Monday, October 5, 2015

Lectures on Radio Repeaters

After having someone at a BBQ ask me to explain what a repeater is for about the 30th time in my life, I decided that I may as well record my standard 10 minute talk on what a repeater does.

Introduction to Radio Repeaters:

Of course, once I've got the camera turned on, why not record another 20 minutes on the internals of a repeater?

Internals of a Repeater:

I suspect there are a few more videos in this series, which I'm currently working on and will record time-permitting.

Monday, February 2, 2015

Designing and Building a 2m Low Pass Filter

 I've been playing with the DRA818V modules that have been making quite a stir in the amateur radio world at the moment. I haven't gotten one on a spectrum analyzer yet, but I have reason to believe that it will require a low pass filter to be RF legal. I'll write more about that once I get a look at it, but figured I'd first built myself a low pass filter in case I need it (if not for these modules, but some other VHF project in the future).

My process for building a low pass filter went as follows:

  • Select the type of filter and cutoff frequency desired
  • Look up normalized coefficients in the ARRL Handbook
  • Divide these coefficients by the cutoff frequency
  • Convert the inductances into turns on some core and capacitors into the nearest values
  • Build the filter.
Since I wanted this filter for 2m, the highest frequency I'm interested in passing is 148MHz, so I selected a cutoff frequency of 150MHz. In hind-sight, this was a poor choice, since a -3dB point only 2MHz above the band caused for a lousy insertion loss. A better choice would have been 10% higher than the top of the band, so 148MHz * 1.10 = 162MHz

I decided to build a 5 pole T configuration Chebyshev filter with 0.1dB of ripple.

Looking this filter up in a random copy of the ARRL Handbook (1981, but any recent one will do), it gives the component values needed for a 50 ohm filter at 1MHz. I'm also building this for 50 ohms, so all I need to convert is the frequency by dividing by 162MHz.

  • L1 = 9.126uH / 162 = 56nH -- 3 turns on 1/4" air core
  • L2 = 15.72uH / 162 = 97nH -- 5 turns on 1/4" air core
  • L3 = 9.126uH / 162 = 56nH -- 3 turns on 1/4" air core
  • C1 = 4364.7pF / 162 = 27pF -- 30pF on hand
  • C2 = 4364.7pF / 162 = 27pF -- 30pF on hand
To convert the inductance values into solenoid designs, I have an old magazine article from the 70s that published a whole table of different diameters of air wound inductors.
For extra blog cred, I built the filter in an Altoids tin with SMA connectors on each side. The four solder pads were formed from a strip of copper clad with three hacksaw cuts.
Next step was hooking this up to a vector network analyzer to see how far off I ended up, which is when I realized that 150MHz was a poor corner frequency choice. Reforming the inductors and taking a turn out of L2 got me closer, but I would have ended up with better performance if I had designed it all correctly from the start.

 The SWR at 2m is about where it's expected to land at 1.3
 The insertion loss at 2m isn't so great at 0.8dB. This would be improved by better component layout, but I built this filter kind of sloppy. It does suppress all the harmonics of 2m as expected. I only bothered to note that 200MHz was down ~23dB, so 440MHz will be well below that.
In an ideal world, points 1 and 2 on the smith chart would fall at the center (1,0), but 45+19j ohms is close enough for me.

Now once I get to the point of experimenting with keying up my DRA818V modules, I'll have a nice LPF on hand in case the harmonics do end up too high for on the air testing.

Sunday, January 11, 2015

ARKnet - Cupertino Emergency WiFi Network

So now that I'm graduated and looking for work full time (hey, you hiring in the Silicon Valley?), I've been spending my free time volunteering with the city of Cupertino as the technical lead building out a test network for a project we're calling the ARKnet.

In the name of emergency preparedness, Cupertino maintains a number of shipping containers throughout the city filled with emergency response equipment and supplies. Traditionally, if a disaster happens to be large enough to knock out communications, when citizens respond to activate these ARKs, these teams include Cupertino ARES members who tender communications back to the city Emergency Operations Center (EOC). In addition to voice communications, we also provide 1200 baud packet for long textual messages via the Santa Clara County's amazing AX.25 BBS system (Callsigns W1XSC-W6XSC). 1200 baud is a very useful data rate when compared to zero in the case of an emergency, but we decided that the time is ripe to start running some serious experiments moving data on the 5.8GHz band instead of on 144MHz.
For this pilot, the city gave us funding for a minimalist network linking the city EOC to a single ARK. To accomplish this, we got permission from one of the few tall buildings in the city to erect a 90 degree beam width access point. This sector antenna is pointed towards the EOC and one of the ARKs, both of which have high-gain uplink radios connected to it. The EOC contains an applications server and an edge router to provide Internet access to the entire network.

Since emergency communications is a major part of the charter for ARKnet, we're only considering uses for the network where the entire application can be entirely self-contained within the network. There's no interacting with the cloud when the Internet is down, so all our applications need to be running on a local server with emergency power. This network will be "Cloud-Free!™"
For the point-to-point links, we're using Mikrotik SXT 802.11ac routers (which come in both 90 degree beam width and 28 degree beam width variants). Mikrotik is unusual in that their products combine both good long-haul WiFi and commercial grade routing features in a single package. We looked at using Ubiquiti for the long-haul links (which I've used before), but being able to have the links also speak OSPF to make the network self-healing is attractive.
Our test ARK isn't profoundly far away from our sector AP (about 1.25 miles), but we do need to punch through a large cluster of trees, so the link isn't great. After a few weeks of lab testing, this weekend was the big rollout where we brought up all the gear and testing the throughput from the ARK to the EOC, which ended up being about 10Mbps.

This kind of bandwidth is certainly usable for any of the applications we've come up with so far, and the next big step for ARKnet is that we need to start developing viable applications to run on this network which will be useful in the case of a disaster.
Of course, I haven't been doing this alone. Thanks to all my fellow CARES volunteers who have helped make this first test-link possible!

Monday, December 22, 2014

Completed My Masters Degree in Electrical Engineering

Needless to say, things have been quiet on here for a while now. I've been deep in the thick of finishing my masters thesis at Cal Poly for my MS EE degree, and that has finally been done!

I'm now graduated, and starting on the job hunt! If your company is hiring in the bay area, CA, drop me a line.

Below is recordings of me presenting the first chapter of my thesis at the ARRL/TAPR DCC conference in Austin, TX, and then me in San Luis Obispo defending my entire thesis in front of my committee.

A few small tweaks before I submit this paper to the Cal Poly library for archive there, but it's essentially done.

Now that I'm not spending all of my work time writing, I'm looking forward to maybe spending some of my play time writing. I'm involved in a lot of interesting projects lately, just haven't had the will to write about them until now. You can look forward to that.

Friday, October 3, 2014

Unboxing the Atmel SAM4L8-XSTK Dev Kit


Thanks again to Atmel for giving this to me yesterday. I had a good time at ARM TechCon with them and everyone else.