Steve Reinhardt wrote:
On 7/7/2010 6:00 PM, Jim Lux wrote:
Ralph Mowery wrote:
Over the years I have seen many patterns for horizontal dipoles and
the inverted V antennas,but do not recall seeing any V shaped patterns
in the books. That is a dipole that is supported on the ends, but sag
in the middle. I know the sag is needed to some extent because of the
weight of the wire and feedline, but what happens to the pattern if it
is say 1/2 wavelength or 1/4 wavelengths at the ends and the sag in
the middle is changed. Maybe drop it a few feet, then 10, 15 and 20
feet for 80 meters.

I know that for maximum distance you usually want the high current
portions up high,but maybe it will be better as a close in or
omnidirectional antenna if the center is lowered somewhat.



In free space, there is not much difference in pattern between a dipole
that is perfectly horizontal and one that is drooped or sags (or for
that matter one that is bent horizontally). What changes is the
feedpoint impedance (72 ohms for perfectly flat, getting towards 50 ohms
with a 120 degree included angle).


The other thing that changes is the apparent height above ground. If you
model a drooped dipole and fiddle with the height to try and match the
vertical pattern for a perfectly flat dipole, you find that the
"effective center" of the drooped dipole is around 1/3 of the droop
distance. That is, if you had a dipole where the center were at 100ft,
and it drooped 30 feet on either end, the pattern is about the same as a
flat dipole at 90 ft.

The height above ground has a HUGE effect on the dipole pattern
(especially at low elevation angles) and probably dominates any small
changes.

After all, a perfect dipole has a gain of 2.15dBi and a infinitely short
dipole has a gain of 1.6dBi, and a drooped or sagging dipole pretty much
has to be somewhere in between.

Jim,

My simulations, and some practical experience, has found that an
included angle closer to 90 degrees yields a 50 ohm feedpoint impedance.
These are 'practical' designs, and not in free-space, of course. Is
there a reference you can give me that derives the 120 degree 50 ohm
result?

Steve
W1KF

you might be right.

Some NEC I did a few years ago showed about 57 ohms.

http://home.earthlink.net/~w6rmk/antenna/zdipolev.htm

Since Google is my friend... I found:
J. Earl Jones (probably not the actor) had a paper in IEEE Trans on Ant
and Prop (May 1976) "Analysis of the Symmetric Center-Fed V-Dipole
Antenna" shows, in Figure 6, numbers like you report.. At 90 degrees,
somewhere between 40 and 50 ohms, and at 120, a bit below 60 ohms (for
very thin wires.. h/a = 20,000

The same graph shows that the reactance (for a fixed length) also
changes, which has the practical implication that the "resonant" length
for a V will be different than a flat dipole.

The paper has analytical, models, and actual data in it. He compares
the model data to approximations from King.

And here you go for some more data (although this is more like a half
rhombic)

http://ntrs.nasa.gov/archive/nasa/ca...1967006193.pdf