Yes!
Be sure it has a proper sized ground plane to operate on
(height of antenna = radius of ground plane. I used three wooden dowels
to mount the ground plane disk to the back lower fuselage.) This way the
cable comes out the top and then runs to the transponder.
Jim
> 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.
> >
> > The Europa Forum is supported by Aviators Network UK
> <info@avnet.co.uk>
>
>
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