Tuesday, December 5, 2017

Amateur Satcom Turnstile 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, 137 MHz NOAA and Russian Meteor weather satellites.  Weather satellite reception is described here.  A quadrifilar helical antenna is described here.
 
Engineering, is the art of making what you need, 
from what you can get.

Radiation Pattern of the Three Element Yagi-Uda Antenna

Once one combine and cross two Yagis, the pattern becomes distinctly twisted. The right hand polarization actually becomes visible in the radiation pattern plot, which I found really cool.


Radiation Pattern of Six Element Turnstile Antenna

Only a true RF Geek can appreciate the twisted invisible inner beauty of a herring bone antenna...
Six Element Turnstile Antenna

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.
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 to make them flutter.

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 for training - It is easy to stow and transport in a car for 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.  The boat anchor is a bit light - good for a canoe maybe.

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

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.

 UAE Desert Clouds Picture Made with WxSat

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 = 513 mm (The value used in the TL card).  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 in reality.

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.  Wrap your right hand around the antenna rod, with your thumb pointing to the sky.  You need the wave to travel from your knuckles to your finger tips.  Therefore if you drive the element at your knuckle, the next one towards your fingertip needs a 90 degree (1/4 wave length) delay.


Tape Measure Antenna

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 1 inch wide 5 m Stanley tape measure was sacrificed for the elements.




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 or heat shrink tubing.  

A beach umbrella stand makes a handy upright support.

The NEC2 Card deck:

CM Turnstile Crossed Yagi, three element each, 2 meter band, 146 MHz
CM Elements are made of 6 mm Al tube (r = 3 mm)
CM
CM CocoaNEC Summary:
CM Frequency 146.000 MHz
CM Feedpoint(1) - Z: (21.111 + i 4.130)    I: (0.0456 - i 0.0089)     VSWR(Zo=50 Ω): 2.4:1
CM Antenna is in free space.
CM Directivity:  8.39 dB
CM Max gain: 8.31 dBi (azimuth 90 deg., elevation 0 deg.)
CM Front-to-back ratio: 14.78 dB (elevation 90 deg)
CM Front-to-back ratio: 15.16 dB (elevation of front lobe)
CM Front-to-rear ratio: 13.92 dB
CM
CM Weather and Amateur Cubesats:
CM 147 (Weather) or 146 MHz (Amateur) 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 first Yagi is oriented in the X plane
CM The wire radius alters the impedance of the dipole:
CM Thicker wire has higher impedance
CM 3 mm radius wire gives 25 Ohm and 1.9:1 VSWR
CM 12 mm radius wire gives 40 Ohm and 6.8:1 VSWR
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
CM
CM The second Yagi is offset by 10 mm in Y axis to prevent short circuiting the elements
CM It is oriented in Z plane
CM An ideal 1/4 wavelength transmission line connects the two dipoles
CE
GW 1 5 -0.513 0 0 +0.513 0 0 0.003
GW 2 5 -0.488 0.513 0 +0.488 0.513 0 0.003
GW 3 5 -0.463 0.821 0 +0.463 0.821 0 0.003
GW 4 5 0 0.010 -0.513 0.010 0 +0.513 0.003
GW 5 5 0 0.523 -0.488 0 0.523 +0.488 0.003
GW 6 5 0 0.831 -0.463 0 0.831 +0.463 0.003
GE
TL 2 3 6 3 50 0.513 0 0 0 0
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 dual design, there are many elements and the impedance is about 21 Ohm.  Therefore, it is good to hook the antenna up with RG58, 50 Ohm coaxial cable.

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.  Note that the wrap only affects the shield.  The centre conductor is shielded and therefore is blissfully unaware that it is wrapped around a stick.

Alternatively, clamp a ferrite around the cable.  This will prevent the cable shield from radiating and disrupting the antenna pattern.  Again, the ferrite will only affect the shield, not the inner conductor.


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.

--0O0--

I have subsequently also built an antenna from a laundry dry rack 6 mm aluminium tubes.  This looks rather better and sturdier and there are enough tubes left over for a 70 cm turnstile transmit antenna also.  Once I have both of them done, I'll build an electric screwdriver rotator mount for it.


La voila!

Herman

No comments:

Post a Comment

On topic comments are welcome. Junk will be deleted.