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feedhorn specification ?

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Ex Member
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Feb 24th, 2007 at 4:31am  
Dear All,
i'm new member here, and i need your help to explain about feedhorn.
- usually i'm using  a  feedhon      Polarotor I E/A for C-band
 http://www.chaparral.com/products/feedhorns/polarotor.html
my dish was a 12 feet regular mesh dish.

can i change the feedhorn  with the prodeline , that ussulally install on solid dish 1.8m (offset dish) ? it ussulally for TX and RX link feeds

hope all friend here can help me.
thanks
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« Last Edit: Feb 27th, 2007 at 9:24am by Admin1 »  
 
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Reply #1 - Feb 27th, 2007 at 9:37am  
...
This feed is designed for dishes with F/D ratio of 0.32 to 0.45, which is typical for axisymmetric, circular dishes.

Offset dishes have higher F/D ratios like 0.6 or 0.8 and need larger diameter feeds in order to project the power into the narrower cone towards the dish.

However the feed you have will still work reasonably well and is worth trying.  

The feed will pick up noise from the warm ground rather than wanted signal from the dish and the high brightness at the edge of the dish will mean the sidelobes are higher then normal.  

The edge of the dish has lower power illumination than the middle.  -10 dB is fine for receive only.  -14 dB to -16 dB gives the best combination high gain, low noise from the ground and low sidelobes and is normal for transmit/receive antennas.  In your case the edge illumination will be about -6 to -8 dB.  You will have an extra narrow main beam, but high sidelobes and high ground noise.

The phase centre of this type of feed will be about 1/8 inch inside the circular waveguide aperture. i.e. in the centre of the blue window and just under the plastic.  

If the outer rings are adjustable move them to the 0.45 position, or a bit further if possible.

Best regards, Eric.
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Reply #2 - Mar 1st, 2007 at 11:34am  
thanks lot mr.eric,.

i have 1 quetion again here,.
as u can see on the pic of feedhorn with the polarotor,
the feed throat was cap by plastic.

what if we got the a polarotor feedhorn with no cap ?
i mean, should i put some plastic to close the throath ?

if we put plasctic at feeds throath, is there is the signal is decrease ?

please advice me,
i'm wait for your valueable feedback.

Regard's
Bonivasius B Daandel

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Eric Johnston
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Reply #3 - Mar 1st, 2007 at 4:33pm  
Get it working first without a plastic cover, called a "radome".

Then find some plastic bottles of suitable diameter to fit any of the rings, and cut off the bottom so you have a suitable shaped cap.  Try it on and if it makes no difference to the signal then you are OK.   Try different types of plastic till you get a good one.  Thin PTFE sheet is ideal, but is difficult to obtain.  Any thin polythene sheet is good for an emergency.

If the hole is left wide open and it rains in, then you will have a pool of water and no signal at all and soon serious corrosion.

It is unlikely that you will be able to seal the plastic cover perfectly.  This means that during the day warm air will expand and leak out.  At night cold damp air will go in.  After several weeks water may appear inside.  If you make several small holes with a pin, above and below, this may stop moisture drops forming inside - but no guarantee however.

Best regards, Eric.
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Reply #4 - Mar 5th, 2007 at 1:36pm  
thanks again,.. mr eric,

this is verry good info to me,
i will do some experiment ,

thanks you so much

best regards
Bonivasius
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