Why not take the opposite approach, and select the radiating frequencies from the EUT. Place antennas near the EUT, and select only those frequencies that are most likely coming from the unit. > -----Original Message----- > From: [email protected] [SMTP:[email protected]] > Sent: Friday, May 07, 1999 2:53 PM > To: Brent DeWitt > Cc: Cortland Richmond; Aschenberg, Mat; ieee pstc list > Subject: Re: Phase cancellation > > Brent has a good point. I propose that this could be overcome by using a > communications grade antenna - not just another EMC antenna, and choosing > one > for gain and directivity. > > I'd suggest using a corner reflector, or a corner/yagi hybrid, giving good > gain > and directivity. It's something that can be designed and constructed > fairly > easily, or purchased at the local ham radio swap-fest. These antennas > will have > severely limited bandwidth much like tuned dipoles. They are only > practicable > to build or handle for around 100 MHz or higher. This scheme is feasible > if you > have only a few ambients of concern, or the ambients are closely spaced > (within > 10 MHz). Otherwise you'd need to build seperate antennas if your ambients > are > spaced too far apart, making this solution too troublesome. > > A stepped attenuator will be needed to keep the extra antenna from > dominating > the measurement. > > I'd only connect the phase cancelling antennas as a 2nd step after first > measuring all other emissions in the normal fashion without the phase > cancelling > antenna. These antennas would create some interesting effects on VSWR > that will > drastically affect your measurements as you depart from the design center > frequency. Setting the attenuator to maximum should also hide the VSWR > effects. > Then you'd have the uncertainty introduced by the combiner network.... > > Regards, > Eric Lifsey > Compliance Manager > National Instruments > > > > > Please respond to "Brent DeWitt" <[email protected]> > > To: "Cortland Richmond" <[email protected]>, "Aschenberg, Mat" > <[email protected]>, "ieee pstc list" <[email protected]> > cc: (bcc: Eric Lifsey/AUS/NIC) > Subject: Phase cancellation > > > > > I have also played with phase cancellation, and found one very serious > limitation, multipath. Unless you are out in a situation where the > ambients > look pretty much like point sources, you will be limited in the depth of > the > null that you can create, since you can only cancel one phase front with > one > "reference" antenna. Since many folks build sites in the hills, mountains > or gullies to try to avoid ambients, this puts the site in a worst case > location for using phase cancellation. > > Maybe in Topeka......... > > Best regards, > > Brent DeWitt > > >
--------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to [email protected] with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to [email protected], [email protected], [email protected], or [email protected] (the list administrators).
