Poor ground, better
Discussion on Top Band Antenna list, June 2000
Schoenbohm, Vigin Islands: I read sometime in the past
that the necessary wave tilt that makes a Beverage develop its
directional and performance criteria equates to the propagation
of the incoming signal differently over the wire and the poorer
conducting earth beneath it.
John B. Mitchell, USA: 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
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.
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.