J D Bean wrote:
>
> When listening to my radio in my workshop on FM 101.3 I notice
> that if my GPS aerial is within about 4 feet there is noticable
> interference. Are there any known restrictions on aerial locations
> and distances between differing aerials?
If you're referring to interference heard on FM
---From the GPS, that may be no more than the DC-DC
upverter for the electro-luminescent display
backlight. Not a problem with antennas
remote-mounted from such noise sources.
The general rule is keep all antennas 2 feet from
each other (that's about 1/4 wavelength at VHF),
and a similar distance from any metal item 1/2
wave-length+ in length -- e.g., the pitch push
rod. Some comm mfr's say keep the comm antenna as
far away from the ELT antenna as possible, as some
ELT's re-radiate certain freq's, such as 121.15,
which garbles comm output at that freq.
Theoretically the transponder antenna needs to be
only be 6" from any other (altho the coax itself
may present a detuning element, so back to the 2'
rule, with reference to a nearby VHF antenna).
> I am hoping to fit a GPS aerial in the fuselage roof somewhere near the
> old fuel filler position but this may be a bit close to the VHF aerial.
GPS operates at 1.5 gHz, meaning 1/4 wavelength is
also very small. My UPS GX-65 GPS map/comm manual
makes no big deal about placement (except on top,
in a metal A/C of course). Two-foot rule again
---From any VHF antenna.
> I notice that my present GPS aerial is connected with a small outside
> diameter co-axial cable with SMC (or something like that) co-axial
> connectors. What is the latest thought on suitable aerial co-axial wire
> and connectors and can the more modern lightweight higher performance
> cables be used?
I've seen "TNC" connectors on GPS, but whatever.
Anything you need can be found at mouser.com, no
lower limit on order amount. digi-key.comm, too.
Important, though is the coax cable. UPS
recommends RG-142B coax for its GPS, which is hard
to find. RG-400 should be OK, but it's $2.00/ft
(from Aircraft Spruce).
Ditto for the transponder -- at least RG-400;
don't use RG-58 at gigaHertz frequencies (GPS and
xpndr).
> I have a Bob Archer transponder unit close to the tail bulkhead.
I hate to be finicky here, but is that xpndr
antenna easily movable?
At the tail bulkhead, you're probably busting the
9-foot coax length limit specified by both King
and Narco, and at $2 a foot. When you're checking
in with an Approach Control facility, you're
basically pointed right at their receiving
antenna, so the signal will have to pass by a
number of metal bits, plus thru RF
energy-abosorbers like your body and a spinning
carbon-fiber prop like a Warp Drive. The antenna
I think should be near the lowest point on the
fuselage bottom. There's enough sensitivity in
ATC's receivers to handle a wide range of
variabilty in xpndr installation and in-service
degradation, but for optimum performance, the tail
of an Europa isn't the best place for an xpndr
antenna, IMHO.
Also, when you run coax cables up to the panel,
keep the xpndr one separate from any other.
Hope this helps!
Regards,
Fred Fillinger, A063
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