David -- you are correct -- the mismatch in the system and antenna factors still remain, regardless of the cable length....
If the antenna (or whatever load) is not 50 ohms at some frequency, and the source feeding the antenna (or whatever) is 50 ohms at that frequency, a mis-match will exist. Adding 50 ohm cable between the source and load won't change the mis-match, but depending on the cable length, may make the mis-match "look" better or worse from the source end. (Unless, of course, the cable is infinitely long in which case it will look very much better, but normally, the cable length acts as an impedance transformer.) For specific frequencies, one could use the cable as an impedance matching section between the source and load, but this is clearly unusable for broad-band applications. Mike Hopkins -----Original Message----- From: Ken Javor [mailto:[email protected]] Sent: Friday, November 16, 2001 1:05 PM To: Pommerenke, David; 'ravinder ajmani'; [email protected] Cc: [email protected] Subject: Re: Pre-amps Note I am replying to all. What is the measurement inaccuracy associated with a mismatch if the transmission line is vanishingly short (relative to a wavelength)? ---------- >From: "Pommerenke, David" <[email protected]> >To: "'Ken Javor'" <[email protected]>, "'Ravinder Ajmani'" <[email protected]>, [email protected] >Cc: [email protected] >Subject: RE: Pre-amps >Date: Fri, Nov 16, 2001, 8:10 AM > > Dear Ken, > > I am thinking you are missing something (hope that I am correct). > If you simply shorten the cable to zero, the problem of the missmatch, and > the effect of an error that is not corrected for by the antenna factor still > remains. > > David Pommerenke > > -----Original Message----- > From: Ken Javor [mailto:[email protected]] > Sent: Thursday, November 15, 2001 5:51 PM > To: Pommerenke, David; 'Ravinder Ajmani'; [email protected] > Cc: [email protected] > Subject: Re: Pre-amps > > > If input VSWR for preamp and output VSWR for antenna are both bad, then it > seems a simple solution is to connect preamp input to antenna output and let > 50 Ohm output of preamp drive cable, solving two problems at once. Am I > missing something? > > ---------- >>From: "Pommerenke, David" <[email protected]> >>To: "'Ravinder Ajmani'" <[email protected]>, [email protected] >>Cc: [email protected] >>Subject: RE: Pre-amps >>Date: Thu, Nov 15, 2001, 7:49 AM >> > >> >> On the amps: >> >> For emissions there are the following electrical criteria: >> >> - Noise figure expect about 4 dB for a broadband amp 30 MHz - 2 GHz. >> The noise figure is often larger at the lower frequencies if the >> amplifier goes up to many GHz. >> >> - Input SWR. This is important. Most broadband amps (especially if they > go >> up to many GHz) >> have a bad input match at low frequencies. As the log-per antennas have > a >> bad mismatch too, >> you will have multiple reflections on the cable between the antenna and >> the pre-amp. >> This reflections will influence your measurement and cannot be > corrected >> for by the antenna >> factor. They may be as large as a few dB below 100 MHz. For that > reason, >> you may be forced >> to add a 3 dB attenuatore at the antenna. This increases your noise >> figure by 3 dB. >> >> - Gain. Of course, you need only as much gain as is needed to overcome > the >> cable loss (cable to >> the spectrum analyzer) and the noise figure of the spectrum analyzer. >> More gain will not help >> you. >> >> David Pommerenke >> >> -----Original Message----- >> From: Ravinder Ajmani [mailto:[email protected]] >> Sent: Wednesday, November 14, 2001 1:27 PM >> To: [email protected] >> Cc: [email protected] >> Subject: Re: Pre-amps >> >> >> >> >> Joe, >> HP (Agilent) make good Pre-amps for different frequency ranges. If you > are >> looking for an economical solution then you may try Com Power Corp. at >> (949) 587-9800. >> >> Regards, Ravinder >> PCB Development and Design Department >> IBM Corporation >> Email: [email protected] >> > *************************************************************************** >> Always do right. This will gratify some people and astonish the rest. >> .... Mark Twain >> >> >> >> >> >> MartinJP@appliedbiosyst >> >> ems.com To: >> [email protected] >> Sent by: cc: >> >> owner-emc-pstc@majordom Subject: Pre-amps >> >> o.ieee.org >> >> >> >> >> >> 11/14/2001 10:14 AM >> >> Please respond to >> >> MartinJP >> >> >> >> >> >> >> >> >> >> I am having some difficulties locating manufacturers that provide preamps >> with a 20-22dB gain. >> >> What manufacturer/model do you recommend? Why? >> >> Your assistance is appreciated. >> >> Regards >> >> Joe Martin >> Applied Biosystems >> >> >> ------------------------------------------- >> This message is from the IEEE EMC Society Product Safety >> Technical Committee emc-pstc discussion list. >> >> Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ >> >> To cancel your subscription, send mail to: >> [email protected] >> with the single line: >> unsubscribe emc-pstc >> >> For help, send mail to the list administrators: >> Michael Garretson: [email protected] >> Dave Heald [email protected] >> >> For policy questions, send mail to: >> Richard Nute: [email protected] >> Jim Bacher: [email protected] >> >> All emc-pstc postings are archived and searchable on the web at: >> No longer online until our new server is brought online and the old >> messages are imported into the new server. >> >> >> >> >> >> ------------------------------------------- >> This message is from the IEEE EMC Society Product Safety >> Technical Committee emc-pstc discussion list. >> >> Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ >> >> To cancel your subscription, send mail to: >> [email protected] >> with the single line: >> unsubscribe emc-pstc >> >> For help, send mail to the list administrators: >> Michael Garretson: [email protected] >> Dave Heald [email protected] >> >> For policy questions, send mail to: >> Richard Nute: [email protected] >> Jim Bacher: [email protected] >> >> All emc-pstc postings are archived and searchable on the web at: >> No longer online until our new server is brought online and the old >> messages are imported into the new server. >> >> ------------------------------------------- >> This message is from the IEEE EMC Society Product Safety >> Technical Committee emc-pstc discussion list. >> >> Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ >> >> To cancel your subscription, send mail to: >> [email protected] >> with the single line: >> unsubscribe emc-pstc >> >> For help, send mail to the list administrators: >> Michael Garretson: [email protected] >> Dave Heald [email protected] >> >> For policy questions, send mail to: >> Richard Nute: [email protected] >> Jim Bacher: [email protected] >> >> All emc-pstc postings are archived and searchable on the web at: >> No longer online until our new server is brought online and the old >> messages are imported into the new server. >> > ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson: [email protected] Dave Heald [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: No longer online until our new server is brought online and the old messages are imported into the new server. ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson: [email protected] Dave Heald [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: No longer online until our new server is brought online and the old messages are imported into the new server.
