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