Make More 432MHz EME Contacts
by Polarization Rotation

This is a version of my article in DUBUS 3/1996.

Page 2 of 3

CONTENTS

Top of article 
Top of this page

1. Polarization Problems
2. How to Rotate Polarization
3. The G3SEK 432MHz Array
4. Next page: How to Use Polarization Rotation 

End, Back

 

The G3SEK 432MHz Array

This antenna no longer exists, and we now live in Scotland.

At the present time, the best solution for yagis at 432MHz is to mechanically rotate the whole array on its polarization axis. The engineering problem is that the yagis must be mounted from the rear, forward of the support frame, and this makes the weight and windload very unbalanced. Fortunately these problems are easy to solve at 432MHz with quite simple mechanical engineering.

Several people have built polarization-rotatable yagi arrays for 432MHz. The first to use polarization rotation scientifically to make more QSOs was KL7WE [5, 6]. Tim (SK) devised the procedures described in Section 5, and he used them very successfully with his rotatable array of 16 2.5 yagis from Fairbanks, Alaska, and in a 12-yagi portable array for expeditions [5]. Tim's Alaska array is now being used by KL7HFQ. Since then, WA9FWD [7] and K1FO have built 16-yagi rotatable arrays. K1FO has published detailed mechanical drawings which many people have copied [8]. K1FO has expanded to 24 yagis, NC1I moved ahead to 48, and K5GW has a Texas-style 64-yagi array!

Even 16 yagis would be too large for my very narrow back garden, so I decided to use only 12. This gives an approximately square array which is very compact and easy to rotate. When mounted close to ground level, shielded from the wind, it does not require any 'heavy metal' engineering.

 

This was before 16 houses were built in the field behind the antenna! Click on the picture to see what the view became...

Feeding 12 yagis

With a small EME array, you cannot afford any compromises. The length of each yagi is limited to about 2.5m by mechanical factors (see below), so you need the best little yagis you can find. The yagis in my system are a 15-element 3.2 wavelength design with double reflectors, based on the DL6WU family and computer-optimized by DJ9BV [9]. When the array is horizontally polarized, the yagis are stacked 4 high and 3 wide.

The feed system has also been very carefully designed to produce the minimum loss. If the number of yagis had been 8 or 16, I would have used open-wire all the way from the individual yagis to the centre of the array; but you can only do that with arrays of 2^N yagis. With any number of yagis that is a multiple of 3, you have to use coax somewhere.

The 12-yagi array is divided into six 2-yagi groups. Each yagi has a folded dipole giving a feedpoint impedance of 200 ohms, and each vertical group of two yagis is fed using 200-ohm balanced line made from ¼-inch aluminium tubing (see left). This gives a 100-ohm balanced feedpoint for the group, which is transformed to 50 ohms unbalanced by a 70-ohm quarter-wavelength coaxial section inside a 'bazooka' balun sleeve.

Each of the six 50-ohm feedpoints is joined by the shortest possible length of FSJ4/50 'Superflex' cable to the 3+3-way (¾ + ¾-wavelength) power divider in the centre of the array. The coaxial cable sections are only 1.2m long, so the feeder losses within the array are very low and there is very little scope for improvement.

Rear-mounting Yagis

Yagis for use with polarization rotation have to be quite short, because they must be mounted from the rear. You could mount the yagis part-way along the booms, but this loses the polarization rotation facility at low elevation angles where you need it the most. KL7WE used 2.5-wavelength yagis (1.75m boom) and the practical limit is about 3.5 wavelengths (2.5m). Longer booms will probably break off at the rear when the winds come, because the forces at the mounting point are too high. Also, longer booms will sag under their own weight, causing pointing errors.

It is very important to minimize the unbalanced weight in the array, so the yagis must be self-supporting without extra bracing at the front. Rope bracing is not very useful, because the whole array can be rotated on its side and even upside-down. Therefore the boom diameter and wall thickness are quite critical. If the boom is too slim, the yagis will not support their own weight; if the boom is too large and heavy, it creates extra forces on the mounting frame. For my 3.2 (2.25m) yagis I used 20mm square tubing, strengthened at the rear mounting point by 300mm of hardwood inside the tube.

To minimize pointing errors, the yagis must be rigidly mounted on a flat support frame. I used 75mm diameter heavy-wall tubing for the main support boom, 50mm diameter for the three cross-booms, and 32mm for the two braces. The forces on the yagi mounting clamps are very large, and I found that ordinary mast clamps do not work. After some experiments I made my own clamps from 30mm square steel tube, welded to steel plates with holes for two U-bolts (see photos above and below). The square boom of the yagi slides inside the square steel tube, and is fixed very firmly by hammering in strips of aluminium and steel.

