V antenna paterns
 

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.

V antenna paterns
 

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.

Download the free demo version of EZNEC and plot it.

http://www.eznec.com/demoinfo.htm


--
Jim Pennino

Remove .spam.sux to reply.

V antenna paterns
 

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.

V antenna paterns
 

Jim Lux wrote:

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.

Those gains are for dipoles in free space. A dipole over ground
typically has several dB greater gain.

The absolutely worst gain any efficient antenna can have in free space
in its best direction is 0 dBi. So nothing you can possibly do to the
pattern can reduce it below this value in free space, as long as you
don't introduce loss. But as Jim says, you can mess with a dipole a fair
amount without changing the pattern much -- except for filling in or
moving the nulls, which can be very sensitive to small changes.

Roy Lewallen, W7EL

V antenna paterns
 

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

V antenna paterns
 

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

V antenna paterns
 

On Wed, 7 Jul 2010 14:56:59 -0400, "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.

My initial EZNEC models of this antenna have not encouraged me to
continue.

In addition to other issues, the pattern tends to be straight up.

I will make certain that any dipoles I put up in the future will have
the center at least as high as the ends.

OTH, I will put on my DO LIST to investigate the characteristics of
turning the antenna on its side. If the pattern merits, I will
consider the matching situation.

Very interesting!
John Ferrell W8CCW

V antenna paterns
 

John Ferrell wrote:
On Wed, 7 Jul 2010 14:56:59 -0400, "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.

My initial EZNEC models of this antenna have not encouraged me to
continue.

In addition to other issues, the pattern tends to be straight up.


That's likely because the effective height of the antenna is low. A
low antenna on 80m means cloud/worm warmer. (and low, in this context,
means 20m, which is still way up in the sky).

So the traditional single support inverted V has all sorts of advantages:
only one support needed
puts the important part of the antenna (the middle) up high

V antenna paterns
 

"Jim Lux" wrote in message
...
..


That's likely because the effective height of the antenna is low. A low
antenna on 80m means cloud/worm warmer. (and low, in this context, means
20m, which is still way up in the sky).

So the traditional single support inverted V has all sorts of advantages:
only one support needed
puts the important part of the antenna (the middle) up high

For many it is difficult to get an antenna up more than 60 feet. That is
close to 1/4 wavelength or less. My back yard has trees on each side and
it was easy to string a dipole close to 60 feet on the ends,but no way to
support the antenna in the middle. That was why the question of should I
string the dipole as tight as possiable with out breaking or adjust the drop
in the middle to some optimum distance for what I want to do. I sort of
thought the mort horizontal it is the beter the signal would be at a
distance, but there may be an optimal angle to have a 200 to 300 mile
antenna.

V antenna paterns
 

On Fri, 9 Jul 2010 14:08:17 -0400, "Ralph Mowery"
wrote:

For many it is difficult to get an antenna up more than 60 feet. That is
close to 1/4 wavelength or less. My back yard has trees on each side and
it was easy to string a dipole close to 60 feet on the ends,but no way to
support the antenna in the middle. That was why the question of should I
string the dipole as tight as possiable with out breaking or adjust the drop
in the middle to some optimum distance for what I want to do. I sort of
thought the mort horizontal it is the beter the signal would be at a
distance, but there may be an optimal angle to have a 200 to 300 mile
antenna.

Hi Ralph,

John's suggestion of using EZNEC in its free version would easily
answer all the questions, test all the assertions, and find all the
unstated characteristics in one fell swoop. It would also let you
find the necessary attributes to obtain your goal.

EZNEC would do all this faster than the 12 blind men trying to
describe an elephant.

However, as for your several questions above. You are not going to
discover gold at the end of the rainbow by tightening the droop. In
fact, getting it to an optimal height (as has been hammered home
several times as a principle necessity) won't budge your distant
contact's S-Meter more than a couple dB (about the width of the
needle).

73's
Richard Clark, KB7QHC