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] <mailto:[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] >> <mailto:[email protected]>> on behalf of DAVID DEBOER >> <[email protected] <mailto:[email protected]>> >> Date: Tuesday, April 12, 2016 at 1:54 PM >> To: danny jacobs <[email protected] <mailto:[email protected]>> >> Cc: hera <[email protected] <mailto:[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] >>> <mailto:[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] >>> <mailto:[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] >>>> <mailto:[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] >>>>> <mailto:[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 <tel:%28505%29%20500%204521> >>>>> Homepage: http://loco.lab.asu.edu/danny_jacobs/ >>>>> <http://loco.lab.asu.edu/danny_jacobs/> >>>>> MWA: mwatelescope.org <http://mwatelescope.org/> >>>>> HERA: reionization.org <http://reionization.org/> >>>>> PAPER: eor.berkeley.edu <http://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/ >>> <http://loco.lab.asu.edu/danny_jacobs/> >>> MWA: mwatelescope.org <http://mwatelescope.org/> >>> HERA: reionization.org <http://reionization.org/> >>> PAPER: eor.berkeley.edu <http://eor.berkeley.edu/> >
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