Village Telco » Village Telco

Serval Arkaroola Demo

drowe — Sun, 11 Jul 2010 00:47:04 +0000

Yesterday I was a guest of the Serval team on an exciting field deployment in the Northern Flinders Ranges. It was a very busy day, starting at 4:30am and flying out at first light from Adelaide 700km north to Arkaroola in a delightful Pilatus PC12.

Pilatus PC12 at Aldinga Airport

Flying to Arkaroola

One hour and 20 minutes later we were guests of the Arkaroola Sanctuary who delivered us via bus, 4WD and helicopter to various locations for trials of the Serval technology. The Arkaroola people were very excited and very helpful. One thing I love about the Village Telco is the good will it generates. Once the project is explained people go out of their way to help you. Projects with social rather than commercial aims are so much more satisfying to work on.

The silence at Arkaroola is amazing. Remote and sparsely populated wilderness. Nice terrain for Wifi too – lots of hills and no trees.

Helicopter from Sillers Lookout

Serval is a system for rapid deployment of phone networks in disaster relief scenarios. Serval uses Village Telco technology (mesh telephone networks) combined with several new innovations. One is Distributed Numbering Architecture (DNA), a way to assign existing cell phone numbers to your phone on the mesh network. The Serval team has hacked Android mobile phones so they work as mesh telephony devices. This involved porting Batman and Asterisk to Android. Very impressive and very cool.

Dany Testing at Flinders University

The result (so far) is a cool numbering scheme where you can assign phone numbers in the field without a server and the worlds 2nd mesh telephony devices (and first mobile mesh telephony devices). The “Serval Batphones” integrate seamlessly with Mesh Potatoes. The DNA software can also run on Mesh Potatoes. Future work will involve using OpenBTS to integrate GSM connectivity.

The Serval team is lead by Dr. Paul Gardner-Stephen from Flinders University. Paul, Romana Challans and Dany Rakotopara (a talented French exchange student) have been working crazy hours to get everything coded, integrated, and to organise the amazing 1-day field trip we had yesterday.

Paul and Dany developing Serval

Serval is partially supported by Flinders University (thanks for funding this demo trip) with large chunks of volunteer effort by the Serval team. The story was exciting enough to attract a local ABC TV crew to join us. I was really impressed with the way Paul, Romana, and the University media department coordinated the media side. Serval and the Village Telco is a great story. This is a point I often miss. I freely admit I am “too much of an engineer” and don’t appreciate the strong media value of our project and the leverage it can provide.

The demos involved three scenarios for rapid deployment of telephone networks; (i) disaster relief, (ii) search and rescue, and (iii) deploying networks in difficult terrain.

I learnt a lot yesterday by talking to Paul and the team:

Wifi and Android penetration in handsets is growing rapidly. This means that in a few years all phones could have mesh telephony built in. Every cell phone could be a “Batphone”. Mesh Potatoes in fixed locations can be deployed quickly in strategic locations to relay the phone calls and blanket areas with a mesh network. You can then use your existing phone handset to make calls on an ad-hoc Wifi mesh network.

Serval Bat Phone at Arkaroola

This is a first world twist on the developing world focus of the Village Telco. In the developing world the Mesh Potato will be the telephony device and it’s low cost is an important factor. However in the first world everyone already has a handset. So it makes sense to use them in a disaster relief scenario.

Key points here are high mobile phone handset penetration (everyone carries one) and the low cost of mesh Wifi networks. The demos yesterday used less than $2000 worth of equipment (Android phones from eBay and Mesh Potatoes) to form a phone network covering 1 square km.

Eye of the Potato: A Mesh Potato overlooking Arkaroola Village

I find it very satisfying that Village Telco technology can be quickly combined with other technologies like Serval. Try forking GSM and you will see what I mean. Accelerated, unencumbered development using Village Telco tech is possible because it is is fully open and uses unlicensed spectrum. Derivative projects like Serval are just the beginning. As I flitted over the beautiful arid landscape in a small helicopter the thought “What hath Steve wrought” (apologies to Samuel Morse and Steve Song) flashed through my mind.

