HEIGHT VERSUS TAKE OFF ANGLE
This factor
is one of the most important one for DXers although many are not aware
of this.
Very often, on this site there has been referred to the different take-off
angles from different types of antennas and their heights.
We can learn that through propagation via the F2 layer the maximum distance
reachable in a single hop is around the 2500 km (many information about
propagation is available on Internet or through reading books).
There are two factors which are capable of changing that distance in
our favor. One is the height of the layer, it various say from 200 to
250 km (and the higher the better) and second the take-off angle (the
lower the better it is for far DX)
Above we
see a some simple drawings of the earth and a layer from the ionosphere,
in this case the F2 this was is one we used for DXing most of the time.
The layer is situated about 200 to 250 km above the earth
taking a closer look at the second drawing we see a transmitted wave
coming from the earth, imagine this is your antenna location.
Your radiation pattern from this antenna is drawn in red the angle between
the earth and that red line is called the take-off angle.
What it does is explained in the first drawing where A shows a high
take-off angle which is the case for a horizontal yagi only a few meters
above the earth.
In B the take-off angle is low, this one is produced by a very high
placed yagi.
WHAT
CAN INFLUENCE THE TAKE-OFF ANGLE?
To this question there are only three answers:
- The polarity of antenna we use
- The height of the antenna
- The ground conditions.
POLARIZATION OF A ANTENNA
A vertical placed antenna is capable of producing quite low take-off
angles without being placed high above the ground. For example: a normal
GPA is able to produce angle's in the order of 16 degrees when it is
sitting at the ground, a horizontal placed dipole has to be at 9 meters
to does the same thing.
A horizontal dipole placed lower then 9 meters will have probably worse
DX results as the vertical.
As said
earlier it is not for nothing that for low frequencies (below 27 mhz
say 7/3,5mhz or evan 1,8mhz), HAMs very often
use verticals for DXing
There are stories going around which say that vertical polarized antennas supposed to be worse for DXing cause they're radiation pattern sloops along with the earth.(it continues along the earth further then the horizon) Although I do not have any fundamental prove that this is not true. The opposite did not happen either. I think it is not true, I can not think of a reason why the radiation pattern would not take-off under a low angle.
One bad aspect of verticals however is know, a vertical above bad ground (desert) can't fill the high expectations which we just gave it. Horizontal antennas often outperform verticals under these conditions.
At bad
ground it is a bad DX antenna the reasons for that are to far to go
into at this site. An example was that a horizontal 1 element quad proved
to be 1 to 2 S-units stronger towards Europe from 234 division (very
bad ground).
Above good ground this effect seems to be amplified just think of those
/MM stat ions..
This advantage
of a vertical antenna, just might be where the Quad has got its great
reputation from. A quad has two times a quarter wave radiating section
which is vertical polarized. In the old days on lower frequencies HAMs
reported that the band seemed to be opening earlier on a quad then on
a yagi, the band seemed to close a bit later as well.
So when you can not place your favorite antenna high enough a vertical
can be a solution for you if you want to work some DX.
There are
also disadvantages to vertical,
It produces more noises...so the station you want to hear is often deeper
away
A vertical is more sensitive to QRM at the neighbors
Above 8 to 9 meters a horizontal antenna will produce a lower or the
same take-off angel, thus better for "real" DX.
A Beam polarized vertical will be influenced more by the mast then a
horizontal beam, unless it is far enough away from this mast or the
mast is made out of non-guiding material.
Most are omni-directional so all stations are heard, this can be an
advantage as well.
It performs poor above "bad" ground like desert.
GROUND
This antenna is from Paolo 1DX101,
The 3 elements Delta Loop has a boomlenght of 5,4 meters.
Paolo is located on a little island in Italy
He is situated only 20 meters from the sea on the East front. 25 meters
to the North and about 3 km from the Sea to the west. So, sea all around.
The antenna has a maximum height of 11 meters. With this station he
reaches results everyone can be jealous off. Just take a look at cluster.dk
and wait until he is on!
