First flight with SV - Comm radio blocks ADS-B

[kurtfly]

I performed first flight today with my Dynon SV Touch retrofit. Most everything worked perfectly in my Glasair! My system includes: SV 1000-T, Dual ADHARS, EMS, SV-XPNDR, ABS-B, ARINC, connected to my GNS-430W, and GNS-250XL.

My only squawk is the ADS-B receiver will unlock/ stop receiving whenever I key the #2 transmitter. The ADS-B antenna is only about 1 foot from the #2 Com antenna. I believe my transmitter is saturating the ADS-B receiver. The Receiver works perfect otherwise. Being all composite I have all the antennas mounted inside; including the ADS-B Rx antenna, except the transponder. It is on the belly.

Before I try to relocate the ADS-B antenna a thought I would try installing a hi-pass filter at the ADS-B receiver input. I ordered a Mini-Circuits BHP-500 filter. It has a low insertion loss at frequencies above 500MHz to 3200MHz (about .3 dB at the ADS-B freq.) but about 75dB attenuation at the VHF com frequencies.

Has anyone else experienced this problem?

Kurt Rutkowski
N122KR
Glasair Super II RG

 

[dynonsupport]

Kurt,
I'd be surprised if the 130 MHz signal from the COM messed with the 978MHz ADA-B signal.

What kind of ADS-B antenna are you using and what kind of ground plane does it have? Also, what kind of message are you getting saying the ADS-B is offline?

 

[kurtfly]

I am using a TED 104-12, mono-pol antenna mounted on a sheet of alum. The ground plane is about 8" x 10" square. I am seeing the ADS-B receive status on the lower left map page show "Receiving" but when I key the transmitter it will turns yellow and "Not Receiving" (or something like that). After I un-key and about 5 seconds later it goes back to "Receiving".

I have repeated this several times with the same results.

 

[Carl_Froehlich]

You might want to monitor GPS signals when you do this test and see if there is an issue there as well.
I would first do a simple swap of the GNS-430 comm antenna with the GNS-250 comm antenna and see if this changes the symptoms. If so, look for a bad antenna connector for feed line issue. Here an antenna tester like the MFJ-259B comes in handy.
Harmonics from the comm radios can really screw up GPS and/or ADSB signals. My GTN-650 lost all GPS signal when transmitting on the GTN-650 comm. Comm #2 did not affect it. After swapping out comm #2 the problem went away (new radio is the Dynon one - and physically located a lot further away from the GTN-650 than the original comm #2 radio). I found out the noise in the GTN-650 was being reradiated into comm #2 (in receive mode) when transmitting on the GTN-650 and that hit the GTN-650 GPS system. It seems this type of issue is a known problem with the GTN-650. Perhaps other Garmin units are affected - I don't know.

 

[kurtfly]

Thanks Carl. I have one of the MFJ-259 SWR analizers and I' ll check it out. If I remember correctly, my #2 comm, the GNC-250XL, has a GPS Notch filter installed per Garmin instructions. I have not seen any GPS unlock issues. But if the Garmin notch filter is a true notch filter (1575MHz) the 978 MHz ADS-B may be out of the range of the filter.
I have a hi-pass filter on order - should have it Wed's. I'll post my findings. Ill install this on the input to the ADS-B receiver.

 

[Carl_Froehlich]

One other suggestion, mount your ADSB receiver next to the ADSB antenna if not already done. This should minimize noise cross talk.
On my RV-10 the receiver is in the tail cone, antenna right below it on the belly.
Carl

 

[kurtfly]

I installed the hi-pass filter and flew with it today. It has fixed my problem. My ADS-B receiver will stay "receiving" when I am transmitting on my #2 com radio.
I installed the Mini-Circuits BHP-500 (500MHz Hi-Pass) at the ADS-B input connector.

I have about 3 hours flying time now with the SV. I am impressed how well the system operates flawlessly, right out of the box. What a great product!

 

[Jonas]

just a wild guess, but it could also be a voltage drop depending on the layout and condition of the 12volt wiring, start to transmit and the amperage is suddenly too high for a poor connection... Seen that happen before.