It's somewhat likely the noise is radiated as opposed to
conducted to
the sensor circuit.
Do you have access to a handheld transceiver (with a "rubber
ducky"
antenna)? While transmitting with it (on a locally inactive
frequency),
you may be able to identify a portion of the circuit that is
vulnerable
to the RF radiation. If you don't find any part of the
circuit
responding to the handheld then grounding properly
likely will
improve
things.
There are instruments sold by Van's which are vulnerable to
RF. The
only solutions found have been to shield the instrument
(in your
case,
probably the CTV box) with metal - heavy foil may work,
but a
metal box
might be better. Or ignore it. When the mic is un-keyed,
do the
readings return to normal immediately?
Regards,
Matt-
Matt,
I hadn't considered using a hand-held radio to
trouble-shoot this
problem. I'll give that a try. Once the mic is unkeyed, the
readings
will return to normal, but not immediately since they have a
built in
damping factor.
For now, my most reliable method of tracking remaining fuel is
with the
Real World Solutions EM-2 engine monitor. This unit
calculates
the fuel
used based on the duty-cycle of the injectors. I enter the
number of
gallons added when I refuel, and it counts down (or up if you
wish) from
there. Even though it doesn't track fuel levels in each
individual
tank, it is still a pretty nice backup system.
I also have a 7-gallon low fuel warning light for each tank,
driven by a
GEMS sensor. Again, nice to have a backup system when the
primary
is a
bit quirky.
Mark
Hi Mark,
Matt may be on the right track with the handheld idea. If its
possible
align the antenna of the handheld as parallel to the installed
cap. tube
as you can then key the handheld while someone else watches the
gage.
Get as close as you can to get the greatest and quickest effect.
If you
get no response try aligning perpendicular to the tube and
key the
handheld again. It's possible that you were lucky (?) enough to
get your
installed tube and installed antenna exactly in parallel
geometric planes and the radiated energy is the culprit.
Dave M
The alignment probably isn't all that critical. Within a fairly
large
range of angles of antenna polarization/orientation, energy transfer
will not fall off dramatically. Being off axis even 45deg would
only
cause a relatively small difference. That's why it's okay that many
aircraft comm dipoles are tipped or even bent. It's also why two
aircraft can talk to each other even if both are maneuvering - even
aerobatics.
In the case of the fuel sensor circuit, just the overall field
strength
caused by the transmitting comm is the likely culprit.
Mark, is your comm radio working well? Can people hear your
transmissions clearly (and at a distance)? If not, the fuel
sensors may
be fine and some portion of the comm feed or antenna may not be
up to
snuff...
Matt-
Matt,
I can be heard very well, or so I'm told. My nav/com is an SL-30 with a
Bob Archer antenna in the tail. It was tested by a ham prior to
installation and checked out very well (don't remember the numbers).
Coax is RG-400 and is one piece from the antenna all the way up to the
radio. The fuel gauges are the only thing that seem to be affected by
the radio.
Mark
RG400 is 50 ohm and is for a quarter wave antennas.
http://www.gore.com/en_xx/products/cables/coaxial/optimized_rg400_coaxial_cable.html
Regular Nav/com antennas for metal airplanes are quarter wave.
"Depending upon height above ground, the influence of surrounding
objects
and other factors, our quarter wave antenna with a near perfect ground
exhibits a nominal input impedance of around 36 ohms."
http://www.electronics-tutorials.com/antennas/antenna-basics.htm
I think what you have Mark is a half wave antenna.
"A half wave dipole antenna does not need a ground plane and is
nominally 75 ohms."
Despite the fact it is working well this implies some
reflected energy on the outside of the coax shield.
------------------------------------------------------------
http://support.radioshack.com/support_tutorials/audio_video/cable-coax.htm
Cables carried by RadioShack
93-ohm Cables: RG-62, RG-62U
75-ohm Cables: RG-11, RG-11U, RG-59, RG-59U, RG-6, RG-6U, RG-6QS
51-ohm Cables: RG-9, RG-9U
50-ohm Cables: RG-58, RG-58U, RG-8, RG-8U, RG-8M, RG-8X--
Paul Lamar ...No rotor no motor.
Paul,
Antennas and feed lines need to be matched with the output of the
radio. In this case, everything is designed to be 50ohm. If anything
in the system was the wrong nominal impedance (75 or 93), the radiated
output would be very low (less than 10% of normal) and the reflected
energy (SWR) would be very high (>3), and he wouldn't be able to get out.
Most aircraft com antennas are modified 1/4 wave - 1/2 wave would be
pretty big - each radiator would be something like 46" long. One of the
antennas on this page is probably what Mark has:
http://www.aircraftspruce.com/catalog/avpages/archer_antennas.php
Even if an antenna's length leads to the wrong characteristic impedance,
you can connect the antenna via an impedance matching circuit (inductor
and/or capacitor). Some antennas include this in the mounting base.
Since Mark's radio performance is good (and his installation sounds
normal compared to other aircraft), I'm back to thinking it's a
vulnerability in the sensor circuit(s). Being a fiberglass ship (no
Faraday cage) with the antenna mounted inside the fuselage, and without
a ground plane, the electronics in the cabin will be quite a bit more
vulnerable to disruption from the transmitting comm.
Regards,
Matt-
Those are very interesting antennas and I agree with what
you said about VSWR. Mark did not say what kind of antenna
he was using. I assumed it was a half wave metal tape.
Many composite aircraft use metal tape antennas as mentioned in the
Jan 2008 Sport Aviation article.
A half wave would be about 38 inches long. The quarter wave antennas
are 19 inches long.
If there is a mismatch the coax shield becomes part of the antenna.
Since it is in the same bundle with the fuel tank wiring the RF is probably
being coupled into the fuel tank wiring. It also radiates to the out side of
a composite aircraft probably enhancing the antenna pattern. I was
an antenna technician (among my many careers) in my younger days about
50 years ago before I got to be an engineer so I forgot a lot :)
http://www.microwaves101.com/encyclopedia/vswr.cfm
"Just remember to divide the larger impedance by the smaller impedance,
because VSWR is always greater than 1. Hey, this calculation is so easy
you can do it in your head!!!"
"Let's look at the special case where you mix up 50 ohm parts into
a 75 ohm system (or vice-versa). In either case, the resulting VSWR
is 1.5:1. Yes, we did that without a calculator. While we're at it,
the reflection coefficient is:" "=(75-50)/(75+50)=0.2"
BTW that is less than the 2:1 claimed by the AC Spruce antennas.
One thing you might have to worry about is what the power amplifier
does since it is probably designed for 50 ohms coax.
Paul Lamar
The Rotary Engine NewsLetter. Powered by Linux.
ACRE NL web site.
http://www.rotaryeng.net
Copyright 1998-2007 All world wide rights reserved.