[Moon] [Moon-Net] TX coax position on dish

Donhawbaker donhawbaker at comcast.net
Wed Oct 23 20:15:47 CEST 2019


     
 

 My theory is that the feed support arms only block a portion of the dish that is the same as the width of the arms.    Incoming waves are blocked only for that portion of the dish that is equal to the width and length of the arms.
 

 
If you run the feed cable directly from the horn to the center of the dish, the area of blockage is going to be a triangle much bigger than the blockage the feed support causes.    The blockage would, assuming the feed horn is a point source, be the triangle area with a point at the feed, then as the rays go out, the angle defined by the point feed plus or minus the width of the feed cable.    In other words, the blockage is a shadow that gets wider as you move towards the surface of the dish.    The closer the feed cable gets to the point source, the wider the shadow that it casts upon the dish.    I have not done the math or conducted any experiments.    But it just seems intuitive that the blockage caused by the feed cable is going to be a lot bigger than that caused by a support arm.    However, it might still be so small as to still be negligible.
 

 
The loss of LDF4-50 is pretty small for a 15 foot run on a 3 meter dish.    Since I mount my equipment on the counter weight support arms at the back of the dish, there is no flexing of cable.    It simply runs down a support arm, around to the back of the dish and then into a storage box.    The whole thing moves when the dish moves.    No bending.    The only cable that needs bending is the IF feed and the push to talk.    And the AC that powers the transverter package.
 

 
73
 

 
 

 
 
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> On Oct 23, 2019 at 1:21 PM,  <Bob Atkins via Moon-net (mailto:moon-net at mailman.pe1itr.com)>  wrote:
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>  Phil
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>  I run my TX feed line from the septum though the hub. As far as I can tell there are no problems to doing it this way. I suppose in theory it might affect the pattern slightly or have a small effect on the noise floor, but in practice I can't see any difference from running the coax down one of the feed support legs.
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>  73
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>  Bob, KA1GT
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> From:  Moon-net  <moon-net-bounces at mailman.pe1itr.com>  on behalf of Philippe Mondon via Moon-net  <moon-net at mailman.pe1itr.com>
>   Sent:  Wednesday, October 23, 2019 5:52 AM
>   To:  Moon-Net  <moon-net at mailman.pe1itr.com>; moon at moonbounce.info  <moon at moonbounce.info>
>   Subject:  [Moon-Net] TX coax position on dish  
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>   Hi All,
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>  Wanting to minimize my TX loss here...
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>  I see most of you are making the coax cables attached and running on one leg of the feed support. That is what i did, but i have about 1.4 db loss on that configuration with the TX line (10m fsj 4-50b going to PA at base of the tower)
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>  I was thinking of putting a shorter piece of rigid coax (with less loss) running straight from the septum to the center of the hub, to finally get out from the back of the dish. Then a piece of flexible going again to rigid low loss towards the PA.
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>  What if a TX coax goes straight that way from dish center hub to the septum? Is there any drawback?
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> I'm loosing about 65w of power heating the coax, and only about 150-160w at feed, 225 from PA.
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> With better coax and less length, i may reach 200+ on feed....
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> Tks for your comments.
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> Vy73 - Phil - FR5DN
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