[amsat-bb] Re: Antenna suggestions for mounting on vehicle for sat work.
maisel at lobo.net
Thu May 24 13:00:16 PDT 2012
That is extremely helpful, and most likely what I will do. Now, is
there a computer interface somewhere for these RS rotators?
Bob Bruninga wrote:
>> The company that comes out with an AZ/EL unit that
>> is about $200 is going to sell a million.
> Radioshack already does. Its called a TV rotator, and I would buy one before they are no more!. For LEO satellites one does not need elevation 98% of the time and with a modest beam (ARROW type) you can have a $70 system. Tilt the beam up about 15 degrees so that you still have max gain on the horizon where satellites spend 1/3rd of all their pass times below 20 degrees. Track then in AZ only.
> As the satellite gets above about 30 degrees and starts to roll off a dB or so of beam gain, remember that at that elevation the satellite is HALF as far away so it is now 6 dB stronger! This remains true up to over 45 degrees, where you may be down 3 dB on the beam but the signal is 10 dB closer to you! The break-even point is above about 70 degrees. BUT! Remember, the satellite is only above 70 degrees less than 2% of all pass times. Simply not worth spending another $700 for an elevation rotator for 1 minute a day of better access.
> Also, do NOT be tempted to tilt above about 15 degrees or you are going to lose gain on the horizon where you need it most. Lots of folks put it up higher because it seems logical, but they are ignoring the very significant DISTANCE factor at low elevations (sketched to scale on the web page).
> Please take a look at the plots on http://aprs.org/rotator1.html
> Its an old page, but the drawings are always valid.
> Bob, WB4APR
More information about the AMSAT-BB