[amsat-bb] Re: K5OE antenna projects
bruninga at usna.edu
Sat Dec 6 20:15:31 PST 2008
> Perhaps in the mean time I can come
> up with some more meaningful methods
> to measure the EggbeaterII performance.
> Any suggestions or recommendations welcome.
A very simple method for OMNI anennas is to simply monitor what you hear relative to elevation angle. Done. To gauge how well an omni antenna performs on a given satellite, you only need one number... that is, you can say a given satellite is generally "useable above elevation angle X on my antenna Y". That is all one needs to compare for any LEO.
If you look at the plot of elevation angles and path-loss, shown on www.aprs.org/rotator1.html, you will see a table that shows how LOSS compares to elevation angle.
EL CUM-% RANGE PATH-GAIN
--- ----- ----- ---------
10 32 3030 0
20 67 2440 2
30 84 1827 5
40 92 1460 6
50 96 1190 7
60 98 1020 9
This shows that you get almost 9 dB of gain on an omni antenna depending on how close the satellite is, and that is simply proportional to elevation angle.
The cumulative percent column shows the total access time BELOW that angle. Notice that signals above 30 degrees are 5 or more dB stronger, but you miss 84% of total access times. Above 45 degrees most LEO's are 6 or more dB stronger and can be heard on just about any antenna, but this only occurs less than 5% of the time.
But anyway, it gives a universal single number comparison of omni antenna performance. Now of couse, what does "useable" mean is debatable as well. But most people can agree approximately. Of course there are nulls and fades, but I am not counting that. I am counting "mostly" useable as my criteria.
Said another way, you can -never- hear some LEO's below some angle on an omni because there are very few things that can "enhance" a signal, though there are lots that can deminish it. So again, lowest-workable-elevation-angle is a pretty good discriminant for a given OMNI installation.
Of course vertical gain-omni's can put more gain on the horizon (typical base-station antennas), but then at the expense of higher angle coverage. And since even a vertical gain omni antenna that is great for base station use might just barely hear some satellites low on the horizon, their nulls and poor performance at higher angles (where the satellite is MUCH stronger) is giving up the best part of the omni link to a spaceccraft, and why no one would use one for satelltie work.
So as everyone is getting tired of hearing me say... my preferred antenna is a 19" whip over a ground plane (like the roof of my car) with short coax, that has 5 dBi gain on 2m and almost 7 dBi gain at UHF as a 3/4 wave vertical above 25 degrees or so. This combined with the 6dB of range improvement above 30 degrees for a LEO gives you almost 13 dBi gain performance (like a beam) for a simple omni (but only for the best overhead passes). But hey, it sure is simple, consistent and always works for high passes!
An no moving parts, or tracking required, and usually, no doppler. I usually tune 5 KHz high for UHF birds so that I hear them the soonest that they get high-enough (and close enough) to be heard. But just fixed tuning at the center freq also works since you only hear the center few minutes of a pass anyway.
Just something to consider.
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