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Nav ILS antenna signal, GS gets blacked out by plane

ssturges

Active Member
I have an RV9A and I placed a classic Comant whisker type Nav antenna on the bottom of fuselage under the horizontal stabilizer(near the rear tiedown), a common place for it. However I found that if I am on the glide slope the signal gets blanked by the plane. If I bank away from the approach the Glide Slope shows up as the antenna becomes visible in the path. I really thought this location would work.

I worry that wing tip installations and the fuselage will blank the signal from the far side of the plane in a similar way. My other option to place it on top of HS stabilizer, a bit of work and an eye sore however. This is an IFR plane, it has to work all the time, not a nice to have.

Has anyone else ever seen this type blanking of the NAV/ILS/GS signal?

I am going to take the VNA to the able just to make sure it not a weak signal due to a cable issue. I can get VORs at 40+ miles at 3K feet however.
 
Out of curiosity, what radio are you using for VOR/ILS reception (e.g. SL30, GNS430W, KX-155, etc.). How is the signal "split" between VOR and GS receiver? Some radios (SL 30, Garmin GTN7xx/6xx) have one input from the VOR antenna, some have two (GNS 430W) and require a splitter or similar.

If the latter, it's possible that the splitter is not full bandwidth on each port and is operates as a notch or passband filter in addition to a splitter. An example of this is the Comant CI-507 (One Antenna - VOR output and GS output); the VOR output will only carry 108 - 118MHz and the GS output will only provide 329 - 335MHz. Any receiver connected to the VOR output will not "see" a GS signal unless it's really up close and personal...
 
The ILS signal is relatively strong. It makes little difference where the antenna is located assuming it and its associated feed line are properly installed.

In other words, while I consider under the tail a bad VOR antenna location (as in eyeball threat), you should not be having this issue.

I have a home brew wingtip antenna that picks up ILS way beyond useful range, and VOR stations at 100+nmi.

Carl
 
I have had similar issues with different manufacturers of radios coupled via an antenna coupler to a VOR/GS antenna. The fix in that case was to use a new coupler from Mini Circuits which had far better isolation specs.
 
I have had similar issues with different manufacturers of radios coupled via an antenna coupler to a VOR/GS antenna. The fix in that case was to use a new coupler from Mini Circuits which had far better isolation specs.

Exactly where I was going with this line of questioning. I encountered this issue when I attempted to split a single VOR/GS antenna and feed an SL30 and GNS 430W. The Commant splitter "filtered" the GS frequency from the VOR output going to the SL 30; No GS was visible until I was well inside the OM, almost the MM.

I ended up with a Mini-circuits ZSFC-3-4+
 
Radios have different receiver input impedances so unless nav radios are the same make/model then it takes a coupler with good isolation specs to make it work.
 
This may be a splitter vs. diplexer issue. My understanding is that there is less signal loss through a diplexer.
 
This may be a splitter vs. diplexer issue. My understanding is that there is less signal loss through a diplexer.

Agreed. Using a signal splitter to separate/combine the GS and VOR/LOC is a misapplication, a ~4db loss minimum and maybe a lot more. A good diplexer will be almost lossless for both bands .

Was VNA mentioned? If one is OK using a vna, ginning up a windshield mounted dipole GS antenna would be easy and hard to beat for performance and signal reliability.

Ron
 
Definitely check out the coax and connectors. It should work there. But wear eye protection anytime you’re under the plane.
 
I am using an IFD540, Based on the comments I will look carefully to see if a cable or diplexer issues with the VNA. The cable did meter out with the DMM in the simple check.
 
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I am using an IFD540, Based on the comments I will look carefully to see if a cable or diplexer issues with the VNA. The cable did meter out with the DMM in the simple check.

I have the Avidyne in my RV-10 with the antenna underneath the tail on the bottom of the fuselage, and have never had any problems with blanking. As mentioned, check your splitter, as well as the coax connections. They can be the culprit as well.

Vic
 
The plane is newly mine, an RV8. I have tons of pictures and I have put eyes on every corner that I could including during the prebuy inspection with cowling and inspection plates off.

But I cannot find the diplexer. I see one coax cable coming from the NAV port of the 430W and one from the G/S port. Both of these cables go aft. Then, I can see the coax going to the wing mounted Archer antenna. What I can't see is where the NAV and G/S coax connect to the antenna coax. Where might a builder have put the diplexer?

Unfortunately the builder died a few years ago, not aviation related.
 
The plane is newly mine, an RV8. I have tons of pictures and I have put eyes on every corner that I could including during the prebuy inspection with cowling and inspection plates off.

But I cannot find the diplexer. I see one coax cable coming from the NAV port of the 430W and one from the G/S port. Both of these cables go aft. Then, I can see the coax going to the wing mounted Archer antenna. What I can't see is where the NAV and G/S coax connect to the antenna coax. Where might a builder have put the diplexer?

Unfortunately the builder died a few years ago, not aviation related.

This is odd. Usually the diplexer is close to the radio (why run two coax lines when one will do?). I suspect the original builder had a separate GS antenna somewhere, sort of like 1976 Cessnas that had a small dipole just for GS, up high on the windscreen.
 
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