[amsat-bb] AO-51 mode V/S

Michael Heim kd0ar at sbcglobal.net
Mon Oct 27 04:52:20 PDT 2008

Although I missed a couple days of the satellite in V/S, once I got the antenna up, I had a LOT of fun on the mode.  Learned a lot, some of which I'd like to share here.

I was using a California Amplifier 130099 downconverter with an 18 inch linear polarized yagi which is attached to the converter. The amp was modified according to the instructions given by K0SM on the Rochester VHF Group's website.  Article is at http://www.rvhfg.org/articles/k0sm/basic%20mods.php  There is also a "block downconverter" out there that has an N connector for attaching whatever antenna you might want to put on it.  If its for MMTS, chances are its capable of being modified.

The unit I have is identical except the antenna is part of the unit.  There is about 20 dB of "excess" front end gain, so you can place this at the end of a long piece of lossy cable or put an attenuator on the radio side of the bias tee, but that is not totally necessary.  The IF is 123.200 MHz for the downlink, plus or minus doppler if the crystal in the unit is netted perfectly.  Mine is off about 30 KHz or so (at 2400).

Using the linear polarized antenna in the vertical plane worked very well for me.  I have the antenna at fixed elevation about 20 degrees or so above the horizon, on the same mast as the V/U antenna.  IF is the FT-817nd.  I get one significant fade during the pass which I'll mention later.  Everything was manually operated , tuning and antenna.  And with a VHF uplink, there isnt enough doppler to be concerned about.

First thing I noticed is the very light activity on this transponder.  Lack of S band gear is probably the reason.  These MMTS units are available.  I've seen them at hamfests real cheap - $5 - $10 each.  They are VERY easy to modify,  taking about 15 - 20 minutes each.  The modifications are foolproof and require no test equipment.  After modifying mine, I had it tested at a friend's in Pittsburgh, and found it to be quite sensitive with no additional modification. they do, however require a minimum of 15 volts, so running them portable presents  a few additional problems, but nothing more than a pack of AA batteries wont cure.

Now for the actual operation...
Working the satellite, I found that there is a major fade that lasts about 30 seconds or so as the bird passes by.  This happens at all elevation angles, so I would assume it has to do with the satellite's antenna orientation in regard to my polarization.  There are also fades that vary with each pass and time of day.  My early hypothesis for this might be local tropo effects that reflect the signal.  I noticed no such effects during a rainy, windy pass, which usually produce no conditions such as that.  The lower the bird gets to the horizon, the more pronounced the effect is.  I can work the bird consistently down to 6 degrees elevation, and that may be because of ground clutter or the elevation angle of the antenna.  The antenna beamwidth is wide enough that I can work the bird from about 40 degrees to about 6 degrees above the horizon.  The signals are generally quite strong and 100% full quieting for most of the pass.  Trees absorb the signal almost
 totally, so I can only use the bird for a little more than half a pass, as I have trees to the south.  As the leaves go away, perhaps it will shoot thru the bare trees, but as of this session, I didnt start picking up the bird until the bird was clear of the trees.

Anyway, I had some nice QSO's on mode S while in V/S mode.  I'm building a mode L uplink from scratch, but its a long way from being finished.  I wanted to share my experiences with the group, and perhaps entice some more operators to set up for this mode.  A load of fun, and so many are missing out especially since for V/S, all that is needed is a simple surplus receive converter and a rig that will receive 123 mhz.

 Michael Heim

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