On 2019-01-16 12:43 PM, Rob Atkinson wrote:
> Where does the idea come from that a grounded tower holding up an
> inverted L is a problem.
From the work done by K2AV in conjunction with the FCP:
http://k2av.com/
In particular, pay attention to the information on a *separately*
fed inverted L (with or without the FCP) hung off the side of a
tower *with no ground system* - particularly an inverted L with
raised radials where the tower is "buried" in poor dirt.
> I have a 50 foot mast holding my inverted L--the L wire is out
> about 3 feet from it. The mast has a 15 foot stinger on top to make
> it a 1/4 wave 80 m. vertical. I can ground the mast or float it above
> ground when using the inverted L on 160 but I see no difference in
> performance.
You also have the mast grounded to the best ground system you can
install. That is a *huge* difference compared to no ground system
and "poor" dirt.
Anyone really concerned about this could always drop a second wire on
the opposite side from the inverted L, and tie the two together at
top and bottom with the single inverted L horizontal wire continuing
on, but I doubt if it would make much difference.
If one is going to hang an inverted L off the side of a tower, the
intelligent thing to do is to "cage" the tower (3 to 6 down leads)
tied to the tower at the top and tied together but isolated from the
tower at the bottom. Connect the radial system (at least 30 1/8 wave
on ground radials) to the bottom of the tower, feed between the
bottom of the skirt and the radials, and adjust the horizontal wire
(top loading) as needed.
If you can not install an adequate on/in ground system, *DETUNE* the
tower to keep it from "sucking out" radiated field and dissipating
it in the poor dirt. ALL OF THIS IS CLEAR FROM MODELLING IF ONE
USES NEC4 (which treats ground losses properly).
73,
... Joe, W4TV
On 2019-01-16 12:43 PM, Rob Atkinson wrote:
Where does the idea come from that a grounded tower holding up an
inverted L is a problem. Is this just some notion that took hold?
The tower is some how an RF sponge? Hams have been doing this for
decades. Of course the tower and ground system are bonded to each
other so the tower provides a return current path as with a ground
system. How is this different from an AM broadcast skirt fed grounded
tower. I have a 50 foot mast holding my inverted L--the L wire is out
about 3 feet from it. The mast has a 15 foot stinger on top to make
it a 1/4 wave 80 m. vertical. I can ground the mast or float it above
ground when using the inverted L on 160 but I see no difference in
performance. Anyone really concerned about this could always drop a
second wire on the opposite side from the inverted L, and tie the two
together at top and bottom with the single inverted L horizontal wire
continuing on, but I doubt if it would make much difference.
73
Rob
K5UJ
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