Author Topic: What is the smallest antenna that still performs well?  (Read 8780 times)

WhiteHare

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Re: What is the smallest antenna that still performs well?
« Reply #15 on: October 19, 2016, 05:36:26 PM »
I didn't realize until just now that a 3V CR2477 has around 1000mah capacity, which is more than most AAA's.  So, I may a tabbed one and go that route:


That would keep the pressure on to keep the antenna as small as possible, so its more and more likely I'll end up using an SMD resistor antenna after all.   :) 

captcha

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Re: What is the smallest antenna that still performs well?
« Reply #16 on: October 19, 2016, 05:39:20 PM »
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I'm now trying to maximize range, not get almost no range.  So, for that purpose, are you saying I would be better off with the thru-hole carbon resistors, since they do have some lead length (unlike the SMD)?

Correct, I think it's the small lead length on the through-hole resistor that still acts as a radiator. Using SMD would make the radiation effects much smaller.

If you look at proper dummy loads, you'll see there's no 'leads' to speak of; it's all encapsulated:


50-ohm SMA dummy load


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The carbon film resistors would have the same helical spiral as the metal film resistors you highlighted.

I'm not sure where the carbon comes into play with 'carbon film' resistors, but it's the spiraled laser-cut 'film' bit that turns the resistor into an inductor.

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51 ohm SMD resistor ..., and it transmits fine.

Don't forget that by simply matching the impedances you have also maximised the output power. This is why it's generally a good idea to look at impedance matching after you have a resonant antenna.. :-)

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I suppose this "SMD resistor antenna" could be especially practical on a LoRa module.  That thing has so much potential link budget that for a home environment you could easily workaround the antenna impairment and still have rock solid wireless links everywhere in the home. ]

Don't expect these dummy loads to be a very good antenna though. Their radiation pattern is chaotic at best and may vary greatly from device to device. But, yeah, if you have a strong enough transmitter and a decent antenna for the base station you may very well use this technique to shrink the footprint of your motes around the house. You could even deliberately add some leads to the smd resistor to see if you get some better range out of it. I wouldn't go overboard but maybe adding a cm or two could have some good effects for increasing the range.

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my near-term goal is to fit everything, including 2x AAA batteries, into a 4x AAA battery holder case like this

Just one word of caution. I've heard that black colour in plastics is often achieved by adding carbon to the material. This could adversely influence your signal strength.

WhiteHare

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Re: What is the smallest antenna that still performs well?
« Reply #17 on: October 19, 2016, 06:31:38 PM »
You could even deliberately add some leads to the smd resistor to see if you get some better range out of it. I wouldn't go overboard but maybe adding a cm or two could have some good effects for increasing the range.

Well, if it's only one or two cm, then perhaps it makes sense to add that as a trace to the PCB?  It wouldn't require much PCB real-estate.  Or is that venturing too far in the direction of then needing to do impedance matching?  I just very recently started designing my own PCB's and ordering them from osh-park, so I could add antenna traces easily enough, but I'm not equipped to do impedance matching.
« Last Edit: October 19, 2016, 06:36:33 PM by WhiteHare »

captcha

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Re: What is the smallest antenna that still performs well?
« Reply #18 on: October 19, 2016, 08:36:41 PM »
There are some PCB-trace impedance calculators on the net that will give you an idea what width tracks you'd need (and separation from underlying GND tracks) to come up with 50 ohms impedance.

Something like this:
https://www.eeweb.com/toolbox/microstrip-impedance

WhiteHare

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Re: What is the smallest antenna that still performs well?
« Reply #19 on: October 19, 2016, 09:12:39 PM »
Thanks!  Maybe it's worth a shot:  it doesn't have to be perfect;  it just has to be better than a dummy load "antenna".   ;)

Felix

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Re: What is the smallest antenna that still performs well?
« Reply #20 on: October 20, 2016, 12:57:49 PM »
RE this 1Ah CR2477:
http://www.digikey.com/product-detail/en/panasonic-bsg/CR-2477-HFN/P126-ND/107129
IMHO the cost is prohibitive given the capacity.



