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Re: 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.
----
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