A grossly late insertion to this discussion...apologies. Medical
situations have not been conducive to quick replies.
The FCP is always fed in the center. FCP dimensions should *not* be changed
for tuning SWR. The folds are designed for minimum *net* field at near
field distances. Monkeying with FCP dimensions for other reasons messes up
this very low net field.
Roughly described, with one feedline conductor going to a radiating wire, a
counterpoise is a place to store the energy from the other feed conductor,
and then return it dissipating as little possible of that energy. The FCP
wires all create fields, but the trick with the shape of the wire folds,
given enough distance, causes the fields from the three main wires to sum
to only a few percent of any one wire's field at any takeoff vector. In
free space, an FCP has so little net field at any departure vector, that
the copper wire accounts for %99 percent of energy loss, and NEC4
calculates the best "gain" at -31 dBi. That's minus 31.
In reality down here on planet Earth, nearby dielectric material is often
too close to take significant advantage of the model's free space
all-fields-add-to-zero. Ideally the only nearby dielectric is ground 8 to
10 feet below. But in small lot reality, nearby dielectric may be walls,
fencing, trees and other vegetation, etc.
The FCP may be more simply understood as an opposed pair of 1/16 wave
radials somehow with a much more workable feed impedance. And directly
below the center there is a null in sum of fields because the left radial
has current going one way and the right radial going the other. This center
null is part of the effectiveness near the ground. Make a 90 degree bend at
the center and the sum in the middle is incomplete, not working as well.
So bending at the center is *not* the recommended configuration, as there
will be increased net field at ground, therefore increased ground losses.
The recommended bend for unavoidable corners is at the midpoint of either
side, e.g. at 16 feet either side of center, which retains the
cancellation from the center of the FCP. We do understand that physical
circumstances may not allow the recommended point for a bend. Something is
better than nothing. And even a center-bent FCP will be better than typical
alternatives seen on a small lot.
We now recommend 8-10 feet of height (2.5-3 meters), at least 1 foot from
tree trunks, and as far away from walls, houses or massive dielectric as
from the ground. Do not run the FCP through bushes and trees. Circumstances
may not permit all avoidances, but design your placement with as much of
this separation as you can manage and get away with.
In one high end HOA neighborhood stealth placement, all the wire was
spray-painted black and run through the bushes and trees. L and FCP were
unnoticed by guests from HOA committee eating supper out on the patio, a
mere 50 feet from the wire. The HOA never did catch on. SWR curve was
broader than typical for that combo, most likely indicating added loss. It
worked well compared to other things the ham had tried, and certainly was a
winner vs. no 160 at all. He worked into Europe from southeast US and the
antenna was a source of stealthy pride.
The FCP wires can all be at the same height and spaced horizontally, or
they can be spaced vertically. If they go past close tree trunks, the wires
should be spaced vertically. Supporting an FCP with trees using electric
fence screw-in insulators (e.g. wires an inch or two away from the trunk)
is decidedly *not* recommended.
If you can get away with placing the FCP in the clear, supporting it with a
center PVC pipe support and rope to a support point at the end, then do it.
Every departure from the recommended configuration has some degree of loss
added. You do what you can get away with. It will still be far less loss
than faint and irregular imitations of commercial grade radial fields.
73, Guy K2AV
On Sat, Aug 29, 2015 at 1:29 PM, James Bennett <w6jhb@me.com> wrote:
> Douglas,
>
> My 160 meter antenna is also located in a corner. I put my FCP up along
> the two intersecting fences, such that there is a 90 degree bend, right at
> the center of the FCP. It works just fine in this configuration. So, if you
> are able to use a 90 degree installation of the FCP, give it a try - it
> ought to work for you as mine did for me.
>
> 73, Jim
>
> Jim Bennett / W6JHB
> Folsom, CA
>
> > On Aug 29, 2015, at 9:06 AM, Douglas Ruz (CO8DM) <co8dm@frcuba.co.cu>
> wrote:
> >
> > Hi,
> >
> > Originally, the FCP is connected in the center (33ft each side for
> 160m)...Did somebody try connect it at one END ???.
> >
> > Probably i will install my vertical in a corner of my property and some
> neighbours houses do not allow connect the FCP in the middle.
> >
> > any idea?
> >
> > Thanks,
> >
> > 73....Douglas, CO8DM
> >
> > "No creo que haya alguna emoción más intensa para un inventor que ver
> alguna de sus creaciones funcionando. Esa emoción hace que uno se olvide de
> comer, de dormir, de todo." - Nikola Tesla
> >
> > _________________
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