Author Topic: easy PCB dipoles for 433, 868 and 915MHz  (Read 166898 times)

joelucid

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #60 on: March 18, 2017, 07:31:07 AM »
For comparison here are the same measurements with dipole and l/2 feed line:

Noise level (at 55khz settings) measured just tickling the WDT:

Noise level, Percent of detections
-105.5,  10%
-106.5,  10%
-107.0,  30%
-107.5,  80%
-108.0, 100%

Noise level measured with flash SPI active:

Noise level, Percent of detections
-101.5,  10%
-105.0,  10%
-105.5,  40%
-106.0,  20%
-106.5,  10%
-107.0,  30%
-107.5,  90%
-108.0, 100%

As you can see the new GW is better even though the antenna sits directly connected to the board. Measurements with l/4 monopole over board yielded noise floors in the -90dBm area, so you can see how much of an improvement this new design yields.

VERY HAPPY!  :D :D

Joe

perky

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #61 on: March 18, 2017, 08:39:44 AM »
Excellent results Joe! I wonder how much difference the ESP-12S made in the end.
Mark.

WhiteHare

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #62 on: March 18, 2017, 11:01:47 AM »
As you can see the new GW is better even though the antenna sits directly connected to the board.

It would be great to see a photo of how you've connected the antenna directly to the board.

joelucid

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #63 on: March 18, 2017, 12:35:16 PM »
Quote
It would be great to see a photo of how you've connected the antenna directly to the board.

WH, look a couple posts up - I did attach a photo. The circular PCB has a cut out into which the antenna fits. And I then solder the antenna on all four sides against the PCB.

BTW, captcha, as you can see the perfect antenna for my purposes would just fit into the cut-out and have one leg on each side of the antenna PCB so that one side doesn't have to pass through a via on the dipole.

Joe

captcha

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #64 on: March 18, 2017, 06:41:07 PM »
Interesting idea..

I'm not sure if I fully understand, but wouldn't you always need a via on the parent board to get to at least one leg of the dipole?

I think we may be splitting hairs here with eliminating vias though, as they would be perpendicular to the dipole.

joelucid

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #65 on: March 19, 2017, 04:58:12 AM »
Quote
I wonder how much difference the ESP-12S made in the end.

What I do know now is that the new board works equally well with either the ESP-12S or the ESP12E in the dipole configuration.

The ESP-12S works so well that even if I replace the dipole/balun with a simple l/4 monopole with board as counterpoise I still get fairly low noise and in particular no impact if I access the flash SPI. However with the ESP-12E there is some impairment with monopole when accessing the flash. However it's still no comparison to the effects on old boards - I might see noise at -102 dBm vs -88 dBm with earlier boards.

It appears that my new board is really good at isolating the antenna from any noise the ESP produces. But the ESP-12S does seem quieter. An interesting test would be to drop the 12S into one of the old noisy boards to see if that already fixes the issue - I don't want to solder these feed thru caps any more often than absolutely necessary ...

Joe
« Last Edit: March 19, 2017, 07:42:26 AM by joelucid »

joelucid

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #66 on: March 19, 2017, 07:40:44 AM »
Quote
An interesting test would be to drop the 12S into one of the old noisy boards to see if that already fixes the issue - I don't want to solder these feed thru caps any more often than absolutely necessary ...

And here's the answer. My old gw with l/4 monopole and esp-12e while accessing SPI flash:

Noise level, Percent of detections
-89.5,  10%
-90.0,  10%
-90.5,  20%
-91.0,  10%
-91.5,  40%
-92.5,  30%
-93.0,  10%
-93.5,  40%
-94.0,  10%
-94.5,  30%
-95.5,  60%
-96.0,  30%
-96.5,  40%
-97.0,  20%
-97.5,  40%
-98.0,  40%
-98.5,  40%
-99.0,  50%
-99.5,  50%
-100.0,  70%
-100.5,  30%
-101.0,  50%
-101.5,  80%
-102.0,  80%
-102.5,  40%
-103.0,  50%
-103.5,  80%
-104.0,  90%
-104.5,  40%
-105.0,  50%
-105.5,  40%

Same revision GW, same antenna, but now with ESP-12S, while accessing flash:

Noise level, Percent of detections
-100.5,  20%
-101.0,  30%
-101.5,  80%
-102.0, 100%

MUCH better.

Here's without accessing flash:

Noise level, Percent of detections
-100.0,  10%
-100.5,  20%
-101.0,  10%
-101.5,  80%
-102.0,  90%
-102.5, 100%

So all in all the 12S is a huge improvement. Flash access is no longer an issue. But my other improvements (a) enable using the 12E without impairment and (b) further bring down the remaining noise.

