1420Mhz feed for a stressed dish

[Fong Zi Hao]

Hello all,
I am currently building a 1.4m, f/D 0.345 geodesic dish/stressed dish developed by JA6XKQ from Japan. I am looking for a 'unconventional' cantenna design that isn't too heavy for the dish. There's this excel spreadsheet from SETI attached below calculating the dimensions of the feed, the feedhorn length came out to be 28.1cm, and if I were to add a choke ring, the diameter will be 36.1cm, blocking a considerable portion of the 1.4m dish. Perhaps for a small dish like this, a choke ring can be opted out instead.

This telescope http://antena.fe.uni-lj.si/literatura/Razno/Diplome/Radioteleskop/clanek/radioteleskop.pdf (page 5 on feed design) uses a shortened version of the cantenna optimized for f/D 0.4, has a length of 18cm. If I remember correctly, the important parts are the probe placement and diameter of the feed. Are there some formulas that can calculate the probe placement for a given feedhorn length optimized for a specific f/D? I am also open to any other feedhorn types if you have better ideas. Thanks in advance.

[Alex P]

How about this :  based on the design of OM6AA

http://www.om6aa.eu/Loop_Feed_with_enhanced_performance.pdf

OD of "Can" = 12.5 cm

'

Alex

KK4VB

[mike....@gmail.com]

"Perhaps for a small dish like this, a choke ring can be opted out instead."

I too was wondering how much difference a choke ring made.  So I made this test.

(see attached)

So this improved the  signal from 0.050 to 0.040 which is 0.010 difference.

While drift scanning that may be the whole span of power on a 24 hour scan.

(see attached)

The  orange and blue traces are two successive days.

My problem is why two successive days  of drift scan don't look the same especially during the day(?heat on the LNA?clouds?Wind shaking/deforming dish?)

Good luck with your project.

my 2 cents

mike W9YS

[Marcus D. Leech]

On 2021-12-02 11:46, mike....@gmail.com wrote:

"Perhaps for a small dish like this, a choke ring can be opted out instead."

I too was wondering how much difference a choke ring made.  So I made this test.

(see attached)

So this improved the  signal from 0.050 to 0.040 which is 0.010 difference.

While drift scanning that may be the whole span of power on a 24 hour scan.

(see attached)

The  orange and blue traces are two successive days.

My problem is why two successive days  of drift scan don't look the same especially during the day(?heat on the LNA?clouds?Wind shaking/deforming dish?)

They'll never look exactly the same.  Even in the absence of things like RFI and wind shaking the dish, there will be gain drift due to different ambient temperature
  on the LNA.  This is the real "Achilles Heel" of single-dish total-power measurements.  Things like gain drift (and temperature changes in spillover/ground) can
  often be larger than the objects you're trying to measure.

Good luck with your project.

my 2 cents

mike W9YS

On Thursday, December 2, 2021 at 8:30:22 AM UTC-6 a_pet...@yahoo.com wrote:

How about this :  based on the design of OM6AA

http://www.om6aa.eu/Loop_Feed_with_enhanced_performance.pdf

OD of "Can" = 12.5 cm

'

Alex

KK4VB

On Thursday, December 2, 2021 at 5:22:50 AM UTC-5 fong...@gmail.com wrote:

Hello all,
I am currently building a 1.4m, f/D 0.345 geodesic dish/stressed dish developed by JA6XKQ from Japan. I am looking for a 'unconventional' cantenna design that isn't too heavy for the dish. There's this excel spreadsheet from SETI attached below calculating the dimensions of the feed, the feedhorn length came out to be 28.1cm, and if I were to add a choke ring, the diameter will be 36.1cm, blocking a considerable portion of the 1.4m dish. Perhaps for a small dish like this, a choke ring can be opted out instead.

This telescope http://antena.fe.uni-lj.si/literatura/Razno/Diplome/Radioteleskop/clanek/radioteleskop.pdf (page 5 on feed design) uses a shortened version of the cantenna optimized for f/D 0.4, has a length of 18cm. If I remember correctly, the important parts are the probe placement and diameter of the feed. Are there some formulas that can calculate the probe placement for a given feedhorn length optimized for a specific f/D? I am also open to any other feedhorn types if you have better ideas. Thanks in advance.

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