Author Topic: Best external antenna/cable for Moteino?  (Read 3746 times)

d00m178

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Best external antenna/cable for Moteino?
« on: November 04, 2016, 06:17:00 AM »
Hello,

I'm looking for best variant ext antenna and cable for antenna for my Moteino's and Anarduino's with LoRa 915Mhz devices.
I've used simple TV cable 50Ohm (3m lenght) and such 915Mhz Antenna, 2dBi
http://www.ebay.com/itm/915Mhz-Antenna-2dBi-/171877270861?hash=item2804af054d:g:RvMAAOSw~gRVvDig

I just solder one end of cable to radio module pins (central wire and GND) and another end (central wire) to antenna.
And seems radio-link works not very stable for both modules.

So I want to check another cable and antennas and need advice which cables and antennas works good in yours examples.

So far I wonder if I can use antennas for WiFI (2.4Ghz) or GSM/3G (I suppose more that 1000 Mhz) ?
Or it possible to use only antennas for 915Mhz ?
Also which cable/connector will be the best solution in terms of the least signal loss ? Length of cable need to be 3m.

Thank you.

captcha

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Re: Best external antenna/cable for Moteino?
« Reply #1 on: November 04, 2016, 05:40:41 PM »
The link to the 'antenna' in the ebay page you mentioned is known as a helix antenna and it's only one half of what you need; it is not a complete antenna and I'm not surprised if you have poor performance if you just solder that to one end of a coax cable. What you need for that antenna to work is a ground plane (aka counter poise) and is somewhat provided by soldering the the helix immediately on the antenna port of the PCB radio module. The GND tracks of the circuit board provide a poor, but often sufficient ground plane.

Antennas also have a property called Resonance. That is, they work specifically well around the resonant frequency but can be very poor performers if you go too far away from the design frequency of the antenna. I would therefore not recommend to use a 2.4GHz antenna for a 915MHz radio. Some GSM/3G antennas could have a working range that covers your 915MHz radio, but I would only use these for receive and not for for transmitting. Not even transmitting an ACK packet.

When you say "best variant" and "works good" you may need to specify which area is most important to you, as 'best' for one is not 'best' for all.

- least losses
- furthest range
- smallest antenna
- most immune to interference
- cheapest

TV cable is usually 75 ohms characteristic impedance, but if you got 50 ohms, see if you can find a datasheet for it and check the losses per meter/foot at 915MHz. If you want least signal loss RG-58 is not the answer.

The length of your cable can be chosen arbitrarily if your antenna has a perfect 50 ohms feedpoint impedance, matching the radio and the feedline. If your antenna is not 50 ohms then you may want to calculate the length of cable by taking into account velocity factor and increments of half-wave electrical lengths.

For connectors I would stay away from PL-259, but N-type, BNC and SMA are all 50 ohms at UHF. The N-types are just too big and bulky for me. I have used some SMA connectors and they are nice and small, fairly robust and not too expensive. I like BNC too because it attaches with only a small twist and any strain from misalignments in the two connecting sections can be easily taken out because the cables still allow for rotating while mated.

d00m178

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Re: Best external antenna/cable for Moteino?
« Reply #2 on: November 05, 2016, 07:40:02 AM »
captcha

Wow! Thank you for a lot of so useful info!
It seems I understand were I was wrong.

Some details - in my project I definitely need about 3m coax cable for antenna, because box with my device (based on Moteino) located under snow in alpine zone.
So about helix antenna - if I solder "ground plane" from, for example, piece of textolite board, to another end of coax cable (which is doesn't soldered now) - antenna should work better?
I just googled "helix antenna 915Mhz" and didn't find anything about my guessing about "ground plane".
It seems this little spring designed to work without "ground plane", and as you said - it should be soldered to PCB radio module ((

There are my requirements to the antenna - it should be not large, it should has least loses and furthest range as possible for cheap or simple DIY antenna.
Interference is not big issue in my situation - In alpine zone not so much radio transmitters.

