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.