Correct Antenna Takeoff Angle?
 

Hi, Everybody,

How does one determine the correct antenna takeoff angle to communicate over
a certain distance?

I can't find any references anywhere that discuss this, taking into account
all of the variables: height of ionosphere, frequency, etc.

Here's a practical example of when it would be important to know this. Say
you are going to put up a dipole antenna for 20 meters specifically to
communicate to a friend 2000 miles away during the day. The takeoff angle of
the major lobe varies widely as the antenna is raised from ground level to 1
wavelength above ground, so presumably there is a 'best' height for the
antenna to put the major lobe right where it needs to be to skip to your
friend's QTH.

The trouble is, we do not seem to have a good way to determine this 'best'
height ahead of time. One would, I guess, have to vary the height of the
antenna while the other station monitored your signal strength, and one
would have to do this over several days, weeks, or months to get good,
averaged data. But... isn't there a better way to do this?

Regards,

Al W6LX

On Tue, 7 Jun 2005 19:12:53 -0700, "Al Lorona"
wrote:

Hi, Everybody,

Hi Al,

How does one determine the correct antenna takeoff angle to communicate over
a certain distance?

Ooooh, that's gonna sting!

I can't find any references anywhere that discuss this, taking into account
all of the variables: height of ionosphere, frequency, etc.

Try propagation modelers. VOACAP has been around for a good 10 years
now, with recent upgrade to VOAWIN.

The trouble is, we do not seem to have a good way to determine this 'best'
height ahead of time. One would, I guess, have to vary the height of the
antenna while the other station monitored your signal strength, and one
would have to do this over several days, weeks, or months to get good,
averaged data. But... isn't there a better way to do this?

The better question is could you even hope to achieve this little?
Aside from having your own antenna farm, DX is a spin of the wheel and
most simply try.

Perhaps you should use the propagation modeler to investigate the
range of variation of DX paths and heights associated with what you
can throw into the air.

73's
Richard Clark, KB7QHC

A number of propagation programs can tell you what angle you need. One
that's easy to use, reasonably accurate, and free is W6ELProp, available
from http://www.qsl.net/w6elprop/.

The ideal height for the antenna is a fairly easy geometry problem, but
it's easiest to let a program tell you this, too. There are a number of
antenna modeling programs available, some of which are free, and all
will give very accurate results. Because it's my product, I recommend
the free demo version of EZNEC from http://eznec.com, which is entirely
adequate for this purpose. But any number of other programs would be
suitable also.

One bit of advice -- don't get hung up on the "takeoff angle" reported
by some antenna analysis programs (like EZNEC). It simply means the
angle at which the pattern is maximum, and isn't important for this or
most situations. What counts is the field strength or gain at the angle
which you'll be using for communication -- that's the angle reported by
the propagation program. That's the number you want to maximize.

Roy Lewallen, W7EL

Al Lorona wrote:
Hi, Everybody,

How does one determine the correct antenna takeoff angle to communicate over
a certain distance?

I can't find any references anywhere that discuss this, taking into account
all of the variables: height of ionosphere, frequency, etc.

Here's a practical example of when it would be important to know this. Say
you are going to put up a dipole antenna for 20 meters specifically to
communicate to a friend 2000 miles away during the day. The takeoff angle of
the major lobe varies widely as the antenna is raised from ground level to 1
wavelength above ground, so presumably there is a 'best' height for the
antenna to put the major lobe right where it needs to be to skip to your
friend's QTH.

The trouble is, we do not seem to have a good way to determine this 'best'
height ahead of time. One would, I guess, have to vary the height of the
antenna while the other station monitored your signal strength, and one
would have to do this over several days, weeks, or months to get good,
averaged data. But... isn't there a better way to do this?

Regards,

Al W6LX

Roy, thank you so much for taking the time to respond. I am going to go play
with W6ELProp right away.

I am a very happy EZNEC user... this is how I am finding out the elevation
angle of the major lobe, and because it's easy to 'adjust' by varying the
height of the antenna, especially at a Field Day site, I want to put it up
to maximize coverage of the rest of the country.

Thanks again.

Regards,

Al W6LX

P.S. I should probably start a different thread to make this comment, but I
am still trying to find a good analysis program to enable simulation of an
balanced-line end-fed antenna, sometimes called a Zepp. This evidently is
one tough problem to solve.

"Roy Lewallen" wrote in message
...
A number of propagation programs can tell you what angle you need. One
that's easy to use, reasonably accurate, and free is W6ELProp, available
from http://www.qsl.net/w6elprop/.

The ideal height for the antenna is a fairly easy geometry problem, but
it's easiest to let a program tell you this, too. There are a number of
antenna modeling programs available, some of which are free, and all
will give very accurate results. Because it's my product, I recommend
the free demo version of EZNEC from http://eznec.com, which is entirely
adequate for this purpose. But any number of other programs would be
suitable also.

