Brian Tarmar wrote:
> Having reached the stage of planning cable routes which separate as much as
> possible power from feeders, can I mount the transponder aerial/antenna in an
> outer underseat stowage? Either totally within the airframe or protruding.
>
> I believe this position offers the shortest cable run (in line with design
> requirements) and keeps costs down if the more expensive RG400 co-axial
> is to be used.
Shortest cable run is a very important factor. Typical for RG-142 and
RG-400 is about .14db/foot loss at 1,000 mHz. So 5 feet means about
15% loss; 10 feet - 28% loss; worst-case 20 feet for way back in tail
- whopping 48%. Garden variety RG-58 is much worse and is for VHF.
> I hear tales of "frying the family jewels" with RF (perhaps
> the aerial should go under the passenger seat!) and blanking by the engine
> when the interrogating station is ahead. Your thoughts please.
Old wives' tale I believe. The duty cycle of transponder replies at
1/second is about .0008%, or 2 milliwatts for average radiated power
---From a 250W box. A 900 mHz cordless phone puts out much more than
that - continuous duty cycle, and they allow it to be about 1 inch
---From our brain. A foot or two distance for xponder is certainly safe.
A big factor is whether there are people or large metal objects
between the antenna and ATC's beacon antenna. IOW, draw an imaginary
line about 2-deg. downward from proposed location through the front of
the A/C in flight attitude. Should be clear of big metal or people,
so mounting a 1/4 wave stub antenna through the fuse bottom at/near
the lowest part is best. Drag is minuscule.
Thus the installation can be a matter of compromising convenient
location and coax loss. It is possible to get satisfactory results
with setups that don't look good on paper, but here FAA requires only
70 watts for low altitudes, and if you have 200W out the back of the
box, some coax loss and absorbers in the line of sight (RF will bend
around them if not too close) means it may still work well.
I'm not concerned about coax distance from power wiring; just don't
bundle. Shielding is 98% effective, and xponder freq too high to be
bothered by the weak harmonics of audio frequency noise in power
wiring, unless maybe inductively coupled via wire bundling.
Best,
Fred F.
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