[amsat-bb] Re: Which Mobile Mag Mount?
bruninga at usna.edu
Thu Sep 15 08:51:31 PDT 2011
Everyone's comments are correct and valid within their context. But the
original question was optimizing for the casual mobile operator in motion.
So here are some additional considerations...
> Second, any "vertical" antenna...will have a [null overhead]
> So, think about it: A GREAT pass of 90 degrees goes overhead
> your vertical is valueless during the middle of that great pass.
True, but when we put some actual numbers on that, the loss is quite
insignificant. Using AO51, only one pass every 5 days goes above 82
degrees, and the total duration above 82 degrees is 24 seconds, out of all
in view times. So we agree, the antenna is valueless, but for under 1% of
the time. The tradeoff is that it is 3 dB better most of the rest of the
The measured elevation profile of a 3/4 wave vertical (a 19.5" whip on the
435 downlink) is shown 80% down the page http://aprs.org/astars.html. The
overhead null is only 10 dB down at 82 degrees... BUT the satellite is 10 dB
closer at that point, so you still hear it almost the same as when it came
above 25 degrees. So you get full sky coverage above 25 degrees > 99% of
the time with the vertical.
> Ideally - in a car - ... stopping and parking and getting a
> vertical perpendicular to the orbit of the satellite would
> give best results.
> Then maybe a Larsen mag mount... And bend - er, I mean,
> "re-form" that whip right at the base so that it is about
> 20" bent. you'll be more successful with the FM birds!
Yes, good idea, but now there is some directionality and so the car needs to
remain pointed towards the satellite during the pass. Otherwise there is
still the null, it is just in a different part of the sky.
> 1. If the satellite is circular polarization the 19.5 " whip looses 3 dB
Yes, but most people have observed a polarization shift during overhead
passes. And in that case, the loss of a circularized antenna is much worse
than 3 dB... usually a complete fade. SO I agree, a circular antenna gets 3
dB better half the time, and much worse the other half the time. I prefer
the simplicity of the whip that does not give up more than 3dB no matter
what the polarization is. Mounting a quadrifilar on the roof of the mobile
is also problematic.
> 2. The common Quadrifilar Antenna is 1/2 wave 1/2 turn.
> The 1 wavelength, 1 turn Quadrifilar Antenna has its
> highest gain near or at the horizon depending on the
> length to diameter ratio.
But it is still only a 3 dB gain antenna out there, and so it is not going
to hear the satellite down on the horizon anyway because the satellite is
10 dB farther away and usually blocked for the mobile. So having gain on
the horizon for a mobile omni antenna is wasted. It either is not enough or
it creats additional nulls higher up. Better to move that gain higher up
but smooth where the satellite is closer and then have good contacts, than
waste gain where it is already insufficient or problematic.
Just different perspectives, but the devil is in the details...
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