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



19"?  Is that at 125MHz (period of 8ns)?  Light will go 3*10^8 m/s.  1
wavelength = 8ns * 3*10^8 m/s * 39.37in/m = 94.49in.  1/4 wave =
23.622in.  Is 19in based on 2meter (144-148MHz)?

http://www.csgnetwork.com/antennagpcalc.html

Somehow I thought that much impedance mismatch would lead to a larger
SWR.  Thanks..  An SWR of 1.5 is still getting close to only
transmitting half power, so it's not good.


Matt-

Perhaps they have some sort of device to shorten it. According to the
SA article most UHF antennas are 19 inches. I'll measure the ones on
my 182 next time I am out at the hangar.