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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