Wednesday, December 27, 2017

Parasitic Quadrifilar Helical Antenna

For polar orbit satellites, one needs an antenna with a mushroom shaped radiation pattern.  It needs to have strong gain towards the horizon where the satellites are distant, less gain upwards where they are close and as little as possible downwards, which would be wasted and a source of noise.  Most satellites are spin stabilized and therefore the antenna also needs circular polarization, otherwise the received signal will flutter as the antennas rotate through nulls.

The helical antenna, first proposed by Kraus in 1948, is the natural solution to circular polarized satellite communications.  It is a simple twisted wire - there seems to be nothing to it.  Various papers have been published on helix antennas, so the operation is pretty well understood.

Therefore, it is amazing that after more than half a century, there is still a new twist to the helical antenna...

Backfire Helix

The backfired quadrifilar helix array is especially popular for amateur satellite communications, but the results reported by Chris van Lindt and Julian Moss (G4ILO) regarding the antenna drawing on the right, left me curious and wondering whether we are dealing with an internet myth, or a comedy of errors, or a design that is too sensitive to build easily.   

Chris reported that the QFH exhibits nulls that are useful for tuning out terrestrial interference.

How can an omni-directional antenna have a null?  That comment rang a huge alarm bell in my mind that the commonly used QFH antenna was not designed or built right.  To figure out what is going on, I modeled the QFH in NEC2.

First of all, I don't really like backfire helices, because that is not the way that Kraus intended them to be implemented and because much power is lost in the forward direction which will then hit the ground, while you are trying to talk to the sky.  The Kraus helix design calls for an earth plane / reflector, which will project the back lobe forward.

Without the reflector, the radiation pattern of a helix is very messy, but since that is what lots of people are using, I modeled it this way.

A model is never 100% the same as a real antenna, but the NEC cards presented below allows any true card carrying radio/computer geek (a.k.a radioham) to easily play around with it and get  a feeling for the critical antenna constraints, before building one.

The helical antenna work published by Kraus in 1948, shows that a thin helix radiates in normal mode, while a fat helix radiates in axial mode, as shown in his famous angle vs circumference graph. 
 



Simple Thin Helix in Free Space - No Reflector

The picture above, shows what a single turn thin helix radiation pattern looks like if there is no reflector - an upside down mushroom.  The bulb at the bottom is turned skyward when the thing is flipped over in a backfire configuration, while the twirl at the top is then pointed to the ground.  So while in backfire mode it is nicely circular polarized and nicely omni-directional, there is nevertheless significant radiation towards the ground.

I plotted these with CocoaNEC on a Macbook (since it makes the prettiest plots) and it cannot rotate a helix in the x or y axis, so if you want to flip it, you got to turn your computer around.  CocoaNEC also cannot handle a half turn helix, so I used one full turn.  You could use xnec2c on Linux or BSD for the full set of NEC2 helix options, at the cost of uglier graphics.

Helical Arrays

A monofilar helix is a very long and unwieldy thing.  It is easier to handle a shorter antenna and there are various ways to achieve that.

Every half wavelength, the current in an antenna goes to zero.  When the current goes to zero, it doesn't matter if the wire is open or closed circuit, so one can cut an antenna every half wave length and it will still work the same.  Similarly a long helix could be considered to be an array of identical little helices in a row.  One could even take these little helices and put them side by side and it will still work the same, or one can rotate and interleave them into a multifilar helix.

The main problem with a multifilar helix is hooking the filaments up with the correct phasing.

Bifilar Helix

In a bifilar design, the one helix is rotated through 180 degrees.  It also needs to be driven with a signal that is rotated 180 degrees.  This is easy to do with a balun.

Connect the centre wire to one helix, the shield to the other and then wind five to ten turns in the coax feed to increase the impedance of the sleeve.   That makes a simple infinite balun.

Quadrifilar Helix

A quad design is the same idea as the bifilar, with four helices each rotated by 0, 90, 180 and 270 degrees.  A quad design is nice and compact, but getting the phasing right is much more of a chore.  A 1/4 wave length of coaxial cable will give a 90 degree phase shift.  This is easy to do for a hobyist, since all you need is a calculator and a ruler.

QFH - 4 Phased Driven Elements

Most of the QFH designs on the wild wild web however, use one short and one long loop of wire (As from the design for the OSCAR 7 satellite).  The idea is to make two helices that are too long (inductive) and two helices that are too short (capacitive), then hook them up in parallel.  One loop then leads 45 degrees, while the other one lags 45 degrees electrically, thus giving a 90 degree phase shift.  See this http://www.uhf-satcom.com/sband/sbandQFH.pdf

However, if the wire dimensions are not exactly right, then it will be anything but - especially the capacitance.  Hence that comment about the handy nulls in the omni-directional pattern...