One bit of advice -- don't get hung up on the "takeoff angle" reported
by some antenna analysis programs (like EZNEC). It simply means the
angle at which the pattern is maximum, and isn't important for this or
most situations. What counts is the field strength or gain at the angle
which you'll be using for communication -- that's the angle reported by
the propagation program. That's the number you want to maximize.

Roy Lewallen, W7EL

Al Lorona wrote:
Hi, Everybody,

How does one determine the correct antenna takeoff angle to communicate
over
a certain distance?

I can't find any references anywhere that discuss this, taking into
account
all of the variables: height of ionosphere, frequency, etc.

Here's a practical example of when it would be important to know this.
Say
you are going to put up a dipole antenna for 20 meters specifically to
communicate to a friend 2000 miles away during the day. The takeoff
angle of
the major lobe varies widely as the antenna is raised from ground level
to 1
wavelength above ground, so presumably there is a 'best' height for the
antenna to put the major lobe right where it needs to be to skip to your
friend's QTH.

The trouble is, we do not seem to have a good way to determine this
'best'
height ahead of time. One would, I guess, have to vary the height of the
antenna while the other station monitored your signal strength, and one
would have to do this over several days, weeks, or months to get good,
averaged data. But... isn't there a better way to do this?

Regards,

Al W6LX

I don't know of any program better than EZNEC for this. (Most others are
just as capable.) You have to realize that the feedline is an integral
part of the antenna, so it must of course be part of the model. Because
it radiates, you have can't use the program's transmission line models,
but have to include the feedline as wires. Depending on the length and
orientation of the feedline, there might also be significant current
flowing from the rig to the Earth via whatever path it can take, and
this is also part of the antenna and must be included in the model if
the model is going to predict the antenna performance.

Just becuase we call one part of the radiating system an "antenna",
another a "feedline" and another "ground" doesn't give them magical
properties and prevent them from radiating. All can be part of the
radiating antenna system and, in the case of a Zepp, generally are.

Roy Lewallen, W7EL

Al Lorona wrote:
P.S. I should probably start a different thread to make this comment, but I
am still trying to find a good analysis program to enable simulation of an
balanced-line end-fed antenna, sometimes called a Zepp. This evidently is
one tough problem to solve.

Another problem I see is that the altitude of reflection probably varies
a lot, so, day to day, it might not matter too much about the exact
takeoff angle...

Scott
N0EDV


Al Lorona wrote:

Hi, Everybody,

How does one determine the correct antenna takeoff angle to communicate over
a certain distance?

I can't find any references anywhere that discuss this, taking into account
all of the variables: height of ionosphere, frequency, etc.

Here's a practical example of when it would be important to know this. Say
you are going to put up a dipole antenna for 20 meters specifically to
communicate to a friend 2000 miles away during the day. The takeoff angle of
the major lobe varies widely as the antenna is raised from ground level to 1
wavelength above ground, so presumably there is a 'best' height for the
antenna to put the major lobe right where it needs to be to skip to your
friend's QTH.

The trouble is, we do not seem to have a good way to determine this 'best'
height ahead of time. One would, I guess, have to vary the height of the
antenna while the other station monitored your signal strength, and one
would have to do this over several days, weeks, or months to get good,
averaged data. But... isn't there a better way to do this?

Regards,

Al W6LX

Al Lorona wrote:
"How does one determine the correct antenna takeoff angle to communicate
over a certain sistance?"

Solve the trigonometry / geometry problem involving the target distance
and height of the reflecting layer.

King, Mimno and Wing in "Transmission Lines, Antennas, and Wave
Guides"say:
Elevation angles above 3-degrees are useful because they are not
immediately absorbed by earth loss. Low-angle radiation (above
3-degrees) makes a long distance trip to a distant receiver with the
fewest hops or bounces between the ionosphere and earth. At 3-degree
elevation, the distance per hop is about 3.500 km (2.100 miles). longer
distances are automatically broken up into units not exceeding 3.500 km.
Multipath transmission often exists and causes fading.

For distances less than 3,500 km, elevation angles of more than
3-degrees must be used. Often the antenna radiates a broad vertical
pattern. In any case, the useful ray becomes steeper as the receiver
becomes closer to the transmitter.

Kraus in edition No. 3 of "Antennas" shows how earth reflection affects
the pattern and impedance of a horizontal antenna. At 1/2-wavelength
elevation, the antenna has maximum radiation at 30-degrees over perfect
earth and the center-fed 1/2-wave horizontal dipole has a feedpoint of
73 ohms. The Zepp should have about the same vertical maximum radiation
angle but its impedance is something else.

Elevation angle is an inverse function of antenna height.

Best regards, Richard Harrison, KB5WZI

"Richard Harrison" wrote in message
...

