Doppler on 432 MHz

First what we have to remind on 432 (and up much more) is the Doppler shift.

The Doppler shift can get as high as 1KHz on 432 MHz. So I knew of several qso which failed by not paying attention to this. We have to see different cases:

1.

Station A is calling CQ on 432,010. His program tells him a Doppler shift of +600Hz (he has moonrise and moon in the east). So he is turns his RIT to +600Hz and hears his echoes clear.

Station B has the moon in the west and a Doppler of -300Hz. Hi turns his RIT to -300Hz and hears his echoes.

B now turns his VFO-knob and hears A (with rit at -300Hz) he can reply and A hears B on the qrg where A hears his echoes. ----That’s fine for random qso.

 

 

2. in skeds :

In every sked you should announce your TX qrg. Like 1. A TX on 010 with RIT. B answers as described.

What if you don’t hear A in sked. Set your TX on sked QRG 010 and use your RIT to find A.

VK3UM planner shows you all 3 doppler shifts between you and the DX station (if selected).

For example :

A_Home-A_Home +567         A_Home-B_DX  -165        B_DX-B_DX   -1096

If both have no Rit activated

A hears his echoes 567 Hz above his TX QRG

B hears his echoes 1096Hz below his TX QRG

A hears B & B hears A 165 below their TX qrgs

So for a sked both A and B set their TX on 010 and with the RIT at -165HZ they will hear each other.

The WSJT program shows different Doppler shifts depending if there is a locator put in the field Grid. If grid is blank you see Doppler home-home. If Grid is filled by the grid locator of the DX Station, the Doppler shift home-DX is shown. That’s a bit confusing, but ok.

 

3.

This time we count the frequency like a viewer/listener on the moon (Moon RX) will see it. The qrg on the moon is for everyone added half on the own Doppler effect.

Now we have the following situation:

Two stations will run a sked on 432,1000 MHz

Station A has a dopplershift of +600Hz (for own echoes)

Station B has a dopplershift of -800Hz  (for own echoes)

Station A tunes his TX to 432,0997 MHz   =432,1000 MHz  – ½*600Hz

So his TX signal will appear at the moon on 432,1000 MHz.

To hear his echoes Station A tunes the RX to  432,0997 + 600Hz = 432,1003 MHz

Station B tunes his TX to 432100,4 MHz   =432,1000 MHz  + ½*800Hz

So his TX signal will appear at the moon on 432,1000 MHz.

To hear his echoes Station A tunes the RX to  432100,4 - 800Hz = 432,0996 MHz

 

So both Stations can hear their echoes and on the same qrg they can hear the other station. This situation has all advantages:

  • The QRG is exact defined and independent from location on earth.
  • Both stations can hear their own echoes and the other station at the same QRG.
  • On random we can use the same RIT adjusting to make qso like on tropo.

Using this method the operator has to tune on both RX QRG and TX QRG, while the Doppler changes over the moon pass. Best is to use 100Hz steps while this will be easily heard by ears and on JT is no big deal while it can be seen in waterfall display. Special in JT don’t tune while Txing, use the 5 seconds when the computer is decoding.

 

On 2m this is no big deal, while Doppler is not so high, but on 432 we have up to 1KHz Doppler and I remember several cases where QSOs where nil by wrong Doppler compensation from one or both stations.

 

 

 

Spatial offset

 

Spatial offset and Faraday

 

In my years of EME I realized that I could work very small Stations in Europe and JA, but to the US I always had problems to work horizontal polarized small stations. I believe many of you horizontal polarized stations made the same experience. There are two effects which causing this. First is the spatial offset between two stations, which depends only on the locations of the stations and the position of the moon. The VK3UM planner with the online spatial offset calculator makes it clear. Eu to Eu is always below 20 degree, means losses of about 1 dB. Remember 45° is a loss of 3dB.

JA has mostly 90 degrees offset to EU, but the JA stations are vertical polarized so it fits perfect. The same we have with the US, but they using horizontal as we do in Europe. So we have a polarization offset close to 90 degrees. That means an extra loss of 10 or 20 dB. Not acceptable on EME.

 

But we can make QSOs to the US from time to time. Also reported that the US guys hear us but we don’t, and vice versa.

The reasons are: one of the stations has polarization rotation or cross yagis, or 2nd Mr. Faraday turns the polarization. The earth’s magnetic field causes the wave front from the radio signal to rotate in polarization several times as it passes through the ionosphere on the way to the moon and back.

That causes not only the wishful turning of 90 degree, but all degrees between 0-90° and to make it real complicated this effect is not reciprocal. And the polarized waves can be split of into many different polarizations. So even with pol. Rotation or H+V antennas we have a big loss, what looks like an absorption. As we know so far Murphy turns into a bad angle ever when we have only one sked with THE MEGA-dxpedition. HI !!

The observed fading on EME is caused by faraday and by libration.

As viewed from earth, the moon appears to rock back and forth on its axis. This motion is called “libration”. Both effects cause deep and fast qsb. This qsb can cut in a CW-letter a dash or dit so you hear a station loud but cannot decode the callsign.

 

As observed it needs an optimal speed for CW to make a qso under such condx. The perfect solution is cross polarization like on 23cm, but this causes other problems on 432 for yagi stations. With dishes its so easy. We observed times when libration is less than at other times, but a calculate able solution is not available. Faraday is also not predictable. We know it is a bit less in times when we have sunspot minimum and aurora causes absorption and extra loss, but we cannot calculate this. In my mind, this makes skeds and random more real a challenge, because you cannot predict all. You have to try and retry until a qso is successful.

 

Here we add a practical example to the spatial offset problem. In the chart below is shown the spatial offset during a window EU-US.

 

At US moonrise we have 45° offset / 3 dB of extra loss. At half of the window spatial turns to nearly 90° / >20dB of extra loss. At Eu moonset the angle turns to below 30° / 1 dB loss. Here is the best chance to place a sked if other parameters are fit as well.  Europe to the northwest US is much worse, for example my qth to KA7V in Oregon is always >70° / >10dB loss. This makes it very hard for linear polarized stations to complete a qso. Better is the path EU to VK5 where spatial is always below 30° / 1dB loss.

Moonrise US                             to moonset EU

This antenna above shows how to solve the polarization problem. Or use a dish !

 

N8CQ 16 yagi portable on trailer with pol. rot.

 

 

Libration

 

Libration in latitude variations have showed the greatest effect during the tests from SV3AAF. Libration in longitude variations (thin sine) have not showed significant effect. Anyway for the most of 2009 they almost coincide so it will be more difficult to distinguish for someone that does not have the experience of 2008.

For the moment the most important:

If you make some echo tests of your own for the purpose of libration study please correlate your observations also with lunar declination. At higher positive declinations I expect more clear results of the libration effect highs and lows. Perigee is of lesser relevance and it better be avoided for the purpose of libration tests at second half of 2009 because it doesn’t coincide with high moon at northern hemisphere. Audio spectrum waterfall monitor like Spectran can help the observation always along with our good ears.

Tnx to SV3AAF for this info !!

 

Maybe all of us can check if this fits with own observations. More theory at :

http://en.wikipedia.org/wiki/Libration

 

 

 

 

 

 

 

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Update August 2017