Paolo started in begin 2004 and is now (medio 2006) already on 189 confirmed
DXCC's
This is a great Example of what a great location could bring you with
a great antenna!
DX
LOCATIONS
For maximum
use of the normal space wave it is important to have the antenna as
high as possible above nearby buildings, trees, wires and surrounding
terrain. A hill that rises above the rest of the country is good locationfor
a station of any kind. The highest point is not necessarily the best
location for the antenna. In the drawing above the hilltop would be
a good site all directions but if maximum to the right is the objective,
a point just below the crest might be better.This would reduce the coverage
in opposite direction. An antenna situated on the left side, lower down
the hill, might do well to the left, but almost certainly would be inferior
in performance to the right.
Selecting
a good DX spot (portable or home QTH) is not easy and depends also on
which band will be operated. For our band a wide saltwater horizon,
especially from a high cliff, just smells of DX.
(Source ARRL antenna
book)
THE HEIGHT VERSUS TAKE-OFF ANGLE.
As said and not for nothing, height is important. If you are searching
for a big signal on the other side of the world there is are two solutions
bigger and higher!
The table below is for ALL horizontal placed antenna over normal ground, they will have the following take off angle's (T/A) at named heights. And don not believe any other story it does not mater how many elements you have a Dipole or a 10 elements yagi will produce under the named angle its signal!
The table above shows where the main signal is radiated ( we still have
radiation at other take-off angles as well.)
Do not forget there is a big difference between take-off angle and gain.
If you compare these with the different take-off angles provided in
the chapter verticals, you can see when a horizontal antenna (with the
same gain) will outperform the vertical.
For example if you are NOT able to erect a yagi in the near of 8..9
meters, say when you are on a DX-expedition you just might be better
of with a vertical
(in both cases the antennas nearly the same gain.)
NOT ALWAYS?
There are studies who tell us that radiated angles below 5 degrees are
slightly absorbed by the ground.
Which angle
are you looking for?
Well to give you an impression: Pacific for Europe 1-5 degrees. Those
are the lower ones and for Europe these angles
should be around the 25 degrees.
With programs like VOACAP or with YTAD and The program Yagi Terrain
Analyzer from K6STI Which comes with the ARRL-Antenna book.
You can calculate what angle you want for a certain DXCC!.
With this
knowledge you can understand that in some cases it can be better to
place a high small yagi say a "cheap" 3 elements yagi instead
of a lower big gun 5 elements long john. It just depends what you want:
are you truly searching for that opening into the pacific for that last
DXCC you just might put your money into a higher mast. A 5 elements
(free space 8,5 dBD gain) placed 9 meters above the ground would have
13,06 dBD gain at 16 degrees.
At 7 degrees the same yagi still produces 9,52dBD.
A small 3 elements yagi (free space 6dBD gain)will produce the same
signal (9,6dBD) at 7 degrees with a height of 13 meters. Visa versa
the 3 elements will at 16 degrees take-off angle "only" produces
about 10 dBD and that's a big difference between the 5 elements.
Next example:
Below we see a 4 elements long boom yagi at a height of 12 meters. We
see that this antenna is doing a great job at 12 degrees there it peaks.
12 degrees for my location is say..Africa. (north side) The higher take-off
angle (C) is actually of no use for use DXers, it might come in handy
during Sporadic-E skip but for F2 dx not really interesting.
Next example:.
Now we
are going to take a look at a small 3 elements yagi (half the boomlenght)
Below we can see the results for that antenna at 18 meters height
We are producing 11,55 dBD at 8 degrees...This means your signal will have the same signal strength and you will receive equally well: stations Which are arriving at 8 degrees at your QTH.
The conclusion Which can be made is: for DXing it might be wiser to spent money in height then in boomlenght.
What about
a bigger antenna then?
If you are limited in height then YES go for a bigger yagi a change
from a 3 to a 5 elements is definitely a big improvement! also in the
regions further away, remember your take-off angle peaks for example
at 10 degrees with a 5 elements it would do the same at the same height
but the gain is higher, therefore the gain is also at lower angles higher!.
