Polarization Rotator

2m Yagi Polarization Rotation

Whilst the obvious thought entering your head at the moment is why bother - just buy x-pol yagis. In my case that isn't an option.

 

This antenna needs to fold down and dissapear so it has to fold flat. Imagine something out of thunderbirds......

 

So my only option is to build an antenna where each of the 4 yagi's on the H-Frame can be 'spun' along the long axis to give me continuously variable polarisation, but can rotate the elements into a position where some mechanical origami means it can fold up and dissapear into the roof of the shack.

 

The Issues this raises are

 

  • Loss caused by metal in the plane of the elements
  • Weight of the rotation drives and inertia effect on the az/el rotator.
  • Rotating matching of the 4 antenna
  • Cost.

 

However on the plus side it should give me an average of 1dB improvement over a switchable X-POL assuming I rotate it to within a few degrees...

 

 

Oh and I only need one four port coupler and one less relay before the preamp.

 

But being honest - it's a pain in the backside , but another challenge to take on.

 

 

Basic Specs/Requirements:

 

DC motor Driven

64 position encoder feedback

Low noise

Low weight

Low Cost

Rotates 9 element yagi on 25mm dia 3mm wall tube

 

 

** UPDATE***

 

I'm a bit busy to update the page, but I now have the new collapsible H frame completed - using 44mm glass fibre tubing. This meant fashioning some adapters between the 50mm plastics tees and the 44mm tube - but it saved a lot of weight. The next job is to move the whole assembly from the garden to on top of the shack. At the moment the system is handcranked from flat to raised - I think fitting the electric winch is a job for next year.

 

(update 21/06/17 I have now moved the whole mast/frame/support onto the roof of the shack)

 

I have also had the pcb's back - with one irritating mistake where the encoder connectors were in completely the wrpng place- but they work well enough and I have finished the first antenna and rotator assembly which works exactly as it should. I'm torn between waiting for new pcb's for the final version or just going with what I have.

 

It needed some hacking as the 25mm test boom I used was indeed 25mm - however the actual booms measure 25.3mm and I could not get the 25mm bearings to fit - however much I heated them. I then had to change to bike headstock bearings which are 27mm diameter - so I added adhesive lined heatshrink to pack the boom out.

 

On the plus side I got some 41mm O/D tube with an I/D of 38mm - this cut out the need for mucking about with the lathe as the pulley and bearing just slide in with the aid of a hammer.

 

The only tools you need other than a basic toolkit are a vice, 30mm+ cone cuter or 30mm drill and a typical dremel tool.

 

So I can sit in the shack and dial up the desired polarisation angle and the antenna rotates to it. So just 3 more to build......

 

Update 07/07/2017

 

I have built and tested the handheld controller and had a play with a single yagi and experimenting with adjusting rotation during reception on a couple of nights.

 

Currently I am rebuilding the motor controller box into a bigger box as the original didnt have enough room for the elevation screw jack relays. I am also fitting the handheld controller into a box rather than have the bare PCB lying on the bench

 

 

All 4 antennas and rotators are up - but I'm now fitting inline connectors to the antenna for the motor feed as when I take the antennas down to experiment with, the leads mean I cant move the antenna away from the mast to work on them.

Obstruction Loss

 

The obvious place for the drive is to drive it from behind the reflector - however this meant that the antenna could not fold and I was sticking the biggest weight at the end of the boom, greatly increasing the inertia felt by the main rotator.

 

So Decision 1 was to put the motor in the centre of the boom.

 

 

Whilst mechanically it seems sensible to put the motor drive on the H frame - this means unless the drive is really small it is going to interfere at some position of the elements. Since the elelments have to rotate 180 degrees to get the most benefit this makes it rather difficult.

 

So the second was to put the drive on the boom itself.

 

This means the drive can be mounted under the elements at right angles to the elements.

 

The drive system was designed with the smallest possible profile and the cables then feed along the underside of the boom and out the back with the coax feed to the antenna.

 

 

 

 

So as you can see from the sketch the motor drive rotates round the pole of the H frame meaning it is always out of the way of the elements.

 

The only loss is that the pulley on the boom has a diameter of ~50mm which means it extends 12.5mm outside the boom.

 

However I use Paraclips on my aerials to raise the elements above the boom - which raise the elements up by about 20mm so this shouldn't be an issue.

