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Re: Cross boom length?



Hi Antonio

As you sayd I completely agree

Tanks

73" de

i8CVS Domenico

----- Original Message -----
From: "Franklin Antonio" <antonio@qualcomm.com>
To: "Michael A. Tondee" <mat_62@netcommander.com>
Cc: <amsat-bb@AMSAT.Org>
Sent: Saturday, March 18, 2006 11:45 PM
Subject: Re: [amsat-bb] Cross boom length?


> At 11:43 AM 3/18/2006, Michael  A. Tondee wrote:
> >Generally speaking, what crossboom length is reccomended on az/el rotator
> >systems?
>
> I have to chuckle at your question, because I know from experience
> that you will get a lot of wildly different answers.  A lot of
> opinions out there.  I'll give you my opinion, and a rationale for
> it.  Its based on what I think is a reasonable model of how an antenna
works.
>
> It would be better of course if someone had actually done the right
> experiment, but I don't think anyone has.  Its hard to do.  What you
> would need to do is have a big antenna range, and the typical oscar
> crossboom with two circularly polarized Yagis on it, with a rotor and
> vertical mast in the center.  Then you would measure the patterns of
> the antennas with different spacings.  You would then learn how much
> the pattern of the 2m antenna degrades as that rotor and vertical
> boom get closer and closer.  How much harm do you do to circularity
> of the main beam or sidelobe levels by having that big ugly piece of
> metal only 2.5 feet away?  I've never seen results of such a
> test.  Of course these days it might be easier to do this study in a
> computer simulation.
>
> Imagine you're the 2m Yagi.  You're vibrating the ether, and using
> your resonant arms to control the waves and make them go a certain
> way.  The last thing you want is big pieces of metal nearby.
>
> So how far away is far enough?
>
> One way to get an idea is to consider the equivalent dish
> antenna.  (Ie a dish with the same gain as your Yagi.)  If I asked
> you to mount two dishes on a crossboom, you would immediately check
> to see if the sum of the radiuses of the two dishes was less than the
> length of the crossboom.  You wouldn't mount one so it shadowed the
> other!  Just common sense.  Similarly, you wouldn't mount a dish so
> that the rotor was in front of the dish!
>
> Of course its even worse than that.  Not only would you make sure
> that the area of the dish wasn't shadowed, you'd take care to keep
> metal out of the general vicinity, because you know that the feed
> spills over the dish, so metal near the dish increases sidelobe levels.
>
> Somehow when people put up a Yagi, the fact that it is long and
> skinny confuses people.  The Yagi must make use of a cross sectional
> area of space which is similar in size to the dish.  There's no other
> way to make directivity happen.  Just because the Yagi doesn't have
> metal extending out into a big area doesn't mean that area isn't
> important.  That's wrong.
>
> Lets do a thought experiment.  A common 2M circular Yagi made by M2
> has a spec'd gain of 12.25dBdc.  That's 14.35 dBic.  Lets calculate
> how big the dish of an equivalent dish antenna would be.
>
> gain = 4 pi efficiency area / wavelength^2
>
> I used 145 MHz and 0.55 efficiency.
>
> My calculation comes out 4.5 meter diameter, or 14.8 feet.  The
> radius of this dish is therefore 7.4 feet.  If you mount that even on
> the end of even a 10 foot crossboom, its gonna overlap the rotor and
> vertical mast.  How about that?
>
> Now I'm not claiming this is a fatal conclusion.  This is a thought
> experiment based on a simple model after all.  What I do conclude is
> that it is likely that the 2M Yagi does indeed "feel" the mast and
> rotor nearby, to the detriment of its pattern.  For me that means I'd
> like to keep metal as far away from that 2m antenna as possible.  For
> me that meant a 10 foot crossboom instead of a 5 foot crossboom.
>
> A 5 ft crossboom will certainly work, but if you pay a bunch of money
> for fancy coax and preamps to get that last half of a dB here and
> there, I don't think you want to throw away antenna performance by
> ignoring the environment of the antennas.
>
> As I said, I'm sure some will disagree.
>
>
> >Seems to me I saw somewhere that the standard length, if there is
> >one, was five feet.
>
> You can buy 10 ft fiberglass poles.  Ridout plastics, for example.
>
> 8 foot lengths available from Max-Gain Systems.  He has 1.5" solid!
>
> There are some bad things about long crossbooms.  They sag a
> little.  When the antennas are at 0 degrees elevation that doesn't
> matter, but at 90 degrees elevation it means your two antennas don't
> point quite in the same direction.  The loss caused by this isn't
> large, although for the purist it is a bother.  You can fix the sag
> with a tether between the two antennas in front of the crossboom, or
> you can just let it be.
>
> I recommend solid fiberglass rather than tubular, and recommend that
> you paint it to keep the sun off of it for the first couple of years.
> ----
> Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
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----
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