"Doug" wrote in message
link.net...

"Meindert Sprang" wrote in message
..
So GT15 is ok, but keep it 1" away from any grounded or other conductive
area (like strapping the
GTO15 to the uninsulated lower part of the backstay with tie-raps... BAD
PRACTICE).

Meindert
PE1GRV

Is the 1" just a rule of thumb or is there some engineering calculations
to
support this distance?

It is more a rule of thumb. At 1" the extra capacity caused by the wire
close to the backstay is low enough not to have any bad influence on the
high impedant line.

Here's some math: two wires, 1 mm in diameter, 25.4mm (1") spaced apart, 1
meter long form a capacitor of 0.35pF, which is hardly noticable at HF
frequencies (15kOhm at 30MHz).
The same wires, 1mm apart, form a capacitor of 9pF. With solid insulation
between them, the capacity goes up by a factor of 2 to 10, depending on the
type of insulation used. Coax, for instance, has a typical capacitance of
100pF/meter.

So 1" is a rule of thumb. For SI minded people, 1cm would probably do fine
too.

Meindert