70cms Cebik Moxon Aerial for LEO Satellites

The 70cms (435MHz) version of the Cebik Moxon is now built and shows an SWR of 1.1 in the shack as measured with my AW07A antenna analyser. Hopefully this SWR will not change too much when I put in in the attic.

The elements are made out of #12 AWG wire from RS Components. The sizes are as specified in Cebik’s original article published in the ARRL QST in August 2001 “A Simple Fixed Antenna for VHF/UHF Satellite Work”. The phasing line is built as for the 2m version using RG59U coax cable from BitsBox only using a length for the 70cm band. The matching line is made from old 75Ω cable TV cable again at 70cm band length. The elements and lines are explained in my earlier post about Building the 2m Cebik Moxon.

The elements are stapled to an old wooden curtain pole to keep the driven elements and reflectors apart. At the ‘floating’ end they are kept apart using the insulation from old multi-core telephone cable and the shaft from cotton buds. I’ll hot-glue these in place once the aerial is in the attic. The distance between these floating ends is the most critical in the whole aerial.

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I plan to use this aerial with the 2m version and suitable diplexers to communicate with U/V or V/U LEO satellites. Watch this space!

Building the 2m Cebik Moxon

The Cebik Moxon is made up of four driven elements, two reflectors, a phasing line and a matching line.

The matching line is needed to match the 25Ω impedance presented by the aerial to the 50Ω impedance expected by the coax feed to the radio. It is made up of parallel quarter-wave lengths of 75Ω cable. I modelled this in an excellent application SimSmith by AE6TY to see how critical its length is (the answer being not very much). The SWR is 1.125 at 351mm, and only rises to 1.13 at 330mm and 354.5mm.

I worked out the velocity factor of the RG59U 75Ω cable (bought from Bitsbox who were very prompt and helpful but didn’t know its velocity factor) by building a TDR out of a circuit to make a fast pulse and my HP54615B scope. As usual Alan, W2AEW came to the rescue with this video #88: Cheap and simple TDR using an oscilloscope and 74AC14 Schmitt Trigger Inverter.

Here are my sums to work out the velocity factor of the RG59U cable measured with the TDR circuit.

5m takes 48.8ns between peaks, so the signal takes \frac{48.8}{2} = 24.4ns  to go 5m which is 4.88ns/m, so the speed is  \frac{1}{4.88ns/m} = 204.918 \times 10^{6} m/s, so the Velocity Factor, VF = \frac{204.918 \times 10^{6}}{c} = 0.683.

Using EE Toolkit on iPhone, in air ¼ 𝜆 is 513.7mm at 145.9MHz. So ¼ 𝜆 should be 513.7 x VF = 350.857mm.

SimSmith Matching Line
SimSmith Matching Line

The phasing line is just a quarter wavelength of 50Ω cable which feeds the other element 90° out of phase from the first. Here’s an Lissajous FigureXY scope picture of the phasing. It should be a bit more upright but I think that is probably due to the way I’m connecting it to the scope rather than the phasing line itself.

Now the bit I am particularly bad at: putting it all together.

I made the elements out of 6mm aluminium round tube. The tube is easy to cut with a hacksaw and to bend using a pipe bender. The matching section and phasing line are connected to the rods using solder tags and screws. The driven elements and reflectors are kept apart by whittled pea sticks. This distance between the driven elements and the reflectors is the most critical measurement. The pea sticks are held in place with shrink wrap and friction. The elements are hot-glued to a plastic drain pipe. This is not very robust, but works as long as the aerial isn’t disturbed too much. I’ll have to fix this properly if I want to move the aerial from the attic to the garden.

Aerial in attic
Aerial in attic

2m Omnidirectional Aerial

I thought it was about time I understood aerials more, so I am building an omnidirectional aerial for LEO satellite use.

I’m hopeful that I will be able to use the aerial in my attic, but if not I’ll use it in the garden. The aerial I’ve picked is documented in the ARRL Antenna Book in one of the supplementary PDFs. The article which describes it was published in QST in 2001 and is called “A Simple Fixed Antenna for VHF/UHF Satellite Work” by L. B. Cebik, W4RNL. I can’t link to it because it is copyrighted by ARRL but I explain most of it below. It is a pair of Moxon rectangles at 90° to each other.