Joe

joelucid

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #67 on: March 19, 2017, 08:13:39 AM »
Quote
I'm not sure if I fully understand, but wouldn't you always need a via on the parent board to get to at least one leg of the dipole?

Look at the attached. The balanced output from the balun just goes completely symmetrical to both sides of the dipole. But then one side needs to pass back through the via to the other side on the dipole.

The other thing I'm thinking is to widen the copper legs of the antenna to the entire 5mm or so width I have. That would widen bandwidth quite a bit. If its wide enough one model might work for both 915 and 868 mhz.

Joe

WhiteHare

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #68 on: March 23, 2017, 05:01:31 PM »
Is there any downside if I were to solder a Moteino directly to your antenna PCB without using a feedline?

In an attempt to answer the question empirically, I soldered it directly to the RFM69HW module on an experimental node.  In case others here are curious: it does work, and it gets better RSSI than a regular monopole.  Hence, probably almost any mote could be adapted this way.

[Edit1: I had filed down the edge of the PCB dipole antenna a little so that the trace pads were right on the edge, and my original plan was just to solder bridge the dipole antenna directly to the corresponding pads on the RFM69HW module.  However, the RFM69HW module had a thinner PCB, and for other reasons also, that didn't quite work out.  So, on this first attempt, I resorted to using a bit of wire to make the connections.]

[Edit2: Having tried it the brute force way, the amount of improvement seems a bit underwhelming.  It leaves me more motivated now to try it the proper way using a 1/2 wavelength of coax (as discussed above).  ]



« Last Edit: March 23, 2017, 10:07:32 PM by WhiteHare »

ChemE

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #69 on: April 14, 2017, 04:06:01 PM »
Did you get anywhere with rev2 captcha?

captcha

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #70 on: April 14, 2017, 06:49:29 PM »
I've received the u.fl surface mount connectors and a couple of pigtail leads. I've redesigned the pcb layout to make them a whole lot smaller but haven't yet send them to the fab as I haven't yet made sure the diptrace library footprint for the u.fl connector is indeed covering all pads. I'll print them out on a laser printer next week and see if that lines up.

TomWS

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #71 on: April 14, 2017, 07:47:07 PM »
I've received the u.fl surface mount connectors and a couple of pigtail leads. I've redesigned the pcb layout to make them a whole lot smaller but haven't yet send them to the fab as I haven't yet made sure the diptrace library footprint for the u.fl connector is indeed covering all pads. I'll print them out on a laser printer next week and see if that lines up.
Sorry if I'm out of touch, but are you thinking of having the u.fl connector on the dipole PCB with the pigtail cable soldered to the Mote PCB? 

I like the idea of having your dipole connected via cable and u.fl is a convenient and proven connector, I'm just a bit confused by your comment.

Tom

captcha

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #72 on: April 15, 2017, 07:47:12 PM »
Sorry for the confusion.

As per your post, the idea is indeed to have a surface mount u.fl connector on the board, right in between the two dipole legs. For testing I will need some pigtail leads cut to the appropriate lengths so I can hook them up to a vector network analyzer. Once that's done I'll post the PCB dipole designs back on OSH Park. Those pigtail leads will of course not be part of the design, nor will the u.fl connectors; it will be just the dipole with the smd footprint to hold the u.fl connector, so you'll have to source those connectors and connecting leads yourself.

This is the u.fl connector I'll use to test the new dipoles:

http://www.ebay.com.au/itm/351802783571


This is a preliminary design of the dipole:


There will also be a 433MHz flavour and I'll also do up a design that has two through-holes 0.1" apart (instead of a u.fl connector) for soldering on a cable directly.

Hope this clears things up.

ChemE

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #73 on: April 15, 2017, 09:53:36 PM »
What dimensions are you at right now out of curiosity?

EDIT: Upon closer inspection there is still a good amount of wasted PCB at the ends.  Why not move the outer-most holes towards the center a little and shave off that unneeded PCB beyond the dipoles?
« Last Edit: April 15, 2017, 10:52:26 PM by ChemE »

captcha

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Re: easy PCB dipoles for 433, 868 and 915MHz
« Reply #74 on: April 15, 2017, 11:07:44 PM »
The outside board dimensions as shown above are now 14.7mm x 11.4mm (5.8" x 0.475"). But please bear in mind these are oversized and not tuned at all for any frequency. I have deliberately extended the elements by a few cm to make sure I've got more than enough when trying to cut them to resonance for the various frequencies.

I'll reiterate: do not use these dimensions and expect that the v0.2 prototype will work. This is definitely a work in progress..  :)

As per your suggestions for moving the holes... yes, that can definitely be done. Let's see how small we can get them.
« Last Edit: April 15, 2017, 11:10:07 PM by captcha »