.

I received OSH Park antennas (thanks for the link).  You kindly gave the link for the SMA connectors ("I ordered the SMA connector from RS with the part number 526-5785 (http://au.rs-online.com/web/p/sma-connectors/5265785/)"). 

So, all well and good.  The only thing I need now is the right cable, with connectors, of the correct half-wavelength.  Is there a source where those are already made-up and ready to buy as well?  I could probably throw something together, but something that's professionally made would presumably have less insertion loss at the connector junctions, and I'm guessing the cost wouldn't be much anyway.  Or am I better off skipping the connectors and just soldering the cable at both ends (one end being the antenna and the other end being the Moteino)?  Would that have less insertion loss, or more?  It does have the virtue of being simple: all I would need is some 50-ohm coax of the correct length, is that right?
« Last Edit: February 28, 2017, 08:32:42 PM by WhiteHare »

captcha

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #34 on: February 28, 2017, 09:41:58 PM »
Quote from: WhiteHare
I received OSH Park antennas

Awesome!  :)

Quote from: WhiteHare
cable, with connectors, of the correct half-wavelength.  Is there a source where those are already made-up and ready to buy as well?

I haven't seen them as pre-cut for ISM-band frequencies, but in my projects I have been using the RG-174 (or RG-316) leads with ready-made SMA connectors at either end as found on eBay. I then cut one side off to length and solder that to either the radio or the antenna. I simply haven't had the need yet to have proper connector-terminated radios AND antennas in one application.

Quote
all I would need is some 50-ohm coax of the correct length, is that right?

Correct. Velocity factor for a lot of coax is around 0.66 so that should be all you need to determine how long the feedline needs to be.

As an example for 433MHz:
- Full wave in free space: 300 / 433 = 0.692 m (69.2 cm)
- Half wave: 69.2 / 2 = 34.6 cm
- Compensated for 0.66 velocity factor of RG-174 coax: 34.6 x 0.66 = 22.9 cm

If you look at the first post on the first page of this post you'll see that I ended up with 22.5cm for a half-wave on 433MHz. There I had the benefit of an antenna analyzer to determine more precisely the velocity factor, but in this example 22.9 cm is good enough if you stay below a couple of half-wave multiples.

Soldering to either the Moteino or PCB Dipole should be fine. Just be aware that RG-174 dielectric is not as heat-resistant as RG-316. I wouldn't worry too much about connector insertion loss, but soldering tends to corrode less between connected surfaces. If you are going to place the project outdoors I would investigate sealing compounds but that's a whole nuther story.

WhiteHare

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #35 on: March 01, 2017, 10:32:22 AM »
So, I don't need to subtract the velocity adjusted length of the PCB trace from the velocity adjusted 1/2 wavelength of coax in order to get the proper length of coax?  This is all foreign territory to me, but I had started to imagine maybe they acted together as a single antenna.  The layman's analogy I was working from to try to understand this was: if the entire dipole antenna were built on a PCB, but you constructed a Faraday cage around the middle of it (which would represent the coax part), such that the parts that stick out of the Faraday cage are equal in length to the traces on the PCB you designed.  i.e. from a resonance standpoint, it's as though you built a complete dipole antenna on the PCB, but you're only allowing the protruding ends to be stimulated by the RF in the environment.  Whereas what you're constructing would be that, except the Faraday cage engulfs the entire imaginary 1/2 wavelength PCB antenna (with length adjusted for the velocity of the PCB medium)--which obviously wouldn't work at all as an antenna--so then the length of the PCB antenna traces are extended so as to protrude the all-engulfing Faraday cage by the length of the antenna traces you have on the actual PCB.  However, from a resonance point of view, doesn't that make each leg of the antenna too long?  i.e. don't you want both the parts of the trace (the part inside the Faraday cage and the part extending beyond it) to sum to the velocity adjusted 1/2 wavelength?

I should probably just shut-up and follow the directions, but it seemed like a fair analogy, so I'm just wondering if it has any merit to it, or if I'm (more likely) completely misunderstanding the theory that's driving the design.
« Last Edit: March 01, 2017, 10:48:08 AM by WhiteHare »

perky

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #36 on: March 01, 2017, 11:44:00 AM »
If the feed point for the dipole was perfectly matched, and the output impedance of the transmitter was perfectly matched, and the coax was ideal, then it shouldn't matter how long the coax is. So I think the 1/2 wavelength criteria is about minimising the effects of reflections back from the feedpoint and those re-reflected back from the transmitter. How important that really is compared to other things though is debatable, I don't personally think it matters that much especially if the transmitter output impedance is well matched.

Mark.

