Author Topic: easy PCB dipoles for 433, 868 and 915MHz  (Read 20170 times)

captcha

  • Full Member
  • ***
  • Posts: 108
  • Country: au
easy PCB dipoles for 433, 868 and 915MHz
« on: September 03, 2016, 07:58:02 PM »
This weekend I finally had access to a VNA to tune my PCB dipoles for 433, 868 and 915MHz.

The 433 and 868 antennas were attached to the VNA with a 22.5cm length of RG174. On 433MHz this is a half-wavelength (VF=0.65) and two half-wavelengths on 868MHz. For 915MHz I only trimmed the feedline an extra 5mm to 22cm which makes it close to two half-wavelengths.

During testing, the antennas simply were left hanging down, away from any nearby metallic or organic objects.

The following graphs are the resulting plots for each antenna:



433MHz
----------
The above plot is for the dipole tuned for 433MHz. The blue line is SWR and shows a dip around 434.8MHz. Close enough for using it on 433MHz. The SWR (top left, in blue) shows a value of 1.11:1 for this frequency. The red line shows an impedance at this frequency of 45.62 Ohms, reasonably close to matching it with the radio which expects 50 ohms. The bandwidth of the antenna with an SWR of 1.5:1 or less is about 15MHz (from 427 to 442MHz). More than sufficient to cater for any variations in operating frequency.

- fed with one half wavelength (22.5cm) RG-174
- bandwith below 1.5:1 SWR = 15MHz
- each dipole leg is 13.3cm






868MHz
----------
The above plot is for the dipole tuned for 868MHz. The blue line shows an SWR of 1.1:1 at 867MHz. The dip is much wider than on 433MHz and has a bandwidth of about 51MHz (838 to 889MHz) for an SWR of 1.5:1 or less. The red line shows an impedance of 47.48 Ohms for this frequency, so a fairly close match for the radio.

- fed with one wavelength (22.5cm) RG-174
- bandwith below 1.5:1 SWR = 51MHz
- each dipole leg is 6.4cm






915MHz
----------
The last plot is for 915MHz and shows an SWR of 1.29:1 for this frequency. The lowest SWR is slightly lower in frequency, but negligible for our purposes. I just left the marker at 915MHz. The antenna has a bandwidth of 43MHz (888 - 931MHz) for an SWR of 1.5:1 or less. The red line shows an impedance of 42.64 Ohms at 915MHz.

- fed with one wavelength (22cm) RG-174
- bandwith below 1.5:1 SWR = 43MHz
- each dipole leg is 6.0cm



The tests were performed with the SDR-Kits DG8SAQ Vector Network Analyzer 3E (site: https://www.sdr-kits.net/)


(Prototype PCB dipole attached to the VNA)

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.

On the 868MHz board I have also added cut marks for trimming the antenna for 915MHz use. The 433MHz board has cut marks for both 915 and 868MHz. If you are going to trim the tracks, make sure you remove the excess outside traces.

Based on the test results, I calculated the velocity factor of the FR4 for a single-layer printed dipole (track width 1mm, one once copper) to be about 0.77.

All these designs have solder pads for an edge-style SMA connector:

(Prototype board with SMA edge connector)

I ordered the SMA connector from RS with the part number 526-5785 (http://au.rs-online.com/web/p/sma-connectors/5265785/)

In the PCB designs I have deliberately left a solder mask over of the bottom sma connector ground pad. I did this purely to minimise corrosion in case you wish to solder leads to the pads instead of using the edge SMA connector. The solder mask is fairly easy to scrape off.

Some general things to keep in mind about these designs:

 - This is a half-wave dipole and therefore it does not have a 50 ohm feedpoint impedance. You will have some rf coming back to the radio on transmit but due to the rigid and balanced PCB based design and the accurate calibration of the length of the dipole legs using the VNA this will be minimal.

 - To keep the impedance mismatch from getting worse it is important to only use feedline lengths of (multiples of) a half wavelength (compensated for the appropriate velocity factor) for the frequency of interest. See the picture in this post to see the effects of different feedline lengths: https://lowpowerlab.com/forum/rf-range-antennas-rfm69-library/small-loop-antennas-433-mhz/msg14061/#msg14061

 - coax feedline losses are siginificant at these frequencies, use the lowest number of half-wave multiple lengths possible.

 - I tested these dipoles on 1.6mm thick FR4 substrate PCBs with 1 ounce copper from OSH Park. Feel free to use these designs to manufacture elsewhere but it is not guaranteed that the results will be the same. Heck, even OSH Park could end up supplying you with different materials, but this is what worked for me.

I mainly made these designs for myself so that I could get easy access to a low-cost and good performing omnidirectional antenna for 433MHz. I did the 868 and 915 designs as a bonus for the community as I already had everything set up and it only took me an extra half hour or so to come up with the final designs. I'm hope some of you will appreciate it.