King, Mimno and Wing in "Transmission Lines, Antennas, and Wave
Guides"say:
Elevation angles above 3-degrees are useful because they are not
immediately absorbed by earth loss. Low-angle radiation (above
3-degrees) makes a long distance trip to a distant receiver with the
fewest hops or bounces between the ionosphere and earth. At 3-degree
elevation, the distance per hop is about 3.500 km (2.100 miles). longer
distances are automatically broken up into units not exceeding 3.500 km.
Multipath transmission often exists and causes fading.

For distances less than 3,500 km, elevation angles of more than
3-degrees must be used. Often the antenna radiates a broad vertical
pattern. In any case, the useful ray becomes steeper as the receiver
becomes closer to the transmitter.

Kraus in edition No. 3 of "Antennas" shows how earth reflection affects
the pattern and impedance of a horizontal antenna. At 1/2-wavelength
elevation, the antenna has maximum radiation at 30-degrees over perfect
earth and the center-fed 1/2-wave horizontal dipole has a feedpoint of
73 ohms. The Zepp should have about the same vertical maximum radiation
angle but its impedance is something else.

Elevation angle is an inverse function of antenna height.

Best regards, Richard Harrison, KB5WZI


Hi, Richard,

Yeah, your reference sounds about right. On Roy's suggestion, I downloaded
W6ELprop and was shocked to find what the elevation angles are for even
moderate path lengths. For example, I ran the path from Albuquerque to
Philadelphia and was amazed to find that the signal would have to leave the
antenna and reflect from the F layer at a 4 degree angle!! That's darn near
parallel to the reflecting surface! This is far, far lower than I had ever
thought. Most antennas have radiation at this angle, of course, but it is
usually way down from the main lobe.

Even for double-hop contacts, I believe that the angle was something like 8
or 10 degrees for that same path, but I don't remember exactly right now.

You cite that a 1/2-wave-high horizontal antenna has a maximum lobe at 30
degrees. Starting at Albuquerque, that 30 degrees will get you only to about
Oklahoma City on the first hop. Again, this surprised me. Believe me, I am
becoming disabused of my previous erroneous notions.

These numbers are all for 40 meters and below. If the horizontal antenna in
question is operated on a higher frequency, there's no guarantee (if I am
reading the data in W6ELprop correctly) that a 30 degree incident angle will
be reflected at all.

I tend to grow jaded upon hearing east coast stations (I am in Los Angeles)
on 40, but now I will appreciate the physics necessary to make such a thing
possible.

I am also going to have to put up my Field Day antenna much, much higher
than previously thought.

Regards,

Al W6LX

Al Lorona wrote:
I can't find any references anywhere that discuss this, taking into account
all of the variables: height of ionosphere, frequency, etc.

The ARRL Antenna Book has charts and graphs of such.
--
73, Cecil http://www.qsl.net/w5dxp

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As an alternative to putting your Field Day antenna higher, consider
finding a location that will enhance lower angle radiation. 10 dB
improvement over flat ground isn't too hard to come by in hilly or
mountainous terrain. The best program I've seen for determining this is
TA (Terrain Analysis), a DOS program which I believe you can still get
from its author, Brian Beezley, K6STI. Last I heard, he'll only take
orders by mail.

I have the same general problem here in Oregon, perhaps even more so. My
Field Day antennas and location are chosen as though I were after DX.
You might be interested in the "Field Day Special", an antenna I've used
for many years from here -- it might also be suitable for you. You can
find the info at http://www.eznec.com/Amateur/Articles/.

Roy Lewallen, W7EL

Al Lorona wrote:

Hi, Richard,

Yeah, your reference sounds about right. On Roy's suggestion, I downloaded
W6ELprop and was shocked to find what the elevation angles are for even
moderate path lengths. For example, I ran the path from Albuquerque to
Philadelphia and was amazed to find that the signal would have to leave the
antenna and reflect from the F layer at a 4 degree angle!! That's darn near
parallel to the reflecting surface! This is far, far lower than I had ever
thought. Most antennas have radiation at this angle, of course, but it is
usually way down from the main lobe.

Even for double-hop contacts, I believe that the angle was something like 8
or 10 degrees for that same path, but I don't remember exactly right now.

You cite that a 1/2-wave-high horizontal antenna has a maximum lobe at 30
degrees. Starting at Albuquerque, that 30 degrees will get you only to about
Oklahoma City on the first hop. Again, this surprised me. Believe me, I am
becoming disabused of my previous erroneous notions.

These numbers are all for 40 meters and below. If the horizontal antenna in
question is operated on a higher frequency, there's no guarantee (if I am
reading the data in W6ELprop correctly) that a 30 degree incident angle will
be reflected at all.

I tend to grow jaded upon hearing east coast stations (I am in Los Angeles)
on 40, but now I will appreciate the physics necessary to make such a thing
possible.

I am also going to have to put up my Field Day antenna much, much higher
than previously thought.

Regards,

Al W6LX