Results from this morning

[Marcus D. Leech]

First, the total-power trace in the general area of Cygnus:



Then the spectral data from near the peak:




The interesting thing is that I get a fair amount of variabilty in the SNR of the daily recordings, which I ascribe to both gain drift, and changes in
  ground noise.  My very small dish (95cm!) is hard to feed properly, so my feed is seeing a fair amount of ground noise.

-- 
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org

[David Fields]

Marcus,
That's a nice display of great data!  Suppose you put a flat aluminum or mesh shield behind your dish, so that the ground wouldn't shine around the edge -- wouldn't this be a useful barrier to thermal (earth black body) noise?  The shield would be low-emissivity, and would  reflect the average (dark) sky.

Cheers,
David Fields

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[Marcus D. Leech]

Marcus,
That's a nice display of great data!  Suppose you put a flat aluminum or mesh shield behind your dish, so that the ground wouldn't shine around the edge -- wouldn't this be a useful barrier to thermal (earth black body) noise?  The shield would be low-emissivity, and would  reflect the average (dark) sky.

Cheers,
David Fields

Yes, I've considered doing that.   Might get around to it in the next few weeks.  So many little projects, so little meth....

[Dave Typinski]

Amen to THAT.
--
Dave

[Paul Oxley]

Marcus & David   Don't forget that the "black" body radiation will still be present from the screen. During the day, the ambient air temperature is usually higher than the ground temperature. The small mass in the screen will very quickly go to the ambient air temperature. Thus you likely will have more impact from noise. The reflection from the sky is very small when compared with the black body radiation value.   In winter, when the ground temperature is usually higher than the ambient air temperature, you might see a slight improvement.   Paul

--- On Fri, 5/17/13, Marcus D. Leech <patchv...@gmail.com> wrote:

From: Marcus D. Leech <patchv...@gmail.com>

[David Fields]

Paul,
Wouldn't the black-body radiation from the mesh be lower than ground by the ratio of mesh to ground microwave emissivities, perhaps a factor of e3?

Thanks,
David

-----Original Message-----
From: Paul Oxley <oxl...@att.net>
To: sara-list <sara...@googlegroups.com>

[Marcus D. Leech]

Paul,
Wouldn't the black-body radiation from the mesh be lower than ground by the ratio of mesh to ground microwave emissivities, perhaps a factor of e3?

Thanks,
David

That's what I'm thinking.

If it is true that such a surface "looks" like a blackbody at near ambient, then dish reflectors wouldn't work very well, mostly throwing their
  own blackbody emissions into the feed.  They clearly don't do that.

[Paul Oxley]

David   Don't forget that a good absorber is also a good radiator. That is why the term "black" is used since at light frequencies, the color black absorbs the most light.  The question is the temperature of the body. It also is a question of the relative strength of the signals. The sky is at a very low temperature. The ground and mesh are many times higher.

Paul --- On Fri, 5/17/13, Marcus D. Leech <patchv...@gmail.com> wrote:

From: Marcus D. Leech <patchv...@gmail.com> Subject: Re: [SARA] Results from this morning To: sara...@googlegroups.com

[David Fields]

Paul,
Good point, the sky to ground black body radiation probably scales as (temperature ratio)**4.  But here, we're trying to replace the ground by a smooth conductor, and the ratio of emissivities suggests that we could diminish the ground noise by a factor of 1000.

I hope that Marcus tries this experiment.

Cheers,
David

-----Original Message-----
From: Paul Oxley <oxl...@att.net>
To: sara-list <sara...@googlegroups.com>

[Marcus D. Leech]

Paul,
Good point, the sky to ground black body radiation probably scales as (temperature ratio)**4.  But here, we're trying to replace the ground by a smooth conductor, and the ratio of emissivities suggests that we could diminish the ground noise by a factor of 1000.

I hope that Marcus tries this experiment.ground noise dish skirt

Cheers,
David

The usual way of dealing with this is to engineer the feed to have an "edge taper" of at *least* 10dB, and usually 12-15dB for radio telescope feeds.
  For a large dish it makes sense to have all of your surface structure contribute to reflecting into the feed, rather than having excess structure
  blocking ground noise.  Many larger radio telescopes still have residual ground-noise of 10 to 20K, which is consisent with a engineered feed taper.

The problem is that for a shallow offset-fed dish, constructing a feed with suitable edge-taper is very challenging at 21cm.  I think I've gone as far
  as I can go in terms of feed pattern with the materials and time I have at hand.

The Ku-band feeds that are normally used with this type of dish have a conical-chapparal type arrangement to reduce sidelobes, and give
  the required narrow-angle feed pattern.  Replicating that for L-band, with cheap materials, is quite a bit of work.

So, back to the "skirt" idea.    If a smooth, conducting surface emitted significant blackbody radiation, then waveguide would completely suck
  compared to coax.   It doesn't.  It's very often used in radio astronomy receivers.  Similarly, if your feedhorn surface was emitting significant
  blackbody radiation, your receiver would see a lot of unwanted noise just from the feedhorn itself.  It doesn't.  Good conductors have low
  microwave emissivity, which is why waveguides work so well, and feedhorns, and etc, etc, etc.

[Jim Abshier]

Folks,

One way of looking at the reflector is to treat it as if it is a
feedline. The noise temperature generated by a lossy feedline is:

T = (1 - 1/L) Tf

where L is the loss (unity or greater) and Tf is the actual temperature
of the feedline. If the feedline is lossless (L=1), it won't matter
what Tf is, T will be zero. For a good reflector, L will be close to
unity. The same sort of approach can be used to determine atmospheric
loss temperature.

Jim Abshier

[Cameron Rout]

Marcus. Literally laughed out loud at the meth joke. I might have to steal that.

Cameron Rout, P. Eng.

Innovation Manager

Fidelity Machine and Mould Solutions

Cell: 1-403-689-9767

Sent from my mobile device, please excuse typos and irregular formatting for the sake of brevity.