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Author Topic: corner reflector distance from the dipole in a parabolic grid antenna  (Read 338 times)
KD7RDZI2
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Posts: 422




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« on: April 17, 2019, 08:56:05 AM »

I am modifying a grid parabolic antenna originally made for Meteosats at around 1700Mhz. I cut the dipole exactly for 1296Mhz and when positioned to the focus of the antenna has good SWR. However the antenna has a small (actually not too small) corner reflector. In the original 1700Mhz design it was positioned 4cm behind the dipole. I positioned it at about 5.7cm but the transverter shows high SWR. If I solve a proportion 1700:1296=X:4, X=5.2cm, so I should put it 0.5cm closer. Is this a practical correct way to determine the correct distance of the corner reflector from the dipole? Huh
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W9IQ
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Posts: 3073




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« Reply #1 on: April 17, 2019, 09:04:41 AM »

First you state that it is a parabolic reflector and then in you ask about a corner reflector. If it is the latter, the dipole is typically 1/2 wavelength from the reflector. If the former, it must be at the focal point.

You also state that you cut the 1700 MHz dipole to 1296 MHz. Do you mean you lengthened it?

- Glenn W9IQ
« Last Edit: April 17, 2019, 09:24:41 AM by W9IQ » Logged

- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
KD7RDZI2
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Posts: 422




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« Reply #2 on: April 17, 2019, 09:32:34 AM »

Hi Glenn,

the antenna consists of three parts:
- parabolic grid
- dipole set at the focus, I removed the original one (1700Mz) and made a dipole cut for 1296Mhz (5.5cm each leg, made with the outer and the inner of the coax without balun and without soldering, it is just one piece with the Aircell coax cable) and this shows good SWR without the corner reflector
- a corner reflector, which is behind the dipole, originally at 4cm (for 1700) from the focus, I positioned at 5.7cm but then the transverter shows high SWR. At what distance from the focus should I put the reflector?

Franco KD7RDZ
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KD7RDZI2
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Posts: 422




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« Reply #3 on: April 17, 2019, 09:35:42 AM »

----------------------------------parabolic dish
               I
               I
               I
              --    dipole
               I
            ------ corner reflector
              

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W9IQ
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Posts: 3073




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« Reply #4 on: April 17, 2019, 10:02:03 AM »

Thanks for the clarification, Franco.

The directivity of the corner reflector is maximized when the dipole is 1/2 wavelength from the corner. But this also raises the feedpoint impedance of the dipole. You can adjust the feedpoint impedance back down by gradually moving the dipole towards the reflector at the expense of some directivity.

The reflector length should be at least 1.414 times the length of the dipole.

If you leave the dielectric in place on 1/2 of your dipole, make certain you have factor the velocity factor in your dipole length calculation.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
KD7RDZI2
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Posts: 422




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« Reply #5 on: April 17, 2019, 11:33:59 AM »

Glenn,

The reflector length is just big enough 1.427 times the length of the dipole. Cheesy
As for the dipole I removed the dielectric, it seems 5.5cm per leg is fine, at least it shows low SWR. I will adjust the feedpoint impedance gradually in the way you suggest to have a low SWR with the corner reflector too.

Thanks a lot for these hints!

Franco KD7RDZ

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W9IQ
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« Reply #6 on: April 17, 2019, 11:39:40 AM »

Sounds good, Franco. Let me know how it works out for you.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
KD7RDZI2
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Posts: 422




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« Reply #7 on: April 22, 2019, 09:42:45 AM »

Hi Glenn

I had some time to experiment adjusting the feedpoint impedance by gradually moving the reflector towards the dipole. I confirm that this distance changes the feedpoint impedance of the dipole. Strangely my transverter is happy (low SWR) either when there is no corner reflector, or the reflector placed at 1/2 wavelength! In between, the impedance match could also work but was not perfect. My SG-Lab transverter shows SWR with leds: green (good match), Yellow-orange (so so), red (no, bad match). Setting the reflector at 1/2 wavelength the transverter shows again the green light  (in between 1/4 to 1/2 was yellow-orange)Grin

Thank you very much for the helpful hints!

Franco KD7RDZ

« Last Edit: April 22, 2019, 09:45:57 AM by KD7RDZI2 » Logged
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