Baltimore Radio Amateur Television Society

The Baltimore Radio ATV Society invites all hams and technology enthusiasts to our next meeting on Monday, April 14, at 7 pm at the Pikesville Library in Reisterstown, MD.

The Brats have built a high speed digital backbone connecting 5 sites across 20 miles of Baltimore, replacing our aging analog repeater links with modern commercial wireless gear and in the process joining the forefront of a technological revolution.

Learn about our early trials, how we planned our network, how we selected our gear, why we spent hours a hundred feet above the ground on a frigid winter night - and why we can send 3 gigabytes of data in 300 seconds with wind gusts over 50.

Get an overview of the regulatory environment and concerns. What frequencies can we use? Is encryption allowed? How do we secure the network, and how do we ensure priority traffic gets through?

See how the Brats are currently using our network today, where we're going tomorrow, and how the amateur radio community can leverage this technology in both everyday amateur use and emergency situations.

Tonight's high winds provided a good opportunity to see how the network holds up. Result: the dishes definitely sway enough to affect signal (from +3 dB better than normal to -3 dB worse), but data rates have been consistent.

The following tests are end to end across two hops:

100,000 x 1024 byte pings in 329 seconds:
--- ping statistics ---
100000 packets transmitted, 99999 received, 0% packet loss, time 329014ms
rtt min/avg/max/mdev = 2.511/3.252/14.082/3.625 ms, pipe 18, ipg/ewma 3.290/2.847 ms

And iperf results:
[SUM] 0.0-300.1 sec 3.19 GBytes 91.3 Mbits/sec

Moving 3 gigaBYTES across two wireless hops a total of 13 miles in winds gusting to 50 in 300 seconds isn't too shabby!

Here's the part 15 wifi spectrum as seen from a directional antenna at one of our sites. The strong signal at 5800 is some part of Towson University's wifi network, about a mile away, directly in the path of this dish:

Operating in Part 15 under the Unlicensed National Information Infrastructure (U-NII) allows for outdoor use and user-installable antennas in much of the spectrum above. Operating under amateur rules allows for greater power, and some unlocked/international gear can be used above 5825 MHz.

We have two dishes very close to each other, pointing 120 degrees apart. One is a 25dB ~18" dish:

And the other is a 30dB ~30" dish:

The 25dB dish has one small cluster of trees and a 2.4 mile run, but you can see one building from the other. That's why its signal is -65 with a noise floor of -90. The link is 240 mbps bi-directional, but with 100 mbps ethernet as the limiting factor we can get sustained transfers of 9 megaBYTES per second.

The 30dB dish is aimed at a site 9.4 miles away, with a ridge and a cluster of trees in the middle. We're pretty sure we can aim them better to get the signal into the low 70s, but through the recent snow and ice storms the signal remained constant, and the bandwidth remained above 100 Mbps. Sustained transfers through this link are 8 megaBYTES per second.

K0RYX, N3GXH, and KB3PLX spent a few hours on a chilly, icy roof adjusting the aim of one of the dishes, and a few more hours inside tweaking settings, before finally getting everything set just right.

Our 9.4 mile wifi link now has ping times of 1.1-1.4 ms, and sustained bidirectional transfer rates of 80 Mbps! With a little more tweaking we hope to boost that even more.

This should eliminate the occasional pop/digitization that occurred when voting between sites, and give us a strong, solid signal through the worst weather we can expect.

WB3DZO now has modern, reliable links, battery backups and emergency power, and multiple control points - standing by for the snowy season ahead!

Here's N3GXH holding up the 30dB dish as we mount the radome/shield around it: