Which Feed Design for 1.42GHz H1?

[Alex P]

I am specifying a small 1.42 GHz H1 system for a local university .

It will be portable and use a 1.2m or 1.5m dish from RF_Ham_Design.

They have 3  possible feed configurations : a Horn, Ring, or a LHCP Helix 'specifically for' 1.42GHz Radio Astronomy.... Is the CP Helix the best choice ?

Thanks,

Alex

[Marcus D. Leech]

A helix would be a poor choice, IMHO.   They aren't wonderful antennas, and you'd need a fairly short one for a typical dish F/D.

I'd use a circular-waveguide horn.  Easy to make, just about the right amount of gain for a typical 0.35-0.42 F/D dish.

A full-wave loop is also an OK choice, but is more sensitive to side radiation than a circular waveguide feed of 1.5 lambda or so.   A piece of 6"
  HVAC duct with a end cap on the end makes a more-than-adequate feed.

[b alex pettit jr]

Hello Marcus,

THANKS...  I think I'll go with the Horn they offer.

For this project, I  prefer to purchase rather than build.

Regards,

Alex

[Marcus D. Leech]

On 2022-04-08 20:36, 'b alex pettit jr' via Society of Amateur Radio Astronomers wrote:

Hello Marcus,

THANKS...  I think I'll go with the Horn they offer.

For this project, I  prefer to purchase rather than build.

Regards,

Alex

Who is offering a pre-built 21cm feed horn?

[b alex pettit jr]

Well,,,23cm,  but I was thinking they could mod to 21 (?)

https://www.rfhamdesign.com/products/dish-feeds/single-band-dish-feed/index.php

Who is offering a pre-built 21cm feed horn?

-m

.

[Marcus D. Leech]

On 2022-04-08 22:08, 'b alex pettit jr' via Society of Amateur Radio Astronomers wrote:

Well,,,23cm,  but I was thinking they could mod to 21 (?)

https://www.rfhamdesign.com/products/dish-feeds/single-band-dish-feed/index.php

Looking at the return-loss plot, you could use it as-is, really. 

[b alex pettit jr]

Seems to me, the f/ 0.35 design would be the better choice : less subject to surrounding RFI interference and temp noise ?

I might under-illuminate the feed a bit, but greatly reduce the 300K thermal noise pickup ?

 

[djl]

The rule of thumb is 10 db down at the edge of the dish.

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[Brad Thomas]

Particularly for a smaller dish, a scalar choke ring can be good. See http://www.setileague.org/hardware/feedchok.htm

With my 210 cm dish, I use a septum feed with such a ring. Here's an image from setileague site of a coffee can feed with choke. This may be what you need:

Here is the septum feed I use. It was made in Russia, so international events have made it unavailable just now.

Good luck with your setup. I greatly enjoy mine!

--Brad Thomas

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[b alex pettit jr]

Hello Brad,

Thanks for that information.... The problem is that a Choke Ring scaled to 21cm is  36cm in diameter ?

That is a huge fraction of the dimensions of  a 1.2m dish.

I have a WiFi 1m dish and custom loop_feed working fairly well.

 It might be best I Initially Implement something similar to that with the RFHamDesign 1.2m in the f/0.35 shape. ( as the WiFi )

That way, I have my system as a reference to eval the performance of the UCF system.

Regards,

Alex  KK4VB

On Saturday, April 9, 2022, 10:16:37 AM EDT, Brad Thomas <kbtho...@gmail.com> wrote:

Particularly for a smaller dish, a scalar choke ring can be good. See http://www.setileague.org/hardware/feedchok.htm

With my 210 cm dish, I use a septum feed with such a ring. Here's an image from setileague site of a coffee can feed with choke. This may be what you need:

Here is the septum feed I use. It was made in Russia, so international events have made it unavailable just now.

Good luck with your setup. I greatly enjoy mine!

--Brad Thomas

.

