It seems to me (and I would be more than happy to hear any differing
opinions) that your GPS antenna only needs to be high enough to be able
to see a reasonable slice of sky. i.e. if your workshop were in the
middle of a circular clearing 80 feet in diameter in a forest with an 80
foot tree canopy, 40 feet of elevation gives you a 90 degree slice of
sky. It makes no sense (again, to me) to elevate your antenna in order
to make your choke ring antenna effective against horizontal multipath.
Below the canopy there would be no horizontal multipath to deal with.
Consider your forest as a 'poor mans choke ring antenna upgrade'
As far as fiber isolation of the GPS RF, I have seen many commercial
installations like that, and judging by the cost and complexity of the
equipment used, I would say that solution is pretty non-trivial. Fiber
isolation of the thunderbolt reference clock output and serial control
would be far easier. You might even be able to rig up rudimentary solar
power for the remote portion of the system. That would effectively
eliminate any ground path for lightning.
My 2 cents
Dale Robertson
NV8U
On 4/11/2012 8:50 AM, Michael Baker wrote:
Time-Nutters--
My workshop is surrounded by tall trees (70 to 80 ft). There
is no easy way to place my T-Bolt antenna above the tree-top
foliage. Since choke-ring antennas do not provide much benefit
for dealing with multi-path that originates from directly above
the antenna I have considered putting the antenna on a 10-ft
pole and mounting the pole in the top of the nearby trees so
as to have the antenna just above the tree-top foliage.
However, here in north-central Florida lightning is a serious
problem. In the 12 years we have lived here, 3 trees have
been hit within 75 meters of my workshop building behind
my house.
Here is a DropBox link to a map of lightning-strike-days
in USA locations:
http://dl.dropbox.com/u/60102282/Lightning%20Isokeraunic%20map.JPG
I have a number of VHF and UHF antennas mounted on my
workshop building but when not in use, they are kept
disconnected where they enter the building.
I have thought about finding some way to bring the GPS
RF signal into my workshop via an optical fiber interface
and sacrifice the RF to optical fiber interface if lightning
strikes it in a treetop but have not found a way to implement
this idea.
Two years ago lightning struck a neighbor's TV antenna
mounted on a pole attached to the side of his house and
started a fire in one of their 2nd floor bedrooms which
did a lot of damage before it was put out. The tower
was well grounded and the coax leading into the room
was fed through a grounded lightning protector but none
of these precautions prevented the fire from the lightning
strike.
Any list folks have ideas on this?
Mike Baker WA4HFR
Gainesville/Micanopy, Fla
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