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When the Fox Doesn't Come Out to Play
Sunday, October 31st, 2021 06:04 pmAmateur radio enthusiasts sometimes organize a "fox hunt." A radio transmitter in an undisclosed location periodically broadcasts a signal on some frequency and participants use a directional antenna, a compass, and other tools to triangulate the transmitter's location.
Yesterday, the Boulder Amateur Radio Club organized a bit of a "reverse fox hunt." Two weeks ago, some club members noticed some unexpected transmissions on the club's VHF repeater from radio operators that were not also listening to that frequency. Someone was able to track down someone who'd made some of those transmissions and found that they had an AllStar hotspot operating on UHF simplex frequency 446.175 MHz. A hotspot in an amateur radio context is a low-power transceiver that's connected via the Internet to a wide-area digital radio network. The hot spot listens to a simplex frequency, sending anything it receives to the network. Likewise, any transmissions from the network are rebroadcast on the simplex frequency by the hotspot. This allows someone, perhaps the owner of the hotspot, to talk with people all over the world using a mobile or handheld radio that would normally only reach the local area.
The hotspot owner changed the frequency it was operating on. This stopped the stream of AllStar network chatter on the Boulder repeater, but this wasn't the root problem. Club members continued to notice occasional transmissions from 446.175 coming through on the repeater's frequency, suggesting that someone else was running a cross-band repeater with UHF input and VHF output on the club repeater's 146.100 MHz input frequency.
So some club members organized a "reverse fox hunt." Rather than listening for periodic transmissions from the "fox" we wanted to find locations where we could trigger the cross-band repeater, then triangulate its location. (I assume the final step would be "Knock on the nearest door and see if they're a ham operator.) Five of us gathered in the NCAR parking lot early Saturday afternoon. We weren't able to trip the cross-band repeater from NCAR, which has line of sight to all of Boulder, so we concluded that the fox was either over the hill in the Denver area or wasn't out of its foxhole that day. When the cross-band repeater had been active, we were able to get reports that it got triggered from Denver's Stapleton neighborhood and from an antenna on a mountain near Coal Creek Canyon.
We split up, sending one person to the Coal Creek area, one to Stapleton, one to Federal Heights, and one to Arvada. I headed down to Green Mountain, which was my favorite vantage point over metro Denver when I lived in Jefferson County. I transmitted from a few spots on the trail, neither of which lit up the Boulder repeater. One of our group had wondered if the cross-band repeater might be configured with a PL code&emdash;a tone of a particular frequency which must be transmitted along with the FM wave in order to activate a repeater to make it less likely to accidentally rebroadcast a message you didn't intend to. So I spent about half an hour turning the knob on my radio and making test transmissions with each possible squelch tone.
In the end, we concluded that the fox didn't come out to play yesterday, and we haven't noticed any spurious transmissions on the Boulder repeater this week. But playing radio was a pretty good excuse to go for a hike on a gorgeous late October afternoon, even if I didn't get to play with the directional yagi antenna I borrowed from a coworker.
Yesterday, the Boulder Amateur Radio Club organized a bit of a "reverse fox hunt." Two weeks ago, some club members noticed some unexpected transmissions on the club's VHF repeater from radio operators that were not also listening to that frequency. Someone was able to track down someone who'd made some of those transmissions and found that they had an AllStar hotspot operating on UHF simplex frequency 446.175 MHz. A hotspot in an amateur radio context is a low-power transceiver that's connected via the Internet to a wide-area digital radio network. The hot spot listens to a simplex frequency, sending anything it receives to the network. Likewise, any transmissions from the network are rebroadcast on the simplex frequency by the hotspot. This allows someone, perhaps the owner of the hotspot, to talk with people all over the world using a mobile or handheld radio that would normally only reach the local area.
The hotspot owner changed the frequency it was operating on. This stopped the stream of AllStar network chatter on the Boulder repeater, but this wasn't the root problem. Club members continued to notice occasional transmissions from 446.175 coming through on the repeater's frequency, suggesting that someone else was running a cross-band repeater with UHF input and VHF output on the club repeater's 146.100 MHz input frequency.
So some club members organized a "reverse fox hunt." Rather than listening for periodic transmissions from the "fox" we wanted to find locations where we could trigger the cross-band repeater, then triangulate its location. (I assume the final step would be "Knock on the nearest door and see if they're a ham operator.) Five of us gathered in the NCAR parking lot early Saturday afternoon. We weren't able to trip the cross-band repeater from NCAR, which has line of sight to all of Boulder, so we concluded that the fox was either over the hill in the Denver area or wasn't out of its foxhole that day. When the cross-band repeater had been active, we were able to get reports that it got triggered from Denver's Stapleton neighborhood and from an antenna on a mountain near Coal Creek Canyon.
We split up, sending one person to the Coal Creek area, one to Stapleton, one to Federal Heights, and one to Arvada. I headed down to Green Mountain, which was my favorite vantage point over metro Denver when I lived in Jefferson County. I transmitted from a few spots on the trail, neither of which lit up the Boulder repeater. One of our group had wondered if the cross-band repeater might be configured with a PL code&emdash;a tone of a particular frequency which must be transmitted along with the FM wave in order to activate a repeater to make it less likely to accidentally rebroadcast a message you didn't intend to. So I spent about half an hour turning the knob on my radio and making test transmissions with each possible squelch tone.
In the end, we concluded that the fox didn't come out to play yesterday, and we haven't noticed any spurious transmissions on the Boulder repeater this week. But playing radio was a pretty good excuse to go for a hike on a gorgeous late October afternoon, even if I didn't get to play with the directional yagi antenna I borrowed from a coworker.
