As much as my inner physicist would like to build the impedance match good to one part in 10^5 or 10^6 - Aaron EW's experience with calibrating out the MWA cable reflections at low frequencies makes me think the reality is we *have* to get (all - not just cable) reflections out of the cosmological region of k-space.
Jackie ---------------------------------------------------------------------------------------- Jacqueline N Hewitt Professor of Physics Director, MIT Kavli Institute for Astrophysics and Space Research [email protected] 617-253-3071 (Phone) 617-253-3111 (FAX) ---------------------------------------------------------------------------------------- On Apr 13, 2016, at 2:46 AM, Miguel F Morales <[email protected]> wrote: > I’ll chime in here on the other side. As Nichole showed in her last paper, > unless you can get the cable reflection below -50 dB it will remove the > associated mode (there is no way to calibrate it accurately enough). So > unless you are really good at building cable connectors... > > M > > > > >> On Apr 12, 2016, at 9:29 PM, Aaron Parsons <[email protected]> wrote: >> >> I'm with Nima on this. Better to spend time getting the termination correct >> than jiggering with cable lengths. >> >> On Tue, Apr 12, 2016 at 3:14 PM, Nima Razavi <[email protected]> wrote: >> Guys, I'm not sure we need to go down that path.... You can actually design >> a well matched system and use decent cables. I've been talking to a few >> cable manufacturers about an LMR200 equivalent 50ohm cable ~35m length. On >> the FE and RX side, I was looking at 3-cable combo in a single sheath: 2x >> LMR200 + 1x cat7 for the upfront phase switch/noise source/temp sensor >> control. >> >> Nima >> >> >> On 12/04/2016 22:49, danny jacobs wrote: >>> Can one can get any cheaper than we have right now on PAPER? >>> >>> On Tue, Apr 12, 2016 at 2:45 PM, Chris Carilli <[email protected]> wrote: >>> solution: buy cheap cables (ie. cable with a lot of intrinsic loss). >>> attenuation in the cable cuts down on the return loss dramatically (3 >>> passes down the cable from the FE instead of 1, and attenuation multiplies >>> exponentially. or something like that). >>> >>> you just have to crank up the power into the cable at the FE. or add some >>> gain at the receiverators. >>> >>> the VLA had a horrendous standing wave problem in the waveguide IF system >>> for years, until we put in attenuators. fixed. >>> >>> cc >>> >>> >>> >>> >>> >>> On 04/12/2016 03:32 PM, Aaron Michael Ewall-Wice wrote: >>>> I think it’s also worth considering that long cables may have >>>> sub-reflections in them at delays smaller than the cable length due to >>>> inhomogeneities in the dielectrics and bends. I think our reflectometry >>>> measurements suggested that sub-reflections may be at the ~-50dB level >>>> which could still be a problem for 21cm measurements. That said, we may >>>> gain from the fact that these sorts of sub-reflections should be >>>> uncorrelated between cables to different antenna elements. >>>> >>>> cheers, >>>> >>>> -Aaron >>>> >>>> >>>> >>>> >>>> >>>>> On Apr 12, 2016, at 5:25 PM, DAVID DEBOER <[email protected]> wrote: >>>>> >>>>> Let’s put this on the agenda for tomorrow. We could keep as is, except >>>>> put in 25m everywhere we can (so have 25m and 35m)… >>>>> >>>>> Dave >>>>> >>>>>> On Apr 12, 2016, at 2:22 PM, Nithyanandan Thyagarajan >>>>>> <[email protected]> wrote: >>>>>> >>>>>> Part of me favors having one cables of one kind (it this is an option) >>>>>> than two - better to lose one mode than two besides other complexities >>>>>> which we may not fully grasp yet that come with multiple cable lengths. >>>>>> >>>>>> From: <[email