Project Horus High altitude balloon project


Radio testing

I had a chance to test the range of my 25mW Radiometrix NTX2 last night. Whilst we couldn't simulate the true line of sight conditions that we'll see during balloon flight, a few friends and I tested the range of the payload in various conditions. The payload spend the majority of the time on the backseat of my friends car, which would have no doubt limited its range. I used a 1/4 wave piece of exposed coax core as a receiving antenna, sticking it out of the window of my car.

Incidentally, it turns out my car locking/alarm remote is in the same band as the NTX2: ~433mhz, meaning that I wasn't able to unlock/start my car - even after trying my spare keys & scratching my head for a good while.

On flat, straight road with minimal obstruction (a few trees & streetlights), we achieved about 3km of useful (decodeable) range. Given that the antennas were only about 1m above ground level (and probably the street width - it was lined with buildings), we would have had insufficient Fresnel clearance, which would have been a limiting factor.

We decided to position the payload somewhere which might give it better coverage, so headed to Belair, one of the more elevated suburbs in Adelaide. We hoped to test from Windy Point (an elevated lookout with a good unobstructed view of Adelaide), but after arriving and finding a gentleman who was a little too interested in making conversation, we found the prospect of being stabbed a little grisly, so left to find another spot. We settled on a street which was partially shielded by trees & houses, but gave us the best possible line of sight.

One car set off back down the hill to see what sort of range we could achieve - we ended up at the airport (10km away as the crow flies), before I broke the antenna sticking out of my window & we called it a night. Given the pitiful antenna I was using on the receiver & the fact that the NTX2 has the same sort of transmit power as a garage door opener, I was pretty impressed with the range.

I also received another radio I picked up cheaply on eBay today - an ICOM 490A. It's nowhere near as fully featured as my 706, but it'll do SSB on the 70cm band, so it'll decode data just fine.

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