I have used the same antenna calibration lab for over ten years, and they always send me a full frequency range package of VSWR data as well as the Gain and Antenna Factor data. I had never really thought about the usefulness of the VSWR data until I talked with Ken a few years ago about antenna calibrations.
Of course, measuring the VSWR on a typical horn antenna is complicated by reflections inside your lab, so, if I have a question, I usually go outside to the company parking lot and point my horn vertically into the sky. Maybe someday the Google satellite will catch me in the act. Ed Price [email protected] <blocked::mailto:[email protected]> WB6WSN NARTE Certified EMC Engineer Electromagnetic Compatibility Lab Cubic Defense Applications San Diego, CA USA 858-505-2780 Military & Avionics EMC Is Our Specialty From: Ken Javor [mailto:[email protected]] Sent: Tuesday, August 17, 2010 12:37 AM To: Derek Walton; Price, Edward Cc: [email protected] Subject: Re: Calibration supplier for signal generator with pulse modulation For some reason, the list server is rejecting the following message I posted in response to Ed’s: “One answer to that for the bigger horns is to do a visual, and for the smaller horns (all horns actually) a vswr check will suffice to indicate whether the coax to waveguide adapter is sound. Someone else pointed that out, and they are correct. I normally shy away from that sort of measurement at microwave frequencies, because it is touchy, but it is still more accurate than an ARP 958 or other radiated measurement.” Ken Javor Phone: (256) 650-5261 ________________________________ From: <[email protected]> List-Post: [email protected] List-Post: [email protected] List-Post: [email protected] Date: Mon, 16 Aug 2010 10:14:13 -0400 To: <[email protected]> Subject: Re: Calibration supplier for signal generator with pulse modulation He has a good point about adapters.... From: Price, Edward <[email protected]> To: [email protected] Sent: Mon, Aug 16, 2010 9:00 am Subject: RE: Calibration supplier for signal generator with pulse modulation From: [email protected] [mailto:[email protected] <mailto:[email protected]?> <mailto:[email protected]?> ] On Behalf Of Ken Javor Sent: Thursday, August 12, 2010 6:31 AM To: Untitled Subject: Re: Calibration supplier for signal generator with pulse modulation And similarly, not because of MU but because of 17025 or perhaps ISO 9000, I’ve seen test equipment that could easily have been calibrated in house, such as current probes, LISNs and a 41 inch rod antenna have to be sent to the calibration lab. This is totally unproductive, except for the calibration lab. And I would argue further that it is detrimental to the discipline, because if you do your own calibration, you understand better how things work. Oh, and my favorite, the calibration of microwave horn antennas, whose properties are entirely set by their linear dimensions. The measurement or verification that their dimensions have not changed since their original acquisition is so much more accurate than the direct measurement of their gain. In fact any passive antenna used in the EMI measurement business can be easily checked visually for dimensional abnormalities, and those with a balun, such as a dipole, biconical or logperiodic can be checked for vswr using a directional coupler; no need for expensive and time consuming annual or semi-annual trips to a calibration facility. If it isn’t physically damaged, and the vswr meets original specs, meaning the balun is good, the antenna is good. Ken Javor Phone: (256) 650-5261 Ken: I completely agree, except…. I have seen a few strange and nasty things happen with horn antennas. 1. Many horn antennas have a coupling pin in the throat of the waveguide to provide for coupling of the RF wave from the waveguide end of the antenna throat to the 50-Ohm coaxial system. This “pin” is often a precision machined and positioned extension of the center pin of a female type N coax connector. The efficiency and VSWR of the antenna is very sensitive to the positioning of that pin, but all is normally fine unless you do something dumb like poke something down the throat of the antenna. But consider what happens if you connect an improperly built coax cable to that antenna connector. In the case where the coax cable center pin extends too far out the end of the cable connector, you can wind up displacing the female connector center pin (and that critical launcher pin inside the antenna). 2. If you use that antenna for both transmit and receive, it’s possible for high RF power (and/or very bad reflections) to create arcing at that launcher pin. You can also soften the dielectric of the antenna connector. Both of these will lead to changes in the antenna efficiency. Sometimes, you may use a very basic horn antenna that ends with simply a waveguide flange. Not much can go wrong with the antenna, but almost everyone will use this antenna with a waveguide-to-coax adapter. Again, all the things I said above now apply to your adapter. I make a practice of sending my flanged horn antennas (at least the ones below 18 GHz) out for calibration with a permanently mated coax adapter mounted on them. That way, my cal data includes the adapter. A good object lesson hit me just a few weeks ago. When I sent my 12 GHz to 18 GHz flanged horn antenna out to my cal lab, I put a brand new female-female SMA adapter on the waveguide adapter (that adapter is odd; it has an SMA male coax connector). Later, my cal lab called me to ask how long I had been using that adapter. Turns out that the beautiful gold plated brand new adapter had a faulty center pin, and the VSWR and loss went completely wild (a bad resonance) at around 15 GHz. Ed Price [email protected] WB6WSN NARTE Certified EMC Engineer Electromagnetic Compatibility Lab Cubic Defense Applications San Diego, CA USA 858-505-2780 Military & Avionics EMC Is Our Specialty - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at http://www.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. 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