Helicopter at Sillers Lookout

Testing in the bush

Bus to Arkaroola Village

Romana being interviewed for the Serval documentary

Dany enjoying the helicopter

Paul and The ABC TV crew

Potato Tree - 4AH Lipo battery is a great idea

4WD trip to the Observatory

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Using the Voice Interface to Tune MP WiFi Connection

Steve — Wed, 23 Jun 2010 19:25:04 +0000

One of the challenges in making the Mesh Potato as dead simple to use as possible is the fact that, unlike many modern Internet devices, it has no GUI. Sure, you can plug a laptop into the ethernet port or connect via WiFi to a web interface but the Mesh Potato on its own has no GUI, which presents certain challenges in designing a drop dead easy-to-use device.

However, constraint can be both a barrier and an enabler of innovation and this is certainly true of the Mesh Potato. As it turns out, we are only just beginning to discover the potential of the voice interface to the Mesh Potato to improve user experience. Here is a great example.

The first thing you want to do when you power up a Mesh Potato is make sure it has a good connection to its peers. This is easy if you have a GUI or even a command line but what if you just have a telephone handset? David Rowe developed a script for his deployment of the Dili Village Telco that continuously polled the quality of the mesh link on the MP. Elektra has taken that one further and integrated the script into a voice interface that give continuous audio feedback on link quality making it easy to tune your WiFi connection with a simple handset. Here’s a brief video of me testing out this feature.

Tuning Mesh Potato WiFi Performance with a Voice User Interface from Steve Song on Vimeo.

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Mesh Potato Firmware Recovery

drowe — Fri, 04 Jun 2010 00:51:06 +0000

Paul Gardner-Stephen had one of his MP01s “brick up” when attempting a GUI-based reflash and was having problems recovering. So I offered to take a look at it for him.

Reflashing and recovery can be intimidating the first time around so I thought a short blog post might be useful to explain the steps I took to bring his potato back to life. I also hit a FXS driver/hardware problem. The steps I took show how to debug the FXS part of your potato.

Paul had made several attempts at reflashing and was concerned that he had zapped the boot loader. So to check the boot loader I took the optional (and usually unnecessary) step of connecting a RS232 daughter board to his MP01:

Mesh Potato Serial Port Access

Due to the shape of the RS232 daughter board I had to remove the MP01 PCB from it’s metal box. The RS232 daughter boards are the same as the IP04 so I have many laying around, however they don’t ship as standard with Mesh Potatoes (let me know if you ever need one). The serial settings are 9600 baud, 8 data bits, 1 stop bit, no parity. Anyway I applied power and after a few seconds I saw the expected boot loader output:

+Ethernet eth0: MAC address 00:09:45:57:a8:89 IP: 192.168.1.124/255.255.255.0, Gateway: 192.168.1.1 Default server: 192.168.1.180


RedBoot(tm) bootstrap and debug environment [ROMRAM]

Non-certified release, version v1.3.0 - built 14:07:19, Jun  9 2009
Copyright (C) 2000, 2001, 2002, 2003, 2004 Red Hat, Inc.

Board: ap61 RAM: 0x80000000-0x81000000, [0x8003dd50-0x80fe1000] available FLASH: 0xa8000000 - 0xa87e0000, 128 blocks of 0x00010000 bytes each. == Executing boot script in 5.000 seconds - enter ^C to abort RedBoot> fis load -l vmlinux.bin.l7

The last line is the boot loader loading the Linux kernel. However Linux wasn’t coming up, probably due to a dud image. So the next step was to try reflashing. My favourite reflash method is documented here on the Dili Village Telco Wiki. It has also been discussed at length on the Village Telco Google Group. There are actually several ways to flash a potato but this way is my favourite and has proven very simple and reliable in practice. It’s a very good idea to practice reflashing before you actually need it, as an exercise.

