Spacecat & Breadboarder
Spacecat Spacecat
Breadboarder Breadboarder
Breadboarder Breadboarder
Hey, I’m thinking about building a low‑power radio telescope from old vacuum tubes and a big wire loop. You got any antenna math or star‑hopping code to share before I turn my garage into a mini observatory?
Spacecat Spacecat
Sounds like a fun project. For a simple loop antenna the resonant frequency is roughly f ≈ c / (2πR) where R is the loop radius in metres and c is the speed of light. If you want a decent bandwidth you can add a small capacitor in series to tune it. The impedance at resonance for a single‑turn loop is about 2π f L, so you’ll end up with a few ohms—easy to match with a small transformer or an in‑line resistor. If you need a quick gain estimate for a single‑turn loop, use G ≈ (π A / λ²)² where A is the loop area and λ is the wavelength. That gives you the theoretical directivity; real‑world losses will reduce it. For star‑hopping you can keep it simple with a small script. Here’s a Python snippet that turns right‑ascension/declination into altitude/azimuth for a given latitude and UTC time: ```python import math from datetime import datetime, timezone def ra_dec_to_altaz(ra, dec, latitude, utc_now): # ra, dec, latitude in degrees jd = 2451545.0 + (utc_now - datetime(2000, 1, 1, 12, tzinfo=timezone.utc)).total_seconds() / 86400.0 # Greenwich Mean Sidereal Time in degrees gmst = 280.46061837 + 360.98564736629 * (jd - 2451545.0) lmst = (gmst + longitude) % 360 ha = (lmst - ra) % 360 if ha > 180: ha -= 360 ha_rad = math.radians(ha) dec_rad = math.radians(dec) lat_rad = math.radians(latitude) sin_alt = math.sin(dec_rad)*math.sin(lat_rad) + math.cos(dec_rad)*math.cos(lat_rad)*math.cos(ha_rad) alt = math.degrees(math.asin(sin_alt)) y = -math.sin(ha_rad) x = math.tan(dec_rad)*math.cos(lat_rad) - math.sin(lat_rad)*math.cos(ha_rad) az = (math.degrees(math.atan2(y, x)) + 360) % 360 return alt, az ``` Plug in your telescope’s latitude, and you’ve got a quick pointing routine. Just keep the loop tuned to the band you’re interested in, and you’ll be able to catch some of the faintest galactic signals without draining the power supply. Happy hacking!
Breadboarder Breadboarder
Nice math, but I’ll tell you what: you’re still going to choke on those tiny loops because you never bothered to calculate the self‑inductance properly. Use a coil with many turns of thin gauge wire, not one giant loop. And that script—great, but remember to feed it the actual longitude too, otherwise your azimuth will drift like a lost satellite. Also, keep a 0.1 µF cap across the loop at 50 MHz to tame the Q; otherwise you’ll get a whisper that’s louder than a moth. Happy tinkering, and remember: if you can’t find the right part in the drawer, you’re not digging deep enough.
Spacecat Spacecat
Thanks for the heads‑up. I’ll run the Wheeler formula for a multi‑turn coil to get a solid inductance estimate and add a few turns of 30‑AWG to boost the Q. I’ll plug the longitude into the script too—no drifting, no problem. And that 0.1 µF across the loop at 50 MHz should keep the resonance tidy. I’ll dig into the drawer and make sure I have everything; if not, I’ll get a fresh batch. Happy hunting, and good luck with the garage observatory.
Breadboarder Breadboarder
Sounds like a plan, just remember the 30‑AWG will get a little hot if you keep the power on too long, so keep that breadboard ventilation handy. And if you ever find a 0.1 µF ceramic with a “loose” tolerance, feel free to replace it with a hand‑selected trimmer—you know I love making the same tweak twice. Good luck, and don’t let the loop’s Q get so high it scares the neighborhood cats.
Spacecat Spacecat
Got it—will keep the heat in check with a fan, and I’ll swap that ceramic for a precision trimmer if it drifts. I’ll monitor the Q so the cats stay calm. Thanks for the tip!