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.
            --
            Paul Lamar ...No rotor no motor.


        I suspect there's some loading circuitry in the antenna..  Just like
        rubber duckies are usually 1/4 wave and clearly they aren't
    nearly 1/4
        wave long.  Or, if it's an antenna with dielectric covering,
    maybe they
        have to be tuned with a shorter length - the dielectric changes the
        resonant frequency - making the wire look longer, I think..


        Matt-

        Mark what kind of antennas are you using? If tape was the coax
        connected in the center. I assume they and the coax are bonded in
        so there is little you can do about changing anything having to do
        with that.

        The other thing I am curious about is the new box that converts
        the fuel sensor value to a variable frequency pulse. Please look
    inside
        and tell me the numbers on the chips if they are readable of course.
        Some manufacturers mask the numbers sometimes so it makes it
        more difficult to copy the circuit.

        The diodes Aubrey mentions may be embedded in the chip or chips what
        ever the case may be. In that case the only thing I think you can
        do is put them in a shielded box and move it away from the coax.

        --
        Paul Lamar ...No rotor no motor.


    Paul (and others),
    I made a breakthrough this  weekend on the fuel probe problem.  I
    rewired the right probe with shielded wire from the probe all the way to
    the EFIS.  Power up the EFIS and Nav/Com, hit PTT, and the gauge is
    steady as a rock.  The left gauge (the one that I haven't changed) still
    acted up as always, but it will be next to fix.  But the right one with
    shielded wire appears to be working great.  It was too late in the day
    to fly it, but I feel 99% certain that this has corrected the problem.

    Thanks to everyone that shared their ideas and suggestions on how to fix
    this very annoying (and potentially dangerous) problem.

    Mark S.
    25.5 hrs

    Congratulations Mark. You had a real bear of a problem there for awhile.
    These kinds of problems usually require real expensive instruments
    to trouble shoot like field strength meters and VSWR meters.

    Probably a good plan of attack is shield everything in an airplane
    including all the engine computer wiring.
    --
    Paul Lamar ...No rotor no motor.

Paul,

I have heard of others having this same type of problem with capacitive
fuel gauges.  You would think the manufacturer would provide some
guidance/assistance on how to correct this problem.  Oh well, such is
EXPERIMENTAL aviation.  Thanks for your help.

Mark

They mostly deal with metal airplanes and they provide a better ground
and some shielding. Also RG400 is the correct coax for metal airplanes
with quarter wave antennas.

I am still curious about your antenna. Did you use a tape antenna?
Send me private email if you want to keep it on the QT :)

Here are a couple of shots from that SA mag composite plane antenna article.
I think you can just barely see where the coax is connected to the
half wave antenna in the detail photo.

BTW I notice that Cessna bought Columbia so you and Tom Parkes now own Cessna's :)
I also notice Cessna opened a plant in China for LSA aircraft.