After reflashing Paul’s MP came right up. I telneted 192.168.1.20 and could see Asterisk running (using ps x) however the phone was dead. No evidence of power to the phone and no dial tone. I used “dmesg” to check what the FXS drivers where doing:

type=16550A serial8250: ttyS0 at MMIO 0xb1100003 (irq = 37) is a 16550A mask: 0xbd CR: 0xbd INT: 0xc6 mp: checking reg0 of 3215: mp: reg0.....: 0x0 mp: part number: 0x0 mp: revision...: 0x0 Registered mp char driver on major 34

Hmmm, these zeroes at “reg0″ and “part number” don’t look good. This indicates the FXS driver can’t read registers on the FXS module. After poking around for a while I discovered the J1 jumper was not installed. This jumper connects the clock to the FXS module. I replaced this jumper and rebooted and the driver output looked better:

type=16550A serial8250: ttyS0 at MMIO 0xb1100003 (irq = 37) is a 16550A mask: 0xbd CR: 0xbd INT: 0xc6 mp: checking reg0 of 3215: mp: reg0.....: 0x3 mp: part number: 0x0 mp: revision...: 0x0 ProSLIC module is Si3215 Start manual calibration Module 0: Installed -- AUTO FXS Registered mp char driver on major 34

This is what you should see for a working FXS driver. The phone was now powered up, and dial tone was present. Hmmm, might be a good idea to print an obvious error message when the wrong register values are found!

To configure a MP01 I actually find it easier to telnet 192.168.1.20 and use vi to edit /etc/config/wireless and /etc/config/network, then run /etc/init.d/network/restart. But many people like the GUI, it’s your choice. I changed the Wifi Channel and BSSID to match my local mesh network and made a few test phones calls. Looks good, so I will send the MP01 back to Paul.

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Mesh Potato Production Update

Steve — Thu, 03 Jun 2010 07:16:35 +0000

We’re getting closer and closer to full production of the Mesh Potato. Below you see the 1.3 version of the Mesh Potato motherboard that integrates the FXS module onto the PCB and also includes an integrated antenna on the PCB as opposed to the external antenna for the first Mesh Potato prototypes. We chose to integrate the antenna both to save production costs but also to improve the weather-proofing of the Mesh Potato; one less joint exposed to the elements.

David Rowe has written a post describing some debugging work with the FXS module in the 1.3 design. His description of his debugging thought process is fascinating to read. If Sherlock Holmes existed today, he would probably be a hardware engineer.

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Smoke testing the Mesh-Potato video – Part II

elektra — Fri, 19 Feb 2010 18:00:58 +0000

We have now a new over-voltage protection circuit design for the mass production Mesh-Potatos. We were not satisfied with the previous version. Ideally the over-voltage protection circuit has a snap-on characteristic that triggers the fuse and interrupts the supply voltage without a grey zone. The new circuit triggers at 43 Volts and acts as a powerful crowbar circuit. I have connected the prototype of this circuit to a Mesh-Potato and went through the robustness tests according to our test plan. You can find the test plan and the schematic in the svn respository. A little video at Vimeo.com is documenting some of the tests – thanks to Katrin Lang, who acted as editor and camera operator this time.

Mesh-Potato smoke testing with 230 Volt AC from Elektra Berlin on Vimeo.

The list of tests included a reverse DC voltage test from a unfused 36 Volt source (consisting of three powerful 12 V lead acid batteries in series), excessive DC voltage tests and finally a really scary test involving 230 Volt AC (330 Volt peak) from mains. Please don’t try this at home. Even if the Mesh-Potato survives, there is a 50% chance that you might have mains potential on ground of all components connected to the MP. Also don’t try this with your alpha or beta series Mesh-Potatos, because the previous over-voltage protection circuit is not up to that challenge.

Technically inexperienced people can easily make mistakes, so our idea was to design the Mesh-Potato as robust as possible. In 2005 I was helping to set up a large scale WLAN network in the Sylhet area in Bangladesh. The network consisted of high towers (up to 100 feet tall) and strong directional antennas, interconnecting towns and a school with wireless long shots (up to 32km). One of the trainees damaged a important wireless relay on a tower by taking the open ends of a 12 Volt cable and plugging it straight into the mains socket. Of course the equipment (a Mesh-Cube from 4G Systems) subsequently looked like a lightning strike had hit it, which was actually what I supposed first. However there had been no thunderstorm in the night before. It took me a while to find the reason. The trainee either hadn’t realized what he had done, or he didn’t want to admit it. He watched me trying to find the problem without saying anything. It is common practice in Bangladesh to plug cables into sockets without plugs. The quality of sockets and plugs is miserable, so loose contacts are the rule, not the exception. Now a important relay was down and it was hard to get a replacement. The problem wasn’t so much the financial loss. Shipping and particularly customs can take weeks in Bangladesh. So during the first Villagetelco workshop I suggested to design the MP as robust as possible.