[Brad Thomas]

Hi Alex, 

I see the RFHam Design helix feed is for a more shallow dish F/D: 0.45-0.5. Your F/D: 0.35 reflector will work, I think. Under-illumination of the dish may actually decrease noise - this is described in the SETI article on 21cm feedhorns I cited - so there may be some advantage to using the RFHam Design feed with your dish.  My DIsh has a similar F/D to yours. I'm tempted to check out that feed out of curiosity.

Best,

Brad

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[b alex pettit jr]

The WiFi dish is rectangular , and you can visibly 'see'  the loop from the narrow sides..

So, I tried adding side panels ..

Not to extend the parabola .. but at a steeper angle to block the view of the loop_feed from the 300K ground ...

Here's the results ..  well worth the effort.

The side extensions reduce the feed-point pickup of the 300K ground temp  ...

They do not improve the S but greatly reduce the N  .

( Blue : normal dish   Green : with panels )

[Lester Veenstra]

Nice work !

 

Lester B Veenstra  K1YCM  MØYCM  W8YCM   6Y6Y

les...@veenstras.com

 

452 Stable Ln (HC84 RFD USPS Mail)

Keyser WV 26726

 

GPS: 39.336826 N  78.982287 W (Google)

GPS: 39.33682 N  78.9823741 W (GPSDO)

 

 

Telephones:

Home:                     +1-304-289-6057

US cell                    +1-304-790-9192

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[kl7uw]

A 1296 feed would likely work fine.  For center-feed design a VE4MA design (cylindrical horn with "choke" collar) matches f/d 0.33 to 0.40 pretty well.  I have a 4.9m dish with f/d = 0.5 so use a dual-mode cylindrical horn (W2IMU design) for both 1296 and 1420.  It receives in LHCP.  Polarity is not important for most celestial sources.  I use a 0.24 dB NF LNA which might not be optimum on 1420.  I have another Peltier cooled LNA for 1420 rated at 0.1 dB NF which would have to be mounted for use.  Dish is currently on ground awaiting rebuild of mount.   ~ Ed - KL7UW

[Marcus D. Leech]

On 2022-04-09 10:41, 'b alex pettit jr' via Society of Amateur Radio Astronomers wrote:

Hello Brad,

Thanks for that information.... The problem is that a Choke Ring scaled to 21cm is  36cm in diameter ?

That is a huge fraction of the dimensions of  a 1.2m dish.

I was going to say that below about 2m aperture, a Choke Ring at 21cm is likely going to cause enough aperture blockage that
  whatever you gain in having a "tailored" illumination pattern, you lose in aperture blockage.

[B & MR Randall]

Alex,

A Horn or a Ring will have lower side lobes then a Helix.  Feed side lobes will pick up ground noise.   Off frequency pattern of the Helix is even worse for side lobes.  

 

A horn will reject lower frequencies & keep 850MHz cell & UHF TV transmitters out of your preamp.  These interference sources can cause overload effect in a preamp, even at low levels.

 

It is VERY important feed F/D match the dish F/D.  A horn with an adjustable choke ring allows the feed to adjust for some range of F/D.

 

http://www.setileague.org/hardware/feedchok.htm has some about feed horns and F/D.

 

Marcus Leech has some good information at https://www.ccera.ca/about/

 

Some feeds that work OK for amateur satellite use aren't too good for radio astronomy.  A person walking into a feed side lobe can trash a radio astronomy observation but have only a slight effect on sat comm.  In my case a curious cat near the edge of my dish messed up an observation.  Curiosity did not kill the cat but did kill an observation.

 

Bruce Randall

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[Marcus D. Leech]

On 2022-04-09 22:10, B & MR Randall wrote:

Alex,

A Horn or a Ring will have lower side lobes then a Helix.  Feed side lobes will pick up ground noise.   Off frequency pattern of the Helix is even worse for side lobes.  

 

A horn will reject lower frequencies & keep 850MHz cell & UHF TV transmitters out of your preamp.  These interference sources can cause overload effect in a preamp, even at low levels.

