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this is Martin, dl4nac. together with dl8oh i am using a c31xr model 'h' since september 1998. we have two good things to report and still one little problem. here are the good news: the antenna is mechanically very solid. our location is on a hilltop (600m asl) with heavy storms and often big ice-loads. during the last two winters several hams lost their antennas here in northern bavaria. however we had no trouble with our c31xr. the performance is quite good on all three bands. we dont have a second antenna at the same time to do some real compares. the feeling however is, that the performance is nearly the same as the big quad we had used before (20m 3 ele, 15m 4 ele, 10m 4ele on 8m boom). we are very happy with this antenna. but one problem we havent solved yet: while 15 m and 20 m are ok, the vswr-behaviour on 10 meters (values and frequency of best vswr) depends very much on length of coax-cable. the vswr is not as broadband as expected. the frequency of best vswr does not move in a linear way with changing the length of the 10m driven element. the antenna is mounted 15m above ground (relatively poor ground / mostly rocks / moderate to steep sloping into all directions). 1.5 m above the c31xr is a small vhf-yagi and 3.5 m above the c31xr is a cushcraft 40-2cd 2 ele 40m yagi. all antennas are pointing into the same direction. 2 m below the c31xr there is a little platform which is a 1.6 m by 1.6 m square. the handrail of the platform is about 1 m below the c31xr. i attached a photo of the antennas as a jpg-file. the coax is about 20 to 25 meters long, H100 type 50 ohms. what we have tried by now: - changing from 1 to 3 feedlines: no significant difference - lifting the c31xr approximately 0.75 meter upward to the position it has now: no significant difference - changing from a rf choke to a force12 1:1-balun: no significant difference - simulating the antenna with eznec 2.0 with and without the platform: no significant change in impedance - terminating the 15m and 20m feedlines: open / 50 ohms / closed: big influence to 10m-vswr-behaviour here are some questions. any answers and comments are welcome: - does anybody have the same effects with his c31xr? - what do you think could be the reason for this vswr-behaviour on 10m? - are our simulation results identical to simulations done by others? the 10m-results of the simulation (done by dl6nbc) are attachad as a doc-file - what else would you test? in the attached xls-file the interested reader can find some of the results of our impedance- and vswr-measurements done with a mfj259b, always tuning between 27.5 and 29.4 MHz: the measured values of 12 different situations are recorded in 'measurement values'. '1 a' to '5' show graphs over frequency for the first 5 situations. number 1 to 3 are measurements done in the shack, each time with 3 different cable-lengths (a/b/c). number 1 with rf-choke, 2 with force12-balun, 3 with force 12 balun and additional balun close to mfj. number 4 is in the shack with force12 balun at the feeding point using different terminations of the 15 m and 20m feedlines. number 5 is measured directly at the 10 m feeding point. number 6 to 12 are points of best vswr, measured directly at the feeding point with some changes in termination of 15 m and 20 m feedlines and different lengths of the 10 m driven element. many thanks for your mails in advance 73s de Martin, DL4NAC@QSL.NET |
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