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Village Telco at linux.conf.au (LCA) 2010

drowe — Sat, 30 Jan 2010 20:36:12 +0000

I have just returned from LCA 2010 where I presented on the Village Telco and manned a Village Telco booth on the Open Day. Very good response and lots of people would like Mesh Potatoes! Check out my LCA 2010 blog post for more.

Mesh Potatoes at the LCA 2010 Open Day

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Potato on the Jetty – Range testing PCB Antennas

drowe — Thu, 14 Jan 2010 04:42:47 +0000

Today we made some phone calls over a 400m link using PCB antennas.

We want to use etched PCB Wifi antennas for the Mesh Potato. However we have heard that some companies have had problems with PCB antennas, such as variable results in production. So before committing to PCB antenna we wanted to do some more tests.

Joel is a local hacker here in Adelaide who happens to live at the top of a 4 story apartment block in a beach side suburb called Henley Beach. This gives him good line of sight to points on the ground several hundred meters away. Much easier than testing at my place which is on dead flat terrain and requires masts for any Wifi range testing.

Before heading out to Joel’s place I tried some tests in my backyard. I set up two MP01s about 10m apart. First I connected conventional sleeve dipole (rubber ducky) antennas and measured the signal strength. I then connected a couple of PCB antennas and repeated. To monitor signal levels I wrote a simple script to dump the Received Signal Strength Indication (RSSI) levels (in dBm) from MadWifi:
#!/bin/sh

while [ 1 ]
do
wlanconfig ath0 list | grep 56:ac:90 | awk ‘{ print $6 }’
sleep 1
done

The grep filters out the last few MAC digits of the MP we are interested in, otherwise you get RSSI measurements of all nearby Wifi devices.

However the results were inconclusive and after an hour I became frustrated:

On a good test both MPs would receive about -30dBm, however if I moved a MP 20mm a level could drop to -44dBm. Lots of multipath in my back yard!
One MP was consistently around -30dBm, whereas the other would move between -30 and -48dBm. Maybe they had different diversity antenna settings. Or maybe the RSSI measurements can’t be trusted.

I had much better (and less frustrating) results using the spectrum analyser to measure signal strengths, as explained in the previous post on PCB Wifi antennas.

In the end I figured it was sufficient to test from a system level rather than attempt more measurements of antenna performance and signal strength. The $64 dollar question is can we make phone calls over a reasonable distance with these antennas?

On the Beach

We placed one MP01 on Joel’s balcony. I then walked down to the end of Henley Jetty (Joel has previously managed to pick up his home Wifi there). There is (just) a line of site between the two points – through a gap in a couple of buildings. The distance was 375m. We used two calibrated V1.2 MP01s (same design as the Beta units). An “iwlist ath0 scan” showed 9 other Wifi networks in operation.

Google Map of Range Test - 375m

One MP01 on Joel's Balcony

I tried a bunch of antenna combinations, starting with regular sleeve dipole (rubber ducky) antennas then moving to PCB antennas first at one end, then both ends of the link. From our previous tests both the sleeve dipole and the PCB monopole antennas have an estimated gain of 2dBi.

We had a total of 4 PCB antenna samples (a mixture of the 17mm and 20mm monopoles) as we wanted to get a feel for performance spread over multiple antennas of the same design. Here are the results:

MP01 #36	MP01 #38	Call Quality
Sleeve Dipole	Sleeve Dipole	Great
Sleeve Dipole	#3 17mm PCB monopole	Great
Sleeve Dipole	#2 20mm PCB monopole	Great
Sleeve Dipole	#4 20mm PCB monopole	Great
#1 17mm PCB monopole	#4 20mm PCB monopole	Great
no antenna	#4 20mm PCB monopole	No Call

The final test was just to make sure we weren’t kidding ourselves.

