Seminars
Members of TCARC occasionally do teaching sessions
at the club house. All those interested are welcome to attend. Members and non
members
THE
WEAKEST LINK
Part
1 of 2
Introduction:
Ham radio is about communication from point A to point B using
electromagnetic radio waves. There are two weak links in this process. The first
is the ionosphere. The propagation conditions are set by the sun so we can only
study and work with it. The other weakest link is the antenna. There are many
types of antennas that have been designed over the years that will perform very
well in long distance communication. The dipole is the most popular for many
reasons that I will describe later.
Computer Programs:
Antenna modeling:
I used the 4NEC2X antenna modeling program. It is
freeware and is available at: http://home.ict.nl/~arivoors/
Propagation:
HamCap,
IonoProbe and DXAtlas are three other programs that work together to give lots
of information on DX conditions all over the world. These programs are available
at: http://www.dxatlas.com/
Propagation
Elevation:
The image below
shows how the sky wave skip distance is a function of the radiation angle of the
antenna. If the antenna radiates upward the skip distance will be short and of
course if the radiation angle is low the skip distance will be farther.
The table below shows the relationship between skip distance and the
elevation angle of the radiated wave. This is for the 40M band at noontime.
As you can see the farther the distance the lower the angle. By taking a
closer look we can conclude that we should design an antenna that will have a
radiation pattern lower than 32 degrees. Also the longest single hop distance is
about 3000 miles at 4 degrees. Multiple hop angles will be between 4 to 35
degrees.
Location
Elevation Distance
Degrees Miles
Fredericton
64
95
Woodstock 58
140
Sherbrooke 31
351
Montreal
30
43
Ottawa
25
533
Kingston
25
588
St. Catharine
22
742
Toronto
21
739
Thunder Bay 20
1159
Winnipeg
20
1513
Saskatoon 15
1915
Calgary
13
2239
Vancouver 9
2658
Honolulu
20
5306 = double hop
The Ideal DX Antenna:
We all wish there would be an ideal DX antenna out there that would be
easy to build and perform to our needs but it does not exist and never will.
Every antenna will conform to the laws of physics. By understanding these laws
we can design the antenna that will fit our needs. There are lots of antennas
that are very good for DX but we must understand a few antenna principles first.
Lets
look at a 20M dipole in free space. It looks like a giant doughnut. The maximum
radiation is at 90 degrees from the dipole. The maximum gain is 2.42dbi.
Of
course we don’t live in space so lets look at what happens if we put the same
antenna one foot above perfect ground. Well it sure doesn’t look good. The
radiation energy is straight up and the gain has gone down to -5.9dbi.
Of course we are too close to the ground. Lets bring it up
to one half wavelength which is 34.7 feet. The image below shows great
improvement. The pattern is almost ideal and the gain has gone up to +8.45dbi.
Lets now study the pattern in a two-dimensional chart. The pattern looks
very good but remember we are still above perfect ground.
Lets change it to the average ground that we have in New Brunswick. The
chart below shows a change but not too much. The gain has gone down 1.05dbi but
the take-off angle has decreased to 28 degrees from 30.
Lets
be brave and bring the antenna up to three-quarter wave from the average ground.
This is at 52 feet. The pattern below has now changed. It is hard to know at
first if it is for better or for worst.
Lets
look at it in a two-dimensional vertical view. The take-off angle has gone down
to 18 degrees but there is a big hole at 42 degrees. The top lobe could pick up
some noise from the sun in the daytime. It would need more study if that would
be a problem.
Lets
go up again to a full wave up. This is at 69.4 feet. The main lower lobe gain
has gone up a bit and the take-off angle seems to be lower.
Lets
look at it again in a two-dimensional vertical view. The take-off angle has gone
down to 14 degrees but there is a big hole at 29 degrees. The top lobe has now
split in two.
As
we see in the above chart if the dipole is too high it will perform well in low
angles but could perform poorly in higher angles. All antennas that operate
above the ground will have this characteristic. Careful design is needed to
optimize performance. I will cover that subject in later documents to follow.
Ground
Conditions:
Let’s put our dipole antenna at 34.7 feet again and see what happens
when we change to different ground conditions. The chart below is the dipole
above average ground.
There have been many claims that
salt water will greatly improve performance. Let’s give it a whiz. The chart
below shows how it performs over salt water.
Obviously
things have changed but not by much. The main lobe has gone up by 2 degrees and
the gain has increased from 7.4dbi to 8.42dbi. What does that mean in the real
world? Lets pretend we want to talk to a station in Calgary. The angle needed is
13 degrees. The antenna gain at 13 degrees above average ground is 4.34dbi. The
antenna gain at 13 degrees above salt water is 4.71dbi. The difference is .37db.
Salt water is very close to perfect ground but it hardly made a difference. Why?
The RF electrical current is flowing between the dipole elements and not through
the ground. The ground is only a reflector for part of the radiated energy. The
RF electrical current for vertical antennas does flow through the ground and ground conditions does make a big
difference. This is where the myth crossed over.
Let’s change it to fresh water to see if it is different than salt
water.
The pattern above over fresh water did not change much from average
ground. The main lobe has gone up by one degree and the gain has gone up about
.5db. The gain at 13 degrees is 4.43dbi. This increase from average ground is
very small. The same myth applies for fresh water as with salt water. Fresh
water does not conduct electricity or reflect electromagnetic waves very well.
Let’s
see what happens in the city. As you can see from the chart below the take-off
angle is at 27 degrees. One degree below the average ground pattern. The main
lobe gain is 6.61dbi. This is .74db lower. At 13 degree the gain is 3.93dbi.
This is .41db lower than average ground
Conclusion:
The properties of the dipole antenna can be suitable for long or short
distance communication. The height above the ground will be a major factor in
the dipole’s performance. Every band will have an optimum height of operation
depending on the requirements.
By Bernard Cormier VE9BGC
