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#1
July 29th 07, 12:57 PM
posted to rec.radio.amateur.antenna
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Request EZNEC computation
Owen Duffy wrote: Ed wrote in . 192.196: but I am at a loss as to what additional information one would need to model a center fed 75 foot long half wave antenna, resosnant at 6.2 MHz, positioned 15 feet above a flat metal roof, fed with 100 feet of 50 ohm coax (RG8), and operated on any one of three frequencies.... 3.95MHz, 5.4MHz, and 7.2 MHz. Small correction.... the feedline is also not important since all I am asking for is essentially the feedpoint impedance of this antenna at those three frequencies of operation. If the metal roof was infinite (or very large compared to the dipole), the feedpoint Z looks to be around 3-j711 at 3.95MHz. (This is probably not a good model of your scenario though.) Is that all you really need to assess the outcome? 100' of RG8 with such a load will lose 20.6dB. The ATU will see around 4-j55, so you may well be losing 3dB Radiator loss (with with you seemed concerned) depends on the wire (which you haven't told us about), and it might be a dB with such a low feedpoint R. The coax loss dominates the problem. There is a lot more to quantifying the problem, and designing a solution than knowing feedpoint Z at three spot frequencies for a dipole over an inadequately specified metal roof. However, we do know that a coax fed centre fed dipole is not an efficient multi-frequency antenna if the coax is of significant length (meaning basically more than nearly zero). Owen |
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