How about the smaller 620mAh battery, at less than half the cost:
http://www.digikey.com/product-detail/en/panasonic-bsg/CR-2450-H1AN/P662-ND/2404067

These have a variable influence on the small antenna performance since the board+antenna (most we've seen in the forum) is sandwiched somewhere in between the RFM and this type of battery. Would be great to see a compare between such batteries.

john4444

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Re: What is the smallest antenna that still performs well?
« Reply #21 on: October 20, 2016, 02:49:19 PM »
@ WhiteHare, in general, what kind of usable range do you experience using the SMD resistor for an antenna?
I'm curious to learn how the RSSI level compares to the wire antenna at various distances.
I have all of my Moteinos in use at the moment or I would try it my self.
John AE5HQ

WhiteHare

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Re: What is the smallest antenna that still performs well?
« Reply #22 on: October 20, 2016, 02:51:30 PM »
RE this 1Ah CR2477:
http://www.digikey.com/product-detail/en/panasonic-bsg/CR-2477-HFN/P126-ND/107129
IMHO the cost is prohibitive given the capacity.



How about the smaller 620mAh battery, at less than half the cost:
http://www.digikey.com/product-detail/en/panasonic-bsg/CR-2450-H1AN/P662-ND/2404067

These have a variable influence on the small antenna performance since the board+antenna (most we've seen in the forum) is sandwiched somewhere in between the RFM and this type of battery. Would be great to see a compare between such batteries.
Thanks for the links.  Those are all good points you make.  I simply looked on Wikipedia to find the button cell with the most mah, and apparently it's the CR2477.  So, there's that, and also the hope that it won't have as much voltage droop as the thinner button cell that Joe and Tom have been experimenting with and commenting about.  Not sure if that's a realistic hope or not though.

WhiteHare

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Re: What is the smallest antenna that still performs well?
« Reply #23 on: October 20, 2016, 03:28:47 PM »
@ WhiteHare, in general, what kind of usable range do you experience using the SMD resistor for an antenna?
I'm curious to learn how the RSSI level compares to the wire antenna at various distances.
I have all of my Moteinos in use at the moment or I would try it my self.

I'll give you the range in my house, but it wouldn't necessarily translate to yours.  That's why I expressed it as the amount of RSSI impairment, which should be about the same, and which you could interpret for your own environment.

That said, I live in two story, roughly 4,000 sf house made using commonplace wood stick construction.  If I put a regular moteino gateway with quarter wave antenna in a corner of the lower floor and I then put the impaired node (i.e. the one running the dummy load resistive antenna pictured above) in the opposite diagonal corner on the second story (which I assume to be a worst case scenario, or close to it), I do receive packets, but with a lot of losses.  I suspect I could improve or worsen that by how I orientate the node, but that's what I got just from plunking it down without any thought.  Also, those results are from running Felix's generic gateway and node sketches.  If I were to change the firmware to use a lower OTA bitrate and narrower bandwidth, and/or possibly increase the transmit power (I'm forgetting now what Felix chose as the default power Tx level), I'm sure it would do better.  Even as is, though, it's doing far better than an NRF24L01+ module would.  To be honest, I didn't expect it would do anywhere near as well as it is.

[Edit: In terms of receiving packets from the next room through a wall or something like that, it's definitely no challenge for it. ]
« Last Edit: October 20, 2016, 05:19:59 PM by WhiteHare »

john4444

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Re: What is the smallest antenna that still performs well?
« Reply #24 on: October 20, 2016, 05:34:17 PM »
@ WhiteHare, You are right that my experience would be different than yours.
Ideally a person would have two units side-by side for the comparison.
But, your last comment is especially interesting to me.
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In terms of receiving packets from the next room through a wall or something like that, it's definitely no challenge for it.
For short range or initial testing - the resistor sounds like a good trick.
John AE5HQ

captcha

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Re: What is the smallest antenna that still performs well?
« Reply #25 on: October 20, 2016, 05:58:50 PM »
Here we go, my 50th post on this site.. :-)

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I do receive packets

Fantastic news! Real-world data! :-) I really think this may be an eye opener for people who really want to miniaturise their motes and don't have a lot of distance to cover.

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Even as is, though, it's doing far better than an NRF24L01+ module would.

And that would have everything to with those NRF24L01+ modules working on 2.4GHz.

The lower you go in frequency the higher the level of signal penetration trough objects like wood, chipboard, stone, etc.. As nice as small antennas for 915MHz may be I will stick to 433MHz just for that reason (well.. it also has something to do with the legality of it all of course).

