I know this is going to sound less than brilliant, but do I have to mount the
transponder antenna with the ball end down?
Cleve Lee
A198 Mono XS
On Tue, 30 April 2002, Fred Fillinger wrote
>
> 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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