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]
<mailto:[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/
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/
MWA: mwatelescope.org <http://mwatelescope.org/>
HERA: reionization.org <http://reionization.org/>
PAPER: eor.berkeley.edu <http://eor.berkeley.edu/>
