>-----Original Message----- >From: richwo...@tycoint.com [mailto:richwo...@tycoint.com] >Sent: Wednesday, November 13, 2002 6:00 AM >To: emc-p...@majordomo.ieee.org >Subject: Radiated immunity testing above 1 GHz > > > >I need to make a decision on an amplifier for radiated >immunity testing in >the 1-2 GHz range. The choices appear to be a new solid state >amplifier ( I >have not located a used one) or a used TWT amp. Is anyone using a TWT >amplifier for radiated immunity testing above 1 GHz? Do I need to be >concerned about protecting the tube from damage caused by >excessive VSWR >cause by, perhaps, chamber effects? Is there anything else I >need to know >about TWT amps (yeah, I know - throw it way if the tube dies)? > >Richard Woods >Sensormatic Electronics >Tyco International >
Rich: A 1 Watt TWT octave-band amplifier is fairly common, and I have seen them sell on eBay for a few hundred dollars. But 1 Watt (depending on antenna performance & system losses) just barely gets you in to the lowest end of immunity testing. A 10 Watt amplifier might cost under $7k new, and would be a rare find at an auction or from an inventory reduction at an equipment rental company. But 10 Watts will let you do most commercial immunity work. A 20 Watt amplifier takes the "edge" off of trying to get the fields, and also lets you do military work (with some patience). I use a set of Hughes 8050 20 Watt amps that cover 1 GHz to 18 GHz. In a previous life, I had the luxury of 200 and 300 Watt TWT amplifiers. They were the Varian VZ series, and I seem to recall they cost around $30k each (in 1980 dollars). I don't know if anyone even makes new equipment like that any more; maybe you can't even replace the TWT's in them. Whatever you get, try to be sure that the amps have circulator protection. TWT's don't like reflected power, and immunity setups almost guarantee ranges of lousy VSWR. The circulator routes that reflected power away from the TWT and into a termination. Most lab amplifiers will either come with a circulator as standard, or have it as a factory option. If you get an amp that doesn't have an internal circulator, endeavor to get an external one ASAP. Re TWT amp operation. The tube makes up maybe 80% of the amplifier cost. Every time I flip on the TWT amp switch, my finger twitches a little. TWT amps don't seem to ever die while in use; rather, they die just as you start them up. I'm very cautious about protecting the amp from physical shock, and I always let the main power stay on for about 5 minutes after RF operation to let the TWT cool down as much as possible. One further suggestion; place the TWT amp as close as possible to the antenna. Since a TWT amp will have about 40 dB of gain, you can usually afford to lose a few dB in a long coax between the signal source and the amp, and still be able to drive the TWT to its limit. This arrangement will but maximum power to your antenna. All of the TWT amps I have ever used would withstand whatever RF was inside the shielded room, maybe since they were relatively "dumb" amplifiers. You might have a problem with this technique if your TWT amp has a microprocessor controller or some other fancy features. And, even for the 1 GHz to 2 GHz range, don't use even good coax, like RG-214. Get either semi-rigid solid wall coax, or get the superb flexible stuff from Gore or Storm. Finally, make sure that your leveling pre-amplifier will control the entire frequency range you are going to use. Some systems (like my IFI LPA-5) might require an optional variable-gain amplifier deck for frequencies above 1 GHz. (Maybe this isn't an issue for you, if you are controlling level by computer control of the signal source amplitude.) Regards, Ed Ed Price ed.pr...@cubic.com Electromagnetic Compatibility Lab Cubic Defense Systems San Diego, CA USA 858-505-2780 (Voice) 858-505-1583 (Fax) Military & Avionics EMC Is Our Specialty Shake-Bake-Shock - Metrology - Reliability Analysis ------------------------------------------- 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: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: davehe...@attbi.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list"
