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Re: Cross boom length?



Franklin,

I was going to run the same analysis to produce the RF aperture which is
how one determines stacking distance for arrays.  This is improtant if one
is squeezing every tenth of a dB performance out their system.

Although I really don't want to discourage folks from improving their
satellite stations or trying to achieve perfection, I have to say that it
isn't all that necessary.  How much loss of gain does one suffer if you
stack antennas too close?  Perhaps more important is how is the antenna
pattern distorted?

If you are attempting to hear eme signals or maybe spacecraft signals
several million miles from earth on its way to another planet every 0.1 dB
is crucial.  For satellite operations you probably are not attempting to
copy signals that are a fraction of a dB above noise, though.

The goal in satellite operation is more like signals 10 or 20 dB over noise
(SNR).  So if practical or structural limitations cause a dB or even 2
below maximum performance that probably is not disasterous.

Purist are correct that one should not have any metal object within the
near field of the antenna.  Thus we hear the warnings about using metallic
crossbooms with circular pol yagis.  Yet, one can use them and stack the
antennas (esp 144 with 432) closer than optimum predicted by theory.

On eme there are stern warnings about placing any higher freq. antennas
inside an array (e.g. 432 inside 144 arrays), yet it has been done without
seeming huge effects.  I have one small 432 yagi (6-foot long) and two 1296
yagis (12-foot long) mounted inside my four 2m eme yagis (12x12 foot frame)
and cannot say I have any discernable effect to any of the antennas.

If I mounted four large 432 yagis that probably would not be true.  So
common sense and reasoning should be applied (in addition to sound
engineering analysis).

An example would be mounting your 144 and 432 satellite antennas on a
14-foot crossboom which exerts forces too great for the strength of the
rotator causing damage to it.  Engineering is always a process of tradeoffs.

For fun I looked up M2's satellite antenna specs (the 2MCP22):
gain = 12.25 dBdc (14.35 dBic)
recommended stacking = 9.5 to 10 feet.
boom length = 18' 7"

An old rule of thumb for stacking VHF+ yagis is to not stack closer than
1/2 the boom length.  0.5*(18.58 ft) = 9.29 feet (9ft3.5in).  Looks close
to M2's recommedation.

Now lets see what the M2-436CP42 stacking distance looks like:
gain = 16.8 dBdc (18.9 dBic)
recom. stacking = 67-inches
Boom length = 18"10"

I guess you might achieve good stacking separation on 432 by use of a
~5-foot cross boom, even if it is not good for 144.

Anyway, all this is stuff for the individual to "chew on" when making
decisions about how to set up their antenna system.

73's Ed - KL7UW


At 02:45 PM 3/18/2006 -0800, Franklin Antonio wrote:
>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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----
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