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imp.guru Droplet and 915MHz Long Range Radio to WiFi


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Sorry for the mega post but as you may have already noticed, lately I

This is wrong.  I won't get started about HAM and ARRL, and how they destroy innovation in the modern era.  But at least you can figure out my opinion.   Lower wavelengths represent smaller particle

One common mistake is to assume that such a thing as a 1/4 wave resonant antenna exists.  The truth is, there must be a 1/2 wave dipole, or you must have a ground plane acting as a counterpoise to you

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Woah! sweet. Really had no idea you has submitted to the IMP guys and to C.G and D.J.

Anyway.. good luck. For RF design help @@jpnorair has some good antenna experience or any of the antenna heads.

Thanks! The guys at Electric Imp are pretty cool about community blog posts, if anyone has good project ideas that they would be happy to get more people involved.

 

Had anyone heard anything recently about a energia port to cc430? I know that I've heard things about panstamp but it seems like that is taking a long time to roll out.

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Neat man! Verrry cool. Love the droplets XD . 

Yea, from the RF side was gonna ask how you made your 50 ohm traces, how you planed to verify your discrete balun was working, and how you were going to impedance match. That picture is abit blurly so can't confirm ya did a discrete balun. I'd recommend using the one already designed for the cc11/cc430 915mhz version. Only way I could think to do balun verification would be with a high freq scope? However, thats only if ya want max power output. If you can get your hands on any type of spectrum analyzer you can use guess & check by looking at the magnitude of the output signal per change. However, if it works it works. :biggrin:  In a past life though I had to optimize the crap out of an RF system so I was curious how you'd go about it if you planned on it.

FYI, ya know oshpark lets ya tent vias right? :biggrin:

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@@username the balun that I use is purpose designed for the cc110L family of parts so I'm not as worried about that. It really only becomes a concern for the hub board where there is a little more RF. If I had the equipment, I would use a Vector Network Analyzer (VNA) to measure the input impedance and rerun loss at the rpsma connecton. Then I would use a coax pig tail, solder it to the output of the balun and measure the gain through the cc1190 PA and the same thing for the LNA. If I was feeling very ambitious, I would use a spectrum analyzer and a noise source to get the LNA noise figure.

 

Could do some other fun things as well but that is where I would start.

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One common mistake is to assume that such a thing as a 1/4 wave resonant antenna exists.  The truth is, there must be a 1/2 wave dipole, or you must have a ground plane acting as a counterpoise to your 1/4 monopole, and this needs to be effectively 1/4 wave itself.  Alternatively, there is a loop antenna structure.  Small chip antennas are generally a capacitive load combined with an inductive load.  This can reduce the size of the antenna, but nothing comes for free.  bandwidth is usually less, and efficiency is usually less.

 

There is all kinds of science, engineering, and clever tricks for making good, compact antennas.  Generally speaking, you want to have a 1/4 wave current distribution in your ground plane.  One thing I can tell by looking at the picture is that you will want a larger ground, or at least a cleverly engineered spiral at the edge of the PCB.  You will also want to use a 4-layer board with an internal ground layer.  I bet you are dropping 20 dB in your antenna system (13 at the very least).  For that form factor, I would estimate that -8 dB is about the best efficiency you can achieve over the 915 band, so that leaves 12 dB of room for improvement.

 

I'm out of the office today and tomorrow, but on Thursday I will show some photos.

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@@jpnorair Thanks!  In this case I did want to prioritize form factor over antenna performance however to your comments, I definitely do want to optimize where possible within this constraint, 12 dB is a big improvement.  In general, one thing that I don't understand is why loop antennas are not more published for these devices, it seems like they have a lot of advantages.  In some cases I've seen PCB loops that do not require any additional components:

 

http://www.emmicroelectronic.com/sites/default/files/public/products/datasheets/em9301_ds.pdf   see page 27...

 

In any case I look forward to your pictures, I'm sure that it will be very helpful.

 

As another update, with the help of @spirillis http://forum.43oh.com/topic/6019-pin-map-for-msp430g2553-rhb-package/ I did get my hub up and running so that is very exciting, looking forward to doing some range testing with it!  I also  received my solder stencils and reflowed 7 more droplets.  I think that I will be using this workflow more because the build went pretty well and I know that I can get more efficiently out of it if I take the time to have all of the materials staged and my workspace is more orderly!

