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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: 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: The following “rules of thumb”  are used for the gamma match :
(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))

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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