HI Ken, excuse my nievity here, but I need help understanding this.
A cable is only its characteristic impedance when fed and terminated in matching impedance. That's why I said non-perfect impedance of the cable. So, since the antenna output impedance is all over the place, you can't say that the cable is 50 Ohms. To get around this, we add pads on antennas ( like when we do NSA measurements ) to improve the match between antenna and cable. I would argue that the receiver head, with a pretty good termination right there, is much better than the uncontrolled match between the antenna and Zo of the cable. What am I missing? Cheers, Derek. -----Original Message----- From: Ken Javor <ken.ja...@emccompliance.com> To: EMC-PSTC <EMC-PSTC@LISTSERV.IEEE.ORG> Sent: Wed, Aug 31, 2011 12:15 pm Subject: Re: EMI Receiver - Recommendations? Expansion on this: “Less issues matching a complex source impedance like and antenna to the non-perfect impedance of a cable.” Cable is usually 50 Ohms. It is the antenna that can have a relatively high vswr into 50 Ohms, especially at the low end of use (i.e., biconical below 80 MHz, and in general wideband antennas at their low end). If the rf head has a 50 Ohm output impedance, then the match is better than the antenna alone, and the measurement accuracy increases due to lower vswr. Ken Javor Phone: (256) 650-5261 ________________________________ From: Derek Walton <lfresea...@aol.com> List-Post: emc-pstc@listserv.ieee.org Date: Wed, 31 Aug 2011 12:56:39 -0400 (EDT) To: <j...@jmwa.demon.co.uk>, <EMC-PSTC@LISTSERV.IEEE.ORG> Subject: Re: EMI Receiver - Recommendations? All good points. you could add that the antenna would better represent free space..... I wonder what the effect of Cal factors would be? Less issues matching a complex source impedance like and antenna to the non-perfect impedance of a cable. Elimination of temperature effects of the cable dielectric. I'm sure there are more. Cheers, Derek. -----Original Message----- From: John Woodgate <j...@jmwa.demon.co.uk> To: EMC-PSTC <EMC-PSTC@LISTSERV.IEEE.ORG> Sent: Wed, Aug 31, 2011 11:10 am Subject: Re: EMI Receiver - Recommendations? In message <40be5b4eca254ad7bae8a4d5ef896...@tamuracorp.com>, dated Wed, 31 Aug 2011, Brian Oconnell <oconne...@tamuracorp.com> writes: >Why is it good that the 'RF head' is separate and/or can be attached to >the antenna? No cable loss? -- OOO - Own Opinions Only. Try www.jmwa.demon.co.uk <http://www.jmwa.demon.co.uk> <http://www.jmwa.demon.co.uk> and www.isce.org.uk <http://www.isce.org.uk> <http://www.isce.org.uk> John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK When I point to a star, please look at the star, not my finger. The star will be more interesting. - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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