If you want to listen to Jupiter sing, bounce a message off the Moon, talk to a Satellite, or a little unmanned Aircraft, you need a very high gain antenna. An easy way to make one, is from an old C-band satellite TV, Big Ugly Dish (BUD).

To use an unknown dish, you need to find its focal point and then make a little antenna with a good front to back ratio, to use as a feed.

A Yagi antenna tends to have a very low impedance, while a helical antenna tends to have a very high impedance. The parasitic elements of a Yagi loads the active element, much like resistors in parallel. One can use the same effect with a helical antenna, to reduce its impedance to something closer to a 50 Ohm co-axial feed cable.

A multifilar helical antenna can be tweaked to almost exactly 50 Ohm, by driving the one filament and leaving the other ones floating just like Yagi director elements. The more floating parts, the lower the impedance gets.

I mounted the filaments into a little wood block glued to a circular FR4 PCB reflector. Drill 2.5 mm holes into it, then glue the filaments into the wood.

The parasitic element is just standing there above the ground plane, seemingly doing nothing.

The driven element must be soldered to the centre of a 50 Ohm feed and the screen of the feed must be soldered to the reflector in a cut-out under the wood block. It always requires some improvisations to make a helix, which is a large part of the 'fun'.

Once the helix is assembled, glue the block to the reflector.

Mount the feed at the end of a wooden dowel rod, at the focal point of the dish. That is the easy part!

CM Bifilar 2.450 GHz ISM Band Helical Antenna with Parasitic Element

CM Copyright reserved, Herman Oosthuysen, 2018, GPL v2

CM

CM 2450 MHz helical array

CM c=299792458 m/s

CM Wave length = 2.998x10^8 / 2450 MHz = 0.122 m

CM WL/2 = 0.061 mm

CM WL/4 = 0.030 mm

CM Axial Mode:

CM Circumference = 1.2 x 0.122 = 0.146 m

CM Pitch = 0.4 x 0.122 = 0.049 m

CM Turns = 1

CM

CE

# Helix driven element

# Tag, Segments, Spacing, Length, Rx, Ry, Rx, Rx, d

GH 1 100 4.90E-02 4.90E-02 2.3E-02 2.3E-02 2.3E-02 2.3E-02 2.20E-03

# Parasitic helix element, 180 degrees rotated

GM 1 1 0.00E+00 0.00E+00 1.80E+02 0.00E+00 0.00E+00 0.00E+00 0.00E+00

# Ground plane

SM 20 20 -5.00E-02 -5.00E-02 -1.00E-03 5.00E-02 -5.00E-02 -1.00E-03 0.00E+00

SC 0 0 5.00E-02 5.00E-02 -1.00E-03 0.00E+00 0.00E+00 0.00E+00 0.00E+00

GE

# THIN WIRE KERNEL: NORMAL: 0; EXTENDED: -1

EK -1

# EXCITATION: I1 VOLTAGE: 0, I2 TAG: 1, I3 SEGMENT: 1, I4 ADMITTANCE: 0, F1: 1 VOLT, F2: 0 IMAGINARY

EX 0 1 1 0 1 0

# FREQUENCY: IFRQ LINEAR: 0, NFRQ STEPS: 41, BLANK, BLANK, FMHZ: 2350 MHz, DELFRK: 5 MHz

FR 0 41 0 0 2.35E+03 5

RP 0 91 120 1000 0.000 0.000 2.000 3.000 5.000E+03

EN

Execute the simulation with

$ xnec2c -i filename.nec

The parasitic element does its thing remarkably well, resulting in an impedance of 48 Ohm (inductive), which is a near perfect match to a 50 ohm coaxial line. The inductive impedance doesn't matter - it just causes a phase shift.

Note that the frequency is very high and the wavelength is very short. Therefore, if you change

If you want to use a different size semi rigid co-ax from your cable junk box to make the helix, then you will need to spend a couple hours tweaking the helix parameters (width and spacing), to get the impedance back to about 50 Ohm again.

La voila!

Herman

To use an unknown dish, you need to find its focal point and then make a little antenna with a good front to back ratio, to use as a feed.

### Focal Length

The focal point of a parabola is easy to find using some forgotten high school geometry:- Measure the diameter (D) and the depth (d) of the dish.
- The focal length F = D^2 / 16 x d

**When it is free, take two.**

-- ancient Jewish proverb.

### Feeding a Hungry Dish

Most satellites are rotating slowly, to improve their stability. This means that an antenna needs to be circularly polarized, otherwise the signal will fade and fluctuate twice with each revolution. This requires either a helical antenna, or a turnstile Yagi antenna.A Yagi antenna tends to have a very low impedance, while a helical antenna tends to have a very high impedance. The parasitic elements of a Yagi loads the active element, much like resistors in parallel. One can use the same effect with a helical antenna, to reduce its impedance to something closer to a 50 Ohm co-axial feed cable.

