Forum Members, This is a simple question with a long intro. The question mainly concerns the use of coax at frequencies higher than what it was designed for. What effects are to be expected?
I just completed checking an antenna factor calibration per SAE ARP-958. The antenna separation was one meter. The antenna was a microwave log-conical or log-spiral used from 1 - 10 GHz. Measurements were made at every GHz. Per ARP-958, two identical antennas are set up one meter apart with one driven and the other connected to a receiver. A reading is taken, then the cables are disconnected from the antennas and connected together. The antenna factor can be determined from the difference between the straight through reading and the reading through the antennas. This measurement is pure substitution, it does not rely on traceability to any standard. It does assume the two antennas are identical. I used two similar but slightly different set-ups. Below 5 GHz, I used RG-214 cables, and lots of it. The antennas were about five feet above the ground (grass) and the separation of the antennas from anything above ground was over two meters. The signal source and receivers were directly behind each antenna and close to the ground at at least two meters separation. The antennas have about 2 dBi gain looking forward. With this set-up, I got excellent agreement with manufacturer's generic antenna factors published in their catalog. All measurements were within 1 dB, and only one measurement was close to 1 dB off. But there was too much loss in the RG-214 to go above 6 GHz, and at 6 GHz the error was 1.1 dB (not bad, but the trend was increasing error, which I attributed to lack of measurement dynamic range). So I changed out the RG-214 for lower loss cable, and shorter cables. This necessitated lowering the antennas to about four feet above the ground, and bringing the signal source and receiver within about one meter of their respective antennas, still directly behind the antennas. It also necessitated bring the antennas closer to the concrete driveway, which paralleled the axis between the antennas, but still over one meter away. Some of the low loss cable was good to 5-6 GHz, but some was only good to 3-4 GHz. This cable was very large diameter and stiff with 7/16" DIN connectors. I used the smaller diameter lighter cable to connect to the antennas, and the heavier stuff hung off that, with the connection between the two types of coax near the ground to minimize moment-arm torque on the antennas. The vswr rating is excellent on these antennas below 5 GHz, so I didn't pad the antennas at all when using the RG-214, on account of the RG-214 was all the pad I could stand. But the vswr increases some at higher frequencies, so with the low loss coax I placed a 20 dB pad at the transmit antenna only. The pad was good to 18 GHz. With this set-up, I had plenty of signal to work with and easily made measurements to 10 GHz. The measurement at 6 GHz yielded an antenna factor 0.5 dB closer to manufacturer's specification, so I thought I was on my way. But the errors grew with increasing frequency. The worst was 2.5 dB off at 10 GHz. Now that isn't at all bad - I would have been happy with that if the other errors were random. But they aren't - the errors increase with increasing frequency and the antenna factor is always larger than it is supposed to be. The point of all this information is to show the controls that were in place and see if anyone reading this can find any source of uncontrolled error that would have been systematic, not random. The only thing I could think of that was not perfect was the antennas were closer to the ground for the high frequency portion of the testing then I would have liked and the cables were not spec'd to 10 GHz. But I would think both of those would have contributed errors that did not increase monotonically with increasing frequency. Any suggestions? Thanks in advance, Ken Javor - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas [email protected] Mike Cantwell [email protected] For policy questions, send mail to: Jim Bacher: [email protected] David Heald: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc ______________________________________________________________________ This email has been scanned by the MessageLabs Email Security System. For more information please visit http://www.messagelabs.com/email ______________________________________________________________________