 

The system has sufficient drive to actually fit the support arm of a yagi through the bearings instead of the boom going through the bearings and still rotate a decent sized antenna - allowing through boom elements to be used. However this can lead to vibration so would need some experimentation to make it work reliably.

 

 

This Sketch gives a better view of the arrangement. The antenna boom is supported by two bearings which are fitted to the GRP tube. This has two advantages - one of which is that it spread the loads

and the second of which is the GRP is quite cheap (it was left over from the rest of the H frame) and has useful parts like the long Tee fittings which simplify the construction.

 

Obviously it spreads the load across the boom as well.

 

My antenna are about 4.5m so will self support with 25mm Aluminium tube with a 3mm wall.

 

 

The Drive

 

Initially I wanted to use gears - but there was a lack of good aluminium gears suitable for the task.

Also using a belt drive can simplify the design a little.

 

The biggest issue on this was how to build this without access to a machine shop - but relying on the tools I have lying around.

 

This meant some 'interesting' choices were made.

 

The first choice was stepper motor or DC motor. I chose DC as it offered me a physically smaller motor and substantially cheaper - and easier to drive. Stepper motors need a good reduction gearbox to be useable and missing steps can be an issue.

 

The gear ratio of the pulleys changed many times - In the end I selected a 32 tooth T5 pulley (10mm belt width) mainly because its boss was 38mm - which could be slid into a 40mm tube with 1mm wall thickness that would then slide into the grp of the support arm.

 

I also found some 25mm bearings with 37mm O/D that could similarly be placed in the 40mm tube.

 

One thing to remember with the motor is to fit 3 capacitors across it M+to M- M+ to case, M_ to case.

 

The motor will be running during RX so we need to reduce the noise as much as possible.

Bearing details for internal mounting on 50mm grp tube and for external mounting 32mm tube.. This required a fair amount of hammering and filing!

Position Detection & Motor Control

 

This was my favourite part. My initial design relied on a potentiometer. But This either meant a multi turn pot or needing quite a large pulley to match the rotation of the boom to the 260 degree travel of the pot shaft - either way this would enlarge the metal area and could affect the antenna performance.

 

I thought about rotary encoders - but they have the same issue if driven off a pulley and so I was looking at making my own reflective encoder stuck to the back of one of the existing pulleys as an alternative.

 

Quite by chance I stumbled on a 16 pules (64 positions per rev) encoder on RS for under £2. (RS Stock No.729-5927) And it had an 18mm diameter bore - That sounded familiar - that happens to be the boss size on a 16 tooth gear.

That meant I could mount it on the motor shaft engaging with the boss of the pulley and keep the design to 2 pulleys.

 

This gives me relative position - but not abolute - so by adding a single reflective optocoupler on the motor mounting plate I could detect a mark on the boom pulley to give me a starting point for rotation.

 

Since these components require a pcb, I took the opportunity to add a PCB with pads and a hole so that the cabling could be soldered to the front side of the board on pads and then fed through the mount and out to the back of the antenna.

 

As the motor current is intermittant and <400mA at Stall I found some 6 core screened cable 20SWG for about £0.80 a metre.

 

The wiring is

+5V to reflective coupler LED (red)

0V (screen)

Encoder A (yellow)

Encoder B (white)

Zero datum (black)

Motor + (blue)

Motor - (purple)

 

Each of the drives then feed back to a central controller on the H frame next to the 4 port coupler.

 

A suitable controller was the sparkfun MKII motor controller shield which can drive four DC motors - stick it on the back of a clone Arduino MEGA 2560 in a box and hey presto - one motor control system.

 

https://www.adafruit.com/product/1438

 

I was going to use the Leonardo as it has 4 interrupt pins and each encoder needs at least one interrupt to work correctly, However Idecided to use serial comms between the shack controller and the motor controller so I had to upgrade to the MEGA so I could have 4 interrupts for the encoders and still have interrupt driven serial communication between the two units. This only became necessary as I decided to control the elevation from the same controller.

 

I use the Encoder library for the teensyduino as it supports 4 encoders.

 

https://www.pjrc.com/teensy/td_libs_Encoder.html#optimize

 

Also I added some debouncing on the encoder outputs

 

 

 

 

 

 

 

 

 

 

 

 

The motor controller mast unit. The relays are for controlling the elevation jack and the feedback is from a potentiometer on the A0 input which is belt driven from the main cross piece.

 

 

 

Shack control and Polarization rotation in use

 

For the shack controller I had originally thought of just bringing down pushbuttons to control the motor controller. I decided instead to build a handheld controller with an LCD screen left over from the linear build and some controls.

