VLA/VHF Program Update 2005 Mar 11

Recent and Upcoming Events

Mar. 7		First two prototype antenna/receiver systems arrive at
		NRAO.  

Mar. 9		First prototype mounted on VLA antenna 6.  Start 24x7
		piggy-back monitor of RFI in the VHF band.

Mar. 10		First light - autocorrelation spectra obtained with the
		VLA correlator.  First RFI sweep to identify troublesome 
		sources.

Mar. 16		Second prototype to be mounted on a VLA antenna, with the 
		goal of obtaining first	fringes by week's end. 

Socorro Travel

Mar  7 - 11	Kimberk, Leiker, Greenhill
Mar 15 - 18	Kimberk, Blundell, Greenhill
Mar 22 - 25	Greenhill, TBD

Notes and Details

The strength and distribution of RFI received by the antenna depends on 
pointing and time of day.  As of Mar. 10, we have scanned the horizon and 
the sky close to zenith during the afternoon.  Narrow lines from TV are 
evident, as are quiet spots in the passband (185-205 MHz).  We also see an 
anomalous (sharp) hump, covering ~5 MHz, that corresponds to channel 9 but 
does not look like the transmission signal from any other visible station. 
Locating the source and understanding how it couples to the antenna sidelobes 
is a high priority. 

A 24x7 piggyback RFI monitor, using our receiver and a dedicated spectrum 
analyzer, is now running.  The data will be cross-correlated with antenna 
pointing (az,el) data.  Ultimately it will be necessary to make an RFI survey 
along a grid of positions on the sky.  Night time RFI observations are a 
priority since target sources are up at night during the winter.

Mitigation strategies for RFI may include notch filtering the signal at the 
antennas, establishing zones of avoidance on the sky, modification to the 
dipole feeds to reduce horizon response, and application of new imaging 
algorithms (prototyped by Cornwell) that permit simultaneous "mapping" of 
RFI and astronomical sources during self-calibration.

Measurements of efficiency and impact on other bands will begin in the coming week.