WhiteHare

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Re: What is the smallest antenna that still performs well?
« Reply #26 on: October 20, 2016, 06:46:10 PM »
I do think there's a role it might play.  For instance, my wife has a lot of potted plants, and I'd be interested in making some sensors to monitor each one (i.e. each potted plant gets its own wireless sensor node).  In this case, she would want the sensors to be as small and discrete as possible, and to make any sense it would need to be inexpensive.  So, that might mean running from a button cell and having no visible antenna.  Maybe there are better ways, but a resistor antenna might possibly be a good fit for that.  If it turns out that range is an issue, I could always add another cheap wireless gateway that's within range, but not visible (perhaps in a closet or cabinet or something like that).

Just one word of caution. I've heard that black colour in plastics is often achieved by adding carbon to the material. This could adversely influence your signal strength.

I hadn't heard before that carbon might be a problem, but after reading your warning, I'll try it both with and without the cover.

My inspiration for the idea came from this:
http://johan.kanflo.com/the-aaduino/
so if it were a monster problem, I'm guessing he would have run into it and written about it, but... maybe not.

You could even deliberately add some leads to the smd resistor to see if you get some better range out of it. I wouldn't go overboard but maybe adding a cm or two could have some good effects for increasing the range.

Yes,  I wouldn't be surprised if it led to at least some improvement.
« Last Edit: October 21, 2016, 09:33:45 AM by Felix »

captcha

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Re: What is the smallest antenna that still performs well?
« Reply #27 on: October 20, 2016, 09:52:45 PM »
AAduino..., and I thought it had something to do with monitoring alcohol consumption before clicking the link, hahaha.. ;-)

But yeah, what a nifty idea..

WhiteHare

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Re: What is the smallest antenna that still performs well?
« Reply #28 on: October 20, 2016, 10:45:29 PM »
I'll give you the range in my house, but it wouldn't necessarily translate to yours.  That's why I expressed it as the amount of RSSI impairment, which should be about the same, and which you could interpret for your own environment.

That said, I live in two story, roughly 4,000 sf house made using commonplace wood stick construction.  If I put a regular moteino gateway with quarter wave antenna in a corner of the lower floor and I then put the impaired node (i.e. the one running the dummy load resistive antenna pictured above) in the opposite diagonal corner on the second story (which I assume to be a worst case scenario, or close to it), I do receive packets, but with a lot of losses.  I suspect I could improve or worsen that by how I orientate the node, but that's what I got just from plunking it down without any thought.  Also, those results are from running Felix's generic gateway and node sketches.  If I were to change the firmware to use a lower OTA bitrate and narrower bandwidth, and/or possibly increase the transmit power (I'm forgetting now what Felix chose as the default power Tx level), I'm sure it would do better.  Even as is, though, it's doing far better than an NRF24L01+ module would.  To be honest, I didn't expect it would do anywhere near as well as it is.

[Edit: In terms of receiving packets from the next room through a wall or something like that, it's definitely no challenge for it. ]

I just now re-did the above experiment, but this time using a uSplatch antenna in a vertical orientation.  Wow, what a difference!  This time it looked as though the gateway received every packet the node sent, even in the above "worst case" scenario.  The reason was clear:  received RSSI at the gateway looked to be about 26dB better than with just the SMD resistor antenna.  Because of the uSplatch Felix's demo sketches had plenty of link budget to work within.

Felix

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Re: What is the smallest antenna that still performs well?
« Reply #29 on: October 21, 2016, 09:41:15 AM »
I just now re-did the above experiment, but this time using a uSplatch antenna in a vertical orientation.  Wow, what a difference!  This time it looked as though the gateway received every packet the node sent, even in the above "worst case" scenario.  The reason was clear:  received RSSI at the gateway looked to be about 26dB better than with just the SMD resistor antenna.  Because of the uSplatch Felix's demo sketches had plenty of link budget to work within.
Very nice, I will make a note to buy and try this uSplatch. Was this test at 915mhz?

Even as is, though, it's doing far better than an NRF24L01+ module would.  To be honest, I didn't expect it would do anywhere near as well as it is.
Aww, that's why I love sub-ghz FSK :), it excels where 2.4ghz fails.