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So I was thinking about this over lunch today. So far I have built 12 droplets and 2 Hubs, everything is working quite well. Of course there are optimizations remaining but overall I would say that the project is ready for a Alpha Build.

 

Would anyone be interested in some Alpha hardware? What I was thinking would be to create a panel with one hub PCB and 6 droplet PCB's. Then I will order a stainless steel stencil and toaster over reflow the entire panels with all components.

 

I would just be looking to cover my costs in doing so. These wouldn't be final numbers but my estimate would be

 

if 10 panels total were ordered, $80 per panel

if 20 panels total were ordered, $70 per panel

if 30 panels total were ordered, $65 per panel

 

These prices do not include the electric imp sd card, that would be an additional $25

 

EDIT: if you think that the prices are a little out of the discretionary spending limit. We could do 4 droplets instead of 6. Also after doing some digging maybe I can get a few more bucks out of the costs. In the end I'll share all of the cost invoices with all participants.

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One common mistake is to assume that such a thing as a 1/4 wave resonant antenna exists.  The truth is, there must be a 1/2 wave dipole, or you must have a ground plane acting as a counterpoise to your 1/4 monopole, and this needs to be effectively 1/4 wave itself.  Alternatively, there is a loop antenna structure.  Small chip antennas are generally a capacitive load combined with an inductive load.  This can reduce the size of the antenna, but nothing comes for free.  bandwidth is usually less, and efficiency is usually less.

 

There is all kinds of science, engineering, and clever tricks for making good, compact antennas.  Generally speaking, you want to have a 1/4 wave current distribution in your ground plane.  One thing I can tell by looking at the picture is that you will want a larger ground, or at least a cleverly engineered spiral at the edge of the PCB.  You will also want to use a 4-layer board with an internal ground layer.  I bet you are dropping 20 dB in your antenna system (13 at the very least).  For that form factor, I would estimate that -8 dB is about the best efficiency you can achieve over the 915 band, so that leaves 12 dB of room for improvement.

 

I'm out of the office today and tomorrow, but on Thursday I will show some photos.

 

So true, small ground planes(in comparison to wavelength) combined with a 1/4 monopole antenna destroys efficiency. Hence why if your in the ~433mhz | ~900mhz band, you either need a large gp on your transmitter or your going to have to have a verrry efficient receiver in order to get decent distance. Having low buadrate & long pre-ambles can help a bit but a course its a tradeoff. It is much easier to make efficient 2.4ghz monopole transmitters which is why their range can be very comparable with 433 & 900 mhz systems.      

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It is much easier to make efficient 2.4ghz monopole transmitters which is why their range can be very comparable with 433 & 900 mhz systems.      

Outdoor line-of-sight, yes.  Indoor, or non-line-of-sight, no.  Even a system with a lousy 433 MHz antenna will grossly outperform a system with an ideal 2.4 GHz antenna, non-line-of-sight. 900MHz is somewhere in the middle.

 

The other thing is that you can usually cut a slot in the ground plane, use a small magnetic loop, or do other things to trade bandwidth for efficiency in a compact antenna.  For the 900 MHz band, you need quite a lot of bandwidth to comply with both EU and US regulations, so this is actually the largest antenna.  433 is global and has a narrow band, so you can use a size-optimized design.  If you are targeting only EU (866) or only US (915), then of course you can optimize as well, however, you will need to use DSSS if you want to use the higher-power FCC 15.247 regulation.  15.249 allows narrowband operation in 915, but only -1dBm output power.  The Frequency-Hopping option for 15.247 also necessitates an antenna with reasonably large bandwidth.

 

In any case, I make a coiled dipole for 433 MHz that has 14 MHz bandwidth, is 10cm long, and has very high efficiency.    If you get clever with the design of your device, it can be quite small and still have a very high performing antenna.

 

As for the pictures I promised, I will need to have another week.  I'm moving this week and all my lab is boxed-up.

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Outdoor line-of-sight, yes.  Indoor, or non-line-of-sight, no.  Even a system with a lousy 433 MHz antenna will grossly outperform a system with an ideal 2.4 GHz antenna, non-line-of-sight. 900MHz is somewhere in the middle.

That makes sense though there is a HAM test question that seems to conflict with that: "T3A02-2014: Why are UHF signals often more effective from inside buildings than VHF signals? ", answer: "The shorter wavelength allows them to more easily penetrate the structure of buildings "

Wouldn't it be opposite of that?

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