Is this a good TCAS antenna location?

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peter-h

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peter-h
I have put two pics here

The avionics shop proposes that the rear GPS antenna is replaced with the TCAS antenna, and the front VHF (COM) antenna is replaced with a dual (Comant WAAS 2/3, $1000) antenna.

What concerns me is that the TCAS antenna will be slightly "behind" the roof curvature, for targets which are at your 12 o'clock.

The roof is composite but it has a ground plane in it, which runs all the way from the front edge to the back edge.

The underside is more straightforward, with a fairly flat bottom which is all-metal.
 
The top or bottom locations are good but I vote for the bottom belly location. No need to relocate antennas and you have a better ground plane. The bottom location will give you a better view of traffic below ahead like that ahead of you on final. The top location will give you a more accurate bearing for traffic in the pattern while being on the ground.

José
 
peter-h said:
I have put two pics here

The avionics shop proposes that the rear GPS antenna is replaced with the TCAS antenna, and the front VHF (COM) antenna is replaced with a dual (Comant WAAS 2/3, $1000) antenna.

What concerns me is that the TCAS antenna will be slightly "behind" the roof curvature, for targets which are at your 12 o'clock.

The roof is composite but it has a ground plane in it, which runs all the way from the front edge to the back edge.

The underside is more straightforward, with a fairly flat bottom which is all-metal.
Click to expand...

AFaIK, that curved ground plane will refract the signals enough to make the forward horizon visible to the antenna.
 
I should mention that I will have both top and bottom antennae.

The bottom one will be approximately vertically below the top one.
 
Peter what type of TCAS are you having?

José
 
peter-h said:
Avidyne 605
Click to expand...

It is a good system. The 605 requires the two antennas to determine bearing. The dual blade is for lateral direction and the single blade is for longitudinal direction. On top you have limited ground plane for the lateral antenna (dual blade) while on the bottom the wings provide plenty of ground plane. Make sure the antenna cables are properly matched to insure accurate bearing.

José
 
Peter the shop must perform a skin map to determine where to place the antenna. Without skin mapping you are only guessing where to install and on an composite aircraft the wrong location is a fatal mistake.

Ask the shop to review the skin mapping they performed. If they did not do one I would advise you contact another shop or have an outside shop do the skin mapping. Or contact MFG and they can tell you the best location since they did he skin mapping as part of certification.

Denny (now retired)
 
Thanks Dennis.

I have written to Avidyne (email and fax) and will see what they say.

I have never heard of anybody doing skin mapping on TCAS. What I have seen is mapping out a stormscope, by walking around the aircraft with an electric drill :)
 
For small aircraft you don't have too many options of where to put the antennas. For TCAS ground plane is an important factor to insure good balanced coverage.

José
 
What I find a bit confusing is that there is a variety of installations out there which all seem to work, well enough for people to be happy with them.

AFAICT the ground plane in my roof covers most of the roof. It has to, for lightning protection.

I also see installs where the TCAS antenna is in the correct place for the aircraft shape i.e. at the highest point (ref my pics, where the forward COM antenna is now) and the forward GPS antenna is where it is i.e. about 10" away from it. The installation seems to work fine.

One installer who has done a number of these commented that in most cases it is always impossible to install the antennae in compliance with the Avidyne requirements!

In particular I wonder whether there is any technical reason to mount the bottom one vertically below the top one. That requirement suggests that the system could be confused by the time delay between the two antennae receiving a signal, but is it really using a time delay at all? I don't think so. It is only about 10ns per ft. The installation requires balanced cable lengths but that would be for a balanced attenuation.
 
I got a reply from Avidyne USA who say the proposed upper antenna location is fine.

They want to see where I propose to put the bottom one.
 
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