[amsat-bb] Re: satellite average elevation
gzook at yahoo.com
Tue Apr 12 13:39:33 PDT 2011
Again, back in "those dayes" we did not have the luxury of computer simulation and from trial and error the majority of people found that about 30 degrees above the horizontal worked the best. That is why the "olde tymers" recommend 30 degrees. It worked very well and we made many contacts using the LEO satellites.
After a relatively short time (less than a year), I acquired an Alliance TV rotor and used it as an elevation rotor along with an AR-22 for the azimuth rotor.
Frankly, even in "those dayes" the antennas often had more than enough gain to make it into the satellites. In fact, AMSAT was pleading with people to reduce their transmitting power to keep the linear transponders from overloading on a single signal which would allow many more people to use the satellites at the same time.
One could use a computer to determine the vertical pattern of their antenna and then determine exactly the elevation that is optimum for their antenna. That angle might be something like 17.2345 degrees, it might be something like 10.795 degrees, or, depending on the design it could be considerably greater. There are many variables and determining the optimum angle is going to be difficult.
Back when AMSAT was formed, and well into its life, people were experimenting to find what worked and what did not work. Back then, the majority of people found that about 30 degrees above the horizontal worked well and that became the defacto standard for a fixed elevation. If computer simulation now proves otherwise then so be it.
Whether the operator puts his/her beam at 15 degrees elevation, at 30 degrees elevation, or at another elevation, the answer to the original question remains the same. Yes, using a yagi with a fixed elevation can be used for satellite communication and, in the vast majority of time, a fixed elevation yagi works very well.
As for measuring a true 1 dB that takes some fairly good test equipment. The "S" meter on most equipment these days is not calibrated to the old standard of S-9 = 50 microvolts, 6 dB per "S" unit, etc. With 50 microvolts for S-9 a 20 dB over signal requires 500 microvolts, a 40 dB over signal requires 5,000 microvolts and a 60 dB over signal requires 50,000 microvolts.
Since I am in the business (am retired but "fix" a lot of radios for others), I have had the chance to actually measure "S" meter readings. A goodly number of "modern" radios read S-9 at around 10 microvolts, 20 dB over at around 40 microvolts, 40 dB over at around 100 microvolts. The dB of signal between "S" units varies all over the place.
The reason that S-9 = 50 microvolts is not used is because a while back the various manufacturers got into a "war" claiming that "their" receivers were "more sensitive" than the next manufacturers. However, what the manufacturers did was to reduce the signal strength required for an S-9 reading and to change the "dB over" readings into signal levels that don't even come close to being accurate.
An "S" meter can be used to determine relative signal strengths such as whether or not one antenna is working better under similar conditions. But, unless the actual meter readings have been accurately calibrated using an accurate standard, or if a calibrated attenuator is placed in the signal path, then there is absolutely no way of telling just how much stronger the signal is from one source to another.
FM "S" meters actually read limiter current and not actual signal strength. Since limiters are designed to saturate at a relatively low signal strength, an FM "S" meter reading will increase very rapidly with small signal increases. Then, upon saturation, there will be very little increase in the reading. A limiter is designed to remove any AM component from the signal leave only the FM or PM signal to be demodulated. That is why FM/PM (most of the older "FM" equipment was actually PM - phase modulated) is much less bothered by noise than an AM signal (SSB and CW are AM modes) since noise is amplitude modulated and not frequency or phase modulated.
--- On Tue, 4/12/11, Bob Bruninga <bruninga at usna.edu> wrote:
Sorry to sound like I am quibbling... but that last sentence implies the idea of an equal "trade off". But the tradeoff is not equal at all and may be missing the point here.
A LEO satellite pass does not need gain at "higher angles" because the satellite is by definition 2 or 3 times closer to the ground station (+6 to +9dB stronger). But one does need the gain at lower angles where the satellite is much further away.
An up-tilt of 30 degrees is throwing away excess gain where it is not needed (high angles) at the expense of low angles where every single dB -is- needed. So there is no real tradeoff... A lower angle (about 15 degrees) is more-or-less optimum for LEO's with fixed tilt and modest gain beams.
To actually quantify the exact best angle (which will depend on the actual beam's own beamwidth), it is simply to up-tilt the antenna no more than the angle at which the gain on the horizon LOSES say less than 1 dB. Note, this is not half the published "antenna beamwidth" which is usually a "3 dB" beamwidth. It is much less than that, less than half the 1 dB beam width. You can measure this by setting the beam no higher than the upangle that loses less than 1 dB to a signal on the horizon....
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