 

It is VERY important feed F/D match the dish F/D.  A horn with an adjustable choke ring allows the feed to adjust for some range of F/D.

I'd re-phrase that as it's important that there not be a gross mis-match between the target F/D of the feed and the F/D of the dish.  A feed designed for a
  deep dish will over-illuminate a shallower dish, leading to MUCH more ground pickup.  Conversely a feed designed for a shallow dish will considerably
  *under* illuminate a deep dish--leading to much lower gain than expected.

For example a dish in the 0.38-0.42 F/D range (typical of mesh-type dishes in the 2.5-3m range from the days of the Big Ugly Dish) will be happy with
  a "naked" circular-waveguide feed.  You can get an improvement of a few degrees (perhaps as much as 10) of Tsys by a precisely-adjusted choke ring.
  I've also had luck using a simple flare from 15cm to 20cm on shallower dishes to boost the gain of the feed a little bit, and thus have a narrower illumination angle.
  Not quite as precise as a choke ring, nor as adjustable. But expedient and adequate.

We have a deplorably-deep dish at the CCERA Rideau Ferry observatory that I had to fashion a dipole+reflector feed for, because none of my "usual"
  feeds would illuminate it very well.

To view this discussion on the web visit https://groups.google.com/d/msgid/sara-list/009701d84c80%241a464f90%244ed2eeb0%24%40net.

[b alex pettit jr]

Hello Bruce,

Thanks for the information ...

I think my side panels were somewhat effective in doing this :

" With a dish designed for optimum gain, the antenna noise temperature, dominated by Earth-seeing sidelobes, is about 50 K.

The overall system noise temperature is thus 100 K, and sensitivity (given 10 Hz bin width and 10 second integration) is on the order of 1.3 E-22 W/m^2.

Now under-illuminate for 10 K of antenna noise. Antenna gain decreases 0.8 dB as discussed above, but overall noise temperature reduces to 60 K, a 2.2 dB decrease in noise.

System sensitivity is now 9.4 E-23 W/m^2, a net system improvement of 1.4 dB! "

There are so many options. I now think it may be best to duplicate ( closely ) what I have configured for my home system to verify the UCF system,

and then perhaps try a horn feed .. or leave that as a "student project "   (smile).

Great Feedback   // THANKS EVERYONE ! //

Regards,

Alex

[fasleitung3]

Hi Alex,

A lot of good advice has already been given, so my comments are mostly a re-iteration of things already been said. It is a bit of a summary from my experience.

The standard off the shelf product from RF-Hamdesign has a f/D ratio of 0.45.

For such a dish I would definetly recommend a feed horn rather than other options. An importat adavantage has been pointed out by Bruce which is the suppression of unwanted signals at lower frequencies. This can make the difference between make and break particularly in urban environments.

While a choke ring would improve illumination the penalty is aperture blockage. For a 1.2 to 1.5-m dish the blockage becomes too high, so a simple circular waveguide should be used.

For a simple circular waveguide feed (aka coffee can feed) it is better to use a shallower dish as this better fits the opening angle of the feed. So I would suggest to stick with the standard 0.45 f/D ration of RF-Hamdesign. 

The feed offered for 1296 MHz may not be optimal at 1420 MHz but should still work reasonably. Unfortunately they do not give the dimensions of their feed which would allow a better assesment. Since you want to purchase rather than build your own, that is certainly an option. If you want to improve the performance later on, you can still replace this feed with something more matched to the observing frequency.

As an example of such a small telescope you may want to have a look at our "portable" 1.2-m dish (description attached). In this case it is a dish with 0.5 f/D and a simple circular waveguide feed horn.

Best regards,

Wolfgang

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[b alex pettit jr]

FULL Wave Loop Feed  VS  QUARTER Wave Horn Feed  ??

======================================================

Hello Wolfgang,

First : I have bookmarked your information and will  keep it for future reference  : Thank You.

