Check your Featherweight tracker antenna

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Adrian A

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Two Featherweight Tracker customers have reported issues with LoRa signal strength, that were traced to center pins on the antennas that were not protruding as they should be. (Thanks for your help in finding this) The photo of the antenna on the right shows this clearly.

1669045343319.jpeg

On the good antenna on the left, you can see a metallic ring at the base of the pin which is visible when the pin is positioned correctly.

The antennas where this has been observed so far were shipped in spring of 2022. Each tracker and ground station is tested for RF performance before shipment, but because the test is performed using test cables that are installed in place of the antennas, we don't know if the problem antenna came from the manufacturer this way or if they were susceptible to being pushed in in the field.

I started sourcing from a different antenna manufacturer in August for availability reasons, and the new antennas are a little smaller and perform a bit better. The center pin problem has not been observed in antennas from the new manufacturer. The newer-style antennas are like the lower-left one in the photo below, with a brass ring at the base of the black plastic radome.

1669046675681.jpeg

My recommendations for Featherweight GPS customers are:
  1. If you have the old-style antenna and you are wondering if you have full LoRa performance, unscrew it and check to make sure that the pin looks like it's sticking out and the brass at the base of the pin is visible, like the left antenna in the top photo. If not, let me know at [email protected] and I'll replace it.
  2. It's a good idea to check your signal strength when you are confirming that tracking is working, while the rocket is on the pad. The numbers on the comm page with "RSSI" indicate the received signal strength in dBm. At typical prelaunch pad distances and conditions, you should see -90 dBm or better. More negative numbers indicate a weaker link.

An aside for the geekier among us: You can estimate how far your max link range could be in your head if you memorize two things:
  1. The weakest receivable signal by the Featherweight system is about -130 dB
  2. 20 dB in signal strength change corresponds to 10x in available range.
So if the rocket is 2000 feet away and your RSSI is -90 dB, you have 40 dB of margin. Since 20 dB is 10x ratio, then 40 dB is 100x ratio and your max range is 200,000 feet. RF transmission along the ground is attenuated compared to line-of-sight transmission through the air, so the expected range based on ground measurements will be conservative.
 
Adrian thank you for posting this. I flew my full scale Arcas a couple weeks ago and the ground station was not getting a signal. I'm gonna go check both of my antennas right now.
 
Thanks Adrian. As a general note I would suggest anyone using this SMA-style of connector always visually inspect the connectors before mating. This allows you to find problems such as yours or perhaps broken pins which result in the same problem. The root cause of many problems is that SMA and RP-SMA (reverse polarised) can easily mate together physically, but if you mate two females you get no connection, and if you mate two males you get at least one damaged and broken pin, likely no connection and a needed repair. I have seen this on a couple of occasions from different people. So, be wise and look before mating :) .
 
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