WhiteHare

  • Hero Member
  • *****
  • Posts: 1297
  • Country: us
Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #1 on: September 04, 2016, 11:38:59 AM »
Nice!  Thanks for putting your work on OSH Park.  Is there any downside if I were to solder a Moteino directly to your antenna PCB without using a feedline?

captcha

  • Full Member
  • ***
  • Posts: 108
  • Country: au
Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #2 on: September 04, 2016, 06:33:45 PM »
Quote
Is there any downside if I were to solder a Moteino directly to your antenna PCB without using a feedline?

As usual: 'it depends'.

Although hard to visualize, the common belief is that maximum radiation happens where current is maximum. For a half-wave dipole this current maximum is at the feedpoint (i.e. in the middle of the antenna, where one leg takes a left turn and the other takes a right).

In a quarter wave vertical antenna (what some call the 'monopole'), current maximum is also at the feedpoint. Think of a quarter wave vertical as a dipole but with one of its legs turned into a plane and perpendicular to the other leg. In terms of using it on a moteino, the 'plane' is provided by the GND tracks of the circuit board and the 'monopole' going straight up is just the other leg.

Many people here on this forum (and elsewhere using similar small radio modules) use quarter wave vertical antennas with great success, even though maximum radiation happens in very close proximity to the microcontroller on the pcb. From this point of view I think you should be fine with attaching the dipole antenna directly to the antenna port.

The close proximity of the pcb to the dipole does screw up some of the antenna's balanced characteristic. A balance I specifically wanted to create using a rigid pcb design. This distorting effect would be even more pronounced on higher frequencies (868/915) due to the moteino being a much bigger size relative to the size of the antenna.

Felix

  • Administrator
  • Hero Member
  • *****
  • Posts: 5481
  • Country: us
    • LowPowerLab
Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #3 on: September 05, 2016, 09:01:29 PM »
@captcha nice work, thanks for sharing the research!

captcha

  • Full Member
  • ***
  • Posts: 108
  • Country: au
Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #4 on: September 05, 2016, 10:02:46 PM »
Thanks, happy to share.

I just noticed that I forgot to include the Diptrace design files. Please find attached the files for all three antennas for those who want to have a closer look.

joelucid

  • Hero Member
  • *****
  • Posts: 869
Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #5 on: September 07, 2016, 04:27:22 AM »
Very cool!

raggedyanne

  • Jr. Member
  • **
  • Posts: 80
  • Country: au
Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #6 on: September 21, 2016, 10:54:52 AM »
Nice work  ;)

The right angle traces from the sma need to be ellipse/arc not square 90 , or at least 45 degree.
What is the trace width ?

Trace Curvature Radius >= 3 Trace Width
« Last Edit: September 21, 2016, 11:04:23 AM by raggedyanne »

captcha

  • Full Member
  • ***
  • Posts: 108
  • Country: au
Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #7 on: September 21, 2016, 11:28:58 PM »
Hi raggedyanne,

The copper antenna element traces have a width of 1mm and a thickness of 1.37 mils (at 1 once per square foot). 1.37 thousands of an inch translates to 34.79um.

Excellent suggestion on the bend radius! Never thought of the importance of it, but I'll be sure to do some further research into the matter and incorporate it into the next design.

Great to have you on the forum!

raggedyanne

  • Jr. Member
  • **
  • Posts: 80
  • Country: au
Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #8 on: September 22, 2016, 05:11:28 AM »
It like running through a maze without the ability of slowing down 99% of the time you hit the wall , but with curved walls you can slide on through without bouncing around  :o

Felix

  • Administrator
  • Hero Member
  • *****
  • Posts: 5481
  • Country: us
    • LowPowerLab
Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #9 on: September 22, 2016, 08:26:23 AM »
Right, imagine the noise if the water pipes had 90 angled turns in your house :)

joelucid

  • Hero Member
  • *****
  • Posts: 869
Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #10 on: September 22, 2016, 08:55:09 AM »
Captcha et al,

I'm about to use these antennas as reference in a project. So quick question:

How much directionality will the asymmetries of this antenna cause (dielectric pcb material only on one side, solder mask on the other etc)? Anybody have a sense for that? Captcha maybe you have experience from your use of the antenna?

Joe

captcha

  • Full Member
  • ***
  • Posts: 108
  • Country: au
Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #11 on: September 22, 2016, 08:41:42 PM »
Read up a lot since the recommendation to not use 90 degree angled corners in PCB designs.

Check the links below for further reading pleasure.

In short, some of the drawbacks of using 90 degree corners in pcb tracks:
- during manufacturing, they can trap solution in the corners and over-etch, making corners thinner than expected.
- fewer 'fold areas' which can lead to track breaking due to warping stresses (this is mainly applicable to thin, flexible PCBs)
- less reflection at turns for signal propagation in very high frequency applications.
- very inefficient as they often take up more space to route around components

Note that vias (to connect pcb layers) are in fact 90 degree corners and should be avoided in certain RF conditions.



