Author Topic: easy PCB dipoles for 433, 868 and 915MHz  (Read 29716 times)

TomWS

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #15 on: September 24, 2016, 09:56:25 AM »
@captcha, good experiment for the dipole, thanks for posting!  One question: did you leave AGC on or have the gain fixed at the RX end?

Tom

joelucid

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #16 on: September 24, 2016, 10:49:16 AM »
Quote
did you leave AGC on or have the gain fixed at the RX end?

In theory it shouldn't matter since the RSSI detection circuit is supposed to work independent of selected gain. In practice it almost never matters since RFM69 lib selects the RSSI threshold so low that RSSI detection triggers immediately when the radio goes into RX just on background noise. It then always selects maximum gain.

If your noise floor is below -110 dBm or you use a RSSI threshold higher than the floor then AGC does select different gains and the supposedly gain independent RSSI measurement apparatus of the RFM69 chip would be put to the test.

Joe

joelucid

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #17 on: September 24, 2016, 11:18:15 AM »
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If an antenna is not properly matched, then yes, ideally from transmitter to antenna (the part where the traces separate to make up the elements) should be (multiples of) a half wavelength. There is a slight mismatch with these dipole antennas and you want to be as close to the half wave(s) as possible, purely to minimise rf coming back. Remember, if you were to use an antenna with 50 ohms feedpoint impedance, then the length of (50 ohm characteristic impedance) transmission line would not matter (you still want to keep it as short as possible of course).

I guess it really depends on where the impedance mismatch is. If the pcb feedline from antenna to connector has a characteristic impedance of 50 Ohm then it should be looked at as extension of the coax and be added to the length of the coax. On the other hand if that short feedline has the same characteristic impedance as the dipole you would only count the coax.

Did you design the feedline with 50 Ohm? If not then likely both connections reflect and it's not so clear anymore what to do.

I could just hook the antenna up to the vna including coax and find the best length that way.

captcha

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #18 on: September 24, 2016, 06:46:00 PM »
@TomWS

Quote
did you leave AGC on or have the gain fixed at the RX end?

To be honest, I haven't played around with any of the settings. I would like to lower the bandwidth and see what range improvements I get but all my results have been based on the default RFM69 library settings, so if I read the library correctly, the AGC is always on.

Oh, not sure if I mentioned it, but I have been using the Moteino R4 with the RM69HW module at maximum power output (100mW/20dBm).


@joelucid

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I guess it really depends on where the impedance mismatch is.

Correct, hence my recommendation to use coax at half-wavelength multiples to not have the length of the coax mess things further up. I'm by no means an expert but for those unfamiliar with rf it is the easiesst path to a decent working antenna system.

For those interested, I used the following method to arrive at the lengths for the antenna elements:

- calibrated the vna with the recommended 50 ohm termination resistor
- connected a 25cm piece of RG-174 with an SMA connector, the other end of the cable was just an open circuit
- cut off bits of coax until the plot showed an electrical half wavelength for 433MHz (this was around 22.5cm)
- stripped about 1cm of insulation from the end of the coax and soldered it to the PCB dipole
- I then ran the vna plot and trimmed each leg by the same amount until the lowest SWR for 433MHz was reached
- since 868MHz is practically double the frequency of 433MHz I left the setup exactly as is but now started trimming the dipole legs until I reached the lowest SWR on 868MHz. Of course, the feedline is now two half-waves but for testing SWR this is negligible.
- once the lowest SWR for 868 was found I de-soldered the coax from the antenna and trimmed it about 5mm and re-soldered it back onto the antenna again.
- again using the vna plots I shortened the elements until I had lowest SWR on 915MHz.

 
Quote
Did you design the feedline with 50 Ohm?

You mean the antenna? Yes, depending on the frequency I used either one or two half-wavelengths of coax. The mismatch is therefore at the antenna feedpoint.

I understand this is not optimal, but I've heard people say that 'every antenna is a compromise'. For better matching and rf isolation a 1:1 balun would be a logical option. But this would further complicate the design for newcomers and I'm not too sure whether there is actually a problem that it addresses. Adding a balun would also introduce additional losses and I would not have the equipment to properly evaluate such a design. To honour common sense dipole designs I think it's preferred to use a feedline of at least a half wavelength away and perpendicular to the antenna. This is also why I tested the dipoles with feedline attached; simulating a realistic environment.

Hope this helps.

TomWS

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #19 on: September 24, 2016, 09:11:21 PM »
@TomWS

Quote
did you leave AGC on or have the gain fixed at the RX end?

To be honest, I haven't played around with any of the settings. I would like to lower the bandwidth and see what range improvements I get but all my results have been based on the default RFM69 library settings, so if I read the library correctly, the AGC is always on.

Oh, not sure if I mentioned it, but I have been using the Moteino R4 with the RM69HW module at maximum power output (100mW/20dBm).

I asked because of the seemingly random successful vs unsuccessful packets at the extreme of the range.  If the AGC is turned off with fixed max gain, I'd expect a more stable (and measurable) result. You're not bucking against some auto correction algorithm.  As you have usefully observed, environmental conditions like car traffic will influence the results.  Again, good work.

Tom

captcha

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #20 on: September 25, 2016, 05:29:13 AM »
Thanks Tom,

I wasn't sure what to expect at the edge of reception but the ongoing car traffic seemed to be the most varying factor. I guess I could do another test over a nearby lake with pretty much an unobstructed signal path, which should give me a few km's of range. That won't happen anytime soon but I am curious about maximum range with these PCB dipoles at either end.

In any case, thanks all for your feedback and suggestions.

TomWS

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #21 on: September 25, 2016, 07:40:59 AM »
I guess I could do another test over a nearby lake with pretty much an unobstructed signal path, which should give me a few km's of range.
I would think the lake would act as an RF reflector of sorts and might even increase the range vs dry land.  Not sure about this, but that comparison would be interesting (although not necessarily useful for most people).

Thanks for your efforts,
Tom

joelucid

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #22 on: September 26, 2016, 03:17:56 AM »
Quote
I would think the lake would act as an RF reflector of sorts and might even increase the range vs dry land.

I have a lake thermometer installed on the distant side of a 150m lake with the gateway in a shed behind the weekend house. I get <3% error rates with an RFM69HW sending at 9 dBm and the house between thermometer and gateway. So I can confirm very good reception on the water in line with your theory. Thermometer is a Moteino with wire, gateway an esp8266 based gateway with custom dipole.

RSSI is around -85 now. I initially ran at 2dBm but that led to lost packets when big ships crossed ...
« Last Edit: September 26, 2016, 03:32:56 AM by joelucid »

joelucid

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #23 on: September 26, 2016, 03:27:01 AM »
Quote
Quote
Did you design the feedline with 50 Ohm?

You mean the antenna? Yes, depending on the frequency I used either one or two half-wavelengths of coax. The mismatch is therefore at the antenna feedpoint.

I meant the short PCB trace between the two elements of the dipole and the connector. Unless that's a 50 Ohm micro strip you'll get reflections at the SMA connector, too. And if it's not 50 Ohm and not the impedance of the dipole you'd get a first reflection at the SMA connector and then another one at the antenna.

See http://www.analog.com/media/en/training-seminars/tutorials/MT-094.pdf on characteristic impedance of micro strips.

I think I'll just measure out the coax length by trial and error for the first run.

Joe

captcha

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #24 on: September 28, 2016, 05:47:07 AM »
I got the dimensions for a 50 ohm impedance trace somewhere off the net and believed at the time that they were right (1mm track with, 1.15mm spacing between centre trace edges and GND pad edges). I haven't done much further research into that part of the connection but it fortunately is only a very short section.

Embarrassingly, I'm not able to find the document anywhere and all I can find is something called a 'coplanar strip with ground'. The calculations for the latter don't match up so I'm not sure if I'm on the wrong track there.. (pun not intended)

The values for RealZ on the 433, 868 and 915MHz dipoles (see graphs on first page of this post) are 45.62, 47.48 and 42.64 ohms respectively and these were measured 1x or 2x half-wavelenghts away from the antenna, so it can't be all that bad.

perky

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #25 on: September 28, 2016, 06:55:36 AM »
To get 50R microstrip trace on a two sided PCB of standard 1.6mm thickness the track needs to be about 3mm wide. It's better I think not to have any ground plane under them and treat those short traces as an extension of the dipole arms, which is possibly what really actually happens.
Mark.

raggedyanne

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #26 on: September 29, 2016, 06:47:16 AM »
The pads on the board can have as resistative or capacitance effect.

