Troubleshooting COM / Sirius XM Reception Issue

[lrobichaux]

Trying to troubleshoot poor COM transmission and garbled Sirius XM reception. For those of you with CC with factory build assist and two COM antennas, how far is the placement (distance) from each other and from the GPS antenna? Thinking that this may be a problem on my kit built EX-3. Thanks in advance.

 

[Neal]

The antennas on the inner side of either wing with dual com should be installed with a ground plane. The GPS antenna is over the shoulder harness anchor location above and aft of your head. I had problems with comms on my last plane, probably no different on this newer plane and sometimes had to switch between com 1 and 2 if I experienced troubles. I think the issue stems from the fact our antennas are top side mounted vs bottom of the aircraft and can be subject to blocking.

@Tsquared had an issue with his GNC 355 and had to adjust the gain. I told him what mine was set to but can't remember the discussion. He had dialed his above where mine is set but you may want to experiment with the gain control on the GNC 355 in the config area. Maybe @Tsquared can chime in to help with what he learned.

 

[Cactus Charlie]

Trying to troubleshoot poor COM transmission

What is poor about the transmission? VHF COM uses an amplitude modulated signal. The first step in trouble shooting a "poor transmission" would be to determine if the problem is related to carrier signal strength or to modulation quality.

If you have no RF test equipment then you may learn something from ATC recordings that are available for many airports - https://www.liveatc.net/archive.php

I have single COM with a GTR 200. By listening to recordings of my transmissions I learned that my modulation level was low. I reduced the sidetone level which made me speak louder to hear the same audio level when transmitting. (Speaking louder increases the voice to noise ratio but increasing MIC gain does not.)

Quality of antenna installation will influence signal strength but not level or quality of modulation.

 

[ve6yeq]

I have a factory-built Carbon Cub EX-3 with the "IFR" panel. There are lots of radios onboard (not counting my iPhone, iPad Mini, VHF handheld - Yaesu FTA-850, Insta360 ACE Pro 2 camera, Insta360 X4 camera, Garmin InReach Mini 2, LightSpeed H-Mod headset, etc.):

• 2 VHF voice transceivers: a Garmin GNC 355A and a Garmin GTR-20 (mounted above the passenger's seat); the Comant VHF Communication CI 121 antennas for these are mounted as documented in the Dropbox "SK73000 Antenna installation.pdf" file.
• 4 GPS Receivers: Garmin G3X Touch (GDU-465), Garmin GNC 355A, Garmin G5 and an INTEGRA AF ELT; the GNC 355A external antenna is a Garmin G35A mounted as per the Dropbox "SK73000 Antenna installation.pdf" file; the GDU-465 external antenna is a Garmin GA26X (BNC) mounted inside above the pilot's seat on the frame bracket that holds the seat belt next to the SiriusXM antenna; the Garmin G5 is using it's internal antenna under the panel (works fine) and the ELT is using it's internal antenna under the pilot's seat (works fine).
• 1 Sirius XM Receiver: Garmin G3X Touch (GDU-465); this uses a Garmin GA24TNC antenna mounted inside above the pilot's seat beside the GDU-465 GPS antenna.
• 4 Bluetooth Transceivers: Garmin G3X Touch (GDU-465), Garmin GNC 355A, Garmin GTR-45R, and Garmin GMA-245R (mounted above the passenger's seat); these are all using internal antennas, and I have no issues with reception on my iPad Mini and iPhone.
• a 1090 MHz ADS-B Transponder; a Garmin GTX-45R mounted under the panel, and the Rami AV-74 Transponder antenna is mounted under the boot cowl, as I plan to add a belly pod in the future.
• a 121.5/406 MHz transmitter in the INTEGRA AF ELT; the antenna is mounted horizontally behind the extended baggage.

 

[Cactus Charlie]

the ELT is using it's internal antenna under the pilot's seat (works fine).

My testing, which used a movable GPS antenna temporarily connected to the GDU 465, showed significant attenuation of the GPS signal when that antenna was placed close to the ELT under the pilot seat. Not surprising since CF is known to be an RF attenuator.

I concluded that the ELT's built in GPS receiver would have delayed or no position acquisition after a crash. I had intended to give the ELT a serial data GPS feed but never got round to it.

How did you determine that the ELT internal GPS antenna "works fine"?