Fast data over VHF and UHF repeaters

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ljheidel
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Posts: 29
Joined: Mon Mar 30, 2015 1:11 am
Location: Northern Virginia

Fri Mar 25, 2016 6:31 am

This appears to be a modulation technique that replaces the standard PSK technique which is limited to 1200 bps using the mic and headphone ports on FM UHF/VHF radios. Currently, the most common use for packet is ARPS, but it can and is used for general IP data. It's not for use with "repeaters" per se, but broadcast packet data in the VHF and UHF bands. I'd be interested in testing this with MURS radios. As I understand it, data is permitted using narrowband (12.5 KHz) MURS channels. The Baofengs and the like aren't Part 90 approved for use in the MURS bands, but if you wanted to be legal with it, you could buy MURS radios and interface the soundcard modems. The advantage is that this you wouldn't be limited by Part 97 restrictions, particularly prohibitions on commercial traffic and encryption. However, you'd be limited to 2 watts if you wanted to stay legal. Of course, you couldn't stop someone from using an unapproved Chinese radio (a TYT TH-9800 for instance) at 50 watts in the MURS band and you'd probably get away with it for at least a shirt period. But, I'd never recommend that since it's highly illegal.

I've completed tests using 1200 bps PSK for the extension of HMMS meshes into areas where line of sight wasn't possible (or practical), including getting OLSR routing working properly. However, the speeds were too slow to be useful. Our test protocol was IRC, but due to the unicast nature of the protocol, each node that joined the server required another packet be transmitted, and it quickly grew to be unmanageable for more than a few users. However, in a pinch, it might be effective, especially if you were able to more than double the speed to 3000 bps. You still run into the encryption/commercial use restrictions if you used it in the Ham bands, but for emergency operations, it might be useful.

I've considered that I might have better luck (and throughput) using standard XBee or even 900 MHz Wifi gear for short range (several miles) communications without line of sight without having to deal with Part 97 restrictions, which considering that most data traffic you'd want to transmit needs to be encrypted, and newer protocols are encrypted by default, really kind of puts the use of amateur packet radio in a realm of marginal (at best) usefulness.

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