Monday, January 4, 2016

My Not-quite White Space Radio System

2016 will be Merrick's Year of Communications. My goal is to write a blog post once a week. What will I write about? Communications! My previous post mentioned this. Now it's time to talk about the hardware I'll be using.

Communications is a lot more fun when it's wireless. (Consider "tied down," "strapped to my desk," and "the old ball and chain." Not fun.) But as much fun as wireless is, how can I perform wireless transmissions and receptions from my home computer with full control over the transmitted signal? Well a software-defined radio (SDR) would work perfectly for that.

Let's see. I'll just do a quick check here... Oh, I only need to spend nearly $700? No! Just over $400! Oh, wait. Here's one for about $300. Those are all awesome SDRs if you want to transmit. So what's the problem? I'm cheap. I know computers cost that much and I'll be using two of those but permission to purchase a $300 radio hasn't been granted by my local approving authority (i.e. wife).

How cheap? Like $3-eBay-FM-transmitter-direct-from-China cheap. At least for the receivers I'll be using some of those cool RTL-SDR receivers (that often cost less than $10).

Now I'm not dumb and I know that I'm really not going to be doing any modulation of the RF signal by using an FM transmitter. I'm not likely to get more than 10 kHz of bandwidth in the audio spectrum to play around in. So from a cost/bandwidth perspective, these $3 radios don't really match the SDRs I linked to above by an order of magnitude or so. But they're $3! Feeding myself today will cost more than that.

I'd get more bandwidth by just connecting the two computers with audio cables instead of going through the FM transmitter. But, again, it's wireless. And audio cables aren't that interesting. By using the FM transmitter I can make my channel model more noisy interesting.

The FM transmitters are the same ones you'd use to play audio from your phone to your car stereo. They operate under "47 CFR (Code of Federal Regulations) Section 15.239, and the July 24, 1991 Public Notice (still in effect)." They are supposed to have no more range than approximately 200 ft (for $3, I'm hoping for 20 ft). But I can operate them with no restrictions to content (e.g. encrypted) or broadcast duration.

I call this a "not-quite white space radio system" in the title of this post. If I use my FM transmitter in an unoccupied channel on the radio, I'm effectively using a "white space." Unlike actual TV band white space allocations, however, the FM band isn't allocated for white space radio by the FCC (probably because no one wants that space and no one is asking the FCC to do so).

Now I just have to wait for the slow boat from China to make it's way to American shores.