About cable - mostly my local stores has many types of RG-58 and RG-174 - both with 50 Ohm.
Also there is a bit expensive RG-213.
I think 3m length of cable is not so much to worry for losses of signal?

Also I agree about SMA connectors, but I wonder - is the BNC also good for 915MHz (I remember that I used BNC for old computer local network) ?

So the main question which antenna should I use and if it possible fix my current helix sping..

perky

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Re: Best external antenna/cable for Moteino?
« Reply #3 on: November 05, 2016, 08:29:26 AM »
I think attenuation in the cable over 3m would be significant. If you look at the list below you'll see RG174 has an attenuation at close to the frequencies your using of 23.6dB/30m, or around 2.36dB per 3m. That could reduce your range to about 3/4 of what you could have had. Bear in mind a reduction of 6dB halves your transmission distance. Choose a cable with low loss from this list that fits your budget:

https://en.wikipedia.org/wiki/Coaxial_cable#Standards

Also, a 1/4 wave monopole (i.e. one that requires a ground plane like your helical antenna) has very little radiation pattern below the ground plane level (it tends to direct the radiation slightly upwards) so if you're at altitude and want to receive from a lower altitude you'd be better off with a dipole that has a more favourable pattern.
Mark.
Edit: Actually if your device is on a slope and you want to transmit to a receiver further down the slope you might even be a able to mount your 1/4 wave monopole upside down with the ground plane at the top.
« Last Edit: November 05, 2016, 08:39:45 AM by perky »

d00m178

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Re: Best external antenna/cable for Moteino?
« Reply #4 on: November 05, 2016, 09:12:37 AM »
Thank you..
So I definitely need change my coax cable and change antenna to dipole..
Would you recommend where to buy dipole antennas for LORa 915Mhz ?
Or DIY manual for creating it?

captcha

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Re: Best external antenna/cable for Moteino?
« Reply #5 on: November 05, 2016, 05:27:20 PM »
For an antenna ground plane to work efficiently with a vertical (monopole) antenna, the ground plane must be good at reflecting electromagnetic waves.

Quote from: d00m178
if I solder "ground plane" from, for example, piece of textolite board, to another end of coax cable (which is doesn't soldered now) - antenna should work better?

I'm not familiar with textolite but it seems it doesn't always have a metallic surface?

Metallic surfaces are best (copper, aluminium, steel, iron, etc..). Salt water is surprisingly good for low frequency waves, but I don't think it works well for near-GHz frequencies. Plain, rocky ground (or snow) is very poor at reflecting waves, especially at near-GHz frequencies, so your best bet is to provide an artificial ground plane in the form of a copper, steel or aluminium plate right where the antenna starts:


quarter-wave vertical with a round, copper-clad PCB as the ground plane. You could also solder your helical onto the inner-conductor of the coax instead of going straight up, but I think a straight vertical will have a slightly better range.



You could also replace the ground plane 'sheet' with 4 ground radials that are about 8.1cm long each (for 915MHz). Something like this:


When these radials are bent down about 45 degrees as shown in the picture you will also have a better impedance match with your coax feedline and the radio will be able to output power more efficiently. With better matching, the length of the cable also doesn't so much anymore (but keep in mind the losses purely due to the length of the cable).

Quote from: perky
mount your 1/4 wave monopole upside down with the ground plane at the top

Interesting.. Is this purely because the slope (falling ground) messes less with the radiation pattern on that side? I know it doesn't work if the ground is level.



Quote from: d00m178
but I wonder - is the BNC also good for 915MHz (I remember that I used BNC for old computer local network) ?

Hah, that shows your age a little. Yes I used them too in networking, many moons ago..  :)  BNC is fine for up to 4GHz, so don't worry about that.


Quote from: d00m178
Would you recommend where to buy dipole antennas for LORa 915Mhz ? Or DIY manual for creating it?

