Tuesday, December 5, 2017

Amateur Satcom Rx Antenna for the 2 meter Band

NEC2, 2 m band, 146 MHz, Yagi Turnstile Simulation and Build

This article describes a Turnstile Antenna for the 2 meter band, 146 MHz amateur satcom and NOAA weather satellites. 

The turnstile is made from two 3 element Yagis - crossed - with a 1/4 wave (90 degree phase shift) RG58 coax delay line between them for circular polarization.
Engineering, is the art of making what you need, 
from what you can get.

A 1 inch wide steel roll-up tape measure is self supporting up to about 600 mm - just good enough for the ~500 mm elements - provided that there is no wind.

Another option is to cut up a Carrefour aluminium clothes rack made of 6 mm aluminium tubes, but I like the idea of a roll-up antenna better - It is easy to stow until the next time I get the urge to bark at the moon.

For the rod, get a 1.5 inch by 1.5 m wooden dowel at Ace Hardware - it comes with a free boat anchor at one end, that one has to remove.

So, one can take some implements from the French Revolution and turn it into a modern day satcom antenna.

Radiation Pattern of the 3 Element Yagi-Uda Antenna

Only a true RF Geek can appreciate the invisible inner beauty of a herring bone antenna...


3-Element Turnstile Antenna

The idea of the antenna sketch is to make it less confusing.   Whether that aim was achieved, is not clear! 

Essentially, it is three crosses on a stick.  The driven elements are broken in the middle at the drive points.  The other elements can go straight through if that is convenient, or they can be broken also - it doesn't matter, since the current is zero in the middle.

The middle elements are driven and the electrical field is forced to rotate clockwise, when looking up at the sky, by using a delay line to make a 2 phase motor.



Tape Measure Turnstile Antenna

The reflector is 1/2 wavelength long.  The driven element is 5% shorter and the director is another 5% shorter.  The spacing from the reflector to the driven element is 1/4 wavelength.  The spacing from the driven element to the director is 0.15 wavelength.  This is a typical 3 element Yagi design.  The dimensions are not very critical, since the frequency is low.
  • The overall length of the reflector is 1027 mm.
  • The length of each arm of the driven elements is 488 mm.
  • The overall length of the director is 927 mm.
  • The spacing between the reflector to the driven elements is 513 mm.
  • The spacing between the driven elements and the director is 308 mm.
  • The cabling is RG58, 50 Ohm or similar. 
  • The delay line is RG58, 342 mm in length.
  • The balun is a clip on ferrite, or 5 to 10 wraps around the rod below the driven elements.
The elements can be made from a 24 mm tape measure, or from 6 mm aluminium tubing from a clothes dry rack or whatever tubing you have on hand.  It will work with almost anything, since the frequency is low.  It is easier if the elements are cut 50 mm longer and trimmed after mounting - for finger and eye safety, trim the corners 45 degrees and wrap the ends with tape.  

The rod can be wood or metal.  Wood is easier to work, but here in the desert, it can be harder to get.  I bought a garden rake with a nice wooden varnished handle for 40 Dirhams and cut the rake off

A beach umbrella stand makes a handy upright support.

The NEC2 Card deck:

