I have GPS interferance between some GPS units and Icom radios. I think its
due to the processor and clock. Dave mentioned waving a handset around the
unit. Also try auto scaning the handheld to see if it stops and locks on the
GPS inteferance. Most only play up on a few frequencies. You can have radio
and GPS installed side by side for hours and not know you have a problem
until one day you go to a new field that just happens to use a frequency
that is particularly susceptable.
Jerry
lts@avnet.co.uk
http://www.avnet.co.uk/touchdown
----- Original Message -----
From: Dave Simpson <Dave_Simpson@londonweb.net>
Subject: Re: Aerials
> I have noticed some RF interference on coms radiating from hand-held GPS
> receivers, even with their back lights off. I've checked the Garmin 195
and
> the Lowrance Airmap and both interfere if the coms antenna is close to the
> receiver, ie. within 2 feet. There was also RF radiating from the antenna
> and its lead in both cases.
>
> My suspicion is that switching edges at the high clock speeds used within
> the GPS receivers are the culprit, but I have no way of proving it.
Although
> the case of the GPS itself may be shielded, the radiation can escape via
the
> antenna lead. Trying to shield such radiation is notoriously difficult if
> it's at high frequencies because even short lengths of line to ground
> present high impedances at the frequencies we're talking about. Best bet
is
> to keep the aerial feeds as short as possible and stick to Fred's 2' rule
> for antenna location.
>
> A fast and dirty way of checking if there is likely to be a problem with a
> GPS is to wave a hand held airband radio around the GPS receiver and its
> antenna. Turn the squelch off and you will get some feel for what's going
> on.
>
> Dave Simpson
>
> -----Original Message-----
> From: Fred Fillinger <fillinger@ameritech.net>
> Date: 23 July 2000 07:01
> Subject: Re: Aerials
>
>
> >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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