If I was making the whole support frame again, I would use square tubing which would make the whole frame self-aligning without needing to over-tighten the clamps.

The centre of the support frame is clamped to the main polarization drive shaft. This is a welded component made from steel plate and angle, similar to K1FO's design [8].

Az-El-Pol Rotation

Az-el-pol mechanical sketch

Let's look at this diagram in more detail.

Polarization drive

Like most people, I use an ordinary antenna rotator for polarization rotation. With a speed of about 1 revolution per minute, it takes about 8 seconds to turn though 45, by which time the polarization loss will be less than 3dB. Accuracy of polarization readout is about 5, which is actually better than is needed. 

For this small array, mounted close to the ground, a KR600 rotator has been quite strong enough. K1FO was able to use a T2X rotator with his tower-mounted 16-yagi array, but has had some polarization rotator problems since he increased the array to 24 yagis.

I used a commercial rotator cage, but it would actually be easier to make your own rotator mount as K1FO and the others have done. The polarization drive shaft is 49mm heavy-wall steel tubing, running in a home-made PTFE bearing.

Elevation drive

The elevation drive is an 18-inch TVRO jackscrew - see photo above. It is very simple to use, and good value for money. The elevation pivot is four heavy-duty steel door hinges, carefully mounted in a straight line on two pieces of 75x75mm steel angle. One side of the hinge is welded to the top of the mast, and the other side is welded to the polarization rotator cage.

It is very important to minimize the overhanging weight at the front of the array, so you must position the elevation axis as far forward as possible. I had to add a piece of heavy steel pipe to the back of the rotator cage, to bring the system into balance. Even so, it is only balanced for gravitational forces, not for wind forces.

Azimuth drive

Bottom: fixed 112-tooth gear
Black object, rear right: motor with 9-tooth gear
Grey box, front right: 10-turn azimuth indicator pot, 12-tooth gear (hand-made!)

The azimuth mast is 4m of 80mm heavy-wall steel water pipe, rotating in a socket in the ground (see drawing above). 1m of pipe is below ground, 3m above, so the whole mast is very strong and rigid. The weight of the whole array is supported at ground level on a large pipe flange, welded on to the mast. The ground socket is a 1m length of steel water pipe, 85mm inside diameter. This is sealed at the bottom and screw-threaded at the top, and was concreted into the ground with great care to keep it vertical. 

Screwed on to the top of the ground socket is a female pipe fitting, welded to the starter flywheel from a VW Polo. This is faced with PTFE blocks, and the underside of the rotating flange is also faced with PTFE.

The bottom end of the mast is built up to the same diameter as the inside of the ground socket by a ring of weld metal. Both surfaces were carefully filed and finished to run smoothly and concentrically. When the mast is lowered into the ground socket, all the weight is carried on the PTFE bearings and there is very little sideways rocking movement. Finally the socket is filled up to ground level with automobile anti-corrosion wax. This system is still running well, after more than 4 years.

The VW Polo starter flywheel has a 250mm diameter 112-tooth gear on the outside. To turn the mast, I use the matching 9-tooth gear from an automobile starter motor, connected to a low-speed DC motor. The motor is fixed to the pipe flange, and rotates with the mast. With a final drive of 12.4:1 to a 250mm gear, nothing will stall this rotator!

Rotation limit switches are made from two micro-switches and a piece of hacksaw blade (like a very big keyer paddle) and they allow total azimuth rotation of about 365°.

Position indicators

The elevation indicator is very simple, a pendulum weight and a precision 1-turn servo potentiometer. The azimuth indicator uses a 10-turn potentiometer, fixed to the rotating flange next to the motor, and driven from the fixed 112-tooth gear (see photo above). This requires a special 12-tooth gear which I made by hand out of plastic, using a 1:1 CAD drawing as a template for drilling and sawing. The electronics in the shack are very simple. Each potentiometer is fed from +5V DC, and readout is by LCD digital voltmeters with appropriate zero and scaling resistors.

 

CONTENTS

Top of article 
Top of this page

1. Polarization Problems
2. How to Rotate Polarization
3. The G3SEK 432MHz Array
4. Next page: How to Use Polarization Rotation

Back to: GM3SEK's Amateur Radio Technical Notebook

Updated 18 April 2010
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