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I was wondering how sensitive these systems are. We are planning a modification which will involve either moving these antennas or leaving them in place under a radome (the radome is designed for x-band frequencies)

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    $\begingroup$ Sorry, you're thinking about modifying a safety system that you don't understand,on the basis that somebody on the internet said "Sure, go for it"? That really is staggeringly bad judgement. $\endgroup$ – David Richerby Apr 16 '19 at 9:00
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    $\begingroup$ Who is the "we" in your question? No need to give the specific organization; something like "xx crew at a single-runway regional airport" or "airplane xx-function team" would likely be sufficient. $\endgroup$ – a CVn Apr 16 '19 at 11:01
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    $\begingroup$ You do realize that virtually all TCAS and most MB antennas come with their own 'radome'? That's effectively what that white shell is. Also, antenna placement on an aircraft is not a random process so moving them takes careful consideration of necessary radiation patterns and potential interference. If you're planning on adding an x-band antenna that needs a large radome (that could potentially encompass the TCAS and MB) you need the assistance of a qualified RF engineer and DER, not this site. $\endgroup$ – Gerry Apr 16 '19 at 11:20
  • $\begingroup$ Hi all. Thanks for your comments. David - the answer is no... We're brain storming at very early stages so I wouldn't jump to conclusions about our judgement quite yet. CVn - we're a design team in a small company looking at modifications to a business jet. Gerry - noted, thanks. Delta Lima - perfect. I was hoping to get some extra insight (I did) and maybe hear about specific cases (not yet). $\endgroup$ – user1412867 Apr 16 '19 at 19:32
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Theoretically it can be done; a radome is an antenna enclosure for a radar. It is designed to have sufficient permittivity at the frequencies of interest. For TCAS, you need to look for 1030 MHz and 1090 MHz (IEEE L-Band). Marker beacons work at 75 MHz (VHF)

It is very important to make sure the TCAS antenna has good line-of-sight to all directions. If not, TCAS will be blind in some directions, with potentially dire consequences. If it is the TCAS directional receiver antenna plan to cover, you have to be very careful; near field interference can cause the directionality of the antenna to be affected, causing misleading position indications on the TCAS display.

So whilst in theory the you can put the antennas in a radome, in practice you have to very be careful and you will require many measurements to validate that the setup actually does not degrade the performance of TCAS.

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