CM Yagi, three element, 2 meter band, 146 MHz
CM Elements are made of 1 inch tape measure (r = 12 mm)
CM 40 Ohm, 9 dBi
CM
CM Feedpoint(1) - Z: (12.210 + i 39.873)    I: (0.0070 - i 0.0229)  
CM VSWR(Zo=50 Ω): 6.8:1
CM Max gain: 9.20 dBi (azimuth 90 deg., elevation 0 deg.)
CM Front-to-back ratio: 7.30 dB (elevation 0 deg)
CM
CM Cubesats:
CM 146 MHz Downlink
CM 436 MHz Uplink
CM
CM Speed of light in vacuum = 299792458 m/s
CM Speed factor of RG58/59 = 0.666
CM
CM 2 m band = 146 MHz
CM L = 2053 mm
CM L/2 = 1027 mm
CM L/4 = 513 mm
CM
CM RG58 90 degree phase shifter:
CM L/4 = 513 * 0.666 = 342 mm
CM
CM The wire radius alters the impedance of the dipole:
CM Thicker wire has higher impedance
CM
CM Reflector spacing alters the impedance of the dipole:
CM Closer spacing has lower impedance
CM
CM Length reflector = L/2 = 1027 mm
CM GW 1 5 -0.513 0 0 +0.513 0 0 0.012
CM
CM Spacing = L * 0.25 = 513 mm
CM Length dipole = L/2 * 0.95 = 976 mm
CM GW 2 5 -0.488 0.513 0 +0.488 0.513 0 0.012
CM
CM Spacing = L * 0.15 = 308 mm
CM Y Position = 513 + 308 = 821 mm
CM Length director = Length dipole * 0.95 = 927 mm 
CM GW 3 5 -0.463 0.821 0 +0.463 0.821 0 0.012
CM
CM Excite the 2nd wire in the middle on element 3 of 5 with 1 Volt
CM EX 0 2 3 0 1 0 0 0 0 0 0
CM
CM Frequency 146 MHz
CM FR 0 1 0 0 146 0
CM
CM Radiation plot 360 degrees
CM xnec2c: RP 0,91,120,0,0,0,2,3,0,0,0
CM CocoaNEC: RP 0,91,120,1000,0,0,2,3,5000
CE
GW 1 5 -0.513 0 0 +0.513 0 0 0.012
GW 2 5 -0.488 0.513 0 +0.488 0.513 0 0.012
GW 3 5 -0.463 0.821 0 +0.463 0.821 0 0.012 
GE
EX 0 2 3 0 1 0 0 0 0 0 0
FR 0 1 0 0 146 0
RP 0 91 120 1000 0 0 2 3 5000
EN

Impedance Match

The impedance of a dipole antenna in free space is supposedly 73 Ohm.  The parasitic elements of a Yagi-Uda, reduce the impedance to something closer to 50 Ohm.  You can fine tune the impedance by adjusting the distance between the reflector and the dipole.  The thickness of the elements also affects the bandwidth and the impedance.

In this design, the impedance is about 40 Ohm.  Therefore, it is good to hook the antenna up with RG58, 50 Ohm coaxial cable.

Circular Polarization

Most Satellites spin around to stabilize them (The ISS is an exception).  The result is that the RF transmissions also rotate.  If you would use a fixed dipole antenna to work a satellite, then the signal strength will fluctuate rapidly.  The solution is to use a Right Hand Circular Polarized Antenna.

You can get polarization naturally, with a helical antenna.  Otherwise, you can make a rotating field by setting up a 2 phase electrical motor.  The first phase is the regular signal and the second phase is obtained with a 1/4 wavelength delay line (90 degree phase shift), applied to a second radiator, set at 90 degrees to the first one.  So we use two identical Yagi antennas in a cross/turnstile configuration.

The delay line is simple to calculate using c = L x f, so L = c / f.   The speed of light in RG58 copper wire is 0.666 of c so the 90 degree delay line is shorter: L' = c / f / 4 * 0.666 = 342 mm.

Balun

The coaxial cable is unbalanced, while the dipoles are balanced.  It is therefore necessary to add some inductance to the cable shield, by wrapping five to ten turns around the rod, just below the driven elements.

Alternatively, clamp a ferrite around the cable.  This will prevent the cable shield from radiating and disrupting the antenna pattern.


Conclusion

A tape measure antenna is not rugged and sooner or later a wire connection will break, but the advantage is that one can fold it and get it in and out of a car, making it good for educational use.

Unknown Satellite Signal
A quick check outside showed that it works.  I could see a satellite signal get stronger over a period of time.  Unfortunately it is raining.  It is the middle of the desert and it is a veritable rain storm - a misty drizzle - not good for my computer!

The signal is stable, and doesn't fluctuate, so the circular polarization is working.


La voila!

Herman

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