I made a model of the aerial in the excellent Mac application cocoaNEC by Kok Chen, W7AY. The other aerial modelling applications I looked at were all based on spreadsheet tables. I found the programming approach used by cocoaNEC easier to use. Here’s the shape of the aerial as shown by cocoaNEC.

Cebik Geometry
Cebik Geometry

The driven elements are the top ones, and the reflectors are the bottom ones. Both have vertical parts which make up the Moxon rectangles. One of the driven elements is driven from the feeder through a matching line. The other driven element is driven 90° out of phase through a phasing line. This gives the radiation pattern below. This is the VHF version of the aerial. As you can see it is almost omnidirectional but favours elevations that you might expect to be able to use a satellite at.

Cebik Radiation Pattern

Cebik Radiation Pattern

This was output by this cocoaNEC program of the Cebik aerial.

// Cebik’s Moxon Turnstile as described in “A Simple Fixed Antenna for VHF-UHF Satellite Work”
// note there is a problem with quotes and double quotes — I had to copy them from another model 
// rather than typing them
// so I’ve converted to metres
// this version replaces the driven2 excitation with a 90º phasing line between driven1 and driven2
// so driven1 is excited from the middle, where the phasing line is connected
// driven2 is just connected through the phasing line

model( "Cebik" ) 
{

real driveh, drivev, gap, reflecth, reflectv, h, rad, n, lambda;
element driven1, driven2;

lambda = 2.05478; // c/145.9MHz

drivev = 0.096774; // 3.81"
driveh = 0.73787; // 29.05"

gap = 0.03556; // 1.40"

reflectv = 0.141986; // 5.59"
reflecth = driveh;

rad = 0.003175; // 0.125"
n = 21; 

h = 1.5; // height above ground

// driven 1 (driven directly) 
driven1 = 
wire(0, -driveh/40, h, 0, driveh/40, h, rad, n); // very middle of dipole
wire(0, -driveh/2, h, 0, -driveh/40, h, rad, n); // horizontal parts
wire(0, driveh/2, h, 0, driveh/40, h, rad, n);
wire(0, -driveh/2, h, 0, -driveh/2, h - drivev, rad, n); // dropping vertical parts
wire(0, driveh/2, h, 0, driveh/2, h - drivev, rad, n);

// reflector 1
wire(0, -reflecth/2, h - drivev - gap - reflectv, 0, reflecth/2, h - drivev - gap - reflectv, rad, n);
wire(0, -reflecth/2, h - drivev - gap - reflectv, 0, -reflecth/2, h - drivev - gap, rad, n);
wire(0, reflecth/2, h - drivev - gap - reflectv, 0, reflecth/2, h - drivev - gap, rad, n);

// driven 2 (via phasing line)
driven2 = 
wire(-driveh/40, 0, h, driveh/40, 0, h, rad, n); // split as driven1
wire(-driveh/2, 0, h, -driveh/40, 0, h, rad, n);
wire(driveh/2, 0, h, driveh/40, 0, h, rad, n);
wire(-driveh/2, 0, h, -driveh/2, 0, h - drivev, rad, n);
wire(driveh/2, 0, h, driveh/2, 0, h - drivev, rad, n);

// reflector 2
wire(-reflecth/2, 0, h - drivev - gap - reflectv, reflecth/2, 0, h - drivev - gap - reflectv, rad, n);
wire(-reflecth/2, 0, h - drivev - gap - reflectv, -reflecth/2, 0, h - drivev - gap, rad, n);
wire(reflecth/2, 0, h - drivev - gap - reflectv, reflecth/2, 0, h - drivev - gap, rad, n);

voltageFeed(driven1, 1.0, 0.0); 
longTransmissionLine(driven1, driven2, 50.0, 1.0 *lambda/4); // 𝜆/4 phasing line

averageGround();

setFrequency(145.9);
}