If the antenna is going to be mounted outside in the harsh environments of cold, rain, snow and sun I would definitely go for something rugged. Google 'sleeve dipole' antenna as it doesn't require a ground plane and can be mounted all vertically with no bits sticking out. Also google 'radiation pattern' for the antenna you are interested in so you have an idea where you can get the best reception.

Btw, what sort of range are you hoping to get with these radios? How many radios? All at the same level/altitude? Knowing some of these factors will help members on the forum assist you with the best possible antenna setup for what you're trying to achieve.

« Last Edit: November 05, 2016, 05:37:32 PM by captcha »

d00m178

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Re: Best external antenna/cable for Moteino?
« Reply #6 on: November 05, 2016, 07:16:17 PM »
Thanks again for such useful information.
Now I just need to check every variants.

Some details about my project - this is a "snow pole" - pole with 10 or more thermosensors (d18b20) placed with some step (20 cm usually) and this pole installed vertically on some slopes in mountains.
When snow covers these sensors we can monitor temperature in particular snow layer. This information needed for better monitoring and analyzing avalanches situations in mountains.
Because of cold conditions I have to place box with MC and batteries on ground level which has temperature about 0 Celsius during all winter. It helps to save batteries life.
But in this case I have to put antenna on top of pole - it is about 3m of cable..
Considering that places for installation depends on many factors - steepness or exposition of slope, etc, so these places can be on different altitudes - I need antenna setup which can cover maximum range - sometimes it can be difficult to get to the place where I can receive data from snowpole. 
So far the best result is about 1 km with direct line of sight.
But sometimes, when it is snowfall or rain or another weather issues in mountains - it dificult to receive data from some snowpoles.
So because of this I want to improve this part of device - antenna and cable.

WhiteHare

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Re: Best external antenna/cable for Moteino?
« Reply #7 on: November 05, 2016, 08:28:04 PM »
Thanks for your wonderful post, Captcha.  You are a veritable fountain of great information!

I realize the orientation is different than in your examples, but could the metal band that encircles a  solar garden light (photo attached) function as a ground plane if one soldered GND to it?  Perhaps the antenna could stick up through the center.  For instance, one might drill a hole straight through a PV cell and maybe it still might work....  These solar pucks have the virtue of being very inexpensive (just $1 retail), and perhaps could serve as both power source and enclosure as well.  If it could establish a ground plane too, then it would be a real bonanza.

« Last Edit: November 05, 2016, 08:33:44 PM by WhiteHare »

WhiteHare

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Re: Best external antenna/cable for Moteino?
« Reply #8 on: November 05, 2016, 08:49:03 PM »
By the way, if it were instead to be an indoor dipole, then as noted on a different thread (https://lowpowerlab.com/forum/projects/(update)-rfm69-library-for-esp8266-moteino-compatible!/msg13658/#msg13658), just two pieces of wire (one connected to ANT and the other connected to GND) seems to make for a surprisingly effective dipole antenna on the RFM69.  I presume the same would be true for the LoRa.
« Last Edit: November 05, 2016, 09:00:04 PM by WhiteHare »

captcha

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Re: Best external antenna/cable for Moteino?
« Reply #9 on: November 05, 2016, 10:25:07 PM »
Bonanzas galore!  :) I love those garden lights, they're about $2 here in Australia. Same same, excellent project box.

Quote from: WhiteHare
solar garden light (...)function as a ground plane if one soldered GND to it?

I think so. It would also do much better on 915MHz than on 433MHz. I am stuck with favor 433 because of my ham licence but am considering putting some projects in those solar lights.

The only issue is that many of my boards run at 3.3V and the PV cell in those garden lights is only 1.5 Volts or so; They're meant to charge a single (slightly shorter than AAA) battery.

