Scenario 7 - Margin Call
Session: Thursday, 15:15 UTC
Station: BKYD-YAGI — crossed yagi on the TV rotator
Operator: You
Riley: Here, unusually cheerful about a problem
Riley’s Note
Section titled “Riley’s Note”Two pieces of bad news and I’m delighted about both.
One: something in this neighborhood started spraying hash all over 70cm overnight. The noise floor on this band is visibly worse than it was Tuesday. My money is on the new grow lights three doors down — cheap switching supply, no filtering, running twenty hours a day.
Two: tonight’s only CUBEHOP window tops out at eighteen degrees. In the trees. That is the thinnest margin we’ve ever worked.
Why am I happy? Because a directional antenna is not just a signal collector, it’s a direction FINDER, and I have been waiting for an excuse to teach you that. We’re going to find the noise, delete it from our receiver, and steal a pass that most operators would write off.
Margin is arithmetic. Tonight we audit it.
— Riley
Tonight’s Pass
Section titled “Tonight’s Pass”| Satellite | AOS | Max Elevation | LOS |
|---|---|---|---|
| CUBEHOP-1 | 15:35 | 18.4° at ~15:40 | 15:46 |
The Interference
Section titled “The Interference”| Property | Observed |
|---|---|
| Center frequency | ~435.360 MHz |
| Width | ~80 kHz of ragged hash |
| Behavior | Continuous — no on/off pattern |
| Doppler | None |
| In the Observations list | No |
Three of those lines are the diagnosis. Satellites drift, rise, and set; ground noise sits perfectly still and changes strength with where your beam is pointing, not with time.
Note also that it is continuous. Deliberate interference usually has a duty cycle — somebody keying up, something scanning. Accidents just run until somebody unplugs them.
Link Reference
Section titled “Link Reference”| Signal | Frequency | Bandwidth | Mode |
|---|---|---|---|
| CUBEHOP-1 FM downlink | 435.250 MHz | 15 kHz | QPSK 1/2 |
| Local hash (the intruder) | ~435.360 MHz | 80 kHz | — |
They are 110 kHz apart. That gap is what makes tonight survivable: the hash can be carved out without touching the bird’s channel, even with Doppler walking the downlink ±10 kHz.
Direction Finding with a Yagi
Section titled “Direction Finding with a Yagi”The crossed yagi has 12 dBi forward gain, a 40° beam, and — this is the part that matters tonight — an 18 dB front-to-back ratio.
That means the same antenna reads a ground signal dramatically louder pointed at it than pointed away from it. Sweep the beam around the horizon with the MAN AZ control in the ROTATOR panel, watch the hash strength, and the peak is your bearing. That’s a foxhunt, and it’s the same instrument you’d use on a signal claiming to be in orbit.
Finding the Decibels
Section titled “Finding the Decibels”Three separate places, and you need all three tonight:
- Bearing — sweep the yagi, find where the hash peaks, know your enemy.
- Notch — the FILTER section has a one-knob notch. Center it on the hash and it drops out of your passband. You can’t unplug the neighbor’s gear; you can decline to listen to it.
- IF filter width — the front end is wide open at 200 kHz. The noise gate charges you by the hertz: narrow the IF filter to 100 kHz and the floor comes down with it.
Then fly the pass: program-track on CUBEHOP-1, and hold the lock through eighteen degrees. AFC is allowed tonight — you proved you could do it by hand three sessions ago.
End of Briefing
Section titled “End of Briefing”Most operators look at a low pass on a noisy band and go inside for dinner.
You are going to sit down, work out where every missing decibel went, take them back one at a time, and make the contact anyway. That’s the difference between owning equipment and being an operator.
Also, tomorrow I’m printing the Hendersons a very friendly pamphlet about ballast filters.