Hi Curt,
> One reason for a "U" is since you have dual downleads with the far end
> grounded and the feed end fed against ground you have only one fourth the
> ground losses.
Even if we (incorrectly) assume all losses are "connection" losses
that show up at the feedpoint and the U reduces feedpoint current
by a factor of two, the system losses only drop in half.
The second ground system that must be added to terminate the
second drop wire now has the other 1/2 of the current, so it also
has 1/4 the initial loss. With two ground systems each dissipating
1/4 the initial single system loss, the result is half the initial loss
not 1/4. Of course this ignores field losses, which swamp-out the
changes.
Now here is catch-22.
If you simply doubled the ground system's size to the same size
as you wound up with using the U, you'd also have about half the
loss of the initial system. (This also ignores field losses, which
swamp-out the changes.)
It is very difficult to find a free lunch. The U is no exception.
This is why multiple-drop systems like this were abandoned from
VLF use. They not only didn't help, they actually made things
worse. You can find one example of this discussed on page 19-14
in the first edition of Jasik's Antenna Engineering Handbook.
> The second reason is that when the lightning season comes the antenna is
> grounded. I lost an inverted "L" antenna to lightning back in Cincy in
> about 1969 due to a coming storm out on the horizon. Well it is a good
> way to meet neighbors whom you have never met before. SMILE!!! With the
> inverted "U" antenna lightning struck a tree across the street but did not
> bother the 90 ft high inverted "U" antenna during the 10 years I had it
> up.
Grounding an object does not make it less likely to get hit. If
anything it actually does the opposite, but the change is so small it
isn't worth talking about.
Grounding helps control the path lightning takes if it strikes the
antenna or near the antenna. "DC grounding" sometimes can
reduce the current going into your equipment, but not as much as
we might expect. That's because lightning behaves like RF rather
than DC.
The only time I ever had major equipment damage was when my
grounded shunt-fed tower got hit. A large portion of the lightning
followed the drop wire, and melted the vacuum cap tuning that wire.
It then arced across a bunch of open relays, eliminated a section of
the feedline, and totally vaporized the antenna TR relay board and
TR relay in my T4XC.
All that with 100 or more radials that were buried in very wet black
soil and bonded to the tower!
That doesn't mean shunt-fed towers are more likely to get hit, it
just means I wasn't lucky.
73, Tom W8JI
w8ji@contesting.com
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