You hit the nail on the head Mike.
The reasons local noise sources are predominately vertically
polarized are because horizontally polarized signals are radiated
and received at high angles by most "antennas" (intentional or
accidental antennas) because of their low height. Besides that, any
horizontal content of the signal is attenuated more than vertical
components as it propagates along earth.
Bottom line is nothing makes it along the earth on groundwave any
distance on low frequencies because the earth "shorts out" any
horizontal electric fields, and of course when the time-varying
electric field goes to zero so does the accompanying magnetic
field. You can't have one without the other.
> Assuming this theory has some validity, does the standard strategy for
> top- band receiving antennas of seeking azimuth directivity apply in the
> urban jungle? After all, most of the popular receiving antennas for 160
> meters (bev- erage, EWE, pennant, probe array) are vertically polarized.
> Has anyone played with phasing low horizontal antennas for a combination
> of azimuth directivity and ground wave noise rejection?
The problem with low antennas that are truly horizontally polarized
is they only radiate at higher angles, and that almost always isn't
good for DX. The Beverage is an exception because its length
allows it to respond to the attenuation-caused-tilt of the vertical
wave as it propagates along the earth in line with the wire.
The EWE and other related antennas (flags, "K9AY" loops,etc)
have considerable high angle response from the horizontal
wires...but other than that the pattern is the same as two short
phased verticals. These antennas become somewhat vertically
dominant because the earth reinforces the sensitivity to vertical
signals while the earth attenuates horizontal signals except those
at high angles. The better the earth the better they work, and if the
earth is perfect they work almost like two short phased verticals.
A low horizontal antenna will "fight the earth effects" and lose
sensitivity except at higher angles, and you would have a tough
time phasing the antenna to get directivity. You'd wind up with
something with very poor sensitivity focused up at a very high angle
and a broad response in many directions, not a very good antenna.
If you could get it up in the air some distance (like 1/4 wl or more)
it would likely work OK.
Nearfield noise is a roll of the dice. You never know what will work
because you never really know what is going on. Sometimes
verticals are better, sometimes horizontals. Sometimes small
loops (which are magnetic dominant within ~1/10 wl and electric
field dominate beyond that distance) help, and at other times short
verticals (which are electric field dominant closer than ~1/10 wl and
magnetic field dominant beyond that distance) help in nearfield
noise.
All you can do is try a bunch of stuff when the environment around
the antenna is noisy. What works for you might not work at all for
the next guy.
73, Tom W8JI
w8ji@contesting.com
--
FAQ on WWW: http://www.contesting.com/FAQ/topband
Submissions: topband@contesting.com
Administrative requests: topband-REQUEST@contesting.com
Problems: owner-topband@contesting.com
|