in Research, SDR

Close Encounters With The RTL-SDR

Standing on some mounds in the back yard, laptop in one arm, antenna pointed to the skies. WI5HER, Port Townsend, 2017

My RTL-SDR came in the mail a couple weeks ago. It’s a little $20 USB dongle with a long wire that connects to an antenna. And when I say antenna, I mean THIS is an antenna.

When unfolded, it’s almost as tall as a grown human and just ridiculously large enough to make your family or anyone in your immediate proximity cringe in embarrassment for your own sake.

Anyway, I was out on the Olympic Peninsula and after the fam had all settled to bed I went out on the back porch with my laptop and SDR gear and start scanning the frequencies. For less than the cost of taking a date to the movies, the rtl-sdr allows you to flip around the airwaves between 500 kHz to 1.7 GHz and visually see a ‘waterfall’ of incoming radio frequency traffic.

A typical waterfall

I started by first finding the local FM radio stations and from there chronicling and recording a whole slew of beeps and bloops all over the dial. I pushed the ear buds deeper into my noggin and smiled with delight!

Then, up around the 450 Mhz zone, I heard a voice descend from the æther.

At this point, I started to really flip out – I hadn’t expected to hear anyone talking in this range, and especially not someone seemingly reading what sounded like absurd poetry.

It was too good to be true. Was this a pirate broadcast? A “messenger” ?

Turns out, there’s a much more reasonable explanation. This is not poetry, per se, but something called the ‘Harvard Sentences‘.

A collection of sample phrases that are used for standardized testing of Voice over IP, cellular, and other telephone systems. They are phonetically balanced sentences that use specific phonemes at the same frequency they appear in English.

My understanding is that certain companies may play these sentences on a loop and then drive around and judge signal strength based on how well they can understand the words being spoken.

The sentences are strange, sometimes profound. I think they would make for some amazing “performance art” that would make Yoko Ono proud. Or a beautiful ritualized “spell” script.

Take for instance this list…  can’t you imagine trying to “accomplish” each of these steps in order to unlock some sort of otherworldly “achievement” ?

  1. The boy was there when the sun rose.

  2. A rod is used to catch pink salmon.

  3. The source of the huge river is the clear spring.

  4. Kick the ball straight and follow through.

  5. Help the woman get back to her feet.

  6. A pot of tea helps to pass the evening.

  7. Smoky fires lack flame and heat.

  8. The soft cushion broke the man’s fall.

  9. The salt breeze came across from the sea.

  10. The girl at the booth sold fifty bonds.

A full list can be found here.

 

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  1. You can go down to 500 KHz? I can only get down to 24 MHz, which I think may be CB territory… (not sure)

  2. Yeah, mine is the same. According to the Amazon listing, you can get HF reception below 24 MHz by running in direct sampling mode, ‘cept – I don’t know what that means.