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Two antennas are here described:
a 2-elements Yagi for 20.2 MHz and a MOXON
antenna for 22.4 MHz
One of the design objectives was an easy
installation and disinstallation, this because the observatory is left
unattended when no observations are scheduled.
2-elements Yagi antenna, construction details, Az-El
mount and tuning
Construction details
The antenna is a 2-elements beam (driven element
and director). The operating frequency is 20.2 MHz (lambda = 14.85m).
The lengths, in meters, of the elements
and the spacing are given by:
-
Driven element = 147 / MHz
Spacing = 0.10 lambda (for max gain)
Director = 139 / MHz
This does not take into account the height
of the antenna above ground and that each element has sections with different
diameters.
The above dimensions were used as input
for the simulation using NEC4WIN95, the simulated behaviour of the antenna
was optimized for resonance and maximum gain at 20.2 MHz at an height of
3 meters above real ground. The calculated dimensions are:
-
Driven element = 7.44m
-
Spacing = 1.62m
-
Director = 6.98m
The following “rules of thumb” are used
for the gamma match :
-
Rod diameter =10mm
-
Rod length (0.04 - 0.05 lambda) = 59 - 74
cm
-
Spacing between the rod and the driven element
(0.007 lambda) = 10 cm
-
Capacitor (2100/MHz) =141pF
(click to enlarge)
These are the far-field Elevation (Zenith)
and Azimuth plots with the antenna pointing to the horizon (zero tilt angle).
The antenna height is 6 meters.

And this is a plot of the antenna parameters
(SWR, Impedance,....) measured using the MiniVNA antenna analyzer
The material is aluminum tubing, the mast
is a telescopic flag-pole. Its height can be up to 6 meters.
The whole structure can be assembled
and disassembled in less than fifteen minutes.
The drawback of this antenna is that
the structure is quite sensitive to the wind.
Gamma match
This is the waterproof box of the gamma-match
protecting the variable capacitor, the capacitor is from an old transistor
radio, only one section is used. The N-type female connector is on
the bottom of the box.
te second picture shows the driven element
and the matching section with the spacer and the adjustable tap.
(click the photos to enlarge)

Azimuth-Elevation mount
The antenna can be moved in both Azimuth and
Elevation. A standard low cost rotator is used for the Azimuth rotation
and is remotely controlled from the shack.
The orientation in Elevation is done by
hand, pulling on a rope attached to the driven element side of the boom,
the other end of the rope is then fastened to the base of the mast.
The following pictures show the rotator
, how the boom pivots on the top of the mast and the Yagi pointing to the
sky
The beamwidth of this antenna in the elevation
plane can be very wide, 50 to 80 degrees depending upon the tilt
angle and its height above ground (as from the simulation) so that a precise
aiming is not necessary.
There is no direct reading of the Elevation
angle, but, using a little trigonometry the length of the rope for a given
elevation angle is given by :
L=SQR(D² + H² - 2*H*D*sin(elevation
angle))
-
L = Length of the rope from its boom
attachment to the base of the mast
-
D = Distance between the rope attachment at
the end of the boom and the pivoting point .
-
H = Distance between the pivoting point and
the rope attachment at the base of the mast.
Tuning procedure
The frequency of resonance was measured with
the Yagi at a height of 3 meters and with a tilt angle of 40 degrees. The
coax cable was disconnected and replaced by a small loop loosely coupled
to a Gate-Dip-Meter (GDM) the frequency of resonance is 20.2 MHz.
It has not been possible with such a simple
instrumentation to measure the bandwidth. The simulation shows resonance
from 19.9 MHz to 20.3 MHz.
The gain of such a configuration, as from
the simulation, is 9.5 dBi.
The next step was to match the impedance
of the antenna to the 50 ohm impedance of the cable. A first adjustment
is done "by ear" listening to the galactic background noise in the receiver.The
variable capacitor in the gamma match is adjusted for maximum received
noise and, if needed, the length of the rod element is
also adjusted.
A second more precise procedure
is to connect the input of a resistance bridge, set on 50 ohm, to the antenna
connector and a RF generator to the RF input of the bridge. The gamma match
shall be adjusted for minimum voltage indication.
MOXON antenna, construction
details and tuning procedure
Have a look at Moxon
Antenna Project for the design of Moxon antennas and where you
will find a small program, Moxgen
, to easily calculate the dimensions of the antenna

These are the far-field Elevation (Zenith)
and Azimuth plots.


And this is a plot of the antenna parameters
(SWR, Impedance,....) measured using the MiniVNA antenna analyzer
Standard 3mm wire is used for the reflector
and driven elements. The elements are attached to four wodden masts using,
as isolators, short sections of PVC pipe.
The masts are 6 meters high. This height
was choosen to have the main lobe maximum at an elevation of 30 degrees,
this because during the years 2007 to 2009 the elevation angle of Jupiter
at transit is around 30 degrees at the latitude of the NICEro observatory
(44 degrees North).
The antenna is connected to the receiver
using 50 ohm coax cable one wavelength long (wavelength in the cable).
Toroid cores are slipped over the cable near the feedpoint to restrict
current flow on the outer surface of the coaxial shield and help to improve
antenna performance.
The tuning procedure consists on measuring
the frequency of resonance. The coax cable was disconnected and replaced
by a small loop loosely coupled to a Gate-Dip-Meter (GDM) and since the
frequency of resonance was lower than expected the length of the elements
and spacings had to be shortened until the desired frequency was reached.
Discrepancy between the resonance value
from the simulation and the measured one could be attributed to the
proximity of a metallic fence, an aerial power line at less than 3 meters
from the antenna reflector and other metallic structures.
This could also have an effect on the
antenna beam pattern and on the gain.
This is a view of the antenna. Each wooden
mast has two sections that are assembled together through a short piece
of PVC pipe, the sections fit tightly into the pipes and can be easily
dissembled. The masts are held in place using guy ropes.
(click the photos to enlarge)
Some details of the antenna (quick-and-dirty
construction ! )
Mast sections with the PVC
pipes. The masts are inserted into pipes buried in the ground.
Feed point (weather protection removed)
Two isolators
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