protected]> on behalf of DAVID DEBOER >>>>>> <[email protected]> >>>>>> Date: Tuesday, April 12, 2016 at 1:54 PM >>>>>> To: danny jacobs <[email protected]> >>>>>> Cc: hera <[email protected]> >>>>>> Subject: Re: cable lengths >>>>>> >>>>>> I was going to suggest the same thing. We could have an ‘active’ long >>>>>> cable (i.e. a pre-post-amp right at the balun). And how long do we need >>>>>> — 150m is already uncomfortably long… >>>>>> >>>>>>> On Apr 12, 2016, at 1:50 PM, danny jacobs <[email protected]> >>>>>>> wrote: >>>>>>> >>>>>>> What about making most of them short and then some of them really long? >>>>>>> >>>>>>> On Tue, Apr 12, 2016 at 1:49 PM, Adam Beardsley >>>>>>> <[email protected]> wrote: >>>>>>>> My concern would be pushing any cable reflection contamination to >>>>>>>> modes in the window. Currently only a small subset of MWA antennas >>>>>>>> have 150 meter cables, but the reflection line shows up very clearly. >>>>>>>> 45 meters puts the delay right at 0.2 h/Mpc, which is where we do a >>>>>>>> lot of our sensitivity forecasts. I doubt moving to 0.3 would change >>>>>>>> sensitivity much, but just food for thought. >>>>>>>> >>>>>>>> -Adam >>>>>>>> >>>>>>>> On Tue, Apr 12, 2016 at 1:26 PM danny jacobs <[email protected]> >>>>>>>> wrote: >>>>>>>>> The 35m length has been worrying me a little lately. Would there be >>>>>>>>> any downsides like having a different spectral response for the >>>>>>>>> longer cables? >>>>>>>>> >>>>>>>>> ~D >>>>>>>>> >>>>>>>>> On Tue, Apr 12, 2016 at 1:24 PM, DAVID DEBOER <[email protected]> >>>>>>>>> wrote: >>>>>>>>>> Hi - I was thinking about the analog cable lengths a bit. We have a >>>>>>>>>> spec at 35 m (30m+the vertical part to the feed). This puts the >>>>>>>>>> delay at about 0.15h/Mpc. Would it be advantageous to have, say, >>>>>>>>>> most of the cable lengths at 25m and a smaller subset at 45m? >>>>>>>>>> >>>>>>>>>> Dave >>>>>>>>> >>>>>>>>> >>>>>>>>> >>>>>>>>> -- >>>>>>>>> ================================================================ >>>>>>>>> Daniel C. Jacobs >>>>>>>>> KE7DHQ >>>>>>>>> National Science Foundation Fellow >>>>>>>>> Arizona State University >>>>>>>>> School of Earth and Space Exploration >>>>>>>>> Low Frequency Cosmology >>>>>>>>> Phone: (505) 500 4521 >>>>>>>>> Homepage: http://loco.lab.asu.edu/danny_jacobs/ >>>>>>>>> MWA: mwatelescope.org >>>>>>>>> HERA: reionization.org >>>>>>>>> PAPER: eor.berkeley.edu >>>>>>> >>>>>>> >>>>>>> >>>>>>> -- >>>>>>> ================================================================ >>>>>>> Daniel C. Jacobs >>>>>>> KE7DHQ >>>>>>> National Science Foundation Fellow >>>>>>> Arizona State University >>>>>>> School of Earth and Space Exploration >>>>>>> Low Frequency Cosmology >>>>>>> Phone: (505) 500 4521 >>>>>>> Homepage: http://loco.lab.asu.edu/danny_jacobs/ >>>>>>> MWA: mwatelescope.org >>>>>>> HERA: reionization.org >>>>>>> PAPER: eor.berkeley.edu >>>>>> >>>>> >>>> >>> >>> >>> >>> >>> -- >>> ================================================================ >>> Daniel C. Jacobs >>> KE7DHQ >>> National Science Foundation Fellow >>> Arizona State University >>> School of Earth and Space Exploration >>> Low Frequency Cosmology >>> Phone: (505) 500 4521 >>> Homepage: http://loco.lab.asu.edu/danny_jacobs/ >>> MWA: mwatelescope.org >>> HERA: reionization.org >>> PAPER: eor.berkeley.edu >> >> -- >> Dr Nima Razavi-Ghods >> Senior Research Associate >> >> Cavendish Astrophysics >> University of Cambridge >> JJ Thomson Avenue >> Cambridge CB3 0HE >> >> Tel: +44 (0)1223 766763 >> Fax: +44 (0)1223 337563 >> >> >> >> -- >> Aaron Parsons >> >> 510-406-4322 (cell) >> Campbell Hall 523, UCB >
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