-=> On 11-18-96  12:52, Alex Draper said to Chris Boone,<=-
-=>"About Antenna installations...,"<=-
 
 
 AD> # <-- mast
 AD> #
 AD> #     _
 AD> #    ( )
 AD> #####I I <-- folded dipole
 AD> #    (_)
 AD> #     _
 AD> #    ( )
 AD> #####I I <-- folded dipole
 AD> #    (_)
 AD> #
 CB> The mast is part of the support for the dipoles....also in the
 CB> arrangement you showed, the mast is being used a reflector with
 CB> both dipoles pointing the same direction...this adds 3db to the
 CB> signal....SO the gain of the above antenna would be 6db IN the
 CB> DIRECTION of the dipoles and 0 at the sides and -3 off the back
 CB> side.... 4 dipoles are placed around the mast at 90 degree
 CB> placements to give a circular pattern of 6db..
 AD> Thanks for the reply Chris...  If i put 4 dipoles around the mast
 AD> at 90 degree placement,  this would give me 3db and 8 dipoles
 AD> would give me 6db circular pattern?  Should the dipoles be 1/4 wave
 AD> from the mast?  Also, do you know how they couple these dipoles to
 AD> one feedline?
 
For this arrangement you need to use a matching harness.  IIRC from my
broadcast engineering schooling, (30-odd years ago), you can make one
using 300-ohm twin lead, with each leg 1/4 electrical wavelength long.
The twinlead HAS to be spaced away from the tower, though.
 
Actually, I don't think you would get a full 6 db gain with a 4-bay
antenna, due to coupling losses, but it would still be a signifigant
improvement over a ground plane on weak signals.
 
 CB> The mast DOES provide lightning protection to the antenna dipoles
 CB> since it is taller....and of coruse it is used to support them as
 CB> well...so it has 3 functions overall..
 
For safety, ANTENNA TOWERS MUST BE WELL GROUNDED!  This does NOT mean
run a piece of lamp cord to a cold water pipe, this means ground rod(s),
(preferably copper-plated steel), at least 5 Ft. into the ground at the
base of the tower.  (National Electric Code).  This way, if lightning
hits the tower, the great majority of the bolt will be conducted
directly to ground where it will be harmless. This does NOT guarentee
that your radio or scanner won't be damaged, but it will reduce the
damage.  (Would you rather have a monitor fry or the whole house burn
down?)  There is also a coaxial-type lightning arrestor available for
co-ax.  These should be available from a TV antenna installer for a few
bucks.  (Radio Shack should have them).
 
 CB> BEST bet to protect your radio during lightning?? UNPLUG THE COAX
 CB> and power cord! There is NO 100% safe way with it connected to the
 CB> outside world!
 
AGREED!
 
 AD> I've heard stories about lightning coming down the coax even when
 AD> the coax were disconnected from the radio.  A large sparks would
 AD> leave the connector to say a grounded wall outlet.  Could this be
 AD> true or just an old CB/Ham radio myth?  Take care.
 
It is NOT a myth!  I've seen it happen!  (To OTHER people's equipment.  I
have been lucky... so far!).   :-}
 
Catch you later... Happy scanning.
   Ivy
 
 
 
... Ham radio + computers = no time + no money   :-{
 
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