SL30 glideslope problem

I have a SL30 displayed on Skyview with a Bob Archer wingtip nav antenna and RG142 antenna cable. I can tune VOR’s from beyond the published service volumes, localizers from 40+ miles out and get a stabile localizer signal all the way inbound with headings +- 45 degrees of the localizer course.

The problem is with glideslope. When intercepting the glideslope using a published approach, the glideslope will come in the indication limits and then may either drop intermittently and or become erratic. All the while the localizer is steady as a rock.

The installation is 8 yrs or so old, and the aircraft has always been VFR, but now I am starting to work on an instrument rating. I observed this behavior on my post installation check flights, but somehow never got around to chasing this down.

Typically if there are problems with the antenna, or antenna cable, localizer is the indication that can be squirrely. Since reception of VOR’s and localizers is just fine, I believe that the antenna is ok. I’d appreciate it if anyone knows anything about either a SL30 to Skyview interface problem or has other suggestions, to chime in before I take the SL30 to get checked in a shop.

Well,… As you noted, the antenna is working at 112 MHz. Now the GS frequencies are 3x higher; the antenna should still be resonant (3/4 wave plus ground plane instead of 1/4 wave plus ground plane) although the matching network won’t be optimum. And there’s only one coax (the SL30 has an internal diplexer) so it’s highly unlikely to be the issue. And both GS and LOC data goes out the same RS232 line. So I’d double check the setup settings on the Dynon. If they look okay, time to pull the SL30 and send it to the shop for a test. They should be able to tell you if there’s a problem with less than an hour labor. Now, if there is a problem, you now have a real problem. These are perhaps the best nav-coms ever built (originally from Apollo) but they have not been produced in some time. Some parts may be no longer available. Garmin may decline outright to work on it. And even if they do, their fixed price policy will leave you wondering if you should invest that much in an old radio that likely cannot be repaired at some point not too far from now. Your shop might be willing to give it a try, even though it is probably not legal for them to do so. Good luck.

Bob, thanks for your response, which is pretty much where I am.

For repairs you could try Chris Short. I have no personal experience, but the guys on the GNS-480 group have had success.
The GNS-480/CNX-80 is essentially a SL-30 married to a GPS navigator. Both developed from Apollo/UPSAT.
https://www.palmdr.com/

I just had a crazy idea, easy to check. Is it possible the original coax has a diplexer at its end, with the GS connection not used? This would greatly reduce the GS signal. Easy to check.

BobTurner said:

I just had a crazy idea, easy to check. Is it possible the original coax has a diplexer at its end, with the GS connection not used? This would greatly reduce the GS signal. Easy to check.

Click to expand...

No, I wired the system myself, no diplexer, only a single coax directly to the SL30 nav input. But yes an open diplexer would do that.

I hesitated to jump in here but a friend just went through this with identical equipment and problem on a purchased RV. He finally called Dynon and they told him to set the SL30 menu for indicator choice to none. He did and it all worked as it should. Doesn't sound right for sure but trying this might be a good thing or a call to Dynon might be in order. BTW, I have been told that the SL30 does not use a duplexer like the 530/430 does. It's internal the SL30. something to verify though.
I was hoping he would see this and respond but sometimes work keeps him out of the loop?

When you tune in an ILS, does the GS flag?

BobTurner said:

I just had a crazy idea, easy to check. Is it possible the original coax has a diplexer at its end, with the GS connection not used? This would greatly reduce the GS signal. Easy to check.

Click to expand...

I've been tearing into my panel to do an upgrade, and while I'm new to this, I did notice that there was what I THINK was a diplexer at the end of my cable as I was de-installing things...

...and it wasn't connected...

...and I had a horribly unreliable GS signal....

Suspect this might be why.

This might be helpful https://forum.flydynon.com/threads/ils-rnav.13041/

Post# 14 and #17 might be of interest.

As posted earlier, on my friends RV The Localizer worked fine but the GS was inop until a setting on the SL30 was set to none?

Good luck sorting through everything

It has been awhile, but I finally got motivated to solve the problem. Even though the antenna seemed to be performing well I had concerns that there might be a ground impedance problem at the higher GS freqs.

Applying basic troubleshooting procedures, I removed the wingtip and checked the antenna. The antenna looked good, but I cleaned up the feed and shield connections just because. My antenna is grounded to the airframe via 6 of the wingtip attach screws into nutplates. Having concerns about the ground path through the nutplates, I did a milliohm check and got 2 miliohms. That shouldn’t have been a problem, but just to rule out ground plane issues, I installed an awg 10 ground jumper from the antenna base leg to the wing end rib. Ground resistance was now around 1 milliohm.
After reinstalling the wingtip, i used an antenna analyzer to check both the localizer freqs and gs freqs. The swr was 1.2 min throughout the localizer freqs and 2.3 in the GS freqs. The antenna appears pretty tuned to the localizer freqs as the swr outside of that range increased significantly. That said, gliideslope isn’t exactly on a harmonic of the localizer, and the best SWR was slightly above the GS freq range in the freqs that would be the harmonic. All makes sense.
Went flying and glideslope problem resolved. I believe that it was in fact a ground impedance issue.

RVDan said:

I believe that it was in fact a ground impedance issue.

Click to expand...

Hard to imagine how that affected the GS but not the localizer reception. But you can’t argue with success!