Poor reception flying toward station.

[Hangar 1271]

I'm flying an RV-8 with a GTN650xi as comm 1 and PS Engineering 200A as comm 2. While flying away from an airport, I can receive fine on the GTN but when I get about 25+ miles out and turn around I can no longer hear ATC. They can hear me but I can't hear them. I have found the 200A will receive farther out so I turn comm 2 receive on for a while until I can hear ATC on comm 1. Both comms have the same type of Commant bent antenna and are about 25 in apart from each other on the belly of the plane. Any ideas? Thanks.

[blaplante]

Try swapping the antennas (by moving the cables between the radios). That'll help isolate the issue.

[Hangar 1271]

Good idea. Someone also suggested turning off squelch to see if that is the issue. Much easier than pulling up the floor as a first step.

[ksdflying]

Curious if you have found a root cause and or fix.

I have the same issue with a new gtn 650xi (com1), with my RV8, and since day one. Com2 is a gtr 200.
I have rechecked wiring more times than I can remember. Grounds, both aircraft and shields are all matching the drawings. Pinout checked too. 100% sure now have all wiring correct. I've even replaced BNC connectors. Swapped antennas of course. Nothing seems to help.

Wondering if I need to change to "straight" whip antennas for at least the 650xi com1.

Curious if you have found a fix.

[Carl Froehlich]

Originally Posted by ksdflying

SNIP.

Wondering if I need to change to "straight" whip antennas for at least the 650xi com1.

You would need a very good instrumented test range to measure the very slight performance difference between a bent whip and a straight antenna. In other words there is no practical difference.

Look more toward that amazingly expensive 650XI and the associated install. I’d start with the “desense” connection (Garmin’s work around for an overloaded receiver front end when transmitting on the other radio). Make sure it is not in play when your are just using the 650XI. Try just pulling the GTR-200 out of the tray and see if the 650XI works better.

Carl

[Walt]

Squelch set to ‘0’ ? RG400?, crimped BnC connections? Ground plane resistance (I like to remove paint under antenna).

[ksdflying]

Mr. Carl, Mr. Walt!! Thank you for your help!.
1. I've toyed with the RX Squelch, and Carrier Squelch in the 650xi. I seem to not "hear" its affect whether at "0" or at "100". Still trying to understand these settings and effects. Yes, RG400. I've not checked Ohms resistance. Guess I need to do this. I've done the typical Continuity check after crimping, but you have a point I need to look at for ground plane resistance. Interesting.

2. Appreciate hearing it likely may not be worth spending another $400 to change out my Rami bent whips for Straights. I have the ground clearance. Just wondered. Thanks. HOWEVER, I have been trying multiple times to get a straight brass tacks answer to this whole "desensitivity" function you mention. I have read about that. But asking questions to the almighty maker of this **** 650xi, I can't get an answer if I need the TX Interlock or any other wiring. I'm trying to get tribal wisdom on this. GTR200 calls for it. But "they" say the new 650xi units have it built in. Still, it seems like I can't get clear direction for my combo of 650xi -> gma 245R -> gtr 200. But Carl you might be on to something, I just don't know which wire or connections or where or how to affect this...properly.

Thank you for responding you two. I'm desperate for tribal wisdom. Info is NOT in the manuals. I don't have anything good to say about this 650xi, but I have been schooled to keep my mouth shut. So the verdict is still out. Would love to be proven otherwise.

[Walt]

I have found many shops don't bother to install the xmt intlk wiring, so I think that being the issue is unlikely, but possible.
On the 650 it would be connector P1003, pin 9. When you ground this pin it lowers the receiver sensitivity. The ground can be sourced from the GTR200 or from the key line out of the audio panel.
I've installed many 650/750 radios and have never had a complaint. Antenna location and installation are at the heart of good reciever functionality.
I personally use Comant straight antenna's whenever possible.
If you can swap the antenna cables at the radios and see if there is any change.

The GTN install manual explains how and what the squelch setting do, see page 6-43 for more info: (a "noisy" aircraft for example can effect sq operation).

COM Squelch Functionality
The GTN Xi COM receiver squelch silences the received audio when there is no signal detected above a specific power threshold. RX Squelch is the primary squelch and functions by calculating the ratio of the signal power to the background noise power then compares it to the set threshold. If other aircraft equipment generates noise that consists of pure tones, those tones may be interpreted as signals and used for the calculation. As a result, the squelch breaks when the corresponding channel is selected. In this case, increase the RX squelch percentage or isolate the noise source until the squelch breaks no longer occur.

Carrier Squelch is the secondary squelch and functions by calculating the signal power only and comparing it to the set threshold. If other aircraft equipment is generating noise of a more random nature (wideband), the combined power may cause squelch breaks on many channels. In this case, increase the carrier squelch percentage or isolate the noise source until the squelch breaks no longer occur.

[Carl Froehlich]

Dan,

I suggest you start with the easy stuff:
- Swap antennas between Comm #1 and Comm #2. Easy enough and you can use a length of coax as a jumper if needed.
- Pull the GTR-200 out of the tray - any effect on the 650?
- Don’t get carried away with the whole ground plane thing. This is RF ground, not DC. Typically a star washer under the mounting screw nut is more than enough.
- Coax continuity is a good check - but it does not tell the RF story. For that you need an antenna analyzer. On the continuity check verify center pin to center pin on the coax, shield to shield and no continuity between the center pin and shield. Disconnect the coax before doing this.

Carl

[ksdflying]

Appreciate this.
That extra explanation of the two squelches help. I'll go try to apply this today and see what effects I get.
Yes, I have been staring at the 1003 connector Pin 9. I was shown a screenshot from a TSO install manual that showed that pin 9, but no where shows if I need to connect it. Well, I know that it is NOT connected. So perhaps I am fine. Thank you for explaining this. If I did want to ground it, what you said seems to resonate.
Okay. I will chew on this and see if I can make anything happen today.
Appreciate the focus on the antenna. I fear this is my problem. I fear my ground plane and proximity of the two Coms is my problem. They are 20" apart, mounted a few inches behind the wing spar center box. Not great. and definitely not the 18 sq inches ground plane area that the garmin manual asks for. I really wish today I mounted the two Coms on belly centerline.
* When I only had the gtr 200 com, my reception was FANTASTIC. But just adding this 650xi in May...it is useless and is vastly worse than the 200, and now that install has affected the 200 now. Its like 2 whole steps backwards. Now I don't have either COM working well, when the single one was before.
Darn though I am so very tempted to change out to straight whip antennas...you can tell I am reaching here.
I'll go to work on this.
Thanks. I'll report back if I find anything positive.