MP01 on the end of Henley Jetty

At one stage a fisherman assisted in my propagation experiments by standing right in my line of site and lowering a metal net as I talked to Joel!

For each call I tried moving the MP01 (with PCB antenna attached) around. For example upside down, side to side, rotated it. No break up of signal, good audio in both directions. Only problem was wind noise. Joel suggested we add digital noise suppression but I figure there isn’t much wind noise inside village homes!

Here is the view of the far end from either side of the link. The arrow shows the location of the other end.

View of Jetty from Joel's Balcony

View of Joel's Balcony from Jetty That's him with the brown eyes.

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First Beta Mesh Potato

drowe — Wed, 13 Jan 2010 08:08:20 +0000

Joel is helping my Village Telco presentation at the upcoming LCA 2010 conference so I took a couple of the first Beta MP01s down to his place for a test drive. They are fresh off the production line and not even calibrated as I needed them in a hurry for LCA 2010. Atcom are currently assembling, testing and calibrating the Betas which will be shipping over the next few weeks.

The first Beta Mesh Potato

These early Betas are running revision 203 firmware, which has Afrimesh as the primary GUI with Luci available for “advanced” configuration options. Thanks Elektra and Antoine for your fine work on the Mesh Potato GUI.

Here is Joel’s experience – our #1 Beta MP01 tester! Joel is pretty geeky (he has contributed for several years to the OLPC project) but has never used a Mesh Potato before. I hope this information will be useful for other Beta testers who should be getting their Mesh Potatoes over the next few weeks. I stood back through most of Joel’s beta experience – I wanted to see how other people approach the Mesh Potato.

How Joel Configured His Mesh Potato

The default Ethernet IP is 192.168.1.20. Joel plugged a cross over Ethernet cable into the MP01, although a regular cable should be OK.
He then logged in via telnet, and set the password from the command line using the “passwd” command. Setting the password activated ssh and disabled telnet. This password also becomes the web admin password.
Joel then pointed his browser at 192.168.1.20 and the Web GUI came up.
He changed the 10.130.1.20 “IP Address” to a unique IP on the mesh. Joel selected 10.130.1.123. This is the only change you need to start making phone calls between Mesh Potatoes. This IP address becomes your phone number, for example dialing 123 on the mesh will make Joel’s phone ring. Note this only changes the mesh Wifi IP – the Ethernet IP is still 192.168.1.20. The Ethernet IP can be changed via the Luci Network Menu.
Setting the Wifi IP on the Mesh Potato

Note the settings are applied automatically – there is no “save” button, although I understand that this will be present on later firmware as out of habit we all look for a save button! To apply the settings (i.e. change the actual Wifi settings) we rebooted (power cycled) our MP01.

We also changed the Wifi channel and BSSID to match the mesh settings of a couple of earlier Potatoes. This is not really necessary, the default Wifi channel and BSSID are probably OK for you.

That’s it – the main step is just change the mesh Wifi IP to something other than 10.130.20. Then reboot and you can start making calls between Mesh potatoes.

Easier Configuration

A slightly easier way is:

Point your web browser at 192.168.1.20, login with user/password root/admin.
Simply change the Wifi IP from 10.130.1.x to 10.130.1.yourchoice.
Power cycle your potato.

Configuration without a Web browser

Power up your Potato and connect a phone, after about 1 minute you will get dial tone.
Dial CONF (2663) and you will hear HAL 9000 talking to you!
Enter the new Wifi IP, e.g. 10*130*1*123

Joel asked a good question – what do all the LEDs mean? They are not labeled although from memory it’s power/Ethernet/Wifi activity etc. I asked Edwin@Atcom about this and the reason was the vinyl labels hadn’t arrived when the first few betas shipped. The rest of the Betas will have these labels for the LEDs.

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Simplified Billing Management

aep — Mon, 28 Dec 2009 12:29:47 +0000

Following the release of A2Billing 1.4.4 stable the 14th of December, we have released the Simplified Management Interface for the release. The Installation Wizard and the Simplified Management Interface and part of a larger effort to simplify pre-paid billing for the Village Telco.