(and thanks for the kind words, happy to help)

@d00m178: That sounds like an awesome project! Love the practicality of it. Those DS18B20 work well over longer distances btw. I'm running several sensors around my home for measuring temperatures in the kitchen, the roofspace and outside. They're all connected on the same bus and the furthest sensor would be about 10m away. Maybe to minimise transmission loss in the feedline, could you mount the moteino right underneath the antenna and have the battery connected with a 3m long wire on the ground? Hmm.. not sure how well electronics will work at -15C (5F) but that may be worth an experiment all by itself.

Quote from: @d00m178
So far the best result is about 1 km with direct line of sight.

With a 20dBm Moteino that is on the short side. With line of sight you should be able to get 2-3km easily. I would indeed suspect cabling and antenna.

It does sound like you have some challenging environmental factors to deal with. Freezing temperatures doesn't influence propagation much, but not so sure about snow on an antenna. A sleeve dipole (painted black) (heated?) may be required to prevent ice and snow from building up around it.

Are you setting up a star-topology where all nodes are around one central node? In that case you may want to consider a directional antenna setup for the outer nodes and a good omnidirectional antenna for the central node. Even just a 2-element yagi  (dipole with one reflector element behind it) can significantly improve directionality and could mean the difference between getting a signal or not.

Elsewhere on this forum you may also find information on lowering the transmission data rate. The slower the transmission speed, the easier it is for the receiver to detect a one or a zero. For low throughput sensor data such as temperatures I'd say it would be well worth investigating.



WhiteHare

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Re: Best external antenna/cable for Moteino?
« Reply #10 on: November 06, 2016, 06:27:49 AM »


The only issue is that many of my boards run at 3.3V and the PV cell in those garden lights is only 1.5 Volts or so;


Exactly.  Do you have any suggestions for a suitable boost converter?  For instance, TI's BQ25504 has awesome specs (http://www.ti.com/product/bq25504#descriptions), but it doesn't look very simple to use. 

[Edit:  The LTC3105 looks like it's easier to use and the attached chart looks promising as far as generating good 3.3v currents from low input voltages.]


« Last Edit: November 06, 2016, 12:54:27 PM by WhiteHare »

captcha

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Re: Best external antenna/cable for Moteino?
« Reply #11 on: November 06, 2016, 04:07:47 PM »
I have used the LTC3525 in one my datalogger projects (2x AA batteries providing power to an mcu and 2x16 LCD running at 5V). It only requires three external components (two caps and an inductor) and works very well.

I'll give the LTC3525 10 points for simplicity and effectiveness, but they are a bit pricey. Haven't looked at anything else yet as I'm looking more at doing things with solar powering 1S LiPos and then regulating it down to 3.3V.

If I were to start using the garden lights I would probably look at putting a bigger battery in it, rather than boosting it up.

perky

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Re: Best external antenna/cable for Moteino?
« Reply #12 on: November 06, 2016, 05:01:56 PM »
As a general point, one thing you have to be mindful of is startup current with boost regulators. Some have an enormous current spike at the beginning as they fire up, and as batteries deplete their internal resistance goes up. Ultimately this means the voltage during that spike can drop to below the working voltage of your circuit, so your battery life might be compromised.
Mark. 

WhiteHare

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Re: Best external antenna/cable for Moteino?
« Reply #13 on: November 06, 2016, 08:49:45 PM »
I have used the LTC3525 in one my datalogger projects (2x AA batteries providing power to an mcu and 2x16 LCD running at 5V). It only requires three external components (two caps and an inductor) and works very well.

I'll give the LTC3525 10 points for simplicity and effectiveness, but they are a bit pricey. Haven't looked at anything else yet as I'm looking more at doing things with solar powering 1S LiPos and then regulating it down to 3.3V.

If I were to start using the garden lights I would probably look at putting a bigger battery in it, rather than boosting it up.