Second : You have brought up a topic I've never asked  ..     Here It Is :

Modeling software computes its values of  Antenna  Gain based upon Transmission :

 "  What is the signal intensity as reshaped by the antenna design with respect to an Isotropic Radiator " ..   dBi

Lets look at Reception : There is  a signal energy density being captured by an antenna using typically a 1/4  , 1/2  or Full wavelength element

 ( exclude the parabolic dish or director elements  for this comparison )

IS there not MORE Energy captured by a physically larger Full or Half wave element than a Quarter Wave ?

Is the Full Wave Loop Feed  not significantly more sensitive for receiving signals than a 1/4th Wave Horn Feed ?

Thanks,

Alex

=================================================================================

On Monday, April 11, 2022, 03:44:02 AM EDT, 'fasleitung3' via Society of Amateur Radio Astronomers <sara...@googlegroups.com> wrote:

Hi Alex,

A lot of good advice has already been given, so my comments are mostly a re-iteration of things already been said. It is a bit of a summary from my experience.

The standard off the shelf product from RF-Hamdesign has a f/D ratio of 0.45.

For such a dish I would definitely recommend a feed horn rather than other options. An important advantage has been pointed out by Bruce which is the suppression of unwanted signals at lower frequencies. This can make the difference between make and break particularly in urban environments.

While a choke ring would improve illumination the penalty is aperture blockage. For a 1.2 to 1.5-m dish the blockage becomes too high, so a simple circular wave-guide should be used.

For a simple circular waveguide feed (aka coffee can feed) it is better to use a shallower dish as this better fits the opening angle of the feed. So I would suggest to stick with the standard 0.45 f/D ration of RF-Hamdesign. 

The feed offered for 1296 MHz may not be optimal at 1420 MHz but should still work reasonably. Unfortunately they do not give the dimensions of their feed which would allow a better assessment. Since you want to purchase rather than build your own, that is certainly an option. If you want to improve the performance later on, you can still replace this feed with something more matched to the observing frequency.

As an example of such a small telescope you may want to have a look at our "portable" 1.2-m dish (description attached). In this case it is a dish with 0.5 f/D and a simple circular waveguide feed horn.

Best regards,

Wolfgang

.

[Larry Mayfield]

Wolfgang,

Thanks for the pdf file which was attached to your SARA communication with the transportable  observatory!  Very clean, nice design, and fantastic implementation!  Lots of ideas in the report!

 

Larry

Pahrump, NV

 

From: 'fasleitung3' via Society of Amateur Radio Astronomers <sara...@googlegroups.com>
Sent: Monday, April 11, 2022 12:44 AM
To: sara...@googlegroups.com

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[Marcus D. Leech]

On 2022-04-11 03:43, 'fasleitung3' via Society of Amateur Radio

You might be able to "trim" the illumination pattern onto that dish
without too much extra aperture blockage with something like this:

https://www.rona.ca/en/product/imperial-7-in-to-6-in-diameter-galvanized-steel-reducer-gv0816-1224140?viewStore=55070&cq_src=google_ads&cq_cmp=9897044398&cq_con=101247314340&cq_term=&cq_med=&cq_plac=&cq_net=u&cq_pos=&cq_plt=gp&&cm_mmc=paid_search-_-google-_-aw_smart_shopping_generic_allproducts-_-71700000066353040&gclid=CjwKCAjwo8-SBhAlEiwAopc9W7n6XIhOa0MI9yv9qWvO-T7I7onwA9yY7AoHse3TvsPpt3jhQvSr_hoCM0gQAvD_BwE&gclsrc=aw.ds

Your 15cm "stove pipe" is the same (more or less) as galvanized HVAC
ducting, and there are a huge variety of "plumbing bits" available for
that stuff.

I imagine that a 15cm to 18cm reducer/expander is something your local
HVAC industry in Germany.    I use these myself.  The outer "flat" ring
can be easily removed by
  drilling-out two rivets.  That gives you a reasonable nice "cone"
that you don't have to make yourself....