 

Since I would need to use the controller on the roof whilst supervising the folding, I fitted waterproof RJ45 connectors so I could plug the handset directly into the motor controller - which would also provide 5V DC to the handheld unit.

 

I fitted a joystick to set the polarisation rotation target and the elevation jack target. After a couple of nights of experimentation I found that I needed to quickly change the polarisation in the RX><TX changeover time so I could reverse the effect of the Faraday rotation.

 

I added a potentiometer to set the expected geometric rotation change of the remote station. Then when I found the best RX signal I could immediatly( in the TX/RX changeover period) press the switch on the joystick to rotate the polarisation of the antennas the opposite amount of rotation to the geometric rotation.

 

So lets say the geometric rotation between me an the states at a particular time is 60 degrees.

On RX I get the strongest signal at 90 degrees.

 

So to present the best TX signal to them I press the button and the polarisation changes to 60-(90-60) degrees or 30 degrees.

 

If the geometric was 70 and measured RX 80 then the TX changes to 70-(80-70) or 60 degrees.

 

I also exposed the usb on the handheld so I could commant the elevation rotator from the PC and also put in a P.T.T. RCA connector. The idea being that the RCA connector can supress the encoder inputs during TX so that if I suffer interference when I'm running high power, the antenna won't wobble around! I havent had that problem yet - but I'm only running 400W (UK legal limit) at the moment.

 

 

Protection

 

My current line of thought is to stick the motor body in a length of 40mm waste pipe and seal it - I don't think heat will be a great issue.

 

As for the PCB/Encoders/wiring I think I am just going to pot the lot in resin as one assembly. They sare only five pounds in parts cost so if they break I will just build a new one!

Weight.

 

The weight of the drive,including angle bracket, 2 pulleys, bearings, both bearing housings, u bolts, motor and encoder mechanism is 512g.

Costs

 

In the table below I have detailed the relevant parts cost.

 

As you can see it is not too bad - around £175 all in. You can save more by shipping around - I certainly did so it has cost me less than that so far, Having said that I've only got one prototype and a pile of bits to make the rest with - so It may end up costing more.

 

 

As well as the parts for ther drive system I also had to buy parts for a new GRP H frame and new 25mm booms for the antennas

 

So in addition to the drive parts I needed

 

1 off 6m 44mm GRP tube

4 off GRP long tee

4 off 25mm 3mm wall aluminium tube 5.1m

 

 

 

As an approximate guide for the drives etc:

 

 

 

Part

Quantity

Cost

Total

 

 

 

 

Motor 5rpm 12v

4

£7

£28*

Encoder

4

£1.80

£7.20

Belt 250mmx10mm T5

4

£4

£16*

32T pulley T5 10mm

4

£6.20

£24.80

16T PulleyT5 10mm

4

£3.50

£16

25mm Bearing

8

£2.20

£17.60*

PCB

4

£2.50

£10

Opto

4

£0.80

£3.20

40mm tube

1

£7

£7*

Cable

16

£0.80 /m

£12.80*

Grommets

4

£0.4

£1.60*

Box

1

£6

£6*

Leonardo Clone

1

£4

£4*

Motor controller

1

£5

£5*

Aluminium Angle 3x2"

1

£7

£7*

ubolts 25mm

8

£1

£8*

Prices inc VAT & Delivery *=ebay! total £174.20

Improvements

 

One option is to use a smaller set of pulleys - say 12 teeth and 27 teeth. This would mean the outside diameter of the boom pulley is reduced to 43mm - or if you are really handy with a lathe a 25 tooth pulley could be used and the outside diameter falls to 39mm which keeps the total diameter of metal parts on the top side of the boom to be only 8 mm above the boom surface.

 

Another option is to go the stepper driver route - it does not need any rotational feedback as with a sufficiently sized motor you can count steps - so only the index opto is required.

 

You would however need a second motor driver to drive the extra coils - but luckily the adafruit shields are stackable.

 

 

 

 

Build Photos (these are with a 100mm bearing sleeve rather than 2x50mm bearing sleeves to be used in the final version)

Suppliers

 

Aluminium Tube ( long Lengths) www.simmal.com

50mm GRP Tube and Tees: (V Cheap!) www.lockinex-store.com

32mm GRP tube: www.polyfibre.co.uk

Pulleys (Although cheaper from China!) : www.accu.co.uk