Hi Herb,
Yes, I can attest to the fact that poor ground seems to really help beverage
performance. Here, in SWVA, the Appalachians are mostly limestone, karst
regions, with a thin overlay of mostly decayed matter representing the oak
and maple hardwood forest floor. I have three beverage antennas running
tree-to-tree between 600-800 feet long. I've done some limited modeling and
read most of the literature, and everything confirms what I hear. I find no
need for preamps since most times the signal on the beverage is within 10-15
db of a full size inverted vee at 100-plus feet, or an inverted L, somewhat
lower, which are my references. The signal/noise ratio, of course, usually
makes up at least 30 db of improvement, so the beverages are almost always
better.
If my ground were better, the tilt angle no doubt would be significantly
higher. I estimate from modeling, research, and on-the-air testing that
these beverages are most sensitive to an angle around 20-50 degrees. While
20 degrees would not be considered "low-angle" on 20 M, it certainly is for
Top Band. As a matter of fact, I question whether even TRANSMIT angles much
below this are particularly useful under most practical conditions. (An
exception might be a quiet January night in the middle of the sunspot
minimum.) There is much in the literature which indicates that transmit
angles around 20-40 degrees are optimum for Top Band DX, since lower angles
require the signal to traverse the D layer through a greater distance, thus
attenuating the signal more. We've seen a lot of discussion about "high
angle" conditions on Top Band, and since my Inverted Vee, because of terrain
influence, is down only about 6 db at 30 degrees, it often outperforms the
Inverted L on typical Eu-path DX from Virginia. Of course, efficiency
enters the picture, since the L doesn't have an ideal ground plane.
The individual who posted a few days ago about using a reflector with his
high dipole resonated with me, also. I believe experimentation with
horizontal arrays, even at lower heights (around 100 ft) is warranted, and I
intend to try this. I placed a reflector directly below an 80 meter
inverted vee at 50 feet (close-spaced, around 20 feet) and obtained 5 db
improvement within 1000 miles and no change at 3000. I think this shows
that close-spaced reflectors tend to widen the lobe, which, in this case is
good, since it gives more useful radiation at lower angles than straight up.
So, even if it's tough to calculate the perfect length for a two-element
array with both elements at, say 100 feet, I think it might serve to provide
10-12 db front-to-back at low angles, and lower the main lobe enough to pick
up around 5 db at 30 degrees, which I have stated I believe is a more
important angle than, say 10 degrees. Poor ground actually may help here
too, and sloping terrain always helps, so I'll experiment some next season.
So, since most people have better ground than I do, I'd stay away from
anything that raises the angles at which the receive antenna is most
sensitive. By the way, my beverages are "short" for the Broadcast Band, but
perform amazingly well, allowing the beverages to "capture" different
signals on the same frequency by switching directions.
73, John K4IQ
--
FAQ on WWW: http://www.contesting.com/topband.html
Submissions: topband@contesting.com
Administrative requests: topband-REQUEST@contesting.com
Problems: owner-topband@contesting.com
|