A multifilar helical antenna can be tweaked to almost exactly 50 Ohm, by driving the one filament and leaving the other ones floating just like Yagi director elements. The more floating parts, the lower the impedance gets.

### Bifilar Helical Feed for WiFi ISM Band

An easy way to make a small helical antenna for the S-Band is with semi-rigid coaxial cable of 2.2 mm diameter (3 mm is more stiff, but still doable).### Twist and Shout

Cut two filaments. Clamp them carefully onto a bread board and then glue a bunch of tooth picks across them with a hot glue gun. Once you have finished building the antenna, twist the helix by hand and then remove the tooth picks.I mounted the filaments into a little wood block glued to a circular FR4 PCB reflector. Drill 2.5 mm holes into it, then glue the filaments into the wood.

The parasitic element is just standing there above the ground plane, seemingly doing nothing.

The driven element must be soldered to the centre of a 50 Ohm feed and the screen of the feed must be soldered to the reflector in a cut-out under the wood block. It always requires some improvisations to make a helix, which is a large part of the 'fun'.

Once the helix is assembled, glue the block to the reflector.

Mount the feed at the end of a wooden dowel rod, at the focal point of the dish. That is the easy part!

### Helix Design

From the famous graph of Kraus, we get the following:- Frequency: 2450 MHz helical array
- c=299792458 m/s
- Wave length = 2.998x10^8 / 2450 MHz = 0.122 m
- Axial Mode:
- Circumference = 1.2 x 0.122 = 0.146 m
- Pitch = 0.4 x 0.122 = 0.049 m
- Turns = 1

- Length of filament: sqrt(circumference^2 + pitch^2) x turns = 0.154 m

### NEC2 Model

Here is the NEC2 model of the bifilar WiFi ISM band helical feed:CM Bifilar 2.450 GHz ISM Band Helical Antenna with Parasitic Element

CM Copyright reserved, Herman Oosthuysen, 2018, GPL v2

CM

CM 2450 MHz helical array

CM c=299792458 m/s

CM Wave length = 2.998x10^8 / 2450 MHz = 0.122 m

CM WL/2 = 0.061 mm

CM WL/4 = 0.030 mm

CM Axial Mode:

CM Circumference = 1.2 x 0.122 = 0.146 m

CM Pitch = 0.4 x 0.122 = 0.049 m

CM Turns = 1

CM

CE

# Helix driven element

# Tag, Segments, Spacing, Length, Rx, Ry, Rx, Rx, d

GH 1 100 4.90E-02 4.90E-02 2.3E-02 2.3E-02 2.3E-02 2.3E-02 2.20E-03

# Parasitic helix element, 180 degrees rotated

GM 1 1 0.00E+00 0.00E+00 1.80E+02 0.00E+00 0.00E+00 0.00E+00 0.00E+00

# Ground plane

SM 20 20 -5.00E-02 -5.00E-02 -1.00E-03 5.00E-02 -5.00E-02 -1.00E-03 0.00E+00

SC 0 0 5.00E-02 5.00E-02 -1.00E-03 0.00E+00 0.00E+00 0.00E+00 0.00E+00

GE

# THIN WIRE KERNEL: NORMAL: 0; EXTENDED: -1

EK -1

# EXCITATION: I1 VOLTAGE: 0, I2 TAG: 1, I3 SEGMENT: 1, I4 ADMITTANCE: 0, F1: 1 VOLT, F2: 0 IMAGINARY

EX 0 1 1 0 1 0

# FREQUENCY: IFRQ LINEAR: 0, NFRQ STEPS: 41, BLANK, BLANK, FMHZ: 2350 MHz, DELFRK: 5 MHz

FR 0 41 0 0 2.35E+03 5

RP 0 91 120 1000 0.000 0.000 2.000 3.000 5.000E+03

EN

### Radiation Pattern

The helix made from 2.2 mm semi-rigid coaxial cable, has a good front to back ratio of about 6 dB and a nearly flat frequency response over the 2.4 GHz band.Execute the simulation with

*xnec2c*:$ xnec2c -i filename.nec

The parasitic element does its thing remarkably well, resulting in an impedance of 48 Ohm (inductive), which is a near perfect match to a 50 ohm coaxial line. The inductive impedance doesn't matter - it just causes a phase shift.

Note that the frequency is very high and the wavelength is very short. Therefore, if you change

*anything*by as little as*half a millimeter*, the results will be*completely*different.If you want to use a different size semi rigid co-ax from your cable junk box to make the helix, then you will need to spend a couple hours tweaking the helix parameters (width and spacing), to get the impedance back to about 50 Ohm again.

La voila!

Herman