VSRT MEMO #068 MASSACHUSETTS INSTITUTE OF TECHNOLOGY HAYSTACK OBSERVATORY WESTFORD, MASSACHUSETTS 01886 May 3, 2010 Telephone: 781-981-5407 Fax: 781-981-0590 To: VSRT Group From: Alan E.E. Rogers Subject: Measurements of the fraction of linear polarization With a small amount of additional hardware it is possible to form an interferometer in which one element is sensitive to left circular polarization and the other is sensitive to right circular polarization. The “cross-polarization” interferometer is sensitive to linear polarization and has no response to unpolarized is the LNB feeds are perfectly circular. Figure 1 shows the VSRT configuration and highlights the added hardware needed to be able to switch from “co-pol” to “cross-pol”. The following results were obtained using a GE “Biax” CFL (see memo #6): Orientation of CFL Co-pol Cross-pol CFL axis towards 210 11 intereferometer CFL axis perpendicular to 92 45 The cross-pol response of the “end view” is presumably the result of imperfect circular feeds in the Direct TV feeds since by symmetry the linear polarization fraction should be zero. The approximately 2:1 ratio between the co-pol and cross-pol looking at the “side-view” of the CFL corresponding to a linear polarization fraction of about 50%. The statement in VSRT memo #38 that all the microwave radiation comes from collisions will the walls may not be quite correct as some may comes from collisions in the plasma as the mean free path is estimated to be only about 1 cm (for a nominal 1 mm Hg pressure.) The detailed physics of the microwave radiation from CFLs needs further investigation. 1 Compact Fluorescent :433Lo~U2-FI or equiv Lamp inpubl rrom additional Notest 1] LNE!achange polarlmationat 16v (1Sv=ROP:17v=WP) ,-----------7 (JWe,; 22 ktm signal to change feed (Di$EqCstd,) Radio Shack 16-1170 Ddpower Dd-pa1111ISO-pwr Injector 1-------,-, 11plltter Channel ~ion H$-4 dAE-9220 change voltage to change polarizatfon Adjui;able laboratory supply lnllne amplifieni 600 - 1600 ~Hz __________ .....J Digikey d 1 low orfl!et Schottky Additional hardware Radio phack 16- 1'70 1N6263detector dlodeli Nominal D.d.output (ln,iide) needed to ~witch DC power from R.F. detector Injector --------------1-----i >--~----, .., 0,S V polariza lion 100 pr ~ RadioShack Radio ~hack Ad +16v DC 1000 ma Optional High Speed V$RT voltmeter for _.....__--1 U!JB2.0 video use2.0 PC ..._____ __,1"6404 "cross-pol" "11lngledlsh• grabber Wlndowii diodes teslll ~ 7113 !caste Java ~B to drop E 2620 Interferometer 1.6v vsrtblock_xpoLdwqaeer 21Aprl0 Figure 1. Block diagram of set-up to change polarization of one of the LNBFs. 2.