Fixing the Transponder with Duct Tape and Aluminum Foil

Gerry Caron wrote:


"Ron Wanttaja" wrote in message
...
(answering several responses)
an
opportunity for inductive coupling. Keep the Xpdr antenna cable away from
comm and audio cables. If they have to be close, try to have the routes
cross at 90 deg. The overbraid needs to be grounded because you can get
some pretty big currents induced along a cable.



Also only one end of the cables shield should be grounded or else your open
to some ground loops which can cause some of your problems.

On Sun, 07 Mar 2004 21:30:30 GMT, UltraJohn
wrote:

Gerry Caron wrote:


"Ron Wanttaja" wrote in message
...
(answering several responses)
an
opportunity for inductive coupling. Keep the Xpdr antenna cable away from
comm and audio cables. If they have to be close, try to have the routes
cross at 90 deg. The overbraid needs to be grounded because you can get
some pretty big currents induced along a cable.

Also only one end of the cables shield should be grounded or else your open
to some ground loops which can cause some of your problems.

Thanks, got the same recommendation from others...and contrary advice from
a few.

Fortunately, with a wooden airplane, *not* grounding the other end is a lot
easier than grounding it... :-)

Ron Wanttaja

Every body worries about "GROUND LOOPS" but in many cases they do not
really know what they are talking about.

Antenna coax is shielded wire that is also controlled impedance wire.
You will find for every transmitter that the shield is connected at
BOTH ends of the wire and for most antennas the shield is also
grounded at both ends. If you disconnect the shield at one end and
measure the VSWR you will notice that it goes toward infinity due the
shield not being connected.

CURRENT ALWAYS HAS TO RETURN TO THE SOURCE.

The current may not flow where you want of even where you think it is
flowing but it will seek a way(s) back to the source.

LOW FREQUENCY CURRENT follows the path of least RESISTANCE.
HIGH FREQUENCY CURRENT follows the path of least INDUCTANCE.

The current will divide inversely proportional to the impedance of
each individual path among the many paths available and seek the path
of least total impedance.

Now what is the difference in low frequency and high frequency current
you might ask. The difference is the length of the connecting wire
in terms of wavelengths of the signal. If the wire is longer than
1/20 of a wavelength then the signal is considered high frequency and
you must think of the wire as a transmission line not just as a common
wire.

For LOW frequency shielding the wire shield is only connected at one
end. Low frequency in this case is defined as DC up to the top of the
audio band, 20 KHz. This keeps the magnetic coupling of near by
cables from inducing a voltage in the shield that couples to the
signal. Connecting the shield at only one end will NOT keep out high
frequency signals!

For very high frequency signals where the shield thickness is many
skin depths of the signal frequency you can have different independent
currents on both the outside and the inside of the shield. For this
reason you must have a 360 degree shield connection or the inner and
outer currents on the shield couple and mix due to the inductance of
the shield pigtail connection. To keep out, or in, high frequency
signals you must connect the shield at BOTH ends.

Here is something to think about:
How many GROUND LOOPS do you have if you place a whip antenna in the
center of a X Y grid of 10 wires by 10 wires where each wire is
connected to the crossing wire? 100?

Now we fill in the spaces between the wires with another ten wires so
we have a grid that is the same size but now has 100 wires by 100
wires. Do we now have 10,000 ground loops? Is this better?
What if we now fill in the spaces between the wires so that it is
solid metal. Do we now have an infinite number of ground loops? Is
this better? Yes, except for weight.

The answer is the current will divide among the many paths and follow
the path of least total impedance.

John Frerichs

On Sun, 07 Mar 2004 21:30:30 GMT, UltraJohn
wrote:

Gerry Caron wrote:


"Ron Wanttaja" wrote in message
...
(answering several responses)
an
opportunity for inductive coupling. Keep the Xpdr antenna cable away from
comm and audio cables. If they have to be close, try to have the routes
cross at 90 deg. The overbraid needs to be grounded because you can get
some pretty big currents induced along a cable.



Also only one end of the cables shield should be grounded or else your open
to some ground loops which can cause some of your problems.