Nomographs for Parabolic Reflector Antennas

A75GR · Apr 28th 2020

I have prepared several nomographs for antenna experimenters, during my mandatory corona-virus quarantine. Nomographs are plotted for parabolic reflector antennas and their feeds, with focus on QO100 operational frequencies. Nomographs could be helpful for quick orientation in selection of a suitable parabolic reflector and its feed.

73, Rasto - A75GR/OM6AA

OE3GBB · Apr 30th 2020

Rasto - Congratuliations! I think it is a great work.
I just don't understand Example 2. Why do we have to convert the SA to a PF-dish and design the feed for that converted f/D?
The result in example 2 with 10 turns doesn't seem correct as other literature is proposing 5 turns for a offset dish with f/D = 0,65.
What is the reason for converting to PF?
73's Gerhard - OE3GBB

DD8PL · Apr 30th 2020

Rasto,

very informative!

Could you please - or somebody else - explain the term "phase center" regarding helix antennas and in the context of where is the phase center located and where or how to position a helix in regards to the focal point of a dish antenna.

Thank you!

OE3GBB · Apr 30th 2020

Hi, the phase center is the virtual point where the beam of the electromagnetic field is emerging. This point is depending from the taper of the helix and the form and size of the reflector.
See: Helical Feed Antennas Paul Wade W1GHZ ©2002
The phase center should be adjusted into the focal point of the antenna. Therefore a mechanical means for adjusting a helix in 3 dimensions would be perfect.

A75GR · May 1st 2020

Dear Gerhard,

Thank you for your inquiry. The offset parabolic reflector has asymmetric structure with different added edge taper for upper and lower rim (it depends on the offset height - H). This added edge taper must be compensated by the feed. But how to compensate it, when upper AET is higher than AET on lower rim? To use asymmetric radiation pattern? Or by taking mean AET value for particular reflector size and optimize design of the feed this way? Or by changing feed angle? Or mix all these procedures together?

t is obvious that each design of the feed for particular offset reflector becomes unique. The good compromise seems to be technique as I described in my paper. It is not my invention, it has been described many times in the professional literature. However, this concept works well if the radiation pattern is similar to the 〖cos〗^2N (θ) function. As example of application this technique, I am attaching some results from my project “Low noise antenna for QO100 downlink“ The feed dual-mode horn with gain of 14.5 dB was used associated with offset reflector with f/D = 0.66 and diameter 40 cm (Gibertini OP40E). The efficiency of 73 % was achieve with very low antenna noise temperature. See attachments.

However, radiation pattern of helical feed has different pattern than 〖cos〗^2N (θ) function. I will recalculate Example 2 from my paper for the actual helical feed with 5-10 turns to figure out the optimal turns more precisely. It takes me some time, since one calculation takes about 30 – 40 minutes.

TNX & 73, Rasto

OE3GBB · May 3rd 2020

Hi Rasto,

thank you for all this very interesting informations. It made me get deeper interested in parabolic theory and made me read some technical literature about. So I now understand that for offset dishes the values for f/D and SA in the data sheets do not correlate. I allways thought that we should choose the feed to get -10 dB (or -12dB) for the actual SA. But according to your paper that is only true for H=0. Do you have an easy to understand explanation, why we have to first find the f/D for a virtual dish having same SA, and than the find the optimal gain for that new f/D? Why is it not sufficient just to design the feed for the SA of the dish?

Thanks for your help!

A75GR · May 3rd 2020

Dear Gerhard,

I added Example 3 into my paper. I performed many calculations with actual radiation patterns of helical feeds. Results are published in edition 2 of my paper “Nomographs for Parabolic Reflector Antennas“ You can find the answer on your question regarding of helical turns there.

I try to prepare some simple explanation on your second question. Pls. give me some time, since I was calculating 3 days...

73 & GL,

Rasto A75GR/OM6AA

A75GR · May 4th 2020

Hello Gerhard,

I have tried to explain why it is more convenient to use subsidiary prime-focus reflector technique for a feed design for offset reflector. See attachment.

73, Rasto A75GR/OM6AA

OE3GBB · May 5th 2020

Hi Rasto,

many thanks. Now I am beginning to understand. Before knowing your paper I was thinking that it is sufficent to choose a feed where the -10dB angle of the feed equals the SA of the dish. But it is much more complicated :-). The approach to find a feed for a basic paraboloid and considering AET is clear now. But you also showed that without simulation there is no way to find the least 1/10 of dB Gain. It also shows that the whole setup offset-dish and Helix feed is quite forgiving.
Thanks again.
73's Gerhard

Nomographs for Parabolic Reflector Antennas

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