5 ms·
> Was it difficult to find the exact source, or not? Is there some reason that getting directionality on a constant 315 MHz signal is harder than one would thin
by throw20102010 7y ago
> Was it difficult to find the exact source, or not? Is there some reason that getting directionality on a constant 315 MHz signal is harder than one would think?
Physically, it's easier to find a UHF beacon (315 MHz is considered ultra high frequency) than it is to find a lower frequency beacon (lots of direction finding contests are done on VHF and HF). The higher frequency means a shorter wavelength and better phase (direction) resolution with a smaller aperture (distance between antennas).
However, many (most) hams don't have equipment that covers 315 MHz. UHF isn't popular because it is shorter range and more expensive equipment (you can build HF equipment with much simpler components, and the design is less complex). Plus, 315 MHz isn't in one of the ham bands, so ham direction finding equipment probably won't be designed for it even if it is UHF.
A wide band SDR would include 315 MHz. While there are many younger hams that enjoy cheap SDRs like the RTL-SDR, many of the older, grumpier hams see it as dangerous and letting undisciplined "hackers" into their hobby. To build a direction finder with an SDR you either need to have an expensive multi-channel SDR (I have a Lime SDR that would work), or you need to construct your own mixer to feed a single channel SDR, or you need to wire up two RTL-SDRs with a coherent clock signal, which puts this out of the range of your "common hacker." Here are two ways you could do this on the cheap: https://www.rtl-sdr.com/building-a-passive-radar-system-with-an-rtl-sdr/ https://www.rtl-sdr.com/building-a-passive-radar-system-with... https://www.rtl-sdr.com/building-a-direction-finding-system-with-a-frequency-mixer-sdr/ https://www.rtl-sdr.com/building-a-direction-finding-system-...
So while most armchair HN readers would love to tell you about how easy this is, most people don't have the equipment sitting around to do direction finding on a 315 MHz signal. The best most people could do would be to "wardrive" around the neighborhood and look for where the signal strength peaks, but this will really only narrow it down to a few houses and then you need to start knocking on doors. A good direction finding setup would pinpoint the exact house.
Source: I'm a ham (recently made extra), but no, I won't tell you my callsign to prove it.
- riskable 7y agoCouldn't you just walk around with a laptop and an RTL-SDR device and measure how strong the (near 315MHz) signal is at various locations as you move about?
- throw20102010 7y agoThat's what I meant when I said "wardrive" around the neighborhood and look for where the signal strength peaks. It's easy in theory, but I'd guess you'd probably only narrow it down to plus/minus two houses in every direction- UHF attenuates through walls, but in air it is more gradual, and so you wouldn't get a sharp peak as you walk around. So now you have eight houses to go check. But once you started knocking, probably all the neighbors would either say, "yeah, that's probably Bill, he's does electronic stuff" or maybe they would say, "that's might be my neighbor I've never met." A strong directional antenna would help, but that means that you have to build it yourself, I'm not aware of an off the shelf antenna that has high gain at 315 MHz. So
- crankylinuxuser 7y agoThere's a new piece of inexpensive gear for RDF: KerberosSDR. It's a quad input coherent radio receiver based after the RTL-SDR chips. One such function is it can give direction and strength of signals in a wide area. And I find this electronic kit antenna works well as long as you pull it out for the appropriate frequency: https://www.nooelec.com/store/ratlsnake-m5.html https://www.nooelec.com/store/ratlsnake-m5.html I use 4 of these for my KerberosSDR.
- throw20102010 7y agoI saw that these are shipping and have almost convinced myself to buy one. I think they're pretty neat and now a wideband phased-array direction finder is a project that a smart high-schooler can do, at a price point that many people can reach. Maybe the next time this type of interference occurs the headline will read different: "Local kid solves problem with science fair project, stumped adults for weeks." With that said, these KerberosSDRs (or any multi-channel coherent SDR solution) are not widespread, are considerably more expensive (6-7x more than a single RTL-SDR), and are only recently available. So my previous point still stands about how armchair hackers can talk about how easy this problem is to solve in theory but they lack the actual equipment to do it.