bpnjensen wrote:
...and I'm gonna use existing trees to put it up about 30 feet above
ground, 15 feet above my rooftop on a 5x100 foot suburban lot. Power
lines both in front and back of my house, the ones behind are much
higher voltage, but not real high-tension wires.

All other things being equal, am I better off:

1 - Putting this thing up parallel to, or more perpendicular to, the
powerlines?



A purely horizontal antenna will usually pick up less local noise.
But on the lower bands (say, 6 MHz and below), a low horizontal
antenna will also tend to disfavor stations over 1,000 miles distant
and favor closer (higher-angle) ones.

Generally, perpendicular to the noise source is better. Visualize it
this way: Imagine you are standing with your eyes precisely at the
noise source location. Look at your antenna. If your antenna looks
like a single point (as it would if you were looking directly at the
end of it), there will be relatively little energy induced in the
wire. With a diffuse noise source like a power line, of course, it's
not quite so simple. But even the most diffuse noise source has a
virtual source at each particular frequency, and the concept is
still helpful. Desired signals off the ends are also attenuated.

An end-fed, coax-fed single wire with its feedpoint in the air will
definitely pick up signals on the outside of the coax and act more
like an inverted L, though, and just choking off the currents with a
balun won't help much. For true horizontal polarization, you'd be
better off with a center-fed dipole arrangement, with a balun at the
center not necessary but helpful in minimizing feedline antenna
effects. See

http://www.ehow.com/how_6002278_buil...ntenna-hf.html

and

http://tinyurl.com/2ch7gzw




2 - Having the coax meet the wire at the base of the tree and
grounding it there, or running the coax up the tree and then depending
on the outer braid on the coax for ground purposes? The coax is
grounded at the first termination point at my MFJ antenna phasing unit
using a short, heavy copper wire to a ground rod.

Thanks,
Bruce



An inverted L fed against ground (which is what you're describing as
your second option) is an excellent antenna in my experience, though
it may, like a vertical, pick up more local noise than a properly
balanced horizontal.

An inverted L is virtually omnidirectional over the lower half of
its range, partly due to the fact that it has a combination of
vertical and horizontal polarization. Feeding it with coax at the
base, with a radial or a few radials laying on the earth for a
ground system (the grass roots will ensnare and cover them in a
month or two), it will receive very well on any frequency from half
up to ten times its quarter wave resonance (quarter wave resonance
in MHz is 234 divided by total wire length in feet).

Any antenna that must operate over a wide range of frequencies will
necessarily be mismatched to the coaxial cable's input impedance on
many of them. A tunable matching network will help signal transfer
-- but for lower-band receiving purposes may not really be necessary
because atmospheric noise level is usually the limiting factor.

(Antennas used over a wide frequency range also tend to have complex
patterns, with many nulls in them, once you get into the higher
frequency ranges -- say, more than twice the resonant frequency.)

Here's a simple diagram of an inverted L:

http://www.ly4a.com/hamradio-news/20...enna-for-160m/

Because of potential static buildup from wind and weather, it's good
to make sure that there's a DC path to ground from the antenna --
your receiver may already provide such a path or it can be provided
by an input transformer or a high-value resistor.

Here's another simple inverted L reference:

http://www.bloomington.in.us/~wh2t/invertedl.html

When I put up my first inverted-L antenna (60 feet in the vertical
section and 90 feet in the horizontal section) I was astounded at
how well it worked, both on HF and on the AM broadcast band. From
Hillsboro, West Virginia I was able to receive WLS, Chicago (some
500 miles away) and similarly distant stations via daytime ground
wave with perfect clarity. On 75 meters, European stations could
usually hear me well and I didn't sacrifice short-distance coverage
of the U.S. east coast.

Antennas are a heady combination of craftsmanship, instinct,
intuition, and science. I recommend the _ARRL Antenna Book_ as a
good starting point if you want to learn more.


With all good wishes,



Kevin, WB4AIO.
--
http://kevinalfredstrom.com/