WhiteHare

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #37 on: March 01, 2017, 12:13:27 PM »
So I think the 1/2 wavelength criteria is about minimising the effects of reflections back from the feedpoint and those re-reflected back from the transmitter.

Thanks!  I'll run with that as the explanation.

ChemE

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #38 on: March 04, 2017, 04:21:50 AM »
Might rev2 of this design be available anytime soon?  I'd love to do a 915MHz design myself but my free version of Eagle limits me to a maximum board size of 100mm and I need 120mm for the traces alone.  Also, the boards could be shrunk to 0.25" wide and drop the price by half unless 0.5" is used because of shipping concerns.  Finally, from reading through the thread again it seems the dipole length and feed may benefit from slight tweaking.  Thanks for the work done to this point though!

captcha

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #39 on: March 04, 2017, 06:16:18 AM »
Certainly.

The boards as shared on OSH Park are all 12.2mm wide and this was done to cater for the placement of an SMA connector. With that connector I don't think I could shave off more than perhaps 2mm on the width.

Of course, I could forgo the SMA pads entirely and just have small soldering tabs (probably through-hole for a bit more strength). That would allow me to make them less wide and nullifies the suggested improvements of square corners and 50 Ohm 'stripline' track impedance.

Any suggestions for size, quantity and placement of mounting holes? Perhaps two holes next to the coax for a small cable-tie?

Once I have that I will order a trial run with slightly longer tracks so that I can trim them for lowest swr on 915MHz and then publish the boards for all to grab as per usual.

ChemE

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #40 on: March 04, 2017, 09:01:42 AM »
...
Of course, I could forgo the SMA pads entirely and just have small soldering tabs (probably through-hole for a bit more strength). That would allow me to make them less wide and nullifies the suggested improvements of square corners and 50 Ohm 'stripline' track impedance.

Any suggestions for size, quantity and placement of mounting holes? Perhaps two holes next to the coax for a small cable-tie?

Doing away with the SMA pads in favor of a few vias to solder the coax to sounds great to me.  Fewer parts to buy and less chance of corrosion long term.  Obviously that gives up easy disconnection but I don't see that as much of a feature.  You've already been thinking about this, your suggestions are spot on.  For me, I'd like a board closer to 120mm x 6.35mm wide which is OSH Park's minimum dimension simply to push cost down to the floor.  I know 120mm isn't possible but that board would be $5.91 for 3 shipped which is absolute peanuts.  How small can you go?
« Last Edit: March 04, 2017, 09:04:13 AM by ChemE »

perky

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #41 on: March 04, 2017, 09:39:40 AM »
I would consider using U.FL connectors, you can buy very cheap U.FL coax 'patch' cables for a few dollars (with or without a second U.FL plug on one end). Admittedly this would be for relatively short cables (a few 10s of cm) because those cables have losses, but it would allow putting the antenna in a box very easily.

Mark.
« Last Edit: March 04, 2017, 07:12:15 PM by perky »

captcha

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #42 on: March 04, 2017, 06:49:38 PM »
U.FL.., yep, can do that as well. (hey, it rhymes).

I haven't ventured into using those before, but I've seen them inside laptops for the WiFi and Bluetooth feedlines. Yes, they are really nice and small, it's just that I'm not overly confident they will still 'snap' on after 20x disconnects. But for small, neat and short runs of connections they would be awesome. I'll add them to the designs and share them all (SMA, through-hole and U.FL).

Anything else I could add before I send them off for production? This is the time.. ;-)

ChemE

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #43 on: March 04, 2017, 07:53:54 PM »
Maybe you are already planning on doing so, but I recently used SparkFun's SOIC-8 BoB and really liked that the long pads had vias placed in the center to help them soak up the solder instead of blobbing out to hit the next pad and make a junction.  I'm not accustomed to fine-pitch soldering (like the U.FL connector being discussed) and felt that this design element helped my clumsy hands do the job.

I'll save you the click...


Other than that, the only other thing I could suggest is just to keep the boundary as small as is possible so the cost is minimized since this is meant to be an easy reference design for the masses if you will.
« Last Edit: March 04, 2017, 08:00:45 PM by ChemE »

perky

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #44 on: March 04, 2017, 08:55:17 PM »
Well, not relating to this but if I had the time I'd really like to have a go at laying out one of these cross meandered dipoles, with and without a U.FL patch cable to introduce a 90 degree phase shift between them to make it circularly polarized, as it may well be significantly more omnidirectional than a single dipole. Gain would be less of course

http://cearl.ee.psu.edu/projects/assets/Project3/Project3_3/Stochastic%20Antennas1.pdf

Getting this to simulate in NEC would be great. That's if you've run out of things to do ;)

Mark.