NEC2 model of a QFH with Transmission Line Phasing:
CM Quad Helix Antenna
CM Copyright reserved, Herman Oosthuysen, 2017, GPL v2
CM
CM 2 meter helical dipole array
CM 137 MHz
CM c=299792458 m/s
CM WL = 2188 mm, r=348 mm
CM WL/2 = 1094 mm
CM WL/4 = 547 mm
CM Max Segments is 10,000 / 40 mm = 250
CM Diameter = 378 mm
CM Radius = 189 mm
CM Length = 570 mm
CM Turns = 1
CM Turn spacing = 570 x 2 = 1140 mm
CE
GH 1 50 1.14E+00 1.14E+01 1.89E-01 1.89E-01 1.89E-01 1.89E-01 1.00E-03
GM 1   1        0        0      90        0        0        0        0
GM 1   1        0        0      90        0        0        0        0
GM 1   1        0        0      90        0        0        0        0
GE
TL 1 1 2 1 50 0.547 0 0 0 0
TL 2 1 3 1 50 0.547 0 0 0 0
TL 3 1 4 1 50 0.547 0 0 0 0
FR     0     0     1      0  1.37E+02         0         0         0         0         0
EX     0     0     1      0  1.00E+00  0.00E+00  0.00E+00  0.00E+00  0.00E+00  0.00E+00
RP     0    91   120   1000         0         0         2         3      5000
EN


The NEC model is actually not complicated, but you need to read the manual to understand it.  I defined one helix with a GH card, then replicated and rotated it 3 times with GM cards.  The phasing is done with three transmission line (TL) cards. The first helix is excited with 1 Volt using an EX card and the last thing is the radiation pattern (RP) card.

BTW, the NEC2 manual is here: www.nec2.org/other/nec2prt3.pdf

Quadrifilar Parasitic Helix

Another way to get the phasing right, is to ignore it altogether!

If you make a quad and only drive one helix and leave the other three floating as parasitic elements (same as on a Yagi-Uda antenna), it will work almost exactly the same as when you actively drive them.  Most importantly, it will work much better than if you drive them wrongly!
 
QFH - 1 Driven, 3 Parasitic

The above plot shows a quadrifilar helix in free space with one driven element and three parasitic elements.  This plot doesn't look much different from the one above it and it eliminates a major head-ache, so you can then set your phasers to stun.

The NEC model is the same, just remove the three transmission lines.

Reflector

The antenna god (a.k.a. Kraus) intended helices to work with reflectors.  If we expand the model to include a ground plane, the pattern turns right side up and the stem of the mushroom (almost) disappears, leaving only the bulb, so all the energy goes the right way, providing another dB or two of gain.

QFH - 1 Driven, 3 Parasitic, Reflector

It is the same as the one above, but you don't need to crick your neck.

NEC2 Model Including a Reflector:
CM Quad Helix Antenna with Parasitic Elements
CM Copyright reserved, Herman Oosthuysen, 2017, GPL v2
CM
CM 2 meter helical array
CM 137 MHz
CM c=299792458 m/s
CM WL = 2188 mm, r=348 mm
CM WL/2 = 1094 mm
CM WL/4 = 547 mm
CM Max Segments is 10,000 / 40 mm = 250
CM Diameter = 378 mm
CM Radius = 189 mm
CM Length = 570 mm
CM Turns = 1
CM Turn spacing = 570 x 2 = 1140 mm
CE
GH 1 50 1.14E+00 1.14E+01 1.89E-01 1.89E-01 1.89E-01 1.89E-01 1.00E-03
GM 1   1        0        0      90        0        0        0        0
GM 1   1        0        0      90        0        0        0        0
GM 1   1        0        0      90        0        0        0        0
GE
GN 1
FR     0     0     1      0  1.37E+02         0         0         0         0         0
EX     0     0     1      0  1.00E+00  0.00E+00  0.00E+00  0.00E+00  0.00E+00  0.00E+00
RP     0    91   120   1000         0         0         2         3      5000
EN 


The reflector is modeled here as a ground plane card (GN 1).   This works on CocoaNEC, but with Xnec2c, you need to shift the helix up by one or two millimeters to avoid a short circuit, or define a multi patch surface with two SM and SC cards slightly below z=0.

Impedance

It therefore turns out that a 2 meter band, 146 MHz QFH antenna is actually easy to build, even easier than anyone imagined, simply by ignoring the phasing problem altogether:  

Wind four helices, each 1027 mm long, around a former 300 to 400 mm diameter, connect one up to a 50 Ohm coax and leave the other three floating as parasitic elements.

For good measure, add a reflector, connect it to the screen of the co-ax and put the thing right side up as Kraus intended.

Similarly, you could make a helical array with any number of filaments and get any amount of gain (practically up to about 15 dBi), but the quad neatly solves the impedance matching problem, since it has an impedance of about 40 Ohms and can be hooked up with garden variety RG-58 co-ax without bothering with a tuning element.

Circular Polarization

The electrical field is forced to rotate clockwise, when looking up at the sky, by the helix rotation.  To confirm that you do the right hand polarization correctly, get a nice big wood screw.  If the helix uses a reflector, then it needs to follow a normal right handed screw.  If the helix is backfired without a reflector, then it needs to be opposite to the right handed screw.  

A wrong way polarized antenna will cause a big drop in signal strength.  Opposite polarization is effectively a permanent null pointed at the satellite. 

A right handed bolt will never fit in a left handed nut - unless you use a big hammer...

La Voila!

Herman



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