Which linear polarization for 144 EME?

[VK5PJ]

Hi,
for a long time now I have had a fixed picture of what polarization(s) I should use for EME and why but it was not until about two weeks ago that a series of messages on a EME reflector challenged my picture.

The back ground, a US ham asked about how to best mount his pair of cross pol yagis (M2's I think) and a series of responses including one suggesting he mount them as an "X" and not as a normal "+" to avoid the mounting problems with elements being in the same plane as his crossboom.. Now I thought to myself yep that would work okay as EME is effectively a scattering mode off the surface of the moon and has lots of polarization rotation effects on the path so its always pot luck as to what you get back.

A few other's challenged that mounting scheme to the point where I had to cringe, almost telling the bloke who suggested the X mount that he might as well connect up a dummy load if he mounted them as an "X".

Thinking about this some more, and trying to take a global view of some one in the southern parts, I visualised a tennis ball with some pins sticking out of it in both the northern and southern hemisphere's to represent the towers of some EME stations and then holding it as arms length from my eye, it soon made me think that we keep forgetting to consider our relative positions to other stations when trying to match our polarisations. A ham in the northern part with horizontal antennas may end up looking like a station that has vertical to someone in the southern hemishpere (excluding rotation shift along the the EME path) when at lower elevations.

So maybe everyone in VK should use vertical yagi's for EME when trying to work to the northern hemisphere... or the true moral, I guess we need both horizintal and vertical and every angle inbetween, so an X mounting is probably no worse than any other scheme.

any thoughts from the crowd, have I hade a fundermental mistake in my reasoning?

Regards,
Peter, vk5pj

[VK5APN]

Peter

For 2m EME is does not matter. You can use linear polarisation (either Vertical or Horizontally mounted yagi's ) and still make QSO's. The advantage occurs when you have both V and H (cross polarised) yagi's. See the following website for further explaination.

http://www.ifwtech.co.uk/g3sek/eme/pol4.htm#Intro

Most use Horizontal. Not for any EME particular reason. Just that you can use the yagi/array for terrestrial communication as well.

Hope this helps.

Regards

Wayne

[VK5APN]

Peter

Further info:

One station recently went to these lengths

http://www.df7kf.de/RX_PHASE.pdf

Using 5 RX's, they are able to see V, H, +45, -45 degrees and CR (Circular Right)

Bound to hear/see everyone, HI

[VK2KRR]

Hi Pete

Circular is probably the best.

As Wayne said, you can use any polarisation and it will still work. BUT at any given time your polarity match may be 90 deg out So helps if you can switch from horizontal to vertical etc and use which ever is working best at the time.

I think the X mounting for that particular station would be advantageous for his cross boom mounting situation. But maybe has dissadvantages for terrestrial with the same antenna. Although if there were other stations with X mounting terrestrial it would be fine in my opinion.

[VK2XSO]

Since linear polarisation doesn't really have a meaning in space, you're going to have trouble defining it.
As mentioned, circular polarisation does not have the problem.

So to take a leaf out of satellite technology which does sometimes use linear polarisation we can define vertical and horizontal polarisation as;

Vertical Polarisation is alignment of the polarisation with celestial lines of longitude.

If this doesn't make sense, then draw a straight line across the sky from the horizon at true north through the moon (or the satellite) and continue on to the horizon at true south. This is vertical polarisation. Perpendicular to that is horizontal polarisation.

[VK2DVZ]

Another possible solution:

A mechanical way of varying (I presume linear) polarity exists, as was evident from a QSL card I received recently from Seiichi JR3REX.

His array photo shows four by CC17B2 antennas mounted on a X type frame (frame at 45 degrees to tower) and all yagis are able to revolve within bearings mounted at the ends of the X frame - mechanically controlled ropes appear to be used to perform this task.

To see a couple of photos of his array, see http://eme.dokidoki.ne.jp/photo/jr3rex/index.html

I guess further info could be obtained from Seiichi, JR3REX

Ross, VK2DVZ

[VK3DXE]

It really depends on how complex you want to get with your system. Two X-pol yagis mounted in an "X" configuration can be switched to provide pretty-well whatever polarisation you want with appropriate switching, so the main considerations should be the stacking distance compromise.

If you want to keep your polarisation switching as simple as possible, and still achieve reasonable results for terrestrial work, then a pair of yagis producing an orthaginally polarised signal should perform similarly to a single horizontal or vertically polarised yagi, within +/- a few tenths of a dB. Most people opt for the straight horizontal/vertical configuration. The arguments for and against any given configuration sometimes approach the levels of idiocy seen in the Mac/Windoze/Eunuchs arguments. Regardless, if you pay due attention to quality of workmanship and measurements, any half decent system will allow you to make contacts, from there it's really just system optimisation.

[VK2DVZ]

Last night I worked DL1DWI, who reported that my signals were best at 45 degree polarization.

Gerhard has an antenna array with motorized polarization adjustment, that appears to use an endless rope/chain mechanism . I do not know the details, however he does have a YouTube video of it in operation on this link: http://www.youtube.com/user/DL1DWI

He has a few more photos on QRZ.com also: http://www.qrz.com/db/DL1DWI

An interesting concept.

Ross