RF / Microwave PC Board Design and Layout
https://www.jlab.org/accel/eecad/pdf/050rfdesign.pdf

Basics in RF and EMF screening and shielding
http://incompliancemag.com/article/the-basic-principles-of-shielding/
« Last Edit: September 30, 2016, 03:46:51 PM by Felix »

john4444

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #27 on: September 30, 2016, 10:02:11 AM »
Raggedy,
Very good references.
Thanks
John AE5HQ

joelucid

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #28 on: October 05, 2016, 07:45:28 AM »
I received my dipoles yesterday and started testing a bit. I got both 433 and 868 Mhz ones. Both antennas are a tiny bit too small, the resonance of the 433 Mhz one is at ~443 Mhz and the 868 Mhz one around 875 Mhz. They both measure pretty much on spot at 433 and 868 if I hold the dipole by its feedline in my hand during measurement. 

The bandwidth of the 868 Mhz antenna is wide enough that it is certainly workable and I did first tests with it.

Now the impedance came out even lower than what captcha measured. I got 39 Ohm for the 868 Mhz antenna. That's probably still ok matching wise, but I wonder why it is so much off the ~75 Ohm a dipole should have. Is it the dielectric? Is it the outer coax that's part of the antenna since there's no balun? Anybody have a theory?

Joe

captcha

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #29 on: October 05, 2016, 06:32:09 PM »
Hi Joe,

Thanks for the comments and the thorough testing you did, and what a great starting point to further test and improve these designs! It's amazing to see how well these antennas are already performing, given the vast number of variables and designing these things at such little cost.

Quote
Both antennas are a tiny bit too small, the resonance of the 433 Mhz one is at ~443 Mhz and the 868 Mhz one around 875 Mhz.

I can think of a few things that could add to a probable cause, but without further testing, have nothing substantial yet to back this up.

Some pointers (in no particular order):

* fault in the gerber files
* different production materials at the PCB manufacturer
* different measurement equipment
* different measurement techniques/environment
* something else..?

I have also just received my batch of antennas this week (only the 433MHz ones) and should be ideal for re-confirming (or not) what you see. Thus far I have only held them up next to my first prototype antenna and they seem to be within 1mm of accuracy of each other. A rough calculation shows me that a frequency of 10Mhz higher (to 443MHz) should be the result of a difference of about 2.3mm shorter on each leg! That definitely isn't the case with the new antennas I just received so something else must be going on.

Quote
on spot at 433 and 868 if I hold the dipole by its feedline in my hand during measurement

Interesting.

You probably already know that this is not how I tested these antennas with the VNA (my testing procedure is described a few posts earlier) but it does show that environmental changes can easily affect the performance.

I'm open to any other interpretations others may offer as it seems there are some other variables that have come into play.




Then, things impedance..

Quote
why it is so much off the ~75 Ohm a dipole should have.

Some pointers:

* The effects of ground are quite substantial on a dipole.
* Add (almost lack of) thickness of the antenna elements to the equation as well.
* Then there's the feedline that can make all the difference. (is it really 50 ohms characteristic impedance?)

Obvious question, but I have to ask since it's not specifically mentioned: you did tune the feedline in each scenario for electrical half wave(s)?

Mind you, my setup could have been just as prone to all sorts of misinformation and variances in equipment etc.. so I'm definitely not saying who's right and who's wrong. However, with all the information presented I think we're getting very close to coming up with a low-cost, simple and effective design.



In short, I think the resonance is the thing to chase, I wouldn't worry too much about impedance.

I will hook up my new 433MHz antennas to the VNA and post here what it reveals. It could very well be that changes in the design are required. In the next version I also want to: 1) include the modification to change the right angles into smooth bends to avoid reflections and 2) re-do the tiny pcb transmission line (from SMA connector to antenna elements) so it's closer to a 50 ohms characteristic impedance.