Code for the simplified wizard to be applied over the current A2Billing Administration GUI is available here:

http://www.it46.se/svn/villagetelco/a2billing/gui/release/

Enjoy the winter solstice. From the land of -10 C Happy Yule-tide! http://en.wikipedia.org/wiki/Yule

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Antenna Testing

drowe — Mon, 21 Dec 2009 20:28:56 +0000

Jeff and I have just had an enjoyable day outside testing candidate antennas for the Mesh Potato. Our goal was to evaluate candidates for the internal antenna of the production Mesh Potato.

Jeff designed three types of antennas which I laid out on PCB and had fabricated locally. The three designs were a dipole, a monopole, and a biquad (single loop). We made three versions of each PCB antenna with slightly different dimensions.

I also made a some wire antennas, a monopole, a biquad (dual loop), and a quad (single loop).

Our PCB and wire antennas

PCB biquad design

PCB Monopole Design

Checking the Antenna Impedance

When the PCBs came back the first step was to check the impedance of each antenna. We want roughly 50 ohms impedance to ensure the maximum amount of power is transferred from the Mesh Potato transmitter to the antenna.

A Standing Wave Ratio (SWR) bridge can be used to measure the SWR. I used a version of the design by Erwin Gijzen, a radio Ham and Wifi experimenter. I constructed the SWR head, and measured the DC voltage from the bridge using a multimeter. The bridge compares the impedance of the antennas to a known 50 ohms impedance. If they are equal then the DC output from the bridge should be 0V. Various degrees of mis-match give different output voltages.

SWR head, microwave PCB made with a Dremel tool

I constructed the bridge from double sided PCB and cut the microstrip (3mm wide on 1.6mm thick FR4) with a Dremel tool to save time. When tested it gave sensible results once I fitted a decent microwave detector diode. Unlike Erwin I couldn’t null it down to 0V with a reference 50 ohm load but it did give indicative readings that enabled me to compare our antennas to reference antennas and determine if they had a reasonable match to 50 ohms.

Here are some results:

Load	SWR bridge output (VDC)
50 ohm dummy	0.5
short circuit	1.3
off the shelf router antenna	0.5
17mm PCB monopole	0.5
20mm PCB monopole	0.7
34mm PCB dipole	1.3
64mm PCB biquad dual loop	1.3
68mm PCB biquad dual loop	1.4
72mm PCB biquad dual loop	1.5
wire biquad dual loop	0.8
wire monopole	0.6

The 17mm and 20mm monopoles look good, close to the reference 50 ohm load and commercial off the shelf router antennas (which have sleeve dipole construction internally). The wire antennas also look good. The PCB dipole and PCB biquads don’t look so great.

I tuned the wire monopole to a low SWR by snipping off bits of wire, 0.5mm at a time. I started with a length of 31mm (free space quarter wavelength at 2.4 GHz) but found a good SWR at 26mm. This is probably due to the dielectric constant of the insulation on the wire affecting the wavelength.

Antenna Test Range

I constructed a test range in my back yard, along the lines discussed by Erwin. I used a Nanostation 2 at the transmitter, sending continuous 802.11b broadcast pings as described here. The antenna under test was placed about 6m away and a spectrum analyser used as the receiver. It wasn’t a very good antenna range but after some experimentation we did get surprisingly repeatable results when we compared our antennas to several control antennas.

I fashioned a clamp on a tripod to hold the antennas:

Tripod and clamp

However the tripod and clamp didn’t work very well. When I swapped antennas the results differed wildly in exactly the same position. It’s hard to place a 17mm printed monopole in the same position as a 80cm colinear antenna as their sizes are so different.

So instead I moved each antenna around by hand until I found the peak amplitude, which was captured by the “max hold” function of the spectrum analyser. Sounds a bit rough but gave good repeatable results, and Jeff and I achieved similar results when testing.

Path Loss

The 802.11b signal peaked at about -30dBm on the spec an. Using the Wifi power measurement method described here this means a total received power of -20dBm.

The expected received signal is:

Pr = Tx power + Tx antenna gain – path loss + Rx antenna gain – coax loss

So we plug in the numbers from the Nanostation 2, a 6m path loss and the 8dBi gain Superpass omni reference antenna we get:

Pr = 16 + 12 – 56 + 8 -1 = -19dBm

which is pretty close to what we are measuring using the spectrum analyser. If only all my calculations came out this close!

