Your explanation is rather lengthy and a bit hard to follow so I will only generalize a bit.
First you might want to look up the wave guide effect in coax. Coax has upper limits because it changes modes from coax and starts behaving like waveguide. I don't recall how sudden the effect is. Second you talk about coax but you don't mention connectors. They have a cut off too. It's usually lower than coax! I would be working with SMAs if I was going to 10 GHz. RG-214 is 7.3 db/100' at a GHz and 31.6 db/100' at 10 GHz. That's a 25 db slope! I'd say a slippery slope at that. Third when you eliminated the antennas, did you simply join the coax using a 'barrel' connector. Barrels as all connectors have significant VSWR at 10 GHz. I think I would replace the two coax with one equal to the two combined lengths. Fred Townsend DC to Light Ken Javor wrote: >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 message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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