You may already know this, but if not:  if you go with LiFePo4, you don't have to worry about stepping down the voltage because that battery chemistry's  maximum voltage is 3.6v.

captcha

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Re: Best external antenna/cable for Moteino?
« Reply #14 on: November 06, 2016, 10:41:41 PM »
Haven't looked into Lifepo's yet but they seem very appealing.. Thanks for the tip! :-)

Humancell

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Re: Best external antenna/cable for Moteino?
« Reply #15 on: November 14, 2016, 08:18:41 PM »
I'm curious ... do you think these helical antennas would work well if put directly onto the Moteino PCB?

I've been looking for a way to bend or "coil" the 80mm wire antenna in a more optimal way to fit into an enclosure ... I had not yet run into these.  Are these supposedto be coiled as some optimal size?


The link to the 'antenna' in the ebay page you mentioned is known as a helix antenna and it's only one half of what you need; it is not a complete antenna and I'm not surprised if you have poor performance if you just solder that to one end of a coax cable. What you need for that antenna to work is a ground plane (aka counter poise) and is somewhat provided by soldering the the helix immediately on the antenna port of the PCB radio module. The GND tracks of the circuit board provide a poor, but often sufficient ground plane.

Antennas also have a property called Resonance. That is, they work specifically well around the resonant frequency but can be very poor performers if you go too far away from the design frequency of the antenna. I would therefore not recommend to use a 2.4GHz antenna for a 915MHz radio. Some GSM/3G antennas could have a working range that covers your 915MHz radio, but I would only use these for receive and not for for transmitting. Not even transmitting an ACK packet.

When you say "best variant" and "works good" you may need to specify which area is most important to you, as 'best' for one is not 'best' for all.

- least losses
- furthest range
- smallest antenna
- most immune to interference
- cheapest

TV cable is usually 75 ohms characteristic impedance, but if you got 50 ohms, see if you can find a datasheet for it and check the losses per meter/foot at 915MHz. If you want least signal loss RG-58 is not the answer.

The length of your cable can be chosen arbitrarily if your antenna has a perfect 50 ohms feedpoint impedance, matching the radio and the feedline. If your antenna is not 50 ohms then you may want to calculate the length of cable by taking into account velocity factor and increments of half-wave electrical lengths.

For connectors I would stay away from PL-259, but N-type, BNC and SMA are all 50 ohms at UHF. The N-types are just too big and bulky for me. I have used some SMA connectors and they are nice and small, fairly robust and not too expensive. I like BNC too because it attaches with only a small twist and any strain from misalignments in the two connecting sections can be easily taken out because the cables still allow for rotating while mated.

captcha

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Re: Best external antenna/cable for Moteino?
« Reply #16 on: November 15, 2016, 12:30:51 AM »
Without hooking these helicals up to a vector network analyzer it is anybody's guess whether these things are properly made and tuned for the advertised frequency. However, since many people report reasonable levels of success with all sorts of antenna's it may be worth a try.

Quote from: Humancell
put directly onto the Moteino PCB

That would be my preferred placement for the helical.

Quote from: Humancell
Are these supposedto be coiled as some optimal size?

Very much so. I would not suggest to further bend or stretch or otherwise manipulate the shape of the antenna and only use the antenna for the designated frequency.

Just out of curiousity, what range are you hoping to achieve for your project? Are you very limited to the enclosure size?


perky

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Re: Best external antenna/cable for Moteino?
« Reply #17 on: November 15, 2016, 07:21:19 PM »
Speaking in very simple terms, reducing the length of an antenna decreases capacitance, so to make it resonate at the same frequency as a dipole of 1/4 wave monopole you have to add inductance. A helical antenna is basically a coil, get the dimensions of that coil and number of turns right and you can add just the right amount of inductance to resonate at the same frequency as a dipole or 1/4 wave monopole. However shortening the antenna also reduces its radiation. You'll see some compromises that have bottom coils and the rest is straight, but the result is the same, the coils (and the helix) add inductance.
Mark.