[Antennas] antenna construction?

[Terry Conboy]

At 08:58 AM 2008-05-22, Howard W3CQH wrote:
>The length of the roof is 90ft, it is 15ft from the center line to each
>edge, (roughly 30ft), height of roof is approx 22ft at the peak and
>10ft at the gutter edge.  The tree is approximately 35ft high.
>
>                                        xxxxxxxxxxxxxx roof edge 
> xxxxxxxxxxxxxxx
>                                        x O   x
>                                        x x
>                                        x ---------------peak of 
> oof  --------------------x
>                                        x x
>xxxxxxxxxx                    x O  x
>x                x                    xxxxxxxxxxxx roof edge xxxxxxxxxxxxxxxxx
>x      O       x
>x                x
>xxxxxxxxxx
>       TREE
>
>My question is: do you think it would be better to run a configuration as
>say a Windom from the tree to the points on the roof (the points can be
>moved) or just a simple inverted "V" and the angle of the "V" in any case is
>going to be very narrow.  I would be feeding either one via some Rg8x
>approximately 65ft.
>
>I would like to be able to use the ant from 160m - 6m if possible, I will be
>feeding it via a tuner, and the wire is going to be #26 enameled as it can't
>be seen.

Howard - There are a lot of variables here, so the answer isn't 
perfectly clear.

You didn't say how far the tree was from the building, but I'd be 
inclined to use a mostly horizontal configuration from the tree to 
the nearest part of the roof and then along the peak of the roof, 
keeping it as high above the roof as you can get away with.  Don't be 
afraid to bend the wire in the horizontal plane and to let the ends 
droop down to get close to a half-wavelength of wire for operation on 
the lowest band.  (An 88 foot dipole is probably the shortest that is 
marginally usable for 80m with open wire feed - double that for 
160m.  The typical G5RV dimensions, 102 feet for 80m and twice that 
for 160m, give lower open wire feeder losses.  For an excellent 
discussion, see http://www.w8ji.com/short_dipoles_and_problems.htm)

No matter what horizontally polarized antenna configuration you use 
at under 35 feet high, it will be most effective at shorter distances 
(out to 500 miles or so) on 160m & 80m.

There are some issues that may require some experimentation to 
determine if they are problem: coupling to the gutters (if 
metal),  electrical wiring, and plumbing in the building.  (I've used 
attic antennas at two different houses: one worked fine, the other 
was a nightmare of RF coupling to everything imaginable!)

The feedline of a "Windom" or off-center-fed antennas requires 
decoupling with a very good choke balun to prevent feedline 
radiation.  The impedances on even multiples of resonance are close 
enough to 50 ohms that you won't have too much feedline loss.

With a simple inverted vee, the impedances are only near 50 ohms when 
the wire is an odd multiple of a half-wavelength.  The impedances on 
even-multiples will be very high and the losses in coax cable will be 
significant.  Of course, you can parallel multiple dipoles/inverted 
vees for operation on higher bands, but it gets harder to hide a lot 
of wires.  (#26 wire is my favorite stealth antenna wire, especially 
if it is bare and oxidizes to a nice dull dark brown.  #26 doesn't 
take ice buildup gracefully, however.)

Instead of coax, you can make open wire feedline out of #26 wires 
with a few spacers (where needed) made from plastic bottles.  (I've 
used it successfully.)  It is hard to see and you can splice it to 
lower-loss ladder line with larger conductors for runs in the attic 
or under the eaves where it won't be seen.  With a tuner, the 
impedances of the open wire lines aren't critical and allow operation 
on all bands (within reason).

Good luck.  Let us know what you put up and how it works out.  I'd be 
happy to model any serious candidate configurations.

73, Terry N6RY