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> They inspired me to find out that I have perfect SWR dip at 13.450 MHz. Great. This simply means that the antenna is too long, and your impedance has not been severly impacted by the tower. >And > I did not know that one was 'allowed' to change the lenght of the T-bars. So > now I want the SWR moved up - I shorten the T-bars, right? Yes, you can tune the antenna with the T-bars. When you tune the antenna, you should try and keep all bars the same length. For example, make sure they start at a common length, and then adjust a few CM each way as needed. At a miminum, you should keep the bars on each T the same length (i.e. keep both the bottom bars the same length, or the top ones the same length). If the T-bars are all random lengths, it's hard to predict what each change will do. You might also get a funny VSWR dip. > I think the effect of the tower and 4 x 6 elements 2 meters antennas are > small, because turning the array does not change the SWR on the Sigma. Do > you think I can conclude so from this observation? If it were only the VHF antennas that were interacting, I think you would probably see a VSWR change. If the entire structure is resonant (antennas + tower), I would tend to doubt rotating the "top hat" would matter. But I don't know for sure. > So untill further notice I conclude that I just have to shorten the T-bars. Yes, shorter T-bars will raise the frequency. Longer ones lower the frequency. Just loosen up the clamps and slide the T-bar in or out! When tuning the antenna on 30 or 40, usually the best thing to do is to tune the antenna by spreading or compressing the coils. This also assumes you have coils that are variable... when we go on DXpeditions, we usually make coils that are not fixed. Some of the earlier designs had coils mounted on a PVC core, where it was easier to adjust the T-bars and leave the coil fixed. I think that has been changed on recent production units, but I am not sure. 73 and Happy DXing! You will hear some Sigma antennas on 20 and 40m in about 2 weeks when we head to Jamaica, probably for the last time :-( Kenny -------------------------------------------- Force12Talk mailing list provided as a service by Force 12 Antennas, Inc. Force12 Web Site: http://www.force12inc.com To Submit Message to the List: Force12Talk@qth.com To unsubscribe and view the Message Archive: see http://qth.com/force12/list For problems with the list: contact force12@qth.com |
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