A bend radius of (at least) 3x the trace width is a minimum for very high frequency applications.



For maintaining a 90 degree corner in a very high frequency signal path, a so-called mitre-bend is the superior technique to change direction on a PCB.


From what I've read (the page at the bottom link is most clear on this), the signal reflection effects become an issue at frequencies above 5 Gigahertz. I'm not convinced that these PCB antennas require rf-friendly angles but a case could be made for minimising physical stresses on the tracks. I will update the designs and post the relevant links here.

Quote
How much directionality will the asymmetries of this antenna cause

I don't think it will matter much but because I have the antennas already I expect to run a test this weekend (weather permitting) and post my findings here.

Links:
http://www.pcb3d.com/uploads/Flex_Design_Guidelines.pdf
https://www.maximintegrated.com/en/app-notes/index.mvp/id/5100
http://www.ti.com/lit/an/slla311/slla311.pdf
http://www.microwaves101.com/encyclopedias/mitered-bends

captcha

  • Full Member
  • ***
  • Posts: 108
  • Country: au
Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #12 on: September 24, 2016, 05:18:36 AM »
Hi Joe,

Quote
Captcha maybe you have experience from your use of the antenna?

I have only used them at close range, but your question intrigued me so I got curious and decided to find out for myself. Today was a beautiful day for testing (28 degrees C, 82F) so after uploading some code to two Moteino's and cable-tying the PCB dipole to a wooden dowel I was ready to do some testing.

I just came back from a little walk around my neighbourhood to find out if the PCB causes the dipole to have any kind of directionality.

Short answer: No perceivable directionality.

The base station consisted of a Moteino sending out three encrypted packets at 2-second intervals, with a payload of some telemetry (temperature, voltage, packet count, RSSI). The antenna used on the base station was a quarter wave vertical with 4 ground radials and a vertical whip antenna for 2m (also resonant on 70cm, so no big worries there).

I walked around with another Moteino that had the PCB dipole for 433MHz connected to it with a 22.5cm length of RG-174. This Moteino was battery powered and I taped everything to a wooden dowel with the dipole at one end. This Moteino had a 128x64 oled screen connected to it and gave me instant feedback as to how many packets were received and what the received signal strength was.


The mobile station.

I walked about 1.4km to a point where the signal often got an RSSI between 99 and 103 and reception was intermittent at best. There was absolutely no line of sight between the two stations as many street signs, a big building, trees, metal fences, street lights, overhead power lines and cars were nearby and around (remember, line of sight does not work like a laser; Google: Fresnel Zone for laughs).

Making sure to keep the antenna vertically polarised at all times I alternated between pointing the copper side of the antenna and the pcb (under) side toward the base station 1400m away. I noticed that the signal dropped away completely when there were a lot of cars in between the stations but sometimes the signal would come good again and I could decode three consecutive packets and received the telemetry data fully intact.

At the fringe of reception in this practical experiment I could not see any significant difference between pointing the antenna with the copper side or with the pcb side to the base station. Often I would get decodes with an RSSI of 98, then one with 102, then 99.. It would definitely jump around but I could not see that one orientation would be better than the other.

What I did notice, is that I used this same walk to test quarter wave verticals at both ends and never got this far.

Fun experiment, learned a lot and I got some exercise too! :-)

joelucid

  • Hero Member
  • *****
  • Posts: 869
Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #13 on: September 24, 2016, 06:07:32 AM »
Thanks that is definitely good news! I have some ordered which I will try probably next week to measure some difficult environments. I really like the reproducibility of these antennas. Much better than custom made dipoles for reference testing.

Thank you so much for putting them together!

I have one further question for you: you recommended l/2 multiples for the cable length for understandable reasons. I take it you measure the length from the point where the antenna traces turn away perpendicular from the dipole proper, not from the connector, correct?

And yeah - range testing is one of the best activities to stay in shape  ;)

Joe

captcha

  • Full Member
  • ***
  • Posts: 108
  • Country: au
Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #14 on: September 24, 2016, 07:33:12 AM »
Interesting question; half-wavelength is between where and where?

Quote
from the point where the antenna traces turn away perpendicular from the dipole proper, not from the connector, correct?

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).

In my testing I didn't use connectors but soldered the RG-174 directly onto the pcb. With connectors I would probably take off an extra 2 cm from the coax to cater for the length of the connector and pcb traces towards the antenna, whilst making sure the electrical length between transmitter and antenna is a (multiple of a) half wavelength. It may also depend a bit on the type of connector you're using in case you're crimping them yourselves.


This is how far the SMA connector screws on. Reference scale is in centimeters.

If you're planning on using the SMA connectors, eBay sells RG-174 and RG-316 coax with pre-fitted SMA connectors for next to nothing. A common name for these is 'pigtails'. Velocity factor and attenuation are similar, but the biggest benefit of 316 is that the dielectric is Teflon (PTFE) and keeps shape much better when soldering onto it.