Antenna Gain Results

We used the 8dBi Superpass as a reference. We would first measure the signals from the Superpass, then save that on the screen as signal A. We would then measure the test antennas and calculate the antenna gain based on the known Superpass gain. We moved each antenna around by hand until a peak was found (the max hold function made this straight forward).

We repeated these tests several times over the day, and while the absolute levels would change 1-2dB the relative levels were always similar.

The antennas are listed in order of gain, and I would estimate the measurements have a tolerance of +/- 1dB. The RF level is the peak of the 802.11b signal on the spectrum analyser.

Antenna	Rx Level (dBm)	Gain (dBi)
15dB grid antenna	-24	14
wire (two loop) biquad with reflector	-26	12
8dB Superpass	-30	8
wire (two loop) biquad	-34	4
wire (one loop) quad	-35	3
wire monopole	-36	2
17mm PCB monopole	-36	2
20mm PCB monopole	-36	2
commercial router antennas	-36	2
72mm PCB biquad dual loop	-40	-2

Discussion

The location of the physical position where peak received signal was found was quite “sharp”. This may have been due to lobes in the signal from the Nanostation 2 or multipath.

Several commercial router antennas were tested (sleeve dipole construction), they all measured about the same. The internal design of these antennas is discussed here.

The results from the control antennas (15dB grid, 8dB Superpass, and nominal 2dB sleeve dipole commercial router antennas) are consistent with what we would expect, which gives us some confidence in the other test results.

The impedance match and gain results from the PCB biquad are poor, which suggests the antenna is not resonant at 2.4GHz. It would be nice to test this antenna on a network analyser to find out where they are resonant (please contact me if you have one – I will ship an antenna to you!) Jeff is working up a simulation of the PCB biquad to test the design. We aren’t pursuing the PCB dipole as we have a bunch of antenna candidates that perform just as well (2dBi).

The wire biquad performance with a reflector was remarkable, nearly as good as the grid antenna which is a much larger antenna. The measured gain (12dBi) is consistent with other peoples results for this antenna.

Jeff and I really liked the wire antennas due to their performance and simplicity. They are easy to make: in production they could be bent up on a jig on 10 seconds from stiff copper wire then soldered to the Mesh Potato motherboard. One small problem with the dual loop biquad wire antennas is a feed arrangement – a small piece of coax would be needed to reach the central feed point. We don’t want the antenna wire directly over the PCB, as this would affect performance. The single loop wire quad is simpler in this regard, as it could be attached at one corner to the PCB.

The PCB monopoles perform well and are very simple, just a 17mm x 3mm track on the PCB next to a good chunk of ground plane. Virtually zero cost to add to the Mesh Potato motherboard. Both the 17mm and 20mm versions worked well, which suggests a relatively wide bandwidth and a high tolerance to small variations in manufacture like dieletric constant of the PCB substrate. Antennas fabricated on PCB are physically smaller than their wire cousins as the signals travel slower which means a smaller wavelength for a given frequency.

Wire single and dual loop biquad/quad antennas had above average gain and some directivity, with both peaks and nulls evident as they were rotated. Is directivity a good thing for a mesh router? You might enhance the signal of one node but null out the signal from another. I am not sure.

The higher gains of some antennas look attractive but may not be useful in practical mesh networks. To achieve the highest gain required careful adjustment of the antenna position. This is fine in a traditional point-point Wifi link, but in a mesh network their are multiple nodes we want to talk to. So if you peak the response to one node, you may dip the response to another. I guess it depends on how many nodes you want to talk to.

The reflector was a piece of blank PCB about 20cm x 20cm. It was moved back and forth behind the antenna until a peak was found (usually at around 15-20mm). All antennas improved by at least 4dB with the reflector, the wire biquad improved by 6-8dB. David C has suggested a slide-in reflector arrangement to give a choice between omni and directional antennas. These tests confirm David’s suggestion is a good one, if a precise way to mounting the reflector can be found.

Here are some of the antennas tested grouped by gain.

Antennas grouped by gain, highest gain on the left

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