[Ground-station] Checking some assumptions about dish weight

[Leffke, Zachary]

Quick numbers from RF Ham Design Mesh dish kits with 2.8mm screen for operation up to 10 GHz (which you may already have):
2.4m -> ~40lbs (and is about the weight of my spun aluminum dish at 2.3m, maybe a bit more)
3.0m -> ~70/80 lbs (there are two variants)
4.5m -> ~172 lbs

Those numbers include feed support arm weights.  The numbers do not include mounting brackets (which might be pretty beefy) or the actual weight of a feed and feed mounted electronics (which should be fairly small at 10 GHz).

Those numbers also do not include the (usually required) counterbalance weights.  As a very rough first cut, I would say whatever the dish weight is, add 3dB to account for the counterbalance (lever arm distance, center of mass of the dish and some geometry math could refine this estimate, I think it’s a bit more than required).  So if the rotator is rated for a 100 lbs (static weight), a 2.4m/8ft dish is probably going to be about the largest that can be supported.  Might want to add a little margin there (or maybe the margin is in more refined math for the counterbalance requirements already, and 3dB more weight is too much with reasonable arm length).

2.4m is a lot of gain and a narrow beam (~1 deg at 10GHz).  So depending on the target pointing accuracy requirement of the system (rule of thumb I heard somewhere is 1/10th the beamwdith of the antenna as a minimum requirement), that might be OK.

To summarize, I think 100lbs sounds about right for the total static weight handling of the dish, brackets, feed arms, feed/feed electronics.  If the pointing accuracy is around 0.1 degrees, then the rotator would be able to support up to about a 2.4m reflector, with counterbalance, fairly accurately but might be pushing the limits of the systems.  If the pointing accuracy can be improved better than 0.1 deg, that would increase confidence that a 2.4m size would be reliably handled.  Somewhere in there is probably a needed analysis for accelerations/velocity capability of the system to make sure it can keep up with LEO az/el angular rates fast enough.  Whipping 100 lbs around quickly might be tough.  As a point of reference my personal Spid RAS/HR is slowwww with my 2.3m system, but the VTGS Spid Big-RAS/HRs can keep up with LEO angular rates with an RF Ham design 3m antenna no problem (only tested up to S-Band frequencies though, so not sure how stable it would be for 10 GHz LEO tracking).

Hope this helps.

-Zach, KJ4QLP
--
Research Associate
Aerospace & Ocean Systems Lab
Ted & Karyn Hume Center for National Security & Technology
Virginia Polytechnic Institute & State University
Work Phone: 540-231-4174
Cell Phone: 540-808-6305

From: Ground-Station <ground-station-bounces at lists.openresearch.institute> On Behalf Of KENT BRITAIN via Ground-Station
Sent: Thursday, January 9, 2020 4:52 PM
To: Michelle Thompson via Ground-Station <ground-station at lists.openresearch.institute>; Michelle Thompson <mountain.michelle at gmail.com>
Subject: Re: [Ground-station] Checking some assumptions about dish weight

Usually 60-75% of the weight is in the mount and positioners.




On Thursday, January 9, 2020, 2:20:31 PM CST, Michelle Thompson via Ground-Station <ground-station at lists.openresearch.institute<mailto:ground-station at lists.openresearch.institute>> wrote:


This question is related to the open source tracking station project.

I've looked at a lot of dish catalogs and eBay sales.

For big dishes, for use up to 10GHz or so, would it be reasonable to specify 100lbs (45kg) for the weight? I'm seeing a lot of dishes up to 8ft that are less than that. We have to pick a number for the mechanical designer.

Another breakpoint seems to be 250lbs (115kg).

-Michelle W5NYV
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