GS signal weak

chrisbre

chrisbre

Filing Flight Plan
Joined
Jul 16, 2014
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17
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Hamburg/Germany
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Display name:
chrisbrenner
My GS needle starts moving and the off-flag disappears at the middle marker, or sometimes even later. Nice to practise LOC approaches, but nothing you want to have every day. I checked the radio (KX-155) by using it in another airplane – works fine. I also replaced the antenna/ GS coupler. No change. What is your idea on that? Bad connector between the coupler and the radio? Could the NAV-antenna just work fine for the localizer but not for the GS?
 
LOC and GS might well be on different antennae.
 
chrisbre said:
My GS needle starts moving and the off-flag disappears at the middle marker, or sometimes even later. Nice to practise LOC approaches, but nothing you want to have every day. I checked the radio (KX-155) by using it in another airplane – works fine. I also replaced the antenna/ GS coupler. No change. What is your idea on that? Bad connector between the coupler and the radio? Could the NAV-antenna just work fine for the localizer but not for the GS?
Click to expand...

Not enough information.

  • When was the last time you know for sure that the glideslope worked in your current configuration.
  • Describe the antenna/glideslope coupler, or even better, mfg & model
  • Describe the nav antenna and where placed on the airframe
I'm just preSUMing that the VOR and LOC work just fine?

The nice thing about what the FCC did for us when it assigned VOR/LOC and GS frequencies is that they made the GS almost a perfect 3x of the VOR/LOC band. A quarter wave antenna for LOC is a 3/4 wave antenna for GS and should work just fine, providing that nobody bollixed up the balun match from coax directly to antenna elements.

Jim
 
NAV antenna ist the original one installed in 1975 at the vertival fin of my Piper Arrow II.
I substituted a RAMI AV-570 for the old CI-507. Same result with both oft them.
There is another NAV splitter installed to feed the second KX-155 (without GS), I just don’t remember the brand. Sequence is: antenna, NAV-NAV-coupler, NAV-GS-coupler, radio.
Gideslope reception has always been weak since I bought the aircraft five years ago but further decreased with the years. However, I never cared about it before getting my IR.
VOR/LOC always have been fine.
 
Failing CDI? Borrow a working CDI to verify...
 
Like any other electrical problem, first suspect are connections. That is because, statistically 70% of problems with electrical equipment are connection problems. So look there first.
 
chrisbre said:
NAV antenna ist the original one installed in 1975 at the vertival fin of my Piper Arrow II.
I substituted a RAMI AV-570 for the old CI-507. Same result with both oft them.
There is another NAV splitter installed to feed the second KX-155 (without GS), I just don’t remember the brand. Sequence is: antenna, NAV-NAV-coupler, NAV-GS-coupler, radio.
Gideslope reception has always been weak since I bought the aircraft five years ago but further decreased with the years. However, I never cared about it before getting my IR.
VOR/LOC always have been fine.
Click to expand...

That's not the optimum way to couple off the glideslope signal. As a matter of fact, I'm not sure that would even work. A single coupler box should give you two VOR/LOC outputs and a GS output from a single antenna. They are commonly called "triplexers" as opposed to the box that only gives you a two VOR/LOC outputs from a single antenna (diplexer).

If you can hold on for a couple of months, I'm working on a new triplexer design and I'll let you try one to see if that fixes your problem. Or you can nose around your local airplane boneyard to see if you can find one there.

Just for grins and giggles, for a few bucks in coax adapters you can hook your antenna directly to your GS receiver to see if that makes things any better. As a matter of fact, you can test to see if it is a coax problem with a single BNC female barrel.

Jim
 
bnt83 said:
This setup is in several STC drawings
Click to expand...

Well, follow me along. The first splitter left of the antenna splits the signal in half power, which is a range reduction of 70%. Then the next splitter splits it in half again, which reduces you down to (70% * 70%) 50% of range. That and I'm not all that convinced without going back to the Wilkinson equations whether or not that first splitter will split both LOC and GS. Mostly in a triplexer, we split the LOC down the middle but use a series LC to tap all the GS signal off of the antenna.

Just because something is in an STC drawing is no guarantee that it is being done correctly ESPECIALLY if you don't tell me what the characteristics of the splitters are.

Jim
 
weirdjim said:
Well, follow me along. The first splitter left of the antenna splits the signal in half power, which is a range reduction of 70%. Then the next splitter splits it in half again, which reduces you down to (70% * 70%) 50% of range. That and I'm not all that convinced without going back to the Wilkinson equations whether or not that first splitter will split both LOC and GS. Mostly in a triplexer, we split the LOC down the middle but use a series LC to tap all the GS signal off of the antenna.

Just because something is in an STC drawing is no guarantee that it is being done correctly ESPECIALLY if you don't tell me what the characteristics of the splitters are.

Jim
Click to expand...


They just say to use these (or equivalent):

http://www.aircraftspruce.com/catalog/elpages/garminsplitter11-13045.php?clickkey=438674

http://www.aircraftspruce.com/catalog/avpages/coupler507.php?clickkey=31073

I don't understand why they draw it that way (above) but they do not use triplexors in any of the drawing I've seen so far.
 
I have a quad in my airplane. It feeds two GS, and two Nav, from one box.
 
Hi folks,
just to give you an update on the status of my problem:
I recently tried to remove the Nav/Nav-coupler. That's when I realized that the NAV have been connected like this:



................................._______GS1
................................/
.............................../
............. _________NAVNAV/GS-coupler
............/..................\
ANT-----<NAV/NAV-coupler.........\________NAV2
............\
............ \
..............\_______NAV ______NAV1


So I removed the NAV/NAV-Coupler and made one NAV INOP. That's when things started getting strange:

........_________GS1
....../
ANT<NAV/GS-Coupler
.......\
........\_______NAV2


This didn't affect GS-reception at all.
But when I made NAV 2 INOP:

........_________GS1
....../
ANT<NAV/GS-Coupler
.......\
........\_______NAV1

GS- as well as LOC-reception had been fine. (??????)
Well, that's how I left it for the moment, at least I can do ILS-approaches now.
Any ideas why it makes a difference which NAV radio is connected to the GS/NAV coupler?

Got a triplexer today and will try this next time. I will keep you updeted on the issue, however, as you noticed, it might take a time

Thanks a lo for all